iOS Programming The Big Nerd Ranch Guide, 5th

WOW! eBook www.wowebook.org

iOSProgramming:TheBigNerdRanchGuidebyChristianKeurandAaronHillegass


Copyright©2015BigNerdRanch,LLC

Allrightsreserved.PrintedintheUnitedStatesofAmerica.Thispublicationisprotectedbycopyright,andpermissionmustbeobtainedfromthepublisherpriortoanyprohibitedreproduction,storageinaretrievalsystem,ortransmissioninanyformorbyanymeans,electronic,mechanical,photocopying,recording,orlikewise.Forinformationregardingpermissions,contact

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Theauthorsandpublisherhavetakencareinwritingandprintingthisbookbutmakenoexpressedorimpliedwarrantyofanykindandassumenoresponsibilityforerrorsoromissions.Noliabilityisassumedforincidentalorconsequentialdamagesinconnectionwithorarisingoutoftheuseoftheinformationorprogramscontainedherein.

AppStore,Apple,Cocoa,CocoaTouch,Finder,Instruments,iCloud,iPad,iPhone,iPod,iPodtouch,iTunes,Keychain,Mac,MacOS,Multi-Touch,Objective-C,OSX,Quartz,Retina,Safari,andXcodearetrademarksofApple,Inc.,registeredintheU.S.andothercountries.

Manyofthedesignationsusedbymanufacturersandsellerstodistinguishtheirproductsareclaimedastrademarks.Wherethosedesignationsappearinthisbook,andthepublisherwasawareofatrademarkclaim,thedesignationshavebeenprintedwithinitialcapitallettersorinallcapitals.

ISBN-100134389395

ISBN-13978-0134389394

Fifthedition,firstprinting,December2015ReleaseE.5.1.1


AcknowledgmentsWhileournamesappearonthecover,manypeoplehelpedmakethisbookareality.Wewouldliketotakethischancetothankthem.

FirstandforemostwewouldliketothankJoeConwayforhisworkontheearliereditionsofthisbook.Heauthoredthefirstthreeeditionsandcontributedgreatlytothefourtheditionaswell.Manyofthewordsinthisbookarestillhis,andforthat,weareverygrateful.

Afewpeopleinparticularwentaboveandbeyondwiththeirhelponthisbook.TheyareMikeyWard,JuanPabloClaude,andChrisMorris.

TheotherinstructorswhoteachtheiOSBootcampfeduswithanever-endingstreamofsuggestionsandcorrections.TheyareBenScheirman,BolotKerimbaev,BrianHardy,ChrisMorris,JJManton,JohnGallagher,JonathanBlocksom,JosephDixon,JuanPabloClaude,MarkDalrymple,MattBezark,MattMathias,MikeZornek,MikeyWard,PouriaAlmassi,RodStrougo,ScottRitchie,StepChristopher,ThomasWard,TJUsiyan,andTomHarrington.Theseinstructorswereoftenaidedbytheirstudentsinfindingbookerrata,somanythanksareduetoallthestudentswhoattendtheiOSBootcamp.

ThankstoalloftheemployeesatBigNerdRanchwhohelpedreviewthebook,providedsuggestions,andfounderrata.

Ourtirelesseditor,ElizabethHoladay,tookourdistractedmumblingsandmadethemintoreadableprose.

AnnaBentleyjumpedintoprovideproofing.

EllieVolckhausendesignedthecover.(Thephotoisofthebottombracketofabicycleframe.)

ChrisLoperatIntelligentEnglish.comdesignedandproducedtheprintbookandtheEPUBandKindleversions.

TheamazingteamatPearsonTechnologyGrouppatientlyguidedusthroughthebusinessendofbookpublishing.

Thefinalandmostimportantthanksgoestoourstudentswhosequestionsinspiredustowritethisbookandwhosefrustrationsinspiredustomakeitclearandcomprehensible.


TableofContentsIntroduction

Prerequisites

WhatHasChangedintheFifthEdition?

OurTeachingPhilosophy

HowtoUseThisBook

UsinganeBook

HowThisBookIsOrganized

StyleChoices

TypographicalConventions

NecessaryHardwareandSoftware

1.ASimpleiOSApplication

CreatinganXcodeProject

Model-View-Controller

DesigningQuiz

InterfaceBuilder

BuildingtheInterface

Creatingviewobjects

Configuringviewobjects

Runningonthesimulator

AbriefintroductiontoAutoLayout

Makingconnections

CreatingtheModelLayer

Implementingactionmethods

Loadingthefirstquestion

BuildingtheFinishedApplication

ApplicationIcons

LaunchScreen

2.TheSwiftLanguage


TypesinSwift

UsingStandardTypes

Inferringtypes

Specifyingtypes

Literalsandsubscripting

Initializers

Properties

Instancemethods

Optionals

Subscriptingdictionaries

LoopsandStringInterpolation

EnumerationsandtheSwitchStatement

Enumerationsandrawvalues

ExploringApple’sSwiftDocumentation

3.ViewsandtheViewHierarchy

ViewBasics

TheViewHierarchy

CreatingaNewProject

ViewsandFrames

Customizingthelabels

TheAutoLayoutSystem

Alignmentrectangleandlayoutattributes

Constraints

AddingconstraintsinInterfaceBuilder

Intrinsiccontentsize

Misplacedviews

Addingmoreconstraints

BronzeChallenge:MoreAutoLayoutPractice

4.TextInputandDelegation

TextEditing

Keyboardattributes


Respondingtotextfieldchanges

Dismissingthekeyboard

ImplementingtheTemperatureConversion

Numberformatters

Delegation

Conformingtoaprotocol

Usingadelegate

Moreonprotocols

BronzeChallenge:DisallowAlphabeticCharacters

5.ViewControllers

TheViewofaViewController

SettingtheInitialViewController

UITabBarController

Tabbaritems

LoadedandAppearingViews

Accessingsubviews

InteractingwithViewControllersandTheirViews

SilverChallenge:DarkMode

FortheMoreCurious:RetinaDisplay

6.ProgrammaticViews

CreatingaViewProgrammatically

ProgrammaticConstraints

Anchors

Activatingconstraints

Layoutguides

Margins

Explicitconstraints

ProgrammaticControls

BronzeChallenge:AnotherTab

SilverChallenge:User’sLocation

GoldChallenge:DroppingPins


FortheMoreCurious:NSAutoresizingMaskLayoutConstraint

7.Localization

Internationalization

Formatters

Baseinternationalization

Preparingforlocalization

Localization

NSLocalizedStringandstringstables

BronzeChallenge:AnotherLocalization

FortheMoreCurious:NSBundle’sRoleinInternationalization

FortheMoreCurious:ImportingandExportingasXLIFF

8.ControllingAnimations

BasicAnimations

Closures

AnotherLabel

AnimationCompletion

AnimatingConstraints

TimingFunctions

BronzeChallenge:SpringAnimations

SilverChallenge:LayoutGuides

9.UITableViewandUITableViewController

BeginningtheHomepwnerApplication

UITableViewController

SubclassingUITableViewController

CreatingtheItemClass

Custominitializers

UITableView’sDataSource

Givingthecontrolleraccesstothestore

Implementingdatasourcemethods

UITableViewCells

CreatingandretrievingUITableViewCells


ReusingUITableViewCells

ContentInsets

BronzeChallenge:Sections

SilverChallenge:ConstantRows

GoldChallenge:CustomizingtheTable

10.EditingUITableView

EditingMode

AddingRows

DeletingRows

MovingRows

DisplayingUserAlerts

DesignPatterns

BronzeChallenge:RenamingtheDeleteButton

SilverChallenge:PreventingReordering

GoldChallenge:ReallyPreventingReordering

11.SubclassingUITableViewCell

CreatingItemCell

ExposingthePropertiesofItemCell

UsingItemCell

DynamicCellHeights

DynamicType

Respondingtouserchanges

BronzeChallenge:CellColors

12.StackViews

UsingUIStackView

Implicitconstraints

Stackviewdistribution

Nestedstackviews

Stackviewspacing

Segues

HookingUptheContent


PassingDataAround

BronzeChallenge:MoreStackViews

13.UINavigationController

UINavigationController

NavigatingwithUINavigationController

AppearingandDisappearingViews

DismissingtheKeyboard

Eventhandlingbasics

DismissingbypressingtheReturnkey

Dismissingbytappingelsewhere

UINavigationBar

Addingbuttonstothenavigationbar

BronzeChallenge:DisplayingaNumberPad

SilverChallenge:ACustomUITextField

GoldChallenge:PushingMoreViewControllers

14.Camera

DisplayingImagesandUIImageView

Addingacamerabutton

TakingPicturesandUIImagePickerController

Settingtheimagepicker’ssourceType

Settingtheimagepicker’sdelegate

Presentingtheimagepickermodally

Savingtheimage

CreatingImageStore

GivingViewControllersAccesstotheImageStore

CreatingandUsingKeys

WrappingUpImageStore

BronzeChallenge:EditinganImage

SilverChallenge:RemovinganImage

GoldChallenge:CameraOverlay

FortheMoreCurious:NavigatingImplementationFiles


//MARK:

15.Saving,Loading,andApplicationStates

Archiving

ApplicationSandbox

ConstructingafileURL

NSKeyedArchiverandNSKeyedUnarchiver

Loadingfiles

ApplicationStatesandTransitions

WritingtotheFilesystemwithNSData

ErrorHandling

BronzeChallenge:PNG

FortheMoreCurious:ApplicationStateTransitions

FortheMoreCurious:ReadingandWritingtotheFilesystem

FortheMoreCurious:TheApplicationBundle

16.SizeClasses

AnotherSizeClass

BronzeChallenge:StackedTextFieldandLabels

17.TouchEventsandUIResponder

TouchEvents

CreatingtheTouchTrackerApplication

CreatingtheLineStruct

Structs

Valuetypesvs.referencetypes

CreatingDrawView

DrawingwithDrawView

TurningTouchesintoLines

Handlingmultipletouches

@IBInspectable

SilverChallenge:Colors

GoldChallenge:Circles

FortheMoreCurious:TheResponderChain


FortheMoreCurious:UIControl

18.UIGestureRecognizerandUIMenuController

UIGestureRecognizerSubclasses

DetectingTapswithUITapGestureRecognizer

MultipleGestureRecognizers

UIMenuController

MoreGestureRecognizers

UILongPressGestureRecognizer

UIPanGestureRecognizerandsimultaneousrecognizers

MoreonUIGestureRecognizer

SilverChallenge:MysteriousLines

GoldChallenge:SpeedandSize

PlatinumChallenge:Colors

FortheMoreCurious:UIMenuControllerandUIResponderStandardEditActions

19.WebServices

StartingthePhotoramaApplication

BuildingtheURL

FormattingURLsandrequests

NSURLComponents

SendingtheRequest

NSURLSession

ModelingthePhoto

JSONData

NSJSONSerialization

Enumerationsandassociatedvalues

ParsingJSONdata

DownloadingandDisplayingtheImageData

TheMainThread

BronzeChallenge:PrintingtheResponseInformation

FortheMoreCurious:HTTP

20.CollectionViewsWOW! eBook

www.wowebook.org

DisplayingtheGrid

CollectionViewDataSource

CustomizingtheLayout

CreatingaCustomUICollectionViewCell

DownloadingtheImageData

Extensions

NavigatingtoaPhoto

SilverChallenge:UpdatedItemSizes

GoldChallenge:CreatingaCustomLayout

21.CoreData

ObjectGraphs

Entities

Modelingentities

Transformableattributes

NSManagedObjectandsubclasses

BuildingtheCoreDataStack

NSManagedObjectModel

NSPersistentStoreCoordinator

NSManagedObjectContext

UpdatingItems

Insertingintothecontext

Savingchanges

UpdatingtheDataSource

Fetchrequestsandpredicates

SavingImagestoDisk

BronzeChallenge:PhotoViewCount

22.CoreDataRelationships

Relationships

AddingTagstotheInterface

Parent-ChildContexts

SilverChallenge:Favorites


23.Afterword

WhattoDoNext

ShamelessPlugs

Index


IntroductionAsanaspiringiOSdeveloper,youfacethreemajortasks:

YoumustlearntheSwiftlanguage.SwiftistherecommendeddevelopmentlanguageforiOS.ThefirsttwochaptersofthisbookaredesignedtogiveyouaworkingknowledgeofSwift.

Youmustmasterthebigideas.Theseincludethingslikedelegation,archiving,andtheproperuseofviewcontrollers.Thebigideastakeafewdaystounderstand.Whenyoureachthehalfwaypointofthisbook,youwillunderstandthesebigideas.

Youmustmastertheframeworks.TheeventualgoalistoknowhowtouseeverymethodofeveryclassineveryframeworkiniOS.Thisisaprojectforalifetime:therearehundredsofclassesandthousandsofmethodsavailableiniOS,andAppleaddsmoreclassesandmethodswitheveryreleaseofiOS.Inthisbook,youwillbeintroducedtoeachofthesubsystemsthatmakeuptheiOSSDK,butyouwillnotstudyeachonedeeply.Instead,ourgoalistogetyoutothepointwhereyoucansearchandunderstandApple’sreferencedocumentation.

WehaveusedthismaterialmanytimesatouriOSbootcampsatBigNerdRanch.ItiswelltestedandhashelpedthousandsofpeoplebecomeiOSdevelopers.Wesincerelyhopethatitprovesusefultoyou.


PrerequisitesThisbookassumesthatyouarealreadymotivatedtolearntowriteiOSapps.WewillnotspendanytimeconvincingyouthattheiPhone,iPad,andiPodtoucharecompellingpiecesoftechnology.

Wealsoassumethatyouhavesomeexperienceprogrammingandknowsomethingaboutobject-orientedprogramming.Ifthisisnottrue,youshouldprobablystartwithSwiftProgramming:TheBigNerdRanchGuide.


WhatHasChangedintheFifthEdition?AllofthecodeinthisbookisSwift,andanearlychapterisdevotedtogettingyouuptospeedwiththisnewlanguage.Throughoutthebook,youwillseehowtouseSwift’scapabilitiesandfeaturestowritebetteriOSapplications.WehavecometoloveSwiftatBigNerdRanchandbelieveyouwill,too.

OtheradditionsincludecollectionviewsandsizeclassesandimprovedcoverageofAutoLayout,webservices,andCoreData.

ThiseditionassumesthatthereaderisusingXcode7.1orlaterandrunningapplicationsonaniOS9orlaterdevice.

Besidestheseobviouschanges,wemadethousandsoftinyimprovementsthatwereinspiredbyquestionsfromourreadersandourstudents.Everychapterofthisbookisjustalittlebetterthanthecorrespondingchapterfromthefourthedition.


OurTeachingPhilosophyThisbookwillteachyoutheessentialconceptsofiOSprogramming.Atthesametime,youwilltypeinalotofcodeandbuildabunchofapplications.Bytheendofthebook,youwillhaveknowledgeandexperience.However,alltheknowledgeshouldnot(and,inthisbook,willnot)comefirst.Thatisthetraditionalwaywehaveallcometoknowandhate.Instead,wetakealearn-while-doingapproach.Developmentconceptsandactualcodinggotogether.

HereiswhatwehavelearnedovertheyearsofteachingiOSprogramming:

Wehavelearnedwhatideaspeoplemustgrasptogetstartedprogramming,andwefocusonthatsubset.

Wehavelearnedthatpeoplelearnbestwhentheseconceptsareintroducedastheyareneeded.

Wehavelearnedthatprogrammingknowledgeandexperiencegrowbestwhentheygrowtogether.

Wehavelearnedthat“goingthroughthemotions”ismuchmoreimportantthanitsounds.Manytimeswewillaskyoutostarttypingincodebeforeyouunderstandit.Werealizethatyoumayfeellikeatrainedmonkeytypinginabunchofcodethatyoudonotfullygrasp.Butthebestwaytolearncodingistofindandfixyourtypos.Farfrombeingadrag,thisbasicdebuggingiswhereyoureallylearntheinsandoutsofthecode.Thatiswhyweencourageyoutotypeinthecodeyourself.Youcouldjustdownloadit,butcopyingandpastingisnotprogramming.Wewantbetterforyouandyourskills.

Whatdoesthismeanforyou,thereader?Tolearnthiswaytakessometrust–andweappreciateyours.Italsotakespatience.Asweleadyouthroughthesechapters,wewilltrytokeepyoucomfortableandtellyouwhatishappening.However,therewillbetimeswhenyouwillhavetotakeourwordforit.(Ifyouthinkthiswillbugyou,keepreading–wehavesomeideasthatmighthelp.)Donotgetdiscouragedifyourunacrossaconceptthatyoudonotunderstandrightaway.Rememberthatweareintentionallynotprovidingalltheknowledgeyouwilleverneedallatonce.Ifaconceptseemsunclear,wewilllikelydiscussitinmoredetaillaterwhenitbecomesnecessary.Andsomethingsthatarenotclearatthebeginningwillsuddenlymakesensewhenyouimplementthemthefirst(orthetwelfth)time.

Peoplelearndifferently.Itispossiblethatyouwilllovehowwehandoutconceptsonanas-neededbasis.Itisalsopossiblethatyouwillfinditfrustrating.Incaseofthelatter,herearesomeoptions:

Takeadeepbreathandwaititout.Wewillgetthere,andsowillyou.

Checktheindex.Wewillletitslideifyoulookaheadandreadthroughamoreadvanceddiscussionthatoccurslaterinthebook.

ChecktheonlineAppledocumentation.Thisisanessentialdevelopertool,and


youwillwantplentyofpracticeusingit.Consultitearlyandoften.

IfSwiftorobject-orientedprogrammingconceptsaregivingyouahardtime(orifyouthinktheywill),youmightconsiderbackingupandreadingourSwiftProgramming:TheBigNerdRanchGuide.


HowtoUseThisBookThisbookisbasedontheclassweteachatBigNerdRanch.Assuch,itwasdesignedtobeconsumedinacertainmanner.

Setyourselfareasonablegoal,like“Iwilldoonechaptereveryday.”Whenyousitdowntoattackachapter,findaquietplacewhereyouwillnotbeinterruptedforatleastanhour.Shutdownyouremail,yourTwitterclient,andyourchatprogram.Thisisnotatimeformultitasking;youwillneedtoconcentrate.

Dotheactualprogramming.Youcanreadthroughachapterfirst,ifyoulike.Butthereallearningcomeswhenyousitdownandcodeasyougo.Youwillnotreallyunderstandtheideauntilyouhavewrittenaprogramthatusesitand,perhapsmoreimportantly,debuggedthatprogram.

Acoupleoftheexercisesrequiresupportingfiles.Forexample,inthefirstchapteryouwillneedaniconforyourQuizapplication,andwehaveoneforyou.Youcandownloadtheresourcesandsolutionstotheexercisesfromhttp://www.bignerdranch.com/solutions/iOSProgramming5ed.zip.

Therearetwotypesoflearning.WhenyoulearnaboutthePeloponnesianWar,youaresimplyaddingdetailstoascaffoldingofideasthatyoualreadyunderstand.Thisiswhatwewillcall“EasyLearning.”Yes,learningaboutthePeloponnesianWarcantakealongtime,butyouareseldomflummoxedbyit.LearningiOSprogramming,ontheotherhand,is“HardLearning,”andyoumayfindyourselfquitebaffledattimes,especiallyinthefirstfewdays.Inwritingthisbook,wehavetriedtocreateanexperiencethatwilleaseyouoverthebumpsinthelearningcurve.Herearetwothingsyoucandotomakethejourneyeasier:

FindsomeonewhoalreadyknowshowtowriteiOSapplicationsandwillansweryourquestions.Inparticular,gettingyourapplicationontoadevicethefirsttimeisusuallyveryfrustratingifyouaredoingitwithoutthehelpofanexperienceddeveloper.

Getenoughsleep.Sleepypeopledonotrememberwhattheyhavelearned.


http://www.bignerdranch.com/solutions/iOSProgramming5ed.zip

UsinganeBookIfyouarereadingthisbookonaneReader,wewanttopointoutthatreadingthecodemaybetrickyattimes.Longerlinesofcodewillwraptoasecondlinedependingonyourselectedfontsize.ThisbothersusbecausewearereallyconscientiousatBigNerdRanchaboutthewayourcodeappearsonthepage.Clearvisualpatternsincodemakethatcodeeasiertounderstand.

Thelongestlinesofcodeinthisbookare86monospacecharacters,likethisone.funcprocessRecentPhotosRequest(datadata:NSData?,error:NSError?)->PhotosResult{

YoucanplaywithyoureReaders’ssettingstofindthebestforviewinglongcodelines.

Whenyougettothepointwhereyouareactuallytypingincode,wesuggestopeningthebookonyourMaciniBooksorAdobeDigitalEditions.(AdobeDigitalEditionsisafreeeReaderapplicationyoucandownloadfromhttp://www.adobe.com/products/digitaleditions/.)Maketheapplicationwindowlargeenoughsothatyoucanseethecodewithnowrappinglines.Youwillalsobeabletoseethefiguresinfulldetail.


http://www.adobe.com/products/digitaleditions/

HowThisBookIsOrganizedInthisbook,eachchapteraddressesoneormoreideasofiOSdevelopmentthroughdiscussionandhands-onpractice.Formorecodingpractice,mostchaptersincludechallengeexercises.Weencourageyoutotakeonatleastsomeofthese.TheyareexcellentforfirmingupyourgraspoftheconceptsintroducedinthechapterandformakingyouamoreconfidentiOSprogrammer.Finally,mostchaptersconcludewithoneortwo“FortheMoreCurious”sectionsthatexplaincertainconsequencesoftheconceptsthatwereintroducedearlier.

Chapter1introducesyoutoiOSprogrammingasyoubuildanddeployatinyapplication.YouwillgetyourfeetwetwithXcodeandtheiOSsimulatoralongwithallthestepsforcreatingprojectsandfiles.ThechapterincludesadiscussionofModel-View-ControllerandhowitrelatestoiOSdevelopment.

Chapter2providesanoverviewofSwift,includingbasicsyntax,types,optionals,initialization,andhowSwiftisabletointeractwiththeexistingiOSframeworks.Youwillalsogetexperienceworkinginaplayground,Xcode’snewcodeprototypingtool.

InChapter3,youwillfocusontheiOSuserinterfaceasyoulearnaboutviewsandtheviewhierarchyandcreateanapplicationcalledWorldTrotter.

Chapter4introducesdelegation,animportantiOSdesignpattern.YouwillalsoatextfieldtoWorldTrotter.

InChapter5,youwillexpandWorldTrotterandlearnaboutusingviewcontrollersformanaginguserinterfaces.Youwillgetpracticeworkingwithviewsandviewcontrollersaswellasnavigatingbetweenscreensusingatabbar.

InChapter6,youwilllearnhowtomanageviewsandviewcontrollersincode.YouwilladdasegmentedcontroltoWorldTrotterthatwillletyouswitchbetweenvariousmaptypes.

Chapter7introducestheconceptsandtechniquesofinternationalizationandlocalization.YouwilllearnaboutNSLocale,stringstables,andNSBundleasyoulocalizepartsofWorldTrotter.

InChapter8,youwilllearnaboutandadddifferenttypesofanimationstotheQuizprojectthatyoucreatedinChapter1.

Chapter9introducesthelargestapplicationinthebook–Homepwner.(Bytheway,“Homepwner”isnotatypo;youcanfindthedefinitionof“pwn”atwww.urbandictionary.com.)Thisapplicationkeepsarecordofyouritemsincaseoffireorothercatastrophe.Homepwnerwilltakeeightchapterstocomplete.

InChapter9-Chapter11,youwillworkwithtables.Youwilllearnabouttableviews,theirviewcontrollers,andtheirdatasources.Youwilllearnhowtodisplaydatainatable,howtoallowtheusertoeditthetable,andhowtoimprovetheinterface.

Chapter12introducesstackviewsthatwillhelpyoucreatecomplexinterfacesveryeasily.YouwilluseastackviewtoaddanewscreentoHomepwnerthatdisplaysthedetailsforasingleitem.


http://www.urbandictionary.com

Chapter13buildsonthenavigationexperiencegainedinChapter5.YouwilluseUINavigationControllertogiveHomepwneradrill-downinterfaceandanavigationbar.

Chapter14introducesthecamera.YouwilltakepicturesanddisplayandstoreimagesinHomepwner.

InChapter15,youwilladdpersistencetoHomepwnerusingarchivingtosaveandloadtheapplicationdata.

InChapter16,youwilllearnaboutsizeclasses,andyouwillusethesetoupdateHomepwner’sinterfacetoscalewellacrossvariousscreensizes.

InChapter17andChapter18,youwillcreateadrawingapplicationnamedTouchTrackertolearnabouttouchevents.YouwillseehowtoaddmultitouchcapabilityandhowtouseUIGestureRecognizertorespondtoparticulargestures.Youwillalsogetexperiencewiththefirstresponderandresponderchainconceptsandmorepracticewithusingstructuresanddictionaries.

Chapter19introduceswebservicesasyoucreatethePhotoramaapplication.ThisapplicationfetchesandparsesJSONfromaserverusingNSURLSessionsandNSJSONSerialization.

InChapter20,youwilllearnaboutcollectionviewsasyoubuildaninterfaceforPhotoramausingUICollectionViewandUICollectionViewCell.

InChapter21andChapter22,youwilladdpersistencetoPhotoramausingCoreData.YouwillstoreandloadimagesandassociateddatausinganNSManagedObjectContext.


StyleChoicesThisbookcontainsalotofcode.Wehaveattemptedtomakethatcodeandthedesignsbehinditexemplary.Wehavedoneourbesttofollowtheidiomsofthecommunity,butattimeswehavewanderedfromwhatyoumightseeinApple’ssamplecodeorcodeyoumightfindinotherbooks.Inparticular,youshouldknowup-frontthatwenearlyalwaysstartaprojectwiththesimplesttemplateproject:thesingleviewapplication.Whenyourappworks,youwillknowitisbecauseofyourefforts–notbecausethatbehaviorwasbuiltintothetemplate.


TypographicalConventionsTomakethisbookeasiertoread,certainitemsappearincertainfonts.Classes,types,methods,andfunctionsappearinabold,fixed-widthfont.Classesandtypesstartwithcapitalletters,andmethodsandfunctionsstartwithlowercaseletters.Forexample,“IntheloadView()methodoftheRexViewControllerclass,createaconstantoftypeString.”

Variables,constants,andfilenamesappearinafixed-widthfontbutarenotbold.Soyouwillsee,“InViewController.swift,addavariablenamedfidoandinitializeitto“Rufus”.”

Applicationnames,menuchoices,andbuttonnamesappearinasansseriffont.Forexample,“OpenXcodeandselectNewProject…fromtheFilemenu.SelectSingleViewApplicationandthenclickChoose….”

Allcodeblocksareinafixed-widthfont.Codethatyouneedtotypeinisalwaysbold.Forexample,inthefollowingcode,youwouldtypeinthetwolinesbeginning@IBOutlet.Theotherlinesarealreadyinthecodeandareincludedtoletyouknowwheretoaddthenewlines.importUIKit

classViewController:UIViewController{

@IBOutletvarquestionLabel:UILabel!@IBOutletvaranswerLabel:UILabel!

}


NecessaryHardwareandSoftwareTobuildtheapplicationsinthisbook,youmusthaveaMacrunningOSXYosemite(10.10.5)orlater.YouwillalsoneedXcode,Apple’sIntegratedDevelopmentEnvironment,whichisavailableontheAppStore.XcodeincludestheiOSSDK,theiOSsimulator,andotherdevelopmenttools.

YoushouldjointheAppleDeveloperProgram,whichcosts$99/year,because:

Downloadingthelatestdevelopertoolsisfreeformembers.

Youcannotputanappinthestoreuntilyouareamember.

Ifyouaregoingtotakethetimetoworkthroughthisentirebook,membershipintheAppleDeveloperProgramisworththecost.Gotohttp://developer.apple.com/programs/ios/tojoin.

WhataboutiOSdevices?MostoftheapplicationsyouwilldevelopinthefirsthalfofthebookareforiPhone,butyouwillbeabletorunthemonaniPad.OntheiPadscreen,iPhoneapplicationsappearinaniPhone-sizedwindow.NotacompellinguseofiPad,butthatisOKwhenyouarestartingwithiOS.Intheearlychapters,youwillbefocusedonlearningthefundamentalsoftheiOSSDK,andthesearethesameacrossiOSdevices.Laterinthebook,youwillseehowtomakeapplicationsrunnativelyonbothiOSdevicefamilies.

Excitedyet?Good.Let’sgetstarted.


http://developer.apple.com/programs/ios/

1ASimpleiOSApplication

Inthischapter,youaregoingtowriteaniOSapplicationnamedQuiz.Thisapplicationwillshowaquestionandthenrevealtheanswerwhentheusertapsabutton.Tappinganotherbuttonwillshowtheuseranewquestion(Figure1.1).

Figure1.1Yourfirstapplication:Quiz

WhenyouarewritinganiOSapplication,youmustanswertwobasicquestions:

HowdoIgetmyobjectscreatedandconfiguredproperly?(Example:“IwantabuttonherethatsaysNextQuestion.”)

HowdoImakemyapprespondtouserinteraction?(Example:“Whentheusertapsthebutton,Iwantthispieceofcodetobeexecuted.”)

Mostofthisbookisdedicatedtoansweringthesequestions.

Asyougothroughthisfirstchapter,youwillprobablynotunderstandeverythingthatyouaredoing,andyoumayfeelridiculousjustgoingthroughthemotions.Butgoingthrough


themotionsisenoughfornow.Mimicryisapowerfulformoflearning;itishowyoulearnedtospeak,anditishowyouwillstartiOSprogramming.Asyoubecomemorecapable,youwillexperimentandchallengeyourselftodocreativethingsontheplatform.Fornow,goaheadanddowhatweshowyou.Thedetailswillbeexplainedinlaterchapters.


CreatinganXcodeProjectOpenXcodeand,fromtheFilemenu,selectNew→Project…(IfXcodeopenstoawelcomescreen,selectCreateanewXcodeproject.)

Anewworkspacewindowwillappearandasheetwillslidedownfromitstoolbar.Onthelefthandside,findtheiOSsectionandselectApplication(Figure1.2).Youwillbeofferedseveralapplicationtemplatestochoosefrom.SelectSingleViewApplication.

Figure1.2Creatingaproject

ThisbookwascreatedforXcode7.1.ThenamesofthesetemplatesmaychangewithnewXcodereleases.IfyoudonotseeaSingleViewApplicationtemplate,usethesimplest-soundingtemplate.OrvisittheBigNerdRanchforumforthisbookathttp://forums.bignerdranch.comforhelpworkingwithnewerversionsofXcode.

ClickNextand,inthenextsheet,enterQuizfortheProductName(Figure1.3).Theorganizationnameandcompanyidentifierarerequiredtocontinue.YoucanuseBigNerdRanchandcom.bignerdranch.Oruseyourcompanynameandcom.yourcompanynamehere.

FromtheLanguagepop-upmenu,chooseSwift,andfromtheDevicespop-upmenu,chooseiPhone.MakesurethattheUseCoreData,IncludeUnitTests,andIncludeUITestscheckboxesareunchecked.


http://forums.bignerdranch.com

Figure1.3Configuringanewproject

YouarecreatingQuizasaniPhoneapplication,butyouwillbeabletorunitonaniPadaswell.Forthefirstpartofthisbook,youwillsticktocreatingiPhoneapplicationsandfocusonlearningthefundamentalsoftheiOSsoftwaredevelopmentkit,orSDK.Later,youwilllearnhowtocreateuserinterfacesthatadaptelegantlytothescreensizesofdifferentiOSdevices.

ClickNextand,inthefinalsheet,savetheprojectinthedirectorywhereyouplantostoretheexercisesinthisbook.ClickCreatetocreatetheQuizproject.

Oncetheprojectiscreated,itwillopenintheXcodeworkspacewindow(Figure1.4).


Figure1.4Xcodeworkspacewindow

Thelefthandsideoftheworkspacewindowisthenavigatorarea.Thisareadisplaysdifferentnavigators–toolsthatshowyoudifferentpartsofyourproject.Youcanopenanavigatorbyselectingoneoftheiconsinthenavigatorselector,whichisthebarjustabovethenavigatorarea.

Thenavigatorcurrentlyopenistheprojectnavigator.Theprojectnavigatorshowsyouthefilesthatmakeupaproject(Figure1.5).Youcanselectoneofthesefilestoopenitintheeditorareatotherightofthenavigatorarea.

Thefilesintheprojectnavigatorcanbegroupedintofolderstohelpyouorganizeyourproject.Afewgroupshavebeencreatedbythetemplateforyou.Youcanrenamethem,ifyouwant,oraddnewones.Thegroupsarepurelyfortheorganizationoffilesanddonotcorrelatetothefilesysteminanyway.

Figure1.5Quizapplication’sfilesintheprojectnavigator


Model-View-ControllerBeforeyoubeginyourapplication,let’sdiscussakeyconceptinapplicationarchitecture:Model-View-Controller,orMVC.MVCisadesignpatternusediniOSdevelopment.InMVC,everyinstancebelongstoeitherthemodellayer,theviewlayer,orthecontrollerlayer.(Layerheresimplyreferstooneormoreobjectsthattogetherfulfillarole.)

Themodellayerholdsdataandknowsnothingabouttheuserinterface.InQuiz,themodelwillconsistoftwoorderedlistsofstrings:oneforquestionsandanotherforanswers.

Usually,instancesinthemodellayerrepresentrealthingsintheworldoftheuser.Forexample,whenyouwriteanappforaninsurancecompany,yourmodelwillalmostcertainlycontainacustomtypecalledInsurancePolicy.

Theviewlayercontainsobjectsthatarevisibletotheuser.Examplesofviewobjects,orviews,arebuttons,textfields,andsliders.Viewobjectsmakeupanapplication’suserinterface.InQuiz,thelabelsshowingthequestionandanswerandthebuttonsbeneaththemareviewobjects.

Thecontrollerlayeriswheretheapplicationismanaged.Controllerobjects,orcontrollers,arethemanagersofanapplication.Controllersconfiguretheviewsthattheuserseesandmakesurethattheviewandmodelobjectsstaysynchronized.

Ingeneral,controllerstypicallyhandle“Andthen?”questions.Forexample,whentheuserselectsanitemfromalist,thecontrollerdetermineswhattheuserseesnext.

Figure1.6showstheflowofcontrolinanapplicationinresponsetouserinput,suchastheusertappingabutton.


Figure1.6MVCpattern

Noticethatmodelsandviewsdonottalktoeachotherdirectly;controllerssitsquarelyinthemiddleofeverything,receivingmessagesanddispatchinginstructions.


DesigningQuizYouaregoingtowritetheQuizapplicationusingtheMVCpattern.Hereisabreakdownoftheinstancesyouwillbecreatingandworkingwith:

Themodellayerwillconsistoftwoinstancesof[String].

TheviewlayerwillconsistoftwoinstancesofUILabelandtwoinstancesofUIButton.

ThecontrollerlayerwillconsistofaninstanceofAppDelegateandaninstanceofViewController.

TheseinstancesandtheirrelationshipsarelaidoutinthediagramforQuizshowninFigure1.7.

Figure1.7ObjectdiagramforQuiz

Figure1.7isthebigpictureofhowthefinishedQuizapplicationwillwork.Forexample,whentheNextQuestionbuttonistapped,itwilltriggeramethodinViewController.Amethodisalotlikeafunction–alistofinstructionstobeexecuted.Thismethodwillretrieveanewquestionfromthearrayofquestionsandaskthetoplabeltodisplaythatquestion.

ItisOKifthisdiagramdoesnotmakesenseyet–itwillbytheendofthechapter.ReferWOW! eBook

www.wowebook.org

backtoitasyoubuildtheapptoseehowitistakingshape.

YouaregoingtobuildQuizinsteps,startingwiththevisualinterfacefortheapplication.


InterfaceBuilderYouareusingtheSingleViewApplicationtemplatebecauseitisthesimplesttemplatethatXcodeoffers.Still,thistemplatehasasignificantamountofmagicinthatsomecriticalcomponentshavealreadybeensetupforyou.Fornow,youwilljustusethesecomponents,withoutattemptingtogainadeepunderstandingofhowtheywork.Therestofthebookwillbeconcernedwiththosedetails.

ClickonceontheMain.storyboardfile.Xcodewillopenitsgraphic-styleeditorcalledInterfaceBuilder.

InterfaceBuilderdividestheeditorareaintotwosections:thedocumentoutline,onthelefthandside,andthecanvas,ontheright.

ThisisshowninFigure1.8.Ifwhatyouseeonyourscreendoesnotexactlymatchthefigure,youmayhavetoclickontheShowDocumentOutlinebutton.(Ifyouhaveadditionalareasshowing,donotworryaboutthem.)Youmayalsohavetoclickonthedisclosuretrianglesonthedocumentoutlinetorevealcontent.


Figure1.8InterfaceBuildershowingMain.storyboard

ThelargerectanglethatyouseeintheInterfaceBuildercanvasiscalledasceneandrepresentstheonly“screen”orviewyourapplicationhasatthistime(rememberthatyouusedthesingleviewapplicationtemplatetocreatethisproject).

Inthenextsection,youwilllearnhowtocreateauserinterfaceforyourapplicationusingInterfaceBuilder.InterfaceBuilderletsyoudragobjectsfromalibraryontothecanvastocreateinstances,andalsoletsyouestablishconnectionsbetweenthoseobjectsandyourcode.Theseconnectionscanresultincodebeingcalledbyauserinteraction.

AcrucialfeatureofInterfaceBuilderisthatitisnotagraphicalrepresentationofcodecontainedinotherfiles.InterfaceBuilderisanobjecteditorthatcancreateinstancesofobjectsandmanipulatetheirproperties.Whenyouaredoneeditinganinterface,itdoesnotgeneratecodethatcorrespondstotheworkyouhavedone.A.storyboardfileisanarchiveofobjectinstancestobeloadedintomemorywhennecessary.


BuildingtheInterfaceLet’sgetstartedonyourinterface.YouhaveselectedMain.storyboardtorevealitssinglesceneinthecanvas(Figure1.9).

Figure1.9ThesceneinMain.storyboard

Itistimetoaddyourviewobjectstothatblankslate.

Creatingviewobjects

MakesurethattheutilityareawithinXcode’swindowisvisible.Youmayneedtoclickontherightmostbuttonofthe controlinthetop-rightcornerofthewindow.Theutilityareaistotherightoftheeditorareaandhastwosections:theinspectorandthelibrary.Thetopsectionistheinspector,whichcontainssettingsforthefileorobjectthatisselectedintheeditorarea.Thebottomsectionisthelibrary,whichlistsitemsthatyoucanaddtoafileorproject.

Atthetopofeachsectionintheutilityareaisaselectorfordifferentinspectorsandlibraries(Figure1.10).


Figure1.10Xcodeutilityarea

Yourapplicationinterfacerequiresfourviewobjects:twobuttonstoacceptuserinputandtwotextlabelstodisplayinformation.Toaddthem,firstmakesureyoucanseetheobjectlibrary,asseeninFigure1.10,byselectingthe tabfromthelibraryselector.

Theobjectlibrarycontainstheobjectsthatyoucanaddtoastoryboardfiletocomposeyourinterface.FindtheLabelobjectbyeitherscrollingdownthroughthelistorbyusingthesearchbaratthebottomofthelibrary.Selectthisobjectinthelibraryanddragitontotheviewobjectonthecanvas.Dragthelabelaroundthecanvasandnoticethedashedbluelinesthatappearwhenthelabelisnearthecenterofthecanvas(Figure1.11).Theseguidelineswillhelpyoulayoutyourinterface.

Figure1.11Addingalabeltothecanvas


Usingtheguidelines,positionthelabelinthehorizontalcenteroftheviewandnearthetop,asshowninFigure1.11.Eventually,thislabelwilldisplayquestionstotheuser.Dragasecondlabelontotheviewandpositionitinthehorizontalcenter,closertothemiddle.Thislabelwilldisplayanswers.

Next,findButtonintheobjectlibraryanddragtwobuttonsontotheview.Positiononebeloweachlabel.

YouhavenowaddedfourviewobjectstotheViewController’suserinterface.Noticethattheyalsoappearinthedocumentoutline.YourcompletedinterfaceshouldlooklikeFigure1.12.

Figure1.12BuildingtheQuizinterface

Configuringviewobjects

Nowthatyouhavecreatedtheviewobjects,youcanconfiguretheirattributes.Someattributesofaview,likesize,position,andtext,canbechangeddirectlyonthecanvas.Forexample,youcanresizeanobjectbyselectingitinthecanvasorthedocumentoutlineandthendraggingitscornersandedgesinthecanvas.


Beginbyrenamingthelabelsandbuttons.Double-clickoneachlabelandreplacethetextwith???.Thendouble-clicktheupperbuttonandchangeitsnametoNextQuestion.RenamethelowerbuttontoShowAnswer.TheresultsareshowninFigure1.13.

Figure1.13Renamingthelabelsandbuttons

Youmayhavenoticedthatasaresultofchangingthetextinthelabelsandbuttons,andthereforetheirwidths,theyarenolongerneatlycenteredinthescene.Clickoneachofthemanddragtocenterthemagain,asshowninFigure1.14.

Figure1.14Centeringthelabelsandbuttons

Runningonthesimulator

Totestyouruserinterface,youaregoingtorunQuizonXcode’siOSsimulator.

ToprepareQuiztorunonthesimulator,findthecurrentschemepop-upmenuontheXcodetoolbar(Figure1.15).


Figure1.15iPhone6sschemeselected

IfitsayssomethinggenericlikeiPhone6s,thentheprojectissettorunonthesimulatorandyouaregoodtogo.IfitsayssomethinglikeChristian’siPhone,thenclickandchooseiPhone6sfromthepop-upmenu.TheiPhone6sschemewillbeyoursimulatordefaultthroughoutthisbook.

(Ifyouwouldliketoruntheapplicationonanactualdevice,readouriOSDeviceProvisionGuideathttps://www.bignerdranch.com/we-teach/how-to-prepare/ios-device-provisioning/.)

Clickthetriangularplaybuttoninthetoolbar.Thiswillbuild(compile)andthenruntheapplication.YouwillbedoingthisoftenenoughthatyoumaywanttolearnandusethekeyboardshortcutCommand-R.

Afterthesimulatorlaunchesyouwillseethattheinterfacehasalltheviewsyouadded,buttheyarenotcorrectlyplaced.ThisisbecausethelabelsandbuttonscurrentlyhaveafixedpositiononascreenlargerthaniPhone6s’s,andtheydonotremaincenteredonthemainview.Tocorrectthisproblem,youwilluseatechnologycalledAutoLayout.

AbriefintroductiontoAutoLayout

Asofnow,yourinterfacelooksniceintheInterfaceBuildercanvas.ButiOSdevicescomeinevermorescreensizes,andapplicationsareexpectedtosupportallscreensizesandorientations–andperhapsmorethanonedevicetype.Youneedtoguaranteethatthelayoutofviewobjectswillbecorrectregardlessofthescreensizeororientationofthedevicerunningtheapplication.ThetoolforthistaskisAutoLayout.

AutoLayoutworksbyspecifyingpositionandsizeconstraintsforeachviewobjectinascene.Theseconstraintscanberelativetoneighboringviewsorcontainerviews.Acontainerviewisjustaviewobjectthatcontainsanotherview.Forexample,takealookatthedocumentoutlineforMain.storyboard(Figure1.16).


https://www.bignerdranch.com/we-teach/how-to-prepare/ios-device-provisioning/

Figure1.16Documentlayoutwithacontainerview

YoucanseeinthedocumentoutlinethatthelabelsandbuttonsyouaddedareindentedwithrespecttoaViewobject.Thisviewobjectisthecontainerofthelabelsandbuttons,andtheobjectscanbepositionedandsizedrelativetothisview.

TobeginspecifyingAutoLayoutconstraints,selectthetoplabelbyclickingoniteitheronthecanvasorinthedocumentoutline.Atthebottomofthecanvas,noticetheAutoLayoutmenus,showninFigure1.17.

Figure1.17TheAutoLayoutmenus

Withthetoplabelstillselected,clickonthe icontorevealtheAlignmenushowninFigure1.18.

Figure1.18Centeringthetoplabelinthecontainer


WithintheAlignmenu,checktheHorizontallyinContainercheckboxtocenterthelabelinthecontainer.ThenclicktheAdd1Constraintbutton.Thisconstraintguaranteesthatonanysizescreen,inanyorientation,thelabelwillbecenteredhorizontally.

Nowyouneedtoaddmoreconstraintstocenterthelowerlabelandthebuttonswithrespecttothetoplabelandtolockthespacingbetweenthem.SelectthefourviewsbyCommand-clickingonthemoneafteranotherandthenclickonthe icontoopenthePinmenushowninFigure1.19.

Figure1.19Pinningthecenteringandspacingbetweenviews

AsseeninFigure1.19,clickontheredverticaldashedsegmentnearthetopofthemenu.Whenyouclickonthesegment,itwillbecomesolidred,indicatingthatthedistanceofeachviewispinnedtoitsnearesttopneighbor.Also,checktheAlignboxandthenselectHorizontalCentersfromthepop-upmenu.ForUpdateFrames,makesurethatyouhaveItemsofNewConstraintsselected.Finally,clickontheAdd7Constraintsbuttonatthebottomofthemenu.

Ifyoumadeanymistakeswhileaddingconstraints,youmayseeredororangeconstraintsandframesonthecanvasinsteadofthecorrectbluelines.Ifthatisthecase,youwillwanttocleartheexistingconstraintsandgothroughthestepsaboveagain.Toclearconstraints,firstselectthebackground(container)view.Thenclickthe icontoopentheResolveAutoLayoutIssuesmenu.SelectClearConstraintsundertheAllViewssection(Figure1.20).Thiswillclearawayanyconstraintsthatyouhaveaddedandgiveyouafreshstartonaddingtheconstraintsbackin.


Figure1.20Clearingconstraints

AutoLayoutcanbeadifficulttooltomaster,andthatiswhyyouarestartingtouseitinthefirstchapterofthisbook.Bystartingearly,youwillhavemorechancestouseitandgetusedtoitscomplexity.Also,dealingwithproblemsbeforethingsgettoocomplicatedwillhelpyouunderstandhowtodebuglayoutissueswithmoreconfidence.

Toconfirmthatyourinterfacebehavescorrectly,buildandruntheapplicationonthesimulator.

Makingconnections

Aconnectionletsoneobjectknowwhereanotherobjectisinmemorysothatthetwoobjectscancommunicate.TherearetwokindsofconnectionsthatyoucanmakeinInterfaceBuilder:outletsandactions.Anoutletisareferencetoanobject.Anactionisamethodthatgetstriggeredbyabuttonorsomeotherviewthattheusercaninteractwith,likeasliderorapicker.

Let’sstartbycreatingoutletsthatreferencetheinstancesofUILabel.TimetoleaveInterfaceBuilderandwritesomecode.

Declaringoutlets

Intheprojectnavigator,findandselectthefilenamedViewController.swift.TheeditorareawillchangefromInterfaceBuildertoXcode’scodeeditor.

InViewController.swift,startbydeletinganycodethatthetemplateaddedbetweenclassViewController:UIViewController{and}sothatthefilelookslikethis:importUIKit


}

Next,addthefollowingcodethatdeclarestwoproperties.(Throughoutthisbook,newcodeforyoutoaddwillbeshowninbold.Codeforyoutodeletewillbestruckthrough.)


Donotworryaboutunderstandingthecodeorpropertiesrightnow;justgetitin.importUIKit

classViewController:UIViewController{@IBOutletvarquestionLabel:UILabel!@IBOutletvaranswerLabel:UILabel!}

ThiscodegiveseveryinstanceofViewControlleranoutletnamedquestionLabelandanoutletnamedanswerLabel.TheviewcontrollercanuseeachoutlettoreferenceaparticularUILabelobject(i.e.,oneofthelabelsinyourview).The@IBOutletkeywordtellsXcodethatyouwillconnecttheseoutletstolabelobjectsusingInterfaceBuilder.

Settingoutlets

Intheprojectnavigator,selectMain.storyboardtoreopenInterfaceBuilder.

YouwantthequestionLabeloutlettopointtotheinstanceofUILabelatthetopoftheuserinterface.

Inthedocumentoutline,findtheViewControllerScenesectionandtheViewControllerobjectwithinit.Inyourcase,theViewControllerstandsinforaninstanceofViewController,whichistheobjectresponsibleformanagingtheinterfacedefinedinMain.storyboard.

Control-drag(orright-clickanddrag)fromtheViewControllerinthedocumentoutlinetothetoplabelinthescene.Whenthelabelishighlighted,releasethemouseandkeyboard;ablackpanelwillappear.SelectquestionLabeltosettheoutlet,asshowninFigure1.21.

Figure1.21SettingquestionLabel

(IfyoudonotseequestionLabelintheconnectionspanel,double-checkyourViewController.swiftfilefortypos.)


Now,whenthestoryboardfileisloaded,theViewController’squestionLabeloutletwillautomaticallyreferencetheinstanceofUILabelatthetopofthescreen,whichwillallowtheViewControllertotellthelabelwhatquestiontodisplay.

SettheanswerLabeloutletthesameway:Control-dragfromtheViewControllertothebottomUILabelandselectanswerLabel(Figure1.22).

Figure1.22SettinganswerLabel

Noticethatyoudragfromtheobjectwiththeoutletthatyouwanttosettotheobjectthatyouwantthatoutlettopointto.

Youroutletsareallset.Thenextconnectionsyouneedtomakeinvolvethetwobuttons.

Definingactionmethods

WhenaUIButtonistapped,itcallsamethodonanotherobject.Thatobjectiscalledthetarget.Themethodthatistriggerediscalledtheaction.Thisactionisthenameofthemethodthatcontainsthecodetobeexecutedinresponsetothebuttonbeingtapped.

Inyourapplication,thetargetforbothbuttonswillbetheinstanceofViewController.Eachbuttonwillhaveitsownaction.Let’sstartbydefiningthetwoactionmethods:showNextQuestion(_:)andshowAnswer(_:).

ReopenViewController.swiftandaddthetwoactionmethodsaftertheoutlets.classViewController:UIViewController{


@IBActionfuncshowNextQuestion(sender:AnyObject){

}

@IBActionfuncshowAnswer(sender:AnyObject){

}}


Youwillfleshoutthesemethodsafteryoumakethetargetandactionconnections.The@IBActionkeywordtellsXcodethatyouwillbemakingtheseconnectionsinInterfaceBuilder.

Settingtargetsandactions

SwitchbacktoMain.storyboard.Let’sstartwiththeNextQuestionbutton.YouwantitstargettobeViewControlleranditsactiontobeshowNextQuestion(_:).

Tosetanobject’starget,youControl-dragfromtheobjecttoitstarget.Whenyoureleasethemouse,thetargetisset,andapop-upmenuappearsthatletsyouselectanaction.

SelecttheNextQuestionbuttoninthecanvasandControl-dragtotheViewController.WhentheViewControllerishighlighted,releasethemousebuttonandchooseshowNextQuestion:underSentEventsinthepop-upmenu,asshowninFigure1.23.

Figure1.23SettingNextQuestiontarget/action

NowfortheShowAnswerbutton.SelectthebuttonandControl-dragfromthebuttontotheViewController.ThenchooseshowAnswer:fromthepop-upmenu.

Summaryofconnections

TherearenowfiveconnectionsbetweentheViewControllerandtheviewobjects.YouhavesetthepropertiesanswerLabelandquestionLabeltoreferencethelabelobjects–twoconnections.TheViewControlleristhetargetforbothbuttons–twomore.Theproject’stemplatemadeoneadditionalconnection:theviewpropertyofViewControllerisconnectedtotheViewobjectthatrepresentsthebackgroundoftheapplication.Thatmakesfive.


Youcanchecktheseconnectionsintheconnectionsinspector.SelecttheViewControllerinthedocumentoutline.Then,intheutilitiesarea,clickthe tabtorevealtheconnectionsinspector(Figure1.24).

Figure1.24Checkingconnectionsintheinspector

Yourstoryboardfileiscomplete.Theviewobjectshavebeencreatedandconfiguredandallthenecessaryconnectionshavebeenmadetothecontrollerobject.Let’smoveontocreatingandconnectingyourmodelobjects.


CreatingtheModelLayerViewobjectsmakeuptheuserinterface,sodeveloperstypicallycreate,configure,andconnectviewobjectsusingInterfaceBuilder.Thepartsofthemodellayer,ontheotherhand,aretypicallysetupincode.

Intheprojectnavigator,selectViewController.swift.Addthefollowingcodethatdeclarestwoarraysofstringsandaninteger.classViewController:UIViewController{


letquestions:[String]=["Fromwhatiscognacmade?","Whatis7+7?","WhatisthecapitalofVermont?"]letanswers:[String]=["Grapes","14","Montpelier"]varcurrentQuestionIndex:Int=0

...}

Thearrayswillbeorderedlistscontainingquestionsandanswers.Theintegerwillkeeptrackofwhatquestiontheuserison.

Noticethatthearraysaredeclaredusingtheletkeyword,whereastheintegerisdeclaredusingthevarkeyword.Aconstantisdenotedwiththeletkeyword;itsvaluecannotchange.Thequestionsandanswersarraysareconstants.Thequestionsandanswersinthisquizwillnotchangeand,infact,cannotbechangedfromtheirinitialvalues.

Avariable,ontheotherhand,isdenotedbythevarkeyword;itsvalueisallowedtochange.YoumadethecurrentQuestionIndexpropertyavariablebecauseitsvaluemustbeabletochangeastheusercyclesthroughthequestionsandanswers.

Implementingactionmethods

Nowthatyouhavequestionsandanswers,youcanfinishimplementingtheactionmethods.InViewController.swift,updateshowNextQuestion(_:)andshowAnswer(_:)....@IBActionfuncshowNextQuestion(sender:AnyObject){++currentQuestionIndexifcurrentQuestionIndex==questions.count{currentQuestionIndex=0}

letquestion:String=questions[currentQuestionIndex]questionLabel.text=questionanswerLabel.text="???"}

@IBActionfuncshowAnswer(sender:AnyObject){letanswer:String=answers[currentQuestionIndex]answerLabel.text=answer}


Loadingthefirstquestion

Justaftertheapplicationislaunched,youwillwanttoloadthefirstquestionfromthearrayanduseittoreplacethe???placeholderinthequestionLabellabel.AgoodwaytodothisisbyoverridingtheviewDidLoad()methodofViewController.(“Override”meansthatyouareprovidingacustomimplementationforamethod.)AddthemethodtoViewController.swift.classViewController:UIViewController{...overridefuncviewDidLoad(){super.viewDidLoad()questionLabel.text=questions[currentQuestionIndex]}}

Allthecodeforyourapplicationisnowcomplete!


BuildingtheFinishedApplicationBuildandruntheapplicationontheiPhone6ssimulator,asyoudidearlier.

Ifbuildingturnsupanyerrors,youcanviewthemintheissuenavigatorbyselectingthetabinthenavigatorarea(Figure1.25).

Figure1.25Issuenavigatorwithexampleerrorsandwarnings

Clickonanyerrororwarningintheissuenavigatortobetakentothefileandthelineofcodewheretheissueoccurred.Findandfixanyproblems(i.e.,codetypos!)bycomparingyourcodewiththecodeinthischapter.Thentryrunningtheapplicationagain.Repeatthisprocessuntilyourapplicationcompiles.

Onceyourapplicationhascompiled,itwilllaunchintheiOSsimulator.PlayaroundwiththeQuizapplication.YoushouldbeabletotaptheNextQuestionbuttonandseeanewquestioninthetoplabel;tappingShowAnswershouldshowtherightanswer.Ifyourapplicationisnotworkingasexpected,double-checkyourconnectionsinMain.storyboard.

YouhavebuiltaworkingiOSapp!Takeamomenttobaskintheglory.

OK,enoughbasking.Yourappworks,butitneedssomespitandpolish.


ApplicationIconsWhilerunningQuiz,selectHardware→Homefromthesimulator’smenu.YouwillseethatQuiz’siconisaboring,defaulttile.Let’sgiveQuizabettericon.

AnapplicationiconisasimpleimagethatrepresentstheapplicationontheiOSHomescreen.Differentdevicesrequiredifferent-sizedicons,someofwhichareshowninTable1.1.

Table1.1Applicationiconsizesbydevice

Device Applicationiconsizes

iPhone6sPlusandiPhone6Plus 180x180pixels(@3x)

iPhone6s,iPhone6,andiPhone5 120x120pixels(@2x)

iPadandiPadmini 152x152pixels(@2x)

iPadPro 167x167pixels(@2x)

Wehavepreparedaniconimagefile(size120x120)fortheQuizapplication.Youcandownloadthisicon(alongwithresourcesforotherchapters)fromhttp://www.bignerdranch.com/solutions/iOSProgramming5ed.zip.UnzipiOSProgramming5ed.zipandfindtheIcon@2x.pngfileintheResourcesdirectoryoftheunzippedfolder.

Youaregoingtoaddthisicontoyourapplicationbundleasaresource.Ingeneral,therearetwokindsoffilesinanapplication:codeandresources.Code(likeViewController.swift)isusedtocreatetheapplicationitself.Resourcesarethingslikeimagesandsoundsthatareusedbytheapplicationatruntime.

Intheprojectnavigator,findAssets.xcassets.SelectthisfiletoopenitandthenselectAppIconfromtheresourcelistonthelefthandside(Figure1.26).

Figure1.26ShowingtheAssetCatalog



ThispanelistheAssetCatalog,whereyoucanmanagealloftheimagesthatyourapplicationwillneed.

DragtheIcon@2x.pngfilefromFinderontothemarginsoftheAppIconsection.Thiswillcopythefileintoyourproject’sdirectoryonthefilesystemandaddareferencetothatfileintheAssetCatalog.(YoucanControl-clickonafileintheAssetCatalogandselecttheoptiontoShowinFindertoconfirmthis,asshowninFigure1.27.)

Figure1.27AddingtheappicontotheAssetCatalog

Buildandruntheapplicationagain.Switchtothesimulator’sHomescreeneitherbyclickingHome→Hardware,asyoudidbefore,orbyusingthekeyboardshortcutCommand-Shift-H.Youshouldseethenewicon.

(Ifyoudonotseetheicon,deletetheapplicationandthenbuildandrunagaintoredeployit.Todothis,theeasiestoptionistoresetthesimulatorbyclickingiOSSimulator→ResetContentandSettings….Thiswillremoveallapplicationsandresetthesimulatortoitsdefaultsettings.Youshouldseetheappiconthenexttimeyouruntheapplication.)


LaunchScreenAnotheritemyoushouldsetfortheprojectisthelaunchimage,whichappearswhileanapplicationisloading.ThelaunchimagehasaspecificroleiniOS:itconveystotheuserthattheapplicationisindeedlaunchinganddepictstheuserinterfacethattheuserwillinteractwithoncetheapplicationloads.Therefore,agoodlaunchimageisacontent-lessscreenshotoftheapplication.Forexample,theClockapplication’slaunchimageshowsthefourtabsalongthebottom,allintheunselectedstate.Oncetheapplicationloads,thecorrecttabisselectedandthecontentbecomesvisible.(Keepinmindthatthelaunchimageisreplacedaftertheapplicationhaslaunched;itdoesnotbecomethebackgroundimageoftheapplication.)

AneasywaytoaccomplishthisistoallowXcodetogeneratethepossiblelaunchscreenimagesforyouusingalaunchscreenfile.

Opentheprojectsettingsbyclickingonthetop-levelQuizintheprojectnavigator.UnderAppIconsandLaunchImages,chooseMain.storyboardfromtheLaunchScreenFiledropdown(Figure1.28).LaunchimageswillnowbegeneratedfromMain.storyboard.

Figure1.28Settingthelaunchscreenfile

Congratulations!Youhavewrittenyourfirstapplicationandevenaddedsomedetailstomakeitpolished.


2TheSwiftLanguage

SwiftisanewlanguagethatAppleintroducedin2014.ItreplacesObjective-CastherecommendeddevelopmentlanguageforiOSandMac.Inthischapter,youaregoingtofocusonthebasicsofSwift.Youwillnotlearneverything,butyouwilllearnenoughtogetstarted.Then,asyoucontinuethroughthebook,youwilllearnmoreSwiftwhileyoulearniOSdevelopment.

SwiftmaintainstheexpressivenessofObjective-Cwhileintroducingasyntaxthatissafer,succinct,andreadable.Itemphasizestypesafetyandintroducesadvancedfeaturessuchasoptionals,generics,andsophisticatedstructuresandenumerations.Mostimportantly,Swiftallowstheuseofthesenewfeatureswhilerelyingonthesametested,elegantiOSframeworksthatdevelopershavebuiltuponforyears.

IfyouknowObjective-C,thenthechallengeisrecastingwhatyouknow.Itmayseemawkwardatfirst,butwehavecometoloveSwiftatBigNerdRanchandbelieveyouwill,too.

IfyoudonotthinkyouwillbecomfortablepickingupSwiftatthesametimeasiOSdevelopment,youmaywanttostartwithSwiftProgramming:TheBigNerdRanchGuideorApple’sSwifttutorials,whichyoucanfindathttp://developer.apple.com/swift.Butifyouhavesomeprogrammingexperienceandarewillingtolearn“onthejob,”youcanstartyourSwifteducationhereandnow.


http://developer.apple.com/swift

TypesinSwiftSwifttypescanbearrangedintothreebasicgroups:structures,classes,andenumerations(Figure2.1).Allthreecanhave

properties:valuesassociatedwithatype

initializers:codethatinitializesaninstanceofatype

instancemethods:functionsspecifictoatypethatcanbecalledonaninstanceofthattype

classorstaticmethods:functionsspecifictoatypethatcanbecalledonthetypeitself

Figure2.1Swiftbuildingblocks

Swift’sstructures(or“structs”)andenumerations(or“enums”)aresignificantlymorepowerfulthaninmostlanguages.Inadditiontosupportingproperties,initializers,andmethods,theycanalsoconformtoprotocolsandcanbeextended.

Swift’simplementationoftypically“primitive”typessuchasnumbersandBooleanvaluesmaysurpriseyou:theyareallstructures.Infact,alloftheseSwifttypesarestructures:

Numbers: Int,Float,Double

Boolean: Bool

Text: String,Character

Collections: Array<T>,Dictionary<K:Hashable,V>,Set<T:Hashable>

Thismeansthatstandardtypeshaveproperties,initializers,andmethodsoftheirown.Theycanalsoconformtoprotocolsandbeextended.

Finally,akeyfeatureofSwiftisoptionals.Anoptionalallowsyoutostoreeitheravalueofaparticulartypeornovalueatall.Laterinthechapter,youwilllearnmoreaboutoptionalsandtheirroleinSwift.


UsingStandardTypesInthissection,youaregoingtoexperimentwithstandardtypesinanXcodeplayground.Aplaygroundletsyouwritecodeandseetheresultswithouttheoverheadofmanuallyrunninganapplicationandcheckingtheoutput.

InXcode,selectFile→New→Playground….Youcanacceptthedefaultnameforthisfile;youwillonlybeherebriefly.MakesuretheplatformisiOS(Figure2.2).

Figure2.2Configuringaplayground

ClickNextandsavethisfileinaconvenientplace.

Whenthefileopens,noticethattheplaygroundisdividedintotwosections(Figure2.3).Thelargerwhiteareatotheleftistheeditorwhereyouwritecode.Thegraycolumnontherightisthesidebar.Theplaygroundcompilesandexecutesyourcodeaftereverylineandshowstheresultsinthesidebar.

Figure2.3Aplayground


Intheexamplecode,thevarkeyworddenotesavariable,sothevalueofstrcanbechangedfromitsinitialvalue.Typeinthecodebelowtochangethevalueofstr,andyouwillseetheresultsappearinthesidebartotheright.varstr="Hello,playground""Hello,playground"str="Hello,Swift""Hello,Swift"

(Noticethatweareshowingsidebarresultstotherightofthecodeforthebenefitofreaderswhoarenotactivelydoingtheexercise.)

Theletkeyworddenotesaconstantvalue,whichcannotbechanged.InyourSwiftcode,youshoulduseletunlessyouexpectthevaluewillneedtochange.Addaconstanttothemix:varstr="Hello,playground""Hello,playground"str="Hello,Swift""Hello,Swift"letconstStr=str"Hello,Swift"

BecauseconstStrisaconstant,attemptingtochangeitsvaluewillcauseanerror.varstr="Hello,playground""Hello,playground"str="Hello,Swift""Hello,Swift"letconstStr=str"Hello,Swift"constStr="Hello,world"

Anerrorappears,indicatedbytheredsymboltotheleftoftheoffendingline.Clickthesymboltogetmoreinformationabouttheerror.Inthiscase,theerrorreads“Cannotassigntovalue:‘constStr’isa‘let’constant.”

Anerrorintheplaygroundcodewillpreventyoufromseeinganyfurtherresultsinthesidebar,soyouusuallywanttoaddressitrightaway.RemovethelinethatattemptstochangethevalueofconstStr.varstr="Hello,playground""Hello,playground"str="Hello,Swift""Hello,Swift"letconstStr=str"Hello,Swift"constStr="Hello,world"

Inferringtypes

Atthispoint,youmayhavenoticedthatneithertheconstStrconstantnorthestrvariablehasaspecifiedtype.Thisdoesnotmeantheyareuntyped!Instead,thecompilerinferstheirtypesfromtheinitialvalues.Thisiscalledtypeinference.

YoucanfindoutwhattypewasinferredusingQuickHelp.Option-clickonconstStrtoseetheQuickHelpinformationforthisconstant,showninFigure2.4.


Figure2.4constStrisoftypeString

Option-clickingtorevealQuickHelpwillworkforanysymbol.

Specifyingtypes

Ifyourconstantorvariablehasaninitialvalue,youcanrelyontypeinference.Ifaconstantorvariabledoesnothaveaninitialvalueorifyouwanttoensurethatitisacertaintype,youcanspecifythetypeinthedeclaration.

Addmorevariableswithspecifiedtypes:varstr="Hello,playground""Hello,playground"str="Hello,Swift""Hello,Swift"letconstStr=str"Hello,Swift"

varnextYear:IntvarbodyTemp:FloatvarhasPet:Bool

Notethatthesidebardoesnotreportanyresultsbecausethesevariablesdonotyethavevalues.

Let’sgooverthesenewtypesandhowtheyareused.

NumberandBooleantypes

ThemostcommontypeforintegersisInt.Thereareadditionalintegertypesbasedonwordsizeandsignedness,butApplerecommendsusingIntunlessyoureallyhaveareasontousesomethingelse.

Forfloating-pointnumbers,Swiftprovidesthreetypeswithdifferentlevelsofprecision:Floatfor32-bitnumbers,Doublefor64-bitnumbers,andFloat80for80-bitnumbers.

ABooleanvalueisexpressedinSwiftusingthetypeBool.ABool’svalueiseithertrueorfalse.

Collectiontypes

TheSwiftstandardlibraryoffersthreecollections:arrays,dictionaries,andsets.


Anarrayisanorderedcollectionofelements.ThearraytypeiswrittenasArray<T>,whereTisthetypeofelementthatthearraywillcontain.Arrayscancontainelementsofanytype:astandardtype,astructure,oraclass.

Addavariableforanarrayofintegers:varhasPet:BoolvararrayOfInts:Array<Int>

Arraysarestronglytyped.Onceyoudeclareanarrayascontainingelementsof,say,Int,youcannotaddaStringtothisarray.

Thereisashorthandsyntaxfordeclaringarrays:youcansimplyusesquarebracketsaroundthetypethatthearraywillcontain.UpdatethedeclarationofarrayOfIntstousetheshorthand:varhasPet:BoolvararrayOfInts:Array<Int>vararrayOfInts:[Int]

Adictionaryisanunorderedcollectionofkey-valuepairs.Thevaluescanbeofanytype,includingstructuresandclasses.Thekeyscanbeofanytypeaswell,buttheymustbeunique.Specifically,thekeysmustbehashable,whichallowsthedictionarytoguaranteethatthekeysareuniqueaswellasaccessthevalueforagivenkeymoreefficiently.BasicSwifttypessuchasInt,Float,Character,andStringareallhashable.

LikeSwiftarrays,Swiftdictionariesarestronglytypedandcanonlycontainkeysandvaluesofthedeclaredtype.Forexample,youmighthaveadictionarythatstorescapitalcitiesbycountry.Thekeysforthisdictionarywouldbethecountrynames,andthevalueswouldbethecitynames.Bothkeysandvalueswouldbestrings,andyouwouldnotbeabletoaddakeyorvalueofanyothertype.

Addavariableforsuchadictionary:vararrayOfInts:[Int]vardictionaryOfCapitalsByCountry:Dictionary<String,String>

Thereisashorthandsyntaxfordeclaringdictionaries,too.UpdatedictionaryOfCapitalsByCountrytousetheshorthand:vararrayOfInts:[Int]vardictionaryOfCapitalsByCountry:Dictionary<String,String>vardictionaryOfCapitalsByCountry:[String:String]

Asetissimilartoanarrayinthatitcontainsanumberofelementsofacertaintype.However,setsareunordered,andthemembersmustbeuniqueaswellashashable.Theunorderednessofsetsmakesthemfasterwhenyousimplyneedtodeterminewhethersomethingisamemberofaset.Addavariableforaset:varwinningLotteryNumbers:Set<Int>

Literalsandsubscripting

Standardtypescanbeassignedliteralvalues,orliterals.Forexample,strisassignedthevalueofastringliteral.Astringliteralisformedwithdoublequotes.ContrasttheliteralvalueassignedtostrwiththenonliteralvalueassignedtoconstStr:


varstr="Hello,playground""Hello,playground"str="Hello,Swift""Hello,Swift"letconstStr=str"Hello,Swift"

Addtwonumberliteralstoyourplayground:letnumber=4242letfmStation=91.191.1

Arraysanddictionariescanbeassignedliteralvaluesaswell.Thesyntaxforcreatingliteralarraysanddictionariesresemblestheshorthandsyntaxforspecifyingthesetypes.letcountingUp=["one","two"]["one","two"]letnameByParkingSpace=[13:"Alice",27:"Bob"][13:"Alice",27:"Bob"]

Swiftalsoprovidessubscriptingasshorthandforaccessingarrays.Toretrieveanelementinanarray,youprovidetheelement’sindexinsquarebracketsafterthearrayname.letcountingUp=["one","two"]["one","two"]letsecondElement=countingUp[1]"two"...

Noticethatindex1retrievesthesecondelement;anarray’sindexalwaysstartsat0.

Whensubscriptinganarray,besurethatyouareusingavalidindex.Attemptingtoaccessanout-of-boundsindexresultsinatrap.Atrapisaruntimeerrorthatstopstheprogrambeforeitgetsintoanunknownstate.

Subscriptingalsoworkswithdictionaries–moreonthatlaterinthischapter.

Initializers

Sofar,youhaveinitializedyourconstantsandvariablesusingliteralvalues.Indoingso,youcreatedinstancesofaspecifictype.Aninstanceisaparticularembodimentofatype.Historically,thistermhasbeenonlyusedwithclasses,butinSwiftitisusedtodescribestructuresandenumerations,too.Forexample,theconstantsecondElementholdsaninstanceofString.

Anotherwayofcreatinginstancesisbyusinganinitializeronthetype.Initializersareresponsibleforinitializingthecontentsofanewinstanceofatype.Whenaninitializerisfinished,theinstanceisreadyforaction.Tocreateanewinstanceusinganinitializer,youusethetypenamefollowedbyapairofparenthesesand,ifrequired,arguments.Thissignature–thecombinationoftypeandarguments–correspondstoaspecificinitializer.

Somestandardtypeshaveinitializersthatreturnemptyliteralswhennoargumentsaresupplied.Addanemptystringandanemptyarraytoyourplayground.letemptyString=String()""letemptyArrayOfInts=[Int]()0elementsletemptySetOfFloats=Set<Float>()0elements

Othertypeshavedefaultvalues:letdefaultNumber=Int()0letdefaultBool=Bool()false

Typescanhavemultipleinitializers.Forexample,StringhasaninitializerthatacceptsanIntandcreatesastringbasedonthatvalue.letnumber=4242


letmeaningOfLife=String(number)"42"

Tocreateaset,youusetheSetinitializerthatacceptsanarrayliteral:letavailableRooms=Set([205,411,412]){412,205,411}

Floathasseveralinitializers.Theparameter-lessinitializerreturnsaninstanceofFloatwiththedefaultvalue.Thereisalsoaninitializerthatacceptsafloating-pointliteral.letdefaultFloat=Float()0.0letfloatFromLiteral=Float(3.14)3.14

Ifyouusetypeinferenceforafloating-pointliteral,thetypedefaultstoDouble.Createthefollowingconstantwithafloating-pointliteral:leteasyPi=3.143.14

UsetheFloatinitializerthatacceptsaDoubletocreateaFloatfromthisDouble:leteasyPi=3.143.14letfloatFromDouble=Float(easyPi)3.14

Youcanachievethesameresultbyspecifyingthetypeinthedeclaration.leteasyPi=3.143.14letfloatFromDouble=Float(easyPi)3.14letfloatingPi:Float=3.143.14

Properties

Apropertyisavalueassociatedwithaninstanceofatype.Forexample,StringhasthepropertyisEmpty,whichisaBoolthattellsyouwhetherthestringisempty.Array<T>hasthepropertycount,whichisthenumberofelementsinthearrayasanInt.Accessthesepropertiesinyourplayground:letcountingUp=["one","two"]["one","two"]letsecondElement=countingUp[1]"two"countingUp.count2

...

letemptyString=""emptyString.isEmptytrue

Instancemethods

Aninstancemethodisafunctionthatisspecifictoaparticulartypeandcanbecalledonaninstanceofthattype.Tryouttheappend(_:)instancemethodfromArray<T>.YouwillfirstneedtochangeyourcountingUparrayfromaconstanttoavariable.letcountingUp=["one","two"]varcountingUp=["one","two"]["one","two"]letsecondElement=countingUp[1]"two"countingUp.count

countingUp.append("three")["one","two","three"]

Theappend(_:)methodacceptsanelementofthearray’stypeandaddsittotheendofthearray.Wewilldiscussmethods,includingnaming,inChapter3.


OptionalsSwifttypescanbeoptional,whichisindicatedbyappending?toatypename.varanOptionalFloat:Float?varanOptionalArrayOfStrings:[String]?varanOptionalArrayOfOptionalStrings:[String?]?

Anoptionalletsyouexpressthepossibilitythatavariablemaynotstoreavalueatall.Thevalueofanoptionalwilleitherbeaninstanceofthespecifiedtypeornil.

Throughoutthisbook,youwillhavemanychancestouseoptionals.Whatfollowsisanexampletogetyoufamiliarwiththesyntaxsothatyoucanfocusontheuseoftheoptionalslater.

Imagineagroupofinstrumentreadings:varreading1:Floatvarreading2:Floatvarreading3:Float

Sometimes,aninstrumentmightmalfunctionandnotreportareading.Youdonotwantthismalfunctionshowingupas,say,0.0.Youwantittobesomethingcompletelydifferentthattellsyoutocheckyourinstrumentortakesomeotheraction.

Youcandothisbydeclaringthereadingsasoptionals.varreading1:Float?nilvarreading2:Float?nilvarreading3:Float?nil

Asanoptionalfloat,eachreadingcancontaineitheraFloatornil.Ifnotgivenaninitialvalue,thenthevaluedefaultstonil.

Youcanassignvaluestoanoptionaljustlikeanyothervariable.Assignfloating-pointliteralstothereadings:reading1=9.89.8reading2=9.29.2reading3=9.79.7

However,youcannotusetheseoptionalfloatslikenon-optionalfloats–eveniftheyhavebeenassignedFloatvalues.Beforeyoucanreadthevalueofanoptionalvariable,youmustaddressthepossibilityofitsvaluebeingnil.Thisiscalledunwrappingtheoptional.

Youaregoingtotryouttwowaysofunwrappinganoptionalvariable:optionalbindingandforcedunwrapping.Youwillimplementforcedunwrappingfirst.Thisisnotbecauseitisthebetteroption–infact,itisthelesssafeone.Butimplementingforcedunwrappingfirstwillletyouseethedangersandunderstandwhyoptionalbindingistypicallybetter.

Toforciblyunwrapanoptional,youappenda!toitsname.First,tryaveragingthereadingsasiftheywerenon-optionalvariables:reading1=9.89.8reading2=9.29.2reading3=9.79.7letavgReading=(reading1+reading2+reading3)/3


Thisresultsinanerrorbecauseoptionalsrequireunwrapping.Forciblyunwrapthereadingstofixtheerror:letavgReading=(reading1+reading2+reading3)/3letavgReading=(reading1!+reading2!+reading3!)/39.566667

Everythinglooksfine,andyouseethecorrectaverageinthesidebar.Butadangerlurksinyourcode.Whenyouforciblyunwrapanoptional,youtellthecompilerthatyouaresurethattheoptionalwillnotbenilandcanbetreatedasifitwereanormalFloat.Butwhatifyouarewrong?Tofindout,commentouttheassignmentofreading3,whichwillreturnittoitsdefaultvalue,nil.reading1=9.89.8reading2=9.29.2reading3=9.7//reading3=9.7

Younowhaveanerror.Toseetheconsolewheretheerrorisreportedinaplayground,youmustopenthedebugarea.FromXcode’sViewmenu,selectDebugArea→ShowDebugArea.Theerrorreads:fatalerror:unexpectedlyfoundnilwhileunwrappinganOptionalvalue

Ifyouforciblyunwrapanoptionalandthatoptionalturnsouttobenil,itwillcauseatrap,stoppingyourapplication.

Asaferwaytounwrapanoptionalisoptionalbinding.Optionalbindingworkswithinaconditionalif-letstatement:Youassigntheoptionaltoatemporaryconstantofthecorrespondingnon-optionaltype.Ifyouroptionalhasavalue,thentheassignmentisvalidandyouproceedusingthenon-optionalconstant.Iftheoptionalisnil,thenyoucanhandlethatcasewithanelseclause.

Changeyourcodetouseanif-letstatementthattestsforvalidvaluesinallthreereadings.letavgReading=(reading1!+reading2!+reading3!)/3ifletr1=reading1,r2=reading2,r3=reading3{letavgReading=(r1+r2+r3)/3}else{leterrorString="Instrumentreportedareadingthatwasnil."}

reading3iscurrentlynil,soitsassignmenttor3fails,andthesidebarshowstheerrorstring.

Toseetheothercaseinaction,restorethelinethatassignsavaluetoreading3.Nowthatallthreereadingshavevalues,allthreeassignmentsarevalid,andthesidebarupdatestoshowtheaverageofthethreereadings.

Subscriptingdictionaries

Recallthatsubscriptinganarraybeyonditsboundscausesatrap.Dictionariesaredifferent.Theresultofsubscriptingadictionaryisanoptional:letnameByParkingSpace=[13:"Alice",27:"Bob"][13:"Alice",27:"Bob"]letspace13Assignee:String?=nameByParkingSpace[13]"Alice"


letspace42Assignee:String?=nameByParkingSpace[42]nil

Ifthekeyisnotinthedictionary,theresultwillbenil.Aswithotheroptionals,itiscommontouseif-letwhensubscriptingadictionary:letspace13Assignee:String?=nameByParkingSpace[13]ifletspace13Assignee=nameByParkingSpace[13]{print("Key13isassignedinthedictionary!")}


LoopsandStringInterpolationSwifthasallthecontrolflowstatementsthatyoumaybefamiliarwithfromotherlanguages:if-else,while,for,for-in,repeat-while,andswitch.Eveniftheyarefamiliar,however,theremaybesomedifferencesfromwhatyouareaccustomedto.ThekeydifferencebetweenthesestatementsinSwiftandinC-likelanguagesisthatwhileenclosingparenthesesarenotnecessaryonthesestatements’expressions,Swiftdoesrequirebracesonclauses.Additionally,theexpressionsforifandwhile-likestatementsmustevaluatetoaBool.

ConsiderthisverytraditionalC-styleloop,writteninSwift:forvari=0;i<countingUp.count;++i{letstring=countingUp[i]//Use'string'}

YoucoulddothesamethingalittlemorecleanlyusingSwift’sRangetypeandthefor-instatement:letrange=0..<countingUp.countforiinrange{letstring=countingUp[i]//Use'string'}

Themostdirectroutewouldbetoenumeratetheitemsinthearraythemselves:forstringincountingUp{//Use'string'}

Whatifyouwantedtheindexofeachiteminthearray?Swift’senumerate()functionreturnsasequenceofintegersandvaluesfromitsargument:for(i,string)incountingUp.enumerate(){//(0,"one"),(1,"two")}

Whatarethoseparentheses,youask?Theenumerate()functionactuallyreturnsasequenceoftuples.Atupleisanorderedgroupingofvaluessimilartoanarray,excepteachmembermayhaveadistincttype.Inthisexamplethetupleisoftype(Int,String).WewillnotspendmuchtimeontuplesinthisbookastheyarenotusediniOSAPIs(becauseObjective-Cdoesnotsupporttuples).TheycanbeusefulinyourSwiftcode,however.

Anotherapplicationoftuplesisinenumeratingthecontentsofadictionary:letnameByParkingSpace=[13:"Alice",27:"Bob"]

for(space,name)innameByParkingSpace{letpermit="Space\(space):\(name)"}

Didyounoticethatcuriousmarkupinthestringliteral?ThatisSwift’sstringinterpolation.Expressionsenclosedwithin\(and)areevaluatedandinsertedintothestringatruntime.Inthisexampleyouareusinglocalvariables,butanyvalidSwiftexpression,suchasamethodcall,canbeused.


Toseethevaluesofthepermitvariableintheplayground,Control-clickontheresultandselectValueHistory.Ifyoudonotseeavalue,clickonthecircularShowResultindicator(Figure2.5).Thiscanbeveryusefulforvisualizingwhatishappeninginyourplaygroundcode’sloops.

Figure2.5UsingtheValueHistorytoseetheresultsofstringinterpolation


EnumerationsandtheSwitchStatementAnenumerationisatypewithadiscretesetofvalues.Defineanenumdescribingpies:enumPieType{caseApplecaseCherrycasePecan}

letfavoritePie=PieType.Apple

Swifthasapowerfulswitchstatementthat,amongotherthings,isgreatformatchingonenumvalues:letname:StringswitchfavoritePie{case.Apple:name="Apple"case.Cherry:name="Cherry"case.Pecan:name="Pecan"}

Thecasesforaswitchstatementmustbeexhaustive:eachpossiblevalueoftheswitchexpressionmustbeaccountedfor,whetherexplicitlyorviaadefault:case.UnlikeinC,Swiftswitchcasesdonotfallthrough–onlythecodeforthecasethatismatchedisexecuted.(Ifyouneedthefall-throughbehaviorofC,youcanexplicitlyrequestitusingthefallthroughkeyword.)

Switchstatementscanmatchonmanytypes,evenranges:letosxVersion:Int=...switchosxVersion{case0...8:print("Abigcat")case9:print("Mavericks")case10:print("Yosemite")default:print("Greetings,peopleofthefuture!What'snewin10.\(osxVersion)?")}

Formoreontheswitchstatementanditspatternmatchingcapabilities,seetheControlFlowsectionintheTheSwiftProgrammingLanguageguide.(Moreonthatinjustamoment.)

Enumerationsandrawvalues

Swiftenumscanhaverawvaluesassociatedwiththeircases:enumPieType:Int{caseApple=0caseCherrycasePecan}

Withthetypespecified,youcanaskaninstanceofPieTypeforitsrawValueandtheninitializetheenumtypewiththatvalue.Thisreturnsanoptional,sincetherawvaluemaynotcorrespondwithanactualcaseoftheenum,soitisagreatcandidateforoptional


binding.letpieRawValue=PieType.Pecan.rawValue//pieRawValueisanIntwithavalueof2

ifletpieType=PieType(rawValue:pieRawValue){//Gotavalid'pieType'!}

TherawvalueforanenumisoftenanInt,butitcanbeanyintegerorfloating-pointnumbertypeaswellastheStringandCharactertypes.

Whentherawvalueisanintegertype,thevaluesautomaticallyincrementifnoexplicitvalueisgiven.ForPieType,onlytheApplecaseisgivenanexplicitvalue.CherryandPecanautomaticallygetassignedarawValueof1and2,respectively.

Thereismoretoenumerations.Eachcaseofanenumerationcanhaveassociatedvalues.YouwilllearnmoreaboutassociatedvaluesinChapter19.


ExploringApple’sSwiftDocumentationToexploreApple’sdocumentationonSwift,startathttp://developer.apple.com/swift.Herearetwoparticularresourcestolookfor.Wesuggestbookmarkingthemandvisitingthemwhenyouwanttoreviewaparticularconceptordigalittledeeper.

TheSwiftProgrammingLanguage

ThisguidedescribesmanyfeaturesofSwift.Itstartswiththebasicsandincludesexamplecodeandlotsofdetail.ItalsocontainsthelanguagereferenceandformalgrammarofSwift.

TheSwiftStandardLibraryReference

ThestandardlibraryreferencelaysoutthedetailsofSwifttypes,protocols,andglobal,orfree,functions.

YourhomeworkistobrowsethroughtheTypessectionoftheSwiftStandardLibraryReferenceandthesectionsofTheSwiftProgrammingLanguageguideonTheBasics,StringsandCharacters,andCollectionTypes.Solidifywhatyoulearnedinthischapterandbecomefamiliarwiththeinformationtheseresourcesoffer.Ifyouknowwheretofindthedetailswhenyouneedthem,thenyouwillfeellesspressuretomemorizethem–lettingyoufocusoniOSdevelopmentinstead.


http://developer.apple.com/swift

3ViewsandtheViewHierarchy

Overthenextfivechapters,youaregoingtobuildanapplicationnamedWorldTrotter.Whenitiscomplete,thisappwillconvertvaluesbetweendegreesFahrenheitanddegreesCelsius.Inthischapter,youwilllearnaboutviewsandtheviewhierarchythroughcreatingWorldTrotter’suserinterface.Attheendofthischapter,yourappwilllooklikeFigure3.1.

Figure3.1WorldTrotter

Let’sstartwithalittlebitofthetheorybehindviewsandtheviewhierarchy.


ViewBasicsRecallfromChapter1thatviewsareobjectsthatarevisibletotheuser,likebuttons,textfields,andsliders.Viewobjectsmakeupanapplication’suserinterface.Aview

isaninstanceofUIVieworoneofitssubclasses

knowshowtodrawitself

canhandleevents,liketouches

existswithinahierarchyofviewswhoserootistheapplication’swindow

Let’slookattheviewhierarchyingreaterdetail.


TheViewHierarchyEveryapplicationhasasingleinstanceofUIWindowthatservesasthecontainerforalltheviewsintheapplication.UIWindowisasubclassofUIView,sothewindowisitselfaview.Thewindowiscreatedwhentheapplicationlaunches.Oncethewindowiscreated,otherviewscanbeaddedtoit.

Whenaviewisaddedtothewindow,itissaidtobeasubviewofthewindow.Viewsthataresubviewsofthewindowcanalsohavesubviews,andtheresultisahierarchyofviewobjectswiththewindowatitsroot(Figure3.2).

Figure3.2Anexampleviewhierarchyandtheinterfacethatitcreates

Oncetheviewhierarchyiscreated,itwillbedrawntothescreen.Thisprocesscanbebrokenintotwosteps:

Eachviewinthehierarchy,includingthewindow,drawsitself.Itrendersitselftoitslayer,whichyoucanthinkofasabitmapimage.(ThelayerisaninstanceofCALayer.)

Thelayersofalltheviewsarecompositedtogetheronthescreen.

Figure3.3showsanotherexampleviewhierarchyandthetwodrawingsteps.


Figure3.3Viewsrenderthemselvesandthenarecompositedtogether

ForWorldTrotter,youaregoingtocreateaninterfacecomposedofdifferentviews.TherewillbefourinstancesofUILabelandoneinstanceofUITextFieldthatwillallowtheusertoenterinatemperatureinFahrenheit.Let’sgetstarted.


CreatingaNewProjectInXcode,selectFile→New→Project…(orusethekeyboardshortcutCommand-Shift-N).FromtheiOSsection,selectApplication,choosetheSingleViewApplicationtemplate,andclickNext.

EnterWorldTrotterfortheproductname.MakesurethatSwiftisselectedfromtheLanguagedropdownandthatiPhoneisselectedfromtheDevicesdropdown.AlsomakesuretheUseCoreDataboxisunchecked(Figure3.4).ClickNextandthenCreateonthefollowingscreen.

Figure3.4ConfiguringWorldTrotter


ViewsandFramesWhenyouinitializeaviewprogrammatically,youuseitsinit(frame:)designatedinitializer.Thismethodtakesoneargument,aCGRect,thatwillbecometheview’sframe,apropertyonUIView.varframe:CGRect

Aview’sframespecifiestheview’ssizeanditspositionrelativetoitssuperview.Becauseaview’ssizeisalwaysspecifiedbyitsframe,aviewisalwaysarectangle.

ACGRectcontainsthemembersoriginandsize.TheoriginisastructureoftypeCGPointandcontainstwoCGFloatproperties:xandy.ThesizeisastructureoftypeCGSizeandhastwoCGFloatproperties:widthandheight(Figure3.5).

Figure3.5CGRect

Whentheapplicationislaunched,theviewfortheinitialviewcontrollerisaddedtotheroot-levelwindow.ThisviewcontrollerisrepresentedbytheViewControllerclassdefinedinViewController.swift.WewilldiscusswhataviewcontrollerisinChapter5,butfornow,itissufficienttoknowthataviewcontrollerhasaviewandthattheviewassociatedwiththemainviewcontrollerfortheapplicationisaddedasasubviewofthewindow.

BeforeyoucreatetheviewsforWorldTrotter,youaregoingtoaddsomepracticeviewsprogrammaticallytoexploreviewsandtheirpropertiesandseehowtheinterfacesfor


applicationsarecreated.

OpenViewController.swiftanddeleteanymethodsthatthetemplatecreated.Yourfileshouldlooklikethis:importUIKit


}

(CuriousabouttheimportUIKitline?UIKitisaframework.Aframeworkisacollectionofrelatedclassesandresources.TheUIKitframeworkdefinesmanyoftheuserinterfaceelementsthatyouruserssee,aswellasotheriOS-specificclasses.Youwillbeusingafewdifferentframeworksasyougothroughthisbook.)

Rightaftertheviewcontroller’sviewisloadedintomemory,itsviewDidLoad()methodiscalled.Thismethodgivesyouanopportunitytocustomizetheviewhierarchy,soitisagreatplacetoaddyourpracticeviews.

InViewController.swift,overrideviewDidLoad().CreateaCGRectthatwillbetheframeofaUIView.Next,createaninstanceofUIViewandsetitsbackgroundColorpropertytoblue.Finally,addtheUIViewasasubviewoftheviewcontroller’sviewtomakeitpartoftheviewhierarchy.(Muchofthiswillnotlookfamiliar.Thatisfine.Wewillexplainmoreafteryouenterthecode.)classViewController:UIViewController{

overridefuncviewDidLoad(){super.viewDidLoad()

letfirstFrame=CGRect(x:160,y:240,width:100,height:150)letfirstView=UIView(frame:firstFrame)firstView.backgroundColor=UIColor.blueColor()view.addSubview(firstView)}

}

TocreateaCGRect,youuseitsinitializerandpassinthevaluesfororigin.x,origin.y,size.width,andsize.height.

TosetthebackgroundColor,youusetheUIColorclassmethodblueColor().ThisisaconveniencemethodthatinitializesaninstanceofUIColorthatisconfiguredtobeblue.ThereareanumberofUIColorconveniencemethodsforcommoncolors,suchasgreenColor(),blackColor(),andclearColor().

Buildandruntheapplication(Command-R).YouwillseeabluerectanglethatistheinstanceofUIView.BecausetheoriginoftheUIView’sframeis(160,240),therectangle’stopleftcorneris160pointstotherightand240pointsdownfromthetopleftcornerofitssuperview.Theviewstretches100pointstotherightand150pointsdownfromitsorigin,inaccordancewithitsframe’ssize(Figure3.6).


Figure3.6WorldTrotterwithoneUIView

Notethatthesevaluesareinpoints,notpixels.Ifthevalueswereinpixels,thentheywouldnotbeconsistentacrossdisplaysofdifferentresolutions(i.e.,Retinavs.non-Retina).Asinglepointisarelativeunitofameasure;itwillbeadifferentnumberofpixelsdependingonhowmanypixelsareinthedisplay.Sizes,positions,lines,andcurvesarealwaysdescribedinpointstoallowfordifferencesindisplayresolution.

Figure3.7representstheviewhierarchythatyouhavecreated.


Figure3.7Currentviewhierarchy

EveryinstanceofUIViewhasasuperviewproperty.Whenyouaddaviewasasubviewofanotherview,theinverserelationshipisautomaticallyestablished.Inthiscase,theUIView’ssuperviewistheUIWindow.

Let’sexperimentwiththeviewhierarchy.First,inViewController.swift,createanotherinstanceofUIViewwithadifferentframeandbackgroundcolor.overridefuncviewDidLoad(){super.viewDidLoad()

letfirstFrame=CGRect(x:160,y:240,width:100,height:150)letfirstView=UIView(frame:firstFrame)firstView.backgroundColor=UIColor.blueColor()view.addSubview(firstView)

letsecondFrame=CGRect(x:20,y:30,width:50,height:50)letsecondView=UIView(frame:secondFrame)secondView.backgroundColor=UIColor.greenColor()view.addSubview(secondView)}

Buildandrunagain.Inadditiontothebluerectangle,youwillseeagreensquarenearthetoplefthandcornerofthewindow.Figure3.8showstheupdatedviewhierarchy.


Figure3.8Updatedviewhierarchywithtwosubviewsassiblings

NowyouaregoingtoadjusttheviewhierarchysothatoneinstanceofUIViewisasubviewoftheotherUIViewinsteadoftheviewcontroller’sview.InViewController.swift,addsecondViewasasubviewoffirstView....

letsecondView=UIView(frame:secondFrame)secondView.backgroundColor=UIColor.greenColor()

view.addSubview(secondView)firstView.addSubview(secondView)

Yourviewhierarchyisnowfourlevelsdeep,asshowninFigure3.9.


Figure3.9OneUIViewasasubviewoftheother

Buildandruntheapplication.NoticethatsecondView’spositiononthescreenhaschanged(Figure3.10).Aview’sframeisrelativetoitssuperview,sothetopleftcornerofsecondViewisnowinset(20,30)pointsfromthetopleftcorneroffirstView.


Figure3.10WorldTrotterwithnewhierarchy

(IfthegreeninstanceofUIViewlookssmallerthanitdidpreviously,thatisjustanopticalillusion.Itssizehasnotchanged.)

Nowthatyouhaveseenthebasicsofviewsandtheviewhierarchy,youcanstartworkingontheinterfaceforWorldTrotter.Insteadofbuildinguptheinterfaceprogrammatically,youwilluseInterfaceBuildertovisuallylayouttheinterface,asyoudidinChapter1.

InViewController.swift,startbyremovingyourpracticecode.overridefuncviewDidLoad(){super.viewDidLoad()

letfirstFrame=CGRect(x:160,y:240,width:100,height:150)letfirstView=UIView(frame:firstFrame)firstView.backgroundColor=UIColor.blueColor()view.addSubview(firstView)

letsecondFrame=CGRect(x:20,y:30,width:50,height:50)letsecondView=UIView(frame:secondFrame)secondView.backgroundColor=UIColor.greenColor()firstView.addSubview(secondView)}

Nowlet’saddsomeviewstotheinterfaceandsettheirframes.

OpenMain.storyboard.Noticethattheinterfaceonthescreeniscurrentlyasquare.Whileyouexploreviewsandtheirframes,itwillbenicetohavethesizeoftheinterfaceinXcodematchthescreensizeofthedevicethatyouwillbeusing.


SelecttheViewControllereitherinthedocumentoutlineorbyclickingtheyellowcircleabovetheinterface.Opentheattributesinspector,whichisthefourthtabintheutilitiesarea.YoucanquicklyopenthispaneusingthekeyboardshortcutCommand-Option-4.

Atthetopofthepane,findthesectionlabeledSimulatedMetricsandchangetheSizetobeiPhone4.7-inch.Thiswillresizethesquareinterfacetomatchthedimensionsofthe4.7-inchdevices.

Fromtheobjectlibrary,dragfiveinstancesofUILabelontothecanvas.Spacethemoutverticallyonthetophalfoftheinterfaceandcenterthemhorizontally.SettheirtexttomatchFigure3.11.

Figure3.11Addinglabelstotheinterface

SelectthetoplabelsoyoucanseeitsframeinInterfaceBuilder.Openitssizeinspector–thefifthtabintheutilitiesarea.(Asyoumighthavenoticedbythispoint,thekeyboardshortcutsfortheutilitiestabsareCommand-Optionplusthetabnumber.Sincethesize


inspectoristhefifthtab,itskeyboardshortcutisCommand-Option-5.)

UndertheViewsection,findFrameRectangle.(Ifyoudonotseeit,youmightneedtoselectitfromtheShowpop-upmenu.)Thesevaluesaretheview’sframe,andtheydictatethepositionoftheviewonscreen(Figure3.12).

Figure3.12Viewframevalues

BuildandruntheapplicationontheiPhone6ssimulator.Thiscorrespondstothe4.7-inchsimulatedmetricsthatyouspecifiedinthestoryboard.TheinterfaceonthesimulatorwilllookidenticaltotheinterfacethatyoulaidoutinInterfaceBuilder.

Customizingthelabels

Let’smaketheinterfacelookalittlebitbetterbycustomizingtheviewproperties.

InMain.storyboard,selectthebackgroundview.Opentheattributesinspectorandgivetheappanewbackgroundcolor:FindandclicktheBackgrounddropdownandclickOther.Selectthesecondtab(theColorSliderstab)andenteraHexColor#ofF5F4F1(Figure3.13).Thiswillgivethebackgroundawarm,graycolor.


Figure3.13Changingthebackgroundcolor

Youcancustomizeattributescommontoselectedviewssimultaneously.Youwillusethistogivemanyofthelabelsalargerfontsizeaswellasaburntorangetextcolor.

SelectthetoptwoandbottomtwolabelsbyCommand-clickingtheminthedocumentoutlineandopentheattributesinspector.Updatethetextcolor:UndertheLabelsection,findColorandopenthepop-upmenu.SelecttheColorSliderstabagainandenteraHexColor#ofE15829.

Nowlet’supdatethefont.Selectthe212and100labels.UndertheLabelsectionintheattributesinspector,findFontandclickonthetexticonnexttothecurrentfont.Fromthepopoverthatappears,maketheFontSystem-SystemandtheSize70(Figure3.14).Selecttheremainingthreelabels.OpentheirFontpop-upandmaketheFontSystem-SystemandtheSize36.


Figure3.14Customizingthelabels’font

Nowthatthefontsizeislarger,thetextnolongerfitswithintheboundsofthelabel.Youcouldresizethelabelsmanually,butthereisaneasierway.

Selectthetoplabelonthecanvas.FromXcode’sEditormenu,selectSizetoFitContent(Command-=).Thiswillresizethelabeltoexactlyfititstextcontents.Repeattheprocessfortheotherfourlabels.(Youcanselectallfourlabelstoresizethemallatonce.)Nowmovethelabelssothattheyareagainnicelyalignedverticallyandcenteredhorizontally(Figure3.15).


Figure3.15Updatingthelabelframes

BuildandruntheapplicationontheiPhone6ssimulator.NowbuildandruntheapplicationontheiPhone6sPlussimulator.Noticethatthelabelsarenolongercentered–instead,theyappearshiftedslightlytotheleft.

Youhavejustseentwoofthemajorproblemswithabsoluteframes.First,whenthecontentschange(likewhenyouchangedthefontsize),theframesdonotautomaticallyupdate.Second,theviewdoesnotlookequallygoodondifferentsizesofscreens.

Ingeneral,youshouldnotuseabsoluteframesforyourviews.Instead,youshoulduseAutoLayouttoflexiblycomputetheframesforyoubasedonconstraintsthatyouspecifyforeachview.Forexample,whatyoureallywantforWorldTrotterisforthelabelstoremainthesamedistancefromthetopofthescreenandtoremainhorizontallycenteredwithintheirsuperview.Theyshouldalsoupdateifthefontortextofthelabelschange.Thisiswhatyouwillaccomplishinthenextsection.


TheAutoLayoutSystemBeforeyoucanfixthelabelstohavethemlayoutflexibly,youneedtolearnalittletheoryabouttheAutoLayoutsystem.

AsyousawinChapter1,absolutecoordinatesmakeyourlayoutfragilebecausetheyassumethatyouknowthesizeofthescreenaheadoftime.

UsingAutoLayout,youcandescribethelayoutofyourviewsinarelativewaythatenablestheirframestobedeterminedatruntimesothattheframes’definitionscantakeintoaccountthescreensizeofthedevicethattheapplicationisrunningon.

Alignmentrectangleandlayoutattributes

TheAutoLayoutsystemisbasedonthealignmentrectangle.Thisrectangleisdefinedbyseverallayoutattributes(Figure3.16).

Figure3.16Layoutattributesdefininganalignmentrectangleofaview

Width/Height

Thesevaluesdeterminethealignmentrectangle’ssize.Top/Bottom/Left/Right

Thesevaluesdeterminethespacingbetweenthegivenedgeofthealignmentrectangleandthealignmentrectangleofanotherviewinthehierarchy.

CenterX/CenterY

Thesevaluesdeterminethecenterpointofthealignmentrectangle.Baseline

Thisvalueisthesameasthebottomattributeformost,butnotall,views.Forexample,UITextFielddefinesitsbaselineasthebottomofthetextitdisplaysratherthanthebottomofthealignmentrectangle.Thiskeeps“descenders”(letterslike‘g’and‘p’thatdescendbelowthebaseline)frombeingobscuredbya


viewrightbelowthetextfield.Leading/Trailing

Thesevaluesarelanguage-specificattributes.Ifthedeviceissettoalanguagethatreadslefttoright(e.g.,English),thentheleadingattributeisthesameastheleftattributeandthetrailingattributeisthesameastherightattribute.Ifthelanguagereadsrighttoleft(e.g.,Arabic),thentheleadingattributeisontherightandthetrailingattributeisontheleft.InterfaceBuilderautomaticallyprefersleadingandtrailingoverleftandright,and,ingeneral,youshouldaswell.

Bydefault,everyviewhasanalignmentrectangle,andeveryviewhierarchyusesAutoLayout.

Thealignmentrectangleisverysimilartotheframe.Infact,thesetworectanglesareoftenthesame.Whereastheframeencompassestheentireview,thealignmentrectangleonlyencompassesthecontentthatyouwishtouseforalignmentpurposes.Figure3.17showsanexamplewheretheframeandthealignmentrectanglearedifferent.

Figure3.17Framevs.alignmentrectangle

Youcannotdefineaview’salignmentrectangledirectly.Youdonothaveenoughinformation(likescreensize)todothat.Instead,youprovideasetofconstraints.Takentogether,theseconstraintsenablethesystemtodeterminethelayoutattributes,andthusthealignmentrectangle,foreachviewintheviewhierarchy.

Constraints

Aconstraintdefinesaspecificrelationshipinaviewhierarchythatcanbeusedtodeterminealayoutattributeforoneormoreviews.Forexample,youmightaddaconstraintlike,“Theverticalspacebetweenthesetwoviewsshouldalwaysbe8points,”or,“Theseviewsmustalwayshavethesamewidth.”Aconstraintcanalsobeusedtogiveaviewafixedsize,like,“Thisview’sheightshouldalwaysbe44points.”

Youdonotneedaconstraintforeverylayoutattribute.Somevaluesmaycomedirectlyfromaconstraint;otherswillbecomputedbythevaluesofrelatedlayoutattributes.Forexample,ifaview’sconstraintssetitsleftedgeanditswidth,thentherightedgeisalreadydetermined(leftedge+width=rightedge,always).Asageneralruleofthumb,youneedatleasttwoconstraintsperdimension(horizontalandvertical).


If,afteralloftheconstraintshavebeenconsidered,thereisstillanambiguousormissingvalueforalayoutattribute,thentherewillbeerrorsandwarningsfromAutoLayoutandyourinterfacewillnotlookasyouexpectonalldevices.Debuggingtheseproblemsisimportant,andyouwillgetsomepracticelaterinthischapter.

Howdoyoucomeupwithconstraints?Let’sseehowusingthelabelsthatyouhavelaidoutonthecanvas.

First,describewhatyouwanttheviewtolooklikeindependentofscreensize.Forexample,youmightsaythatyouwantthetoplabeltobe:

8pointsfromthetopofthescreen

centeredhorizontallyinitssuperview

aswideandastallasitstext

ToturnthisdescriptionintoconstraintsinInterfaceBuilder,itwillhelptounderstandhowtofindaview’snearestneighbor.Thenearestneighboristheclosestsiblingviewinthespecifieddirection(Figure3.18).

Figure3.18Nearestneighbor

Ifaviewdoesnothaveanysiblingsinthespecifieddirection,thenthenearestneighborisitssuperview,alsoknownasitscontainer.


Nowyoucanspellouttheconstraintsforthelabel:

1. Thelabel’stopedgeshouldbe8pointsawayfromitsnearestneighbor(whichisitscontainer–theviewoftheViewController).

2. Thelabel’scentershouldbethesameasitssuperview’scenter.

3. Thelabel’swidthshouldbeequaltothewidthofitstextrenderedatitsfontsize.

4. Thelabel’sheightshouldbeequaltotheheightofitstextrenderedatitsfontsize.

Ifyouconsiderthefirstandfourthconstraints,youcanseethatthereisnoneedtoexplicitlyconstrainthelabel’sbottomedge.Itwillbedeterminedfromtheconstraintsonthelabel’stopedgeandthelabel’sheight.Similarly,thesecondandthirdconstraintstogetherdeterminethelabel’srightandleftedges.

Nowthatyouhaveaplanforthetoplabel,youcanaddtheseconstraints.ConstraintscanbeaddedusingInterfaceBuilderorincode.ApplerecommendsthatyouaddconstraintsusingInterfaceBuilderwheneverpossible,andthatiswhatyouwilldohere.However,ifyourviewsarecreatedandconfiguredprogrammatically,thenyoucanaddconstraintsincode.InChapter6,youwillpracticethatapproach.

AddingconstraintsinInterfaceBuilder

Let’sgetstartedconstrainingthattoplabel.

Selectthetoplabelonthecanvas.Inthebottomrighthandcornerofthecanvas,findtheAutoLayoutconstraintmenu(Figure3.19).


Figure3.19UsingtheAutoLayoutconstraintmenu

Clickthe icon(thethirdfromtheleft)torevealthePinmenu.Thismenushowsyouthecurrentsizeandpositionofthelabel.

AtthetopofthePinmenuarefourvaluesthatdescribethelabel’scurrentspacingfromitsnearestneighboronthecanvas.Forthislabel,youareonlyinterestedinthetopvalue.

Toturnthisvalueintoaconstraint,clickthetopredstrutseparatingthevaluefromthesquareinthemiddle.Thestrutwillbecomeasolidredline.

Inthemiddleofthemenu,findthelabel’sWidthandHeight.ThevaluesnexttoWidthandHeightindicatethecurrentcanvasvalues.Toconstrainthelabel’swidthandheighttothecurrentcanvasvalues,checktheboxesnexttoWidthandHeight.ThebuttonatthebottomofthemenureadsAdd3Constraints.Clickthisbutton.

Atthispoint,youhavenotspecifiedenoughconstraintstofullydeterminethealignmentrectangle.InterfaceBuilderwillhelpyoudeterminewhattheproblemis.

InthetoprightcornerofInterfaceBuilder,noticetheyellowwarningsign(Figure3.20).Clickonthisicontorevealtheissue:“Horizontalpositionisambiguousfor“212”.”


Figure3.20Horizontalambiguity

Youhaveaddedtwoverticalconstraints(atopedgeconstraintandaheightconstraint),butyouhaveonlyaddedonehorizontalconstraint(awidthconstraint).Havingonlyoneconstraintmakesthehorizontalpositionofthelabelambiguous.Youwillfixthisissuebyaddingacenteralignmentconstraintbetweenthelabelanditssuperview.

Withthetoplabelstillselected,clickthe icon(thesecondfromtheleftintheAutoLayoutconstraintsmenu)torevealtheAlignmenu.Ifyouhavemultipleviewsselected,thismenuwillallowyoutoalignattributesamongtheviews.Sinceyouhaveonlyselectedonelabel,theonlyoptionsyouaregivenaretoaligntheviewwithinitscontainer.

IntheAlignmenu,selectHorizontallyinContainer(donotclickAdd1Constraintyet).Onceyouaddthisconstraint,therewillbeenoughconstraintstofullydeterminethealignmentrectangle.Toensurethattheframeofthelabelmatchestheconstraintsspecified,opentheUpdateFramespop-upmenufromtheAlignmenuandselectItemsofNewConstraints.Thiswillrepositionthelabeltomatchtheconstraintsthathavebeenadded.NowclickonAdd1Constrainttoaddthecenteringconstraintandrepositionthelabel.

Thelabel’sconstraintsareallbluenowthatthealignmentrectangleforthelabelisfullyspecified.Additionally,thewarningatthetoprightcornerofInterfaceBuilderisnowgone.

BuildandruntheapplicationontheiPhone6ssimulatorandtheiPhone6sPlussimulator.Thetoplabelwillremaincenteredinbothsimulators.

Intrinsiccontentsize

Althoughthetoplabel’spositionisflexible,itssizeisnot.Thisisbecauseyouhaveaddedexplicitwidthandheightconstraintstothelabel.Ifthetextorfontweretochange,youwouldbeinthesamepositionyouwereinearlier.Thesizeoftheframeisabsolute,sotheframewouldnothugtothecontent.

Thisiswheretheintrinsiccontentsizeofaviewcomesintoplay.Youcanthinkoftheintrinsiccontentsizeasthesizethataview“wants”tonaturallybe.Forlabels,thissizeisthesizeofthetextrenderedatthegivenfont.Forimages,thisisthesizeoftheimageitself.

Aview’sintrinsiccontentsizeactsasimplicitwidthandheightconstraints.Ifyoudonotspecifyconstraintsthatexplicitlydeterminethewidth,theviewwillbeitsintrinsicwidth.Thesamegoesfortheheight.

Withthisknowledge,letthetoplabelhaveaflexiblesizebyremovingtheexplicitwidthandheightconstraints.

InMain.storyboard,selectthewidthconstraintonthelabel.YoucandothisbyWOW! eBook

www.wowebook.org

clickingontheconstraintonthecanvas.Alternatively,inthedocumentoutline,youcanclickonthedisclosuretrianglenexttothe212label,thendisclosethelistofconstraintsforthelabel(Figure3.21).Onceyouhaveselectedthewidthconstraint,presstheDeletekey.Dothesamefortheheightconstraint.

Figure3.21Selectingthewidthconstraint

Noticethattheconstraintsforthelabelarestillblue.Sincethewidthandheightarebeinginferredfromthelabel’sintrinsiccontentsize,therearestillenoughconstraintstodeterminethelabel’salignmentrectangle.

Misplacedviews

Asyouhaveseen,blueconstraintsindicatethatthealignmentrectangleforaviewisfullyspecified.Orangeconstraintsoftenindicateamisplacedview.ThismeansthattheframefortheviewinInterfaceBuilderisdifferentthantheframethatAutoLayouthascomputed.


Amisplacedviewisveryeasytofix.Thatisgood,becauseitisalsoaverycommonissuethatyouwillencounterwhenworkingwithAutoLayout.

Giveyourtoplabelamisplacedviewsothatyoucanseehowtoresolvethisissue.Resizethetoplabelonthecanvasusingtheresizecontrolsandlookfortheyellowwarninginthetoprightcornerofthecanvas.Clickonthiswarningicontorevealtheproblem:“Framefor“212”willbedifferentatruntime”(Figure3.22).

Figure3.22Misplacedviewwarning

Asthewarningsays,theframeatruntimewillnotbethesameastheframespecifiedonthecanvas.Ifyoulookclosely,youwillseeanorangedottedlinethatindicateswhattheruntimeframewillbe.

Buildandruntheapplication.NoticethatthelabelisstillcentereddespitethenewframethatyougaveitinInterfaceBuilder.Thismightseemgreat–yougettheresultthatyouwant,afterall.ButthedisconnectbetweenwhatyouhavespecifiedinInterfaceBuilderandtheconstraintscomputedbyAutoLayoutwillcauseproblemsdownthelineasyoucontinuetobuildyourviews.Let’sfixthemisplacedview.

Backinthestoryboard,selectthetoplabelonthecanvas.Clickthe icon(theright-mosticon)torevealtheResolveAutoLayoutIssuesmenu.SelectUpdateFramesfromtheSelectedViewssection.Thiswillupdatetheframeofthelabeltomatchtheframethattheconstraintswillcompute.

YouwillgetveryusedtoupdatingtheframesofviewsasyouworkwithAutoLayout.Onewordofcaution:ifyoutrytoupdatetheframesforaviewthatdoesnothaveenoughconstraints,youwillalmostcertainlygetunexpectedresults.Ifthathappens,undothechangeandinspecttheconstraintstoseewhatismissing.

Atthispoint,thetoplabelisingoodshape.Ithasenoughconstraintstodetermineitsalignmentrectangle,andtheviewislayingoutthewayyouwant.

BecomingproficientwithAutoLayouttakesalotofexperience,sointhenextsectionyouWOW! eBook

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aregoingtoremovetheconstraintsfromthetoplabelandthenaddconstraintstoallofthelabels.

Addingmoreconstraints

Let’sfleshouttheconstraintsfortherestoftheviews.Beforeyoudothat,youwillfirstremovetheexistingconstraintsfromthetoplabel.

Selectthetoplabelonthecanvas.OpentheResolveAutoLayoutIssuesmenuandselectClearConstraintsfromtheSelectedViewssection(Figure3.23).

Figure3.23Clearingconstraints

Youaregoingtoaddtheconstraintstoalloftheviewsintwosteps.Firstyouwillcenterthetoplabelhorizontallywithinthesuperview.Thenyouwilladdconstraintsthatpinthetopofeachlabeltoitsnearestneighborwhilealigningthecentersofallofthelabels.

Selectthetoplabel.OpentheAlignmenuandchooseHorizontallyinContainerwithaconstantof0.MakesurethatUpdateFrameshasNoneselected;rememberthatyoudonotwanttoupdatetheframeofaviewthatdoesnothaveenoughconstraints,andthisoneconstraintwillcertainlynotprovideenoughinformationtocomputethealignmentrectangle.GoaheadandAdd1Constraint.

Nowselectallfivelabelsonthecanvas.Itcanbeveryconvenienttoaddconstraintstomultipleviewssimultaneously.OpenthePinmenuandmakethefollowchoices:

1. Selectthetopstrutandmakesureithasaconstantof8.

2. FromtheAlignmenu,chooseHorizontalCenters.

3. FromtheUpdateFramesmenu,chooseItemsofNewConstraints.

YourmenushouldmatchFigure3.24.Onceitdoes,clickAdd9Constraints.ThiswilladdtheconstraintstotheviewsandupdatetheirframestoreflecttheAutoLayoutchanges.


Figure3.24AddingmoreconstraintswiththePinmenu

BuildandruntheapplicationontheiPhone6ssimulator.Theviewswillbecenteredwithintheinterface.NowbuildandruntheapplicationontheiPhone6sPlussimulator.Unlikeearlierinthechapter,allofthelabelsremaincenteredonthelargerinterface.

AutoLayoutisacrucialtechnologyforeveryiOSdeveloper.Ithelpsyoucreateflexiblelayoutsthatworkacrossarangeofdevicesandinterfacesizes.Italsotakesalotofpracticetomaster.YouwillgetalotofexperienceworkingwithAutoLayoutasyouworkthroughthisbook.

NowthattheinterfaceforWorldTrotterisusingAutoLayouttoadapttovariousscreensizes,thereisnoneedforyoutospecifyaniPhonescreensizewhenworkinginthestoryboard.

InMain.storyboard,selecttheViewControllerandopenitsattributesinspector.FindtheSimulatedMetricssectionandchangetheSizetoInferred.Theinterfaceupdatestobethesquareshapethatitwasinitially.Noticethatthelabelsstillremaincenteredinthissquareinterfaceduetotheconstraintsthatyouadded.

Designinginterfacesusingtheinferredsquareshapehelpstoforceyoutothinkaboutdesigningadaptiveinterfacesthatworkwithavarietyofscreensizesinsteadofdesigningforoneparticularscreensize.


BronzeChallenge:MoreAutoLayoutPracticeRemovealloftheconstraintsfromtheViewControllerinterfaceandthenaddthembackin.Trytodothiswithoutconsultingthebook.


4TextInputandDelegation

WorldTrotterlooksgood,butsofaritdoesnotdoanything.Inthischapter,youaregoingtoaddaninstanceofUITextFieldtoWorldTrotter.ThetextfieldwillallowtheusertotypeinatemperatureinFahrenheitthatwillthenbeconvertedtoCelsiusanddisplayedontheinterface(Figure4.1).

Figure4.1WorldTrotterwithaUITextField


TextEditingThefirstthingyouaregoingtodoisaddaUITextFieldtotheinterfaceandsetuptheconstraintsforthattextfield.Thistextfieldwillreplacethetoplabelintheinterfacethatcurrentlyhasthetext“212.”

OpenMain.storyboard.SelectthetoplabelandpresstheDeletekeytoremovethissubview.Theconstraintsforalloftheotherlabelswillturnredsincetheywerealldirectlyorindirectlyanchoredtothattoplabel(Figure4.2).ThatisOK;youwillfixthemshortly.

Figure4.2Ambiguousframesforthelabels

OpentheobjectlibraryanddragaTextFieldtothetopofthecanvaswherethelabelyoudeletedwaspreviouslyplaced.

Nowsetuptheconstraintsforthistextfield.Withthetextfieldselected,opentheAlignmenuandaligntheviewHorizontallyinContainerwithaconstantof0.MakesurethatUpdateFramesissettoNoneandthenAdd1Constraint.

NowopenthePinmenu.Givethetextfieldatopedgeconstraintof8points,abottomedgeconstraintof8points,andawidthof250(Figure4.3).Addthesethreeconstraints.


Figure4.3TextfieldPinmenu

Finally,selectthetextfieldandthelabelrightbelowit.OpentheAlignmenu,selectHorizontalCenterswithaconstantof0,UpdateFramesforAllFramesinContainer,andthenAdd1Constraint(Figure4.4).

Figure4.4Aligningthetextfield

Next,customizesomeofthetextfieldproperties.OpentheattributesinspectorforthetextWOW! eBook

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fieldandmakethefollowingchanges:

Setthetextcolor(fromtheColormenu)toburntorange.

SetthefonttoSystemwithasizeof70.

SettheAlignmenttocentered.

Settheplaceholdertexttobevalue.Thisisthetextthatwillbedisplayedwhentheuserhasnotenteredanytext.

SettheBorderStyletobenone,whichisthefirstelementofthesegmentedcontrolwiththedottedlines.

TheattributesinspectorforyourtextfieldshouldlooklikeFigure4.5.

Figure4.5Textfieldattributesinspector

Sincethetextfield’sfontchanged,theviewsonthecanvasnowaremisplaced.Selectthegraybackgroundview,opentheResolveAutoLayoutIssuesmenu,andselectUpdateFramesfromtheAllViewsinViewControllersection.Thetextfieldandlabelswillberepositionedtomatchtheirconstraints(Figure4.6).


Figure4.6Updatedframes

Buildandruntheapplication.Taponthetextfieldandentersometext.Ifyoudonotseethekeyboardappear,clickthesimulator’sHardwaremenuandselectKeyboard→ToggleSoftwareKeyboardorusethekeyboardshortcutCommand-K.Bydefault,thesimulatortreatsyourcomputer’skeyboardasaBluetoothkeyboardconnectedtothesimulator.Thisisnotusuallywhatyouwant.Instead,youwantthesimulatortomimicaniOSdevicerunningwithoutanyaccessoriesattachedbyusingtheonscreenkeyboard.

Keyboardattributes

Whenatextfieldistapped,thekeyboardautomaticallyslidesupontothescreen.(Youwillseewhythishappenslaterinthischapter.)Thekeyboard’sappearanceisdeterminedbyasetoftheUITextField’spropertiescalledtheUITextInputTraits.Oneofthesepropertiesisthetypeofkeyboardthatisdisplayed.Forthisapplication,youwanttousethedecimalpad.

Intheattributesinspectorforthetextfield,findtheattributenamedKeyboardTypeandchooseDecimalPad.Inthesamesection,youcanseesomeoftheothertextinputtraitsthatyoucancustomizeforthekeyboard.ChangebothCorrectionandSpellCheckingtoNo(Figure4.7).


Figure4.7Keyboardtextinputtraits

Buildandruntheapplication.Tappingonthetextfieldwillnowrevealthedecimalpad.

Respondingtotextfieldchanges

ThenextstepoftheprojectwillbetoupdatetheCelsiuslabelwhentextistypedintothetextfield.Youaregoingtoneedtowritesomecodetodothis.Specifically,thiscodewillgointotheviewcontrollersubclassassociatedwiththisinterface.

Currently,thatcorrespondswiththeViewControllerclassdefinedinViewController.swift.However,ViewControllerisnotaverydescriptivenameforaviewcontrollerthatmanagestheconversionbetweenFahrenheitandCelsius.Havingdescriptivetypenamesallowsyoutomoreeasilymaintainyourprojectsastheygrowlarger.Youaregoingtodeletethisfileandreplaceitwithamoredescriptiveclass.

Intheprojectnavigator,findViewController.swiftanddeleteit.ThencreateanewfilebyselectingFile→New→File…(orpressCommand-N).IntheiOSsection,selectSource,chooseSwiftFile,andclickNext.

Onthenextpane,namethisfileConversionViewController.SavethefileintheWorldTrottergroupwithintheWorldTrotterprojectandmakesurethattheWorldTrottertargetischecked,asshowninFigure4.8.ClickCreate,andXcodewillopenConversionViewController.swiftintheeditor.


Figure4.8SavingaSwiftfile

InConversionViewController.swift,importUIKitanddefineanewviewcontrollernamedConversionViewController.importFoundationimportUIKit

classConversionViewController:UIViewController{

}

NowyouneedtoassociatetheinterfaceyoucreatedinMain.storyboardwiththisnewviewcontrollerthatyoudefined.

OpenMain.storyboardandselecttheViewController,eitherinthedocumentoutlineorbyclickingtheyellowcircleabovetheinterface.

Opentheidentityinspector,whichisthethirdtabintheutilitiesview(Command-Option-3).Atthetop,findtheCustomClasssectionandchangetheClasstoConversionViewController(Figure4.9).YouwilllearnwhatallofthisisdoinginChapter5.

Figure4.9Changingthecustomclass

YousawinChapter1thatabuttoncansendeventstoacontrollerwhenthebuttonis


tapped.Textfieldsareanothercontrol(bothUIButtonandUITextFieldaresubclassesofUIControl)andcansendaneventwhenthetextchanges.

Togetthisallworking,youwillneedtocreateanoutlettotheCelsiustextlabelandcreateanactionforthetextfieldtocallwhenthetextchanges.

OpenConversionViewController.swiftanddefinethisoutletandaction.Fornow,thelabelwillbeupdatedwithwhatevertexttheusertypesintothetextfield.classConversionViewController:UIViewController{

@IBOutletvarcelsiusLabel:UILabel!

@IBActionfuncfahrenheitFieldEditingChanged(textField:UITextField){celsiusLabel.text=textField.text}}

OpenMain.storyboardtomaketheseconnections.TheoutletwillbeconnectedjustasyoudidinChapter1.Control-dragfromtheConversionViewControllertotheCelsiuslabel(theonethatcurrentlysays“100”)andconnectittothecelsiusLabel.

Connectingtheactionwillbealittledifferentsinceyouwanttheactiontobetriggeredwhentheeditingchanges.

Selectthetextfieldonthecanvasandopenitsconnectionsinspectorfromtheutilitypane(theright-mosttab,orCommand-Option-6).Theconnectionsinspectorisagreatplacetomakeconnectionsandseewhatconnectionshavealreadybeenmade.

YouaregoingtohavechangestothetextfieldtriggertheactionyoudefinedinConversionViewController.Intheconnectionsinspector,locatetheSentEventsandtheEditingChangedevent.ClickanddragfromthecircletotherightofEditingChangedtotheConversionViewControllerandclickthefahrenheitFieldEditingChanged:actioninthepop-upmenu(Figure4.10).

Figure4.10Connectingtheeditingchangedevent


Buildandruntheapplication.Tapthetextfieldandtypesomenumbers.TheCelsiuslabelwillmimicthetextthatistypedin.Nowdeletethetextinthetextfieldandnoticehowthelabelseemstogoaway.Alabelwithnotexthasanintrinsiccontentwidthandheightof0,sothelabelsbelowitmoveup.Let’sfixthisissue.

InConversionViewController.swift,updatefahrenheitFieldEditingChanged(_:)todisplay“???”ifthetextfieldisempty.@IBActionfuncfahrenheitFieldEditingChanged(textField:UITextField){celsiusLabel.text=textField.text

iflettext=textField.textwhere!text.isEmpty{celsiusLabel.text=text}else{celsiusLabel.text="???"}}

Ifthetextfieldhastextandthattextisnotempty,itwillbesetonthecelsiusLabel.Ifeitherofthoseconditionsarenottrue,thenthecelsiusLabelwillbegiventhestring“???”.

Buildandruntheapplication.Addsometext,deleteit,andthenconfirmthatthecelsiusLabelispopulatedwith“???”oncethetextfieldisempty.

Dismissingthekeyboard

Currently,thereisnowaytodismissthekeyboard.Let’saddthatfunctionality.OnecommonwayofdoingthisisbydetectingwhentheusertapstheReturnkeyandusingthatactiontodismissthekeyboard;youwillusethisapproachinChapter13.SincethedecimalpaddoesnothaveaReturnkey,youwillallowtheusertotaponthebackgroundviewtotriggerthedismissal.

Whenthetextfieldistapped,themethodbecomeFirstResponder()iscalledonthetextfield.Thisisthemethodthat,amongotherthings,causesthekeyboardtoappear.Todismissthekeyboard,youcallthemethodresignFirstResponder()onthetextfield.YouwilllearnmoreaboutthesemethodsinChapter13.

Toaccomplishthis,youwillneedanoutlettothetextfieldandamethodthatistriggeredwhenthebackgroundviewistapped.ThismethodwillcallresignFirstResponder()onthetextfieldoutlet.Let’stakecareofthecodefirst.

OpenConversionViewController.swiftanddeclareanoutletnearthetoptoreferencethetextfield.@IBOutletvarcelsiusLabel:UILabel!@IBOutletvartextField:UITextField!

Nowimplementanactionmethodthatwilldismissthekeyboardwhencalled.

(Inthecodeabove,weincludedexistingcodesothatyoucouldpositionthenewcodecorrectly.Inthecodebelow,wedonotprovidethatcontextbecausethepositionofthenewcodeisnotimportantsolongasitiswithinthecurlybracesforthetypebeing


implemented–inthiscase,theConversionViewControllerclass.Whenacodeblockincludesallnewcode,wesuggestthatyouputitattheendofthetype’simplementation,justinsidethefinalclosingbrace.InChapter14,youwillseehowtoeasilynavigatewithinanimplementationfilewhenyourfilesgetlongerandmorecomplex.)@IBActionfuncdismissKeyboard(sender:AnyObject){textField.resignFirstResponder()}

Twothingsarestillneeded:thetextFieldoutletneedstobeconnectedinthestoryboardfile,andyouneedawayoftriggeringthedismissKeyboard(_:)methodyouadded.

Totakecareofthefirstitem,openMain.storyboardandselecttheConversionViewController.Control-dragfromtheConversionViewControllertothetextfieldonthecanvasandconnectittothetextFieldoutlet.

Nowyouneedawayoftriggeringthemethodyouimplemented.Youwilluseagesturerecognizertoaccomplishthis.

AgesturerecognizerisasubclassofUIGestureRecognizerthatdetectsaspecifictouchsequenceandcallsanactiononitstargetwhenthatsequenceisdetected.Therearegesturerecognizersthatdetecttaps,swipes,longpresses,andmore.Inthischapter,youwilluseaUITapGestureRecognizertodetectwhentheusertapsthebackgroundview.YouwilllearnmoreaboutgesturerecognizersinChapter18.

InMain.storyboard,findTapGestureRecognizerintheobjectlibrary.DragthisobjectontothebackgroundviewfortheConversionViewController.Youwillseeareferencetothisgesturerecognizerinthescenedock,therowoficonsabovethecanvas.

Control-dragfromthegesturerecognizerinthescenedocktotheConversionViewControllerandconnectittothedismissKeyboard:method(Figure4.11).

Figure4.11Connectingthegesturerecognizeraction


ImplementingtheTemperatureConversionWiththebasicsoftheinterfacewiredup,let’simplementtheconversionfromFahrenheittoCelsius.YouaregoingtostorethecurrentFahrenheitvalueandcomputetheCelsiusvaluewheneverthetextfieldchanges.

InConversionViewController.swift,addapropertyfortheFahrenheitvalue.Thiswillbeanoptionaldouble(aDouble?).@IBOutletvarcelsiusLabel:UILabel!

varfahrenheitValue:Double?

Thereasonthispropertyisoptionalisbecausetheusermightnothavetypedinanumber,similartotheemptystringissueyoufixedearlier.

NowaddacomputedpropertyfortheCelsiusvalue.ThisvaluewillbecomputedbasedontheFahrenheitvalue.varfahrenheitValue:Double?

varcelsiusValue:Double?{ifletvalue=fahrenheitValue{return(value-32)*(5/9)}else{returnnil}}

FirstyouchecktoseeifthereisaFahrenheitvalue.Ifthereis,youconvertthisvaluetotheequivalentvalueinCelsius.IfthereisnoFahrenheitvalue,thenyoucannotcomputeaCelsiusvalueandsoyoureturnnil.

AnytimetheFahrenheitvaluechanges,theCelsiuslabelneedstobeupdated.

AddamethodtoConversionViewControllerthatupdatesthecelsiusLabel.funcupdateCelsiusLabel(){ifletvalue=celsiusValue{celsiusLabel.text="\(value)"}else{celsiusLabel.text="???"}}

YouwantthismethodtobecalledwhenevertheFahrenheitvaluechanges.Todothis,youwilluseapropertyobserver,whichisachunkofcodethatgetscalledwheneveraproperty’svaluechanges.

Apropertyobserverisdeclaredusingcurlybracesimmediatelyafterthepropertydeclaration.Insidethebraces,youdeclareyourobserverusingeitherwillSetordidSetdependingonwhetheryouwanttobenotifiedimmediatelybeforeorimmediatelyafterthepropertyvaluechanges,respectively.

AddapropertyobservertofahrenheitValuethatgetscalledafterthepropertyvaluechanges.varfahrenheitValue:Double?{didSet{


updateCelsiusLabel()}}

(Onesmallnote:propertyobserversarenottriggeredwhenthepropertyvalueischangedfromwithinaninitializer.)

Withthatlogicinplace,youcannowupdatetheFahrenheitvaluewhenthetextfieldchanges(which,inturn,willtriggeranupdateoftheCelsiuslabel).

InfahrenheitFieldEditingChanged(_:),deleteyourearliernonconvertingimplementationandinsteadupdatetheFahrenheitvalue.@IBActionfuncfahrenheitFieldEditingChanged(textField:UITextField){iflettext=textField.textwhere!text.isEmpty{celsiusLabel.text=text}else{celsiusLabel.text="???"}

iflettext=textField.text,value=Double(text){fahrenheitValue=value}else{fahrenheitValue=nil}}

Firstyoucheckwhetherthetextfieldhassometext.Ifso,youchecktoseewhetherthattextcanberepresentedbyaDouble.Forexample,“3.14”canberepresentedbyaDouble,butboth“three”and“1.2.3”cannot.Ifbothofthosecheckspass,thentheFahrenheitvalueissettothatDoublevalue.Ifeitherofthosechecksfails,thentheFahrenheitvalueissettonil.

Buildandruntheapplication.TheconversionbetweenFahrenheitandCelsiusworksgreat–solongasyouenteravalidnumber.

Intheremainderofthischapter,youwillupdateWorldTrottertoaddresstwoissues:youwillformattheCelsiusvaluetoshowaprecisionuptoonefractionaldigit,andyouwillnotallowtheusertotypeinmorethanonedecimalseparator.

Thereareacoupleofotherissueswithyourapp,butyouwillfocusonthesetwofornow.Someoftheotherissueswillbepresentedaschallengesattheendofthischapter.Let’sstartwithupdatingtheprecisionoftheCelsiusvalue.

Numberformatters

Youuseanumberformattertocustomizethedisplayofanumber.Thereareotherformattersforformattingdates,energy,mass,length,andmore.

CreateaconstantnumberformatterinConversionViewController.swift.letnumberFormatter:NSNumberFormatter={letnf=NSNumberFormatter()nf.numberStyle=.DecimalStylenf.minimumFractionDigits=0nf.maximumFractionDigits=1returnnf}()


Hereyouareusingaclosuretoinstantiatethenumberformatter.YouarecreatinganNSNumberFormatterwiththe.Decimalstyleandconfiguringittodisplaynomorethanonefractionaldigit.YouwilllearnmoreaboutthisnewsyntaxfordeclaringpropertiesinChapter15.

NowmodifyupdateCelsiusLabel()tousethisformatter.funcupdateCelsiusLabel(){ifletvalue=celsiusValue{celsiusLabel.text="\(value)"celsiusLabel.text=numberFormatter.stringFromNumber(value)}else{celsiusLabel.text="???"}}

Buildandruntheapplication.PlayaroundwithFahrenheitvaluestoseetheformatteratwork.YoushouldneverseemorethanonefractionaldigitontheCelsiuslabel.

Inthenextsection,youwillupdatetheapplicationtoacceptamaximumofonedecimalseparatorinthetextfield.Todothis,youwilluseacommoniOSdesignpatterncalleddelegation.


DelegationDelegationisanobject-orientedapproachtocallbacks.Acallbackisafunctionthatissuppliedinadvanceofaneventandiscalledeverytimetheeventoccurs.Someobjectsneedtomakeacallbackformorethanoneevent.Forinstance,thetextfieldwantsto“callback”whentheuserenterstextaswellaswhentheuserpressestheReturnkey.

However,thereisnobuilt-inwayfortwo(ormore)callbackfunctionstocoordinateandshareinformation.Thisistheproblemaddressedbydelegation–yousupplyasingledelegatetoreceivealloftheevent-relatedcallbacksforaparticularobject.Thisdelegateobjectcanthenstore,manipulate,acton,andrelaytheinformationfromthecallbacksasitseesfit.

Whentheusertypesintoatextfield,thattextfieldwillaskitsdelegateifitwantstoacceptthechangesthattheuserhasmade.ForWorldTrotter,youwanttodenythatchangeiftheuserattemptstoenteraseconddecimalseparator.ThedelegateforthetextfieldwillbetheinstanceofConversionViewController.

Conformingtoaprotocol

ThefirststepisenablinginstancesoftheConversionViewControllerclasstoperformtheroleofUITextFielddelegatebydeclaringthatConversionViewControllerconformstotheUITextFieldDelegateprotocol.Foreverydelegaterole,thereisacorrespondingprotocolthatdeclaresthemethodsthatanobjectcancallonitsdelegate.

TheUITextFieldDelegateprotocollookslikethis:protocolUITextFieldDelegate:NSObjectProtocol{optionalfunctextFieldShouldBeginEditing(textField:UITextField)->BooloptionalfunctextFieldDidBeginEditing(textField:UITextField)optionalfunctextFieldShouldEndEditing(textField:UITextField)->BooloptionalfunctextFieldDidEndEditing(textField:UITextField)optionalfunctextField(textField:UITextField,shouldChangeCharactersInRangerange:NSRange,replacementStringstring:String)->BooloptionalfunctextFieldShouldClear(textField:UITextField)->BooloptionalfunctextFieldShouldReturn(textField:UITextField)->Bool}

Thisprotocol,likeallprotocols,isdeclaredwithprotocolfollowedbyitsname,UITextFieldDelegate.TheNSObjectProtocolafterthecolonreferstotheNSObjectprotocolandtellsyouthatUITextFieldDelegateinheritsallofthemethodsintheNSObjectprotocol.ThemethodsspecifictoUITextFieldDelegatearedeclarednext.

Youcannotcreateinstancesofaprotocol;itissimplyalistofmethodsandproperties.Instead,implementationislefttoeachtypethatconformstotheprotocol.

Inaclass’sdeclaration,theprotocolsthattheclassconformstoareinacomma-delimitedlistfollowingthesuperclass(ifthereisone).InConversionViewController.swift,declarethatConversionViewControllerconformstotheUITextFieldDelegate


protocol.classConversionViewController:UIViewController,UITextFieldDelegate{

Protocolsusedfordelegationarecalleddelegateprotocols,andthenamingconventionforadelegateprotocolisthenameofthedelegatingclassplusthewordDelegate.Notallprotocolsaredelegateprotocols,however,andyouwillseeanexampleofadifferentkindofprotocolinChapter15.TheprotocolswehavementionedsofararepartoftheiOSSDK,butyoucanalsowriteyourownprotocols.

Usingadelegate

NowthatyouhavedeclaredConversionViewControllerasconformingtotheUITextFieldDelegateprotocol,youcansetthedelegatepropertyofthetextfield.

OpenMain.storyboardandControl-dragfromthetextfieldtotheConversionViewController.ChoosedelegatefromthepopovertoconnectthedelegatepropertyofthetextfieldtotheConversionViewController.

Next,youaregoingtoimplementtheUITextFieldDelegatemethodthatyouareinterestedin–textField(_:shouldChangeCharactersInRange:replacementString:)

Becausethetextfieldcallsthismethodonitsdelegate,youmustimplementitinConversionViewController.swift.

InConversionViewController.swift,implementtextField(_:shouldChangeCharactersInRange:replacementString:)

toprintthetextfield’scurrenttextaswellasthereplacementstring.Fornow,justreturntruefromthismethod.functextField(textField:UITextField,shouldChangeCharactersInRangerange:NSRange,replacementStringstring:String)->Bool{

print("Currenttext:\(textField.text)")print("Replacementtext:\(string)")

returntrue}

NoticethatXcodewasabletoautocompletethismethodbecauseConversionViewControllerconformstoUITextFieldDelegate.ItisagoodideatodeclareaprotocolbeforeimplementingmethodsfromtheprotocolsothatXcodecanofferthissupport.

Buildandruntheapplication.EnterseveraldigitsinthetextfieldandwatchXcode’sconsole(Figure4.12).Itprintsoutthecurrenttextofthetextfieldaswellasthereplacementstring.


Figure4.12Printingtotheconsole

Considerthis“currenttext”and“replacementtext”informationinlightofyourgoalofpreventingmultipledecimalseparators.Logically,iftheexistingstringhasadecimalseparatorandthereplacementstringhasadecimalseparator,thechangeshouldberejected.

UpdatetextField(_:shouldChangeCharactersInRange:replacementString:)

tousethislogic.functextField(textField:UITextField,shouldChangeCharactersInRangerange:NSRange,replacementStringstring:String)->Bool{

print("Currenttext:\(textField.text)")print("Replacementtext:\(string)")

returntrue

letexistingTextHasDecimalSeparator=textField.text?.rangeOfString(".")letreplacementTextHasDecimalSeparator=string.rangeOfString(".")

ifexistingTextHasDecimalSeparator!=nil&&replacementTextHasDecimalSeparator!=nil{returnfalse}else{returntrue}}

Buildandruntheapplication.Attempttoentermultipledecimalseparators;theapplicationwillrejecttheseconddecimalseparatorthatyouenter.

Moreonprotocols

IntheUITextFieldDelegateprotocol,therearetwokindsofmethods:methodsthathandleinformationupdatesandmethodsthathandlerequestsforinput.Forexample,the


textfield’sdelegateimplementsthetextFieldDidBeginEditing(_:)methodifitwantstoknowwhentheusertapsonthetextfield.

Ontheotherhand,textField(_:shouldChangeCharactersInRange:replacementString:)

isarequestforinput.Atextfieldcallsthismethodonitsdelegatetoaskwhetherthereplacementstringshouldbeacceptedorrejected.ThemethodreturnsaBool,whichisthedelegate’sanswer.

Methodsdeclaredinaprotocolcanberequiredoroptional.Bydefault,protocolmethodsarerequired,meaningthataclassconformingtotheprotocolmusthaveanimplementationofthosemethods.Ifaprotocolhasoptionalmethods,theseareprecededbythedirectiveoptional.LookingbackattheUITextFieldDelegateprotocol,youcanseethatallofitsmethodsareoptional.Thisistypicallytrueofdelegateprotocols.


BronzeChallenge:DisallowAlphabeticCharactersCurrently,theusercanenteralphabeticcharacterseitherbyusingaBluetoothkeyboardorbypastingcopiedtextintothetextfield.Fixthisissue.Hint:youwillwanttousetheNSCharacterSetclass.


5ViewControllers

AviewcontrollerisaninstanceofasubclassofUIViewController.Aviewcontrollermanagesaviewhierarchy.Itisresponsibleforcreatingviewobjectsthatmakeupthehierarchyandforhandlingeventsassociatedwiththeviewobjectsinitshierarchy.

Sofar,WorldTrotterhasasingleviewcontroller,ConversionViewController.Inthischapter,youwillupdateittousemultipleviewcontrollers.Theuserwillbeabletoswitchbetweentwoviewhierarchies–oneforviewingtheConversionViewControllerandanotherfordisplayingamap(Figure5.1).

Figure5.1ThetwofacesofWorldTrotter


TheViewofaViewControllerAssubclassesofUIViewController,allviewcontrollersinheritanimportantproperty:varview:UIView!

ThispropertypointstoaUIViewinstancethatistherootoftheviewcontroller’sviewhierarchy.Whentheviewofaviewcontrollerisaddedasasubviewofthewindow,theviewcontroller’sentireviewhierarchyisadded,asshowninFigure5.2.

Figure5.2ObjectdiagramforWorldTrotter

Aviewcontroller’sviewisnotcreateduntilitneedstoappearonthescreen.Thisoptimizationiscalledlazyloading,anditcanconservememoryandimproveperformance.

Therearetwowaysthataviewcontrollercancreateitsviewhierarchy:

programmatically,byoverridingtheUIViewControllermethodloadView()

inInterfaceBuilder,byusinganinterfacefilesuchasastoryboard

YouhavealreadyseenbothofthesemethodsinChapter3.First,youcreatedasampleviewhierarchyprogrammatically,thenyouswitchedtoInterfaceBuildertocreatetheinterfaceforConversionViewControllerusingastoryboardfile.YouwillcontinuetouseInterfaceBuilderinthischapterasyoufurtherexploreviewcontrollers.InChapter6,youwillgetmoreexperiencecreatingprogrammaticviewsusingloadView().


SettingtheInitialViewControllerAlthoughastoryboardcanhavemanyviewcontrollers,eachstoryboardfilehasexactlyoneinitialviewcontroller.Theinitialviewcontrolleractsasanentrypointintothestoryboard.Youaregoingtoaddandconfigureanotherviewcontrollertothecanvasandsetittobetheinitialviewcontrollerforthestoryboard.

OpenMain.storyboard.Fromtheobjectlibrary,dragaViewControllerontothecanvas(Figure5.3).Ifyouarerunningoutofavailablespace,youcanControl-clickonthebackgroundtochooseadifferentzoomlevel.

Figure5.3Addingaviewcontrollertothecanvas

YouwantthisviewcontrollertodisplayanMKMapView–aclassdesignedtodisplayamap–insteadoftheexistingwhiteUIView.

SelecttheviewoftheViewController–nottheViewControlleritself!–andpressDeletetoremovethisviewfromthecanvas.ThendragaMapKitViewfromtheobjectlibraryontotheviewcontrollertosetitastheviewforthisviewcontroller(Figure5.4).


Figure5.4Addingamapviewtothecanvas

NowselecttheViewControllerandopenitsattributesinspector.UndertheViewControllersection,checktheboxnexttoIsInitialViewController(Figure5.5).DidyounoticethatthegrayarrowonthecanvasthatwaspointingattheConversionViewControllerisnowpointingtotheViewController?Thearrow,asyouhaveprobablysurmised,indicatestheinitialviewcontroller.Anotherwaytoassigntheinitialviewcontrolleristodragthatarrowfromoneviewcontrollertoanotheronthecanvas.

Figure5.5Settingtheinitialviewcontroller

Thereisaquirkthatwouldcauseproblemsifyouweretobuildandruntheapprightnow.(Tryit,ifyoulike.)MKMapViewisinaframeworkthatisnotcurrentlybeingloadedintotheapplication.Aframeworkisasharedlibraryofcodethatincludesassociatedresources


suchasinterfacefilesandimages.YoubrieflylearnedaboutframeworksinChapter3,andyouhavebeenusingacoupleofframeworksalready:UIKitandFoundationarebothframeworks.

Sofar,youhavebeenincludingframeworksinyourappbyusingtheimportkeyword,likeso:importUIKit

NowyouneedtoimporttheMapKitframeworkfortheMKMapViewtoload.However,ifyouimporttheMapKitframeworkusingtheimportkeywordwithoutincludinganycodethatusesthatframework,thecompilerwilloptimizeitout–eventhoughyouareusingamapviewinyourstoryboard.

Instead,youneedtomanuallylinktheMapKitframeworktotheapp.

Withtheprojectnavigatoropen,clickontheWorldTrotterprojectatthetopofthelisttoopentheprojectsettings.FindandopentheGeneraltabinthesettings.ScrolldowntothebottomandfindthesectionlabeledLinkedFrameworksandLibraries.Clickonthe+atthebottomandsearchforMapKit.framework.SelectthisframeworkandclickAdd(Figure5.6).

Figure5.6AddingtheMapKitframework

Nowyoucanbuildandruntheapplication.Sinceyouhavechangedtheinitialviewcontroller,themapshowsupinsteadoftheviewoftheConversionViewController.

Asmentionedabove,therecanonlyeverbeoneinitialviewcontrollerassociatedwithagivenstoryboard.YousawthisearlierwhenyousettheViewControllertobetheinitialviewcontroller.Atthatpoint,theConversionViewControllerwasnolongertheinitialviewcontrollerforthisstoryboard.Let’stakealookathowthisrequirementworkswiththerootlevelUIWindowtoaddtheinitialviewcontroller’sviewtothewindowhierarchy.

UIWindowhasarootViewControllerproperty.Whenaviewcontrollerissetasthewindow’srootViewController,thatviewcontroller’sviewgetsaddedtothewindow’sviewhierarchy.Whenthispropertyisset,anyexistingsubviewsonthewindowareremovedandviewcontroller’sviewgetsaddedtothewindowwiththeappropriateAutoLayoutconstraints.

Eachapplicationhasonemaininterface,areferencetoastoryboard.Whentheapplicationlaunches,theinitialviewcontrollerforthemaininterfacegetssetastherootViewControllerofthewindow.


Themaininterfaceforanapplicationissetintheprojectsettings.StillintheGeneraltaboftheprojectsettings,findtheDeploymentInfosection.HereyouwillseetheMainInterfacesetting(Figure5.7).ThisissettoMain,whichcorrespondstoMain.storyboard.

Figure5.7Anapplication’smaininterface


UITabBarControllerViewcontrollersbecomemoreinterestingwhentheuserhasawaytoswitchbetweenthem.Throughoutthebook,youwilllearnanumberofwaystopresentviewcontrollers.Inthischapter,youwillcreateaUITabBarControllerthatwillallowtheusertoswapbetweentheConversionViewControllerandtheUIViewControllerdisplayingthemap.

UITabBarControllerkeepsanarrayofviewcontrollers.Italsomaintainsatabbaratthebottomofthescreenwithatabforeachviewcontrollerinitsarray.Tappingonatabresultsinthepresentationoftheviewoftheviewcontrollerassociatedwiththattab.

OpenMain.storyboardandselecttheViewController.FromtheEditormenu,chooseEmbedIn→TabBarController.ThiswilladdtheViewControllertotheviewcontrollersarrayoftheTabBarController.YoucanseethisrepresentedbytheRelationshiparrowpointingfromtheTabBarControllertotheViewController(Figure5.8).Additionally,InterfaceBuilderknowstomaketheTarBarControllertheinitialviewcontrollerforthestoryboard.

Figure5.8Tabbarcontrollerwithoneviewcontroller

Atabbarcontrollerisnotveryusefulwithjustoneviewcontroller.AddtheConversionViewControllertotheTabBarController’sviewcontrollersarray.

Control-dragfromtheTabBarControllertotheConversionViewController.FromtheRelationshipSeguesection,chooseviewcontrollers(Figure5.9).


Figure5.9Addingaviewcontrollertothetabbarcontroller

Buildandruntheapplication.Taponthetwotabsatthebottomtoswitchbetweenthetwoviewcontrollers.Atthemoment,thetabsjustsay“Item,”whichisnotveryhelpful.Inthenextsection,youwillupdatethetabbaritemstomakethetabsmoredescriptiveandobvious.

UITabBarControllerisitselfasubclassofUIViewController.AUITabBarController’sviewisaUIViewwithtwosubviews:thetabbarandtheviewoftheselectedviewcontroller(Figure5.10).


Figure5.10UITabBarControllerdiagram

Tabbaritems

Eachtabonthetabbarcandisplayatitleandanimage,andeachviewcontrollermaintainsatabBarItempropertyforthispurpose.WhenaviewcontrolleriscontainedbyaUITabBarController,itstabbaritemappearsinthetabbar.Figure5.11showsanexampleofthisrelationshipiniPhone’sPhoneapplication.


Figure5.11UITabBarItemexample

First,youneedtoaddafewfilestoyourprojectthatwillbetheimagesforthetabbaritems.Intheprojectnavigator,opentheAssetCatalogbyopeningAssets.xcassets.Then,findConvertIcon.png,[email protected],[email protected],MapIcon.png,[email protected],andMapIcon@3x.pngintheResourcesdirectoryofthefilethatyoudownloadedearlier(http://www.bignerdranch.com/solutions/iOSProgramming5ed.zip).DragthesefilesintotheimagessetlistontheleftsideoftheAssetCatalog(Figure5.12).



Figure5.12AddingimagestotheAssetCatalog

Thetabbaritempropertiescanbeseteitherprogrammaticallyorinastoryboard.Sinceyourdataisstatic,thestoryboardwillbethebestplacetosetthetabbaritemproperties.

InMain.storyboard,locatetheViewController.Noticethatatabbarwiththetabbariteminitwasaddedtotheinterfacesincetheviewcontrollerwillbepresentedwithinatabbarcontroller.Thisisveryusefulwhenlayingoutyourinterface.

Selectthistabbaritemandopenitsattributesinspector.UndertheBarItemsection,changetheTitleto“Map”andchooseMapIconfromtheImagemenu.Youcanalsochangethetextofthetabbaritembydouble-clickingonthetextonthecanvas.Thetabbarwillbeupdatedtoreflectthesevalues(Figure5.13).

Figure5.13ViewController’stabbaritem

NowfindtheConversionViewControllerandselectitstabbaritem.SettheTitletobe“Convert”andtheImagetobeConvertIcon.

Let’salsochangethefirsttabtobetheConvertViewController.Theorderofthetabsisdeterminedbytheorderoftheviewcontrollerswithinthetabbarcontroller’sviewControllersarray.YoucanchangetheorderinastoryboardbydraggingthetabsatthebottomoftheTabBarController.


FindtheTabBarControlleronthecanvas.DragtheConverttabtobeinthefirstposition.

Buildandruntheapplication.Notonlyarethetabbaritemsatthebottommoredescriptive,buttheConvertViewControllerisnowthefirstviewcontrollerthatisdisplayed(Figure5.14).

Figure5.14Tabbaritemswithlabelsandicons


LoadedandAppearingViewsNowthatyouhavetwoviewcontrollers,thelazyloadingofviewsmentionedearlierbecomesmoreimportant.

Whentheapplicationlaunches,thetabbarcontrollerdefaultstoloadingtheviewofthefirstviewcontrollerinitsarray,whichistheConvertViewController.ThismeansthattheMapViewController’sviewisnotneededandwillonlybeneededwhen(orif)theusertapsthetabtoseeit.

Youcantestthisbehaviorforyourself.Whenaviewcontrollerfinishesloadingitsview,viewDidLoad()iscalled,andyoucanoverridethismethodtomakeitprintamessagetotheconsole,allowingyoutoseethatitwascalled.

Youaregoingtoaddcodetobothviewcontrollers.However,thereisnocodecurrentlyassociatedwiththeviewcontrollerdisplayingthemapbecauseeverythinghasbeenconfiguredusingthestoryboard.Nowthatyouwanttoaddcodetothatviewcontroller,youaregoingtocreateaviewcontrollersubclassandassociateitwiththatinterface.

CreateanewSwiftfile(Command-N)andnameitMapViewController.OpenMapViewController.swiftanddefineaUIViewControllersubclassnamedMapViewController.importFoundationimportUIKit

classMapViewController:UIViewController{

}

NowopenMain.storyboardandselectthemap’sviewcontroller.OpenitsidentityinspectorandchangetheClasstoMapViewController.

NowthatyouhaveassociatedtheMapViewControllerclasswiththeviewcontrolleronthecanvas,youcanaddcodetobothConversionViewControllerandMapViewControllertoprinttotheconsolewhentheirviewDidLoad()methodiscalled.

InConversionViewController.swift,overrideviewDidLoad()toprintastatementtotheconsole.overridefuncviewDidLoad(){//AlwayscallthesuperimplementationofviewDidLoadsuper.viewDidLoad()

print("ConversionViewControllerloadeditsview.")}

InMapViewController.swift,overridethesamemethod.overridefuncviewDidLoad(){//AlwayscallthesuperimplementationofviewDidLoadsuper.viewDidLoad()

print("MapViewControllerloadeditsview.")}

Buildandruntheapplication.Theconsolereportsthat


ConversionViewControllerloadeditsviewrightaway.TapMapViewController’stab,andtheconsolewillreportthatitsviewisnowloaded.Atthispoint,bothviewshavebeenloaded,soswitchingbetweenthetabsnowwillnolongertriggerviewDidLoad().(Tryitandsee.)

Accessingsubviews

Often,youwillwanttodosomeextrainitializationorconfigurationofsubviewsdefinedinInterfaceBuilderbeforetheyappeartotheuser.Sowherecanyouaccessasubview?Therearetwomainoptions,dependingonwhatyouneedtodo.ThefirstoptionistheviewDidLoad()methodthatyouoverrodetospotlazyloading.Thismethodiscalledaftertheviewcontroller’sinterfacefileisloaded,atwhichpointalloftheviewcontroller’soutletswillreferencetheappropriateobjects.ThesecondoptionisanotherUIViewControllermethod,viewWillAppear(_:).Thismethodiscalledjustbeforeaviewcontroller’sviewisaddedtothewindow.

Whichshouldyouchoose?OverrideviewDidLoad()iftheconfigurationonlyneedstobedoneonceduringtherunoftheapp.OverrideviewWillAppear(_:)ifyouneedtheconfigurationtobedoneeachtimetheviewcontroller’sviewappearsonscreen.


InteractingwithViewControllersandTheirViewsLet’slookatsomemethodsthatarecalledduringthelifecycleofaviewcontrolleranditsview.Someofthesemethodsyouhavealreadyseen,andsomearenew.

init(coder:)istheinitializerforUIViewControllerinstancescreatedfromastoryboard.

Whenaviewcontrollerinstanceiscreatedfromastoryboard,itsinit(coder:)getscalledonce.YouwilllearnmoreaboutthismethodinChapter15.

init(nibName:bundle:)isthedesignatedinitializerforUIViewController.

Whenaviewcontrollerinstanceiscreatedwithouttheuseofastoryboard,itsinit(nibName:bundle:)getscalledonce.Notethatinsomeapps,youmayendupcreatingseveralinstancesofthesameviewcontrollerclass.Thismethodwillgetcalledonceoneachviewcontrollerasitiscreated.

loadView()isoverriddentocreateaviewcontroller’sviewprogrammatically.

viewDidLoad()isoverriddentoconfigureviewscreatedbyloadinganinterfacefile.Thismethodgetscalledaftertheviewofaviewcontrolleriscreated.

viewWillAppear(_:)isoverriddentoconfigureviewscreatedbyloadinganinterfacefile.

ThismethodandviewDidAppear(_:)getcalledeverytimeyourviewcontrollerismovedonscreen.viewWillDisappear(_:)andviewDidDisappear(_:)getcalledeverytimeyourviewcontrollerismovedoffscreen.


SilverChallenge:DarkModeWhenevertheConversionViewControllerisviewed,updateitsbackgroundcolorbasedonthetimeofday.Intheevening,thebackgroundshouldbeadarkcolor.Otherwise,thebackgroundshouldbealightcolor.YouwillneedtooverrideviewWillAppear(_:)toaccomplishthis.(Ifthatisnotenoughexcitementinyourlife,youcanchangethebackgroundcoloreachtimetheviewcontrollerisviewed.)


FortheMoreCurious:RetinaDisplayWiththereleaseofiPhone4,AppleintroducedtheRetinadisplayforiPhoneandiPodtouch.TheRetinadisplayhasmuchhigherresolutioncomparedtoearlierdevices.Let’slookatwhatyoushoulddotomakegraphicslooktheirbestonbothdisplays.

Forvectorgraphics,youdonotneedtodoanything;yourcodewillrenderascrisplyasthedeviceallows.However,ifyoudrawusingCoreGraphicsfunctions,thesegraphicswillappeardifferentlyondifferentdevices.InCoreGraphics(alsocalledQuartz),lines,curves,text,etc.aredescribedintermsofpoints.Onanon-Retinadisplay,apointis1x1pixel.OnmostRetinadisplays,apointis2x2pixels(Figure5.15).TheexceptionsareiPhone6PlusandiPhone6sPlus,whichhaveahigherresolutionRetinadisplaywhereapointis3x3pixels.

Figure5.15Renderingtodifferentresolutions

Giventhesedifferences,bitmapimages(likeJPEGorPNGfiles)willbeunattractiveiftheimageisnottailoredtothedevice’sscreentype.Sayyourapplicationincludesasmallimageof25x25pixels.Ifthisimageisdisplayedona2xRetinadisplay,thentheimagemustbestretchedtocoveranareaof50x50pixels.Atthispoint,thesystemdoesatypeofaveragingcalledanti-aliasingtokeeptheimagefromlookingjagged.Theresultisanimagethatisnotjagged–butitisfuzzy(Figure5.16).


Figure5.16Fuzzinessfromstretchinganimage

Youcouldusealargerfileinstead,buttheaveragingwouldthencauseproblemsintheotherdirectionwhentheimageisshrunkforanon-Retinadisplay.Theonlysolutionistobundletwoimagefileswithyourapplication:oneatapixelresolutionequaltothenumberofpointsonthescreenfornon-RetinadisplaysandonetwicethatsizeinpixelsforRetinadisplays.

Fortunately,youdonothavetowriteanyextracodetohandlewhichimagegetsloadedonwhichdevice.Allyouhavetodoissuffixthehigher-resolutionimagefor2xRetinadisplayswith@2xandthehigher-resolutionimagefor3xRetinadisplayswith@3x.Makesurethe@2x(or@3x)goesbeforethefileextension:[email protected],[email protected],whenyouuseUIImage’sinit(named:)initializertoloadtheimage,thismethodlooksinthebundleandgetstheappropriatefilefortheparticulardevice.


6ProgrammaticViews

Inthischapter,youwillupdateWorldTrottertocreatetheviewforMapViewControllerprogrammatically(Figure6.1).Indoingso,youwilllearnmoreaboutviewcontrollersandhowtosetupconstraintsandcontrols(suchasUIButtons)programmatically.

Figure6.1WorldTrotterwithprogrammaticviews

Currently,theviewforMapViewControllerisdefinedinthestoryboard.Thefirststep,then,istoremovethisviewfromthestoryboardsoyoucaninsteadcreateitprogrammatically.

InMain.storyboard,selectthemapviewassociatedwithMapViewControllerandpressDelete(Figure6.2).


Figure6.2Deletingtheview


CreatingaViewProgrammaticallyYoulearnedinChapter5thattocreateaviewcontroller’sviewprogrammatically,youoverridetheUIViewControllermethodloadView().

OpenMapViewController.swiftandoverrideloadView()tocreateaninstanceofMKMapViewandsetitastheviewoftheviewcontroller.Youwillneedareferencetothemapviewlateron,socreateapropertyforitaswell.importUIKitimportMapKit

classMapViewController:UIViewController{

varmapView:MKMapView!

overridefuncloadView(){//CreateamapviewmapView=MKMapView()

//Setitas*the*viewofthisviewcontrollerview=mapView}


print("MapViewControllerloadeditsview.")}

}

Whenaviewcontrolleriscreated,itsviewpropertyisnil.Ifaviewcontrollerisaskedforitsviewanditsviewisnil,thentheloadView()methodiscalled.

Buildandruntheapplication.Althoughtheapplicationlooksthesame,themapviewisbeingcreatedprogrammaticallyinsteadofthroughInterfaceBuilder.


ProgrammaticConstraintsInChapter3,youlearnedaboutAutoLayoutconstraintsandhowtoaddthemusingInterfaceBuilder.Inthissection,youwilllearnhowtoaddconstraintstoaninterfaceprogrammatically.

ApplerecommendsthatyoucreateandconstrainyourviewsinInterfaceBuilderwheneverpossible.However,ifyourviewsarecreatedincode,thenyouwillneedtoconstrainthemprogrammatically.TheinterfaceforMapViewControlleriscreatedprogrammatically,soitisagreatcandidateforprogrammaticconstraints.

Tolearnaboutprogrammaticconstraints,youaregoingtoaddaUISegmentedControltoMapViewController’sinterface.Asegmentedcontrolallowstheusertochoosebetweenadiscretesetofoptions,andyouwilluseonetoallowtheusertoswitchbetweenmaptypes:standard,hybrid,andsatellite.

InMapViewController.swift,updateloadView()toaddasegmentedcontroltotheinterface.overridefuncloadView(){//CreateamapviewmapView=MKMapView()

//Setitas*the*viewofthisviewcontrollerview=mapView

letsegmentedControl=UISegmentedControl(items:["Standard","Hybrid","Satellite"])segmentedControl.backgroundColor=UIColor.whiteColor().colorWithAlphaComponent(0.5)segmentedControl.selectedSegmentIndex=0

segmentedControl.translatesAutoresizingMaskIntoConstraints=falseview.addSubview(segmentedControl)}

Thelineofcoderegardingtranslatingconstraintshastodowithanoldersystemforscalinginterfaces–autoresizingmasks.BeforeAutoLayoutwasintroduced,iOSapplicationsusedautoresizingmaskstoallowviewstoscalefordifferent-sizedscreensatruntime.

Everyviewhasanautoresizingmask.Bydefault,iOScreatesconstraintsthatmatchtheautoresizingmaskandaddsthemtotheview.Thesetranslatedconstraintswilloftenconflictwithexplicitconstraintsinthelayoutandcauseanunsatisfiableconstraintsproblem.ThefixistoturnoffthisdefaulttranslationbysettingthepropertytranslatesAutoresizingMaskIntoConstraintstofalse.(ThereismoreaboutAutoLayoutandautoresizingmasksattheendofthischapter.)

Anchors

WhenyouworkwithAutoLayoutprogrammatically,youwilluseanchorstocreateyourconstraints.Anchorsarepropertiesontheviewthatcorrespondtoattributesthatyoumightwanttoconstraintoananchoronanotherview.Forexample,youmightconstraintheleadinganchorofoneviewtotheleadinganchorofanotherview.Thiswouldhavethe


effectofthetwoviews’leadingedgesbeingaligned.

Let’screatesomeconstraintstodothefollowing.

Thetopanchorofthesegmentedcontrolshouldbeequaltothetopanchorofitssuperview.

Theleadinganchorofthesegmentedcontrolshouldbeequaltotheleadinganchorofitssuperview.

Thetrailinganchorofthesegmentedcontrolshouldbeequaltothetrailinganchorofitssuperview.

InMapViewController.swift,createtheseconstraintsinloadView().letsegmentedControl=UISegmentedControl(items:["Standard","Hybrid","Satellite"])segmentedControl.backgroundColor=UIColor.whiteColor().colorWithAlphaComponent(0.5)segmentedControl.selectedSegmentIndex=0

segmentedControl.translatesAutoresizingMaskIntoConstraints=falseview.addSubview(segmentedControl)

lettopConstraint=segmentedControl.topAnchor.constraintEqualToAnchor(view.topAnchor)letleadingConstraint=segmentedControl.leadingAnchor.constraintEqualToAnchor(view.leadingAnchor)lettrailingConstraint=segmentedControl.trailingAnchor.constraintEqualToAnchor(view.trailingAnchor)

(Notethatduetopagesizerestrictionsweareshowingeachconstant’sdeclarationsplitacrosstwolines.Youshouldentereachdeclarationonasingleline.)

Xcodewillalertyoutoaproblemwitheachlineyouhaveentered.Youwillfixtheminamoment.

AnchorshaveamethodconstraintEqualToAnchor(_:)thatwillcreateaconstraintbetweenthetwoanchors.ThereareafewotherconstraintcreationmethodsonNSLayoutAnchor,includingonethatacceptsaconstantasanargument:funcconstraintEqualToAnchor(anchor:NSLayoutAnchor!,constantc:CGFloat)->NSLayoutConstraint!

Activatingconstraints

YounowhavethreeNSLayoutConstraintinstances.However,theseconstraintswillhavenoeffectonthelayoutuntilyouexplicitlyactivatethembysettingtheiractivepropertiestotrue.ThiswillresolveXcode’scomplaint.

InMapViewController.swift,activatetheconstraintsattheendofloadView().lettopConstraint=segmentedControl.topAnchor.constraintEqualToAnchor(view.topAnchor)letleadingConstraint=segmentedControl.leadingAnchor.constraintEqualToAnchor(view.leadingAnchor)lettrailingConstraint=segmentedControl.trailingAnchor.constraintEqualToAnchor(view.trailingAnchor)

topConstraint.active=trueleadingConstraint.active=truetrailingConstraint.active=true


Constraintsneedtobeaddedtothemostrecentcommonancestorfortheviewsassociatedwiththeconstraint.Figure6.3showsaviewhierarchyalongwiththecommonancestorfortwoviews.

Figure6.3Commonancestor

Ifaconstraintonlyhasoneview(suchaswhenaddingawidthorheightconstrainttoaview),thenthatviewisconsideredthecommonancestor.

Bysettingtheactivepropertyonaconstrainttotrue,theconstraintwillworkitswayupthehierarchyfortheitemstofindthecommonancestortoaddtheconstraintto.ItwillthencallthemethodaddConstraint(_:)ontheappropriateview.SettingtheactivepropertyispreferabletocallingaddConstraint(_:)orremoveConstraint(_:)yourself.

BuildandruntheapplicationandswitchtotheMapViewController.Thesegmentedcontrolisnowpinnedtothetop,leading,andtrailingedgesofitssuperview(Figure6.4).


Figure6.4Segmentedcontroladdedtothescreen

Althoughtheconstraintsaredoingtherightthing,theinterfacedoesnotlookgood.Thesegmentedcontrolisunderlappingthestatusbar,anditwouldlookbetterifthesegmentedcontrolwasinsetfromtheleadingandtrailingedgesofthescreen.Let’stacklethestatusbarissuefirst.

Layoutguides

Viewcontrollersexposetwolayoutguidestoassistwithlayoutcontent:thetopLayoutGuideandthebottomLayoutGuide.Thelayoutguidesindicatetheextenttowhichtheviewcontroller’sviewcontentswillbevisible.UsingtopLayoutGuidewillallowyourcontenttonotunderlapthestatusbarornavigationbaratthetopofthescreen.(YouwilllearnaboutnavigationbarsinChapter13.)UsingthebottomLayoutGuidewillallowyourcontenttonotunderlapthetabbaratthebottomofthescreen.

Thelayoutguidesexposethreeanchorsthatyoucanusetoaddconstraints:topAnchor,bottomAnchor,andheightAnchor.Sinceyouwantthesegmentedcontroltobeunderthestatusbar,youwillconstrainthebottomanchorofthetoplayoutguidetothetopanchorofthesegmentedcontrol.

InMapViewController.swift,updatethesegmentedcontrol’sconstraintsinloadView().Makethesegmentedcontrolbe8pointsbelowthetoplayoutguide.lettopConstraint=segmentedControl.topAnchor.constraintEqualToAnchor(view.topAnchor)lettopConstraint=segmentedControl.topAnchor.constraintEqualToAnchor(topLayoutGuide.bottomAnchor,constant:8)letleadingConstraint=segmentedControl.leadingAnchor.constraintEqualToAnchor(view.leadingAnchor)lettrailingConstraint=segmentedControl.trailingAnchor.constraintEqualToAnchor(view.trailingAnchor)


Buildandruntheapplication.Thesegmentedcontrolnowappearsbelowthestatusbar.Byusingthelayoutguidesinsteadofahardcodedconstant,theviewswilladaptbasedonthecontexttheyappearin.


Nowlet’supdatethesegmentedcontrolsothatitisinsetfromtheleadingandtrailingedgesofitssuperview.

Margins

Althoughyoucouldinsetthesegmentedcontrolusingaconstantontheconstraint,itismuchbettertousethemarginsoftheviewcontroller’sview.

EveryviewhasalayoutMarginspropertythatdenotesthedefaultspacingtousewhenlayingoutcontent.ThispropertyisaninstanceofUIEdgeInsets,whichyoucanthinkofasatypeofframe.Whenaddingconstraints,youwillusethelayoutMarginsGuide,whichexposesanchorsthataretiedtotheedgesofthelayoutMargins.

Theprimaryadvantageofusingthemarginsisthatthemarginscanchangedependingonthedevicetype(iPadoriPhone)aswellasthesizeofthedevice(iPhone6soriPhone6sPlus).Usingthemarginswillgiveyoucontentthatlooksgoodonanydevice.

Updatethesegmentedcontrol’sleadingandtrailingconstraintsinloadView()tousethemargins.lettopConstraint=segmentedControl.topAnchor.constraintEqualToAnchor(topLayoutGuide.bottomAnchor,constant:8)

letleadingConstraint=segmentedControl.leadingAnchor.constraintEqualToAnchor(view.leadingAnchor)lettrailingConstraint=segmentedControl.trailingAnchor.constraintEqualToAnchor(view.trailingAnchor)

letmargins=view.layoutMarginsGuideletleadingConstraint=segmentedControl.leadingAnchor.constraintEqualToAnchor(margins.leadingAnchor)lettrailingConstraint=segmentedControl.trailingAnchor.constraintEqualToAnchor(margins.trailingAnchor)


Buildandruntheapplicationagain.Thesegmentedcontrolisnowinsetfromtheview’smargins(Figure6.5).

Figure6.5Segmentedcontrolwithupdatedconstraints


Explicitconstraints

Itishelpfultounderstandhowthesemethodsthatyouhaveusedcreateconstraints.NSLayoutConstrainthasthefollowinginitializer:convenienceinit(itemview1:AnyObject,attributeattr1:NSLayoutAttribute,relatedByrelation:NSLayoutRelation,toItemview2:AnyObject?,attributeattr2:NSLayoutAttribute,multiplier:CGFloat,constantc:CGFloat)

Thisinitializercreatesasingleconstraintusingtwolayoutattributesoftwoviewobjects.Themultiplieristhekeytocreatingaconstraintbasedonaratio.Theconstantisafixednumberofpoints,likeyouhaveusedinyourspacingconstraints.

ThelayoutattributesaredefinedasconstantsintheNSLayoutConstraintclass:

NSLayoutAttribute.Left

NSLayoutAttribute.Leading

NSLayoutAttribute.Trailing

NSLayoutAttribute.Top

NSLayoutAttribute.Bottom

NSLayoutAttribute.Width

NSLayoutAttribute.Height

NSLayoutAttribute.CenterX

NSLayoutAttribute.CenterY

NSLayoutAttribute.FirstBaseline

NSLayoutAttribute.LastBaseline

Thereareadditionalattributesthathandlethemarginsassociatedwithaview,suchasNSLayoutAttribute.LeadingMargin.

Let’sconsiderahypotheticalconstraint.Sayyouwantedtheimageviewtobe1.5timesaswideasitistall.Youcouldcreateitwiththefollowingcode.(Donottypethishypotheticalconstraintinyourcode!Itwillconflictwithothersyoualreadyhave.)letaspectConstraint=NSLayoutConstraint(item:imageView,attribute:.Width,relatedBy:.Equal,toItem:imageView,attribute:.Height,multiplier:1.5,constant:0.0)

Tounderstandhowthisinitializerworks,thinkofthisconstraintastheequationshowninFigure6.6.


Figure6.6NSLayoutConstraintequation

Yourelatealayoutattributeofoneviewtothelayoutattributeofanotherviewusingamultiplierandaconstanttodefineasingleconstraint.


ProgrammaticControlsNowlet’supdatethesegmentedcontroltochangethemaptypewhentheusertapsonasegment.

AUISegmentedControlisasubclassofUIControl.YouhaveworkedwithanotherUIControlsubclassinChapter1,theUIButtonclass.Controlsareresponsibleforcallingmethodsontheirtargetinresponsetosomeevent.

ControleventsareoftypeUIControlEvents.Hereareafewofthecommoncontroleventsthatyouwilluse:

UIControlEvents.TouchDown

Atouchdownonthecontrol.UIControlEvents.TouchUpInside

Atouchdownfollowedbyatouchupwhilestillwithintheboundsofthecontrol.

UIControlEvents.ValueChanged

Atouchthatcausesthevalueofthecontroltochange.

UIControlEvents.EditingChanged

AtouchthatcausesaneditingchangeforaUITextField.

Youused.TouchUpInsidefortheUIButtoninChapter1(itisthedefaulteventwhenyouControl-dragtoconnectactionsinInterfaceBuilder),andyousawthe.EditingChangedeventinChapter4.Forthesegmentedcontrol,youwillusethe.ValueChangedevent.

InMapViewController.swift,updateloadView()toaddatarget-actionpairtothesegmentedcontrolandassociateitwiththe.ValueChangedevent.overridefuncloadView(){

mapView=MKMapView()view=mapView

letsegmentedControl=UISegmentedControl(items:["Standard","Satellite","Hybrid"])segmentedControl.backgroundColor=UIColor.whiteColor().colorWithAlphaComponent(0.5)segmentedControl.selectedSegmentIndex=0

segmentedControl.addTarget(self,action:"mapTypeChanged:",forControlEvents:.ValueChanged)

...

Next,implementtheactionmethodthattheeventwilltrigger.Thismethodwillcheckwhichsegmentwasselectedandupdatethemapaccordingly.funcmapTypeChanged(segControl:UISegmentedControl){switchsegControl.selectedSegmentIndex{case0:mapView.mapType=.Standardcase1:


mapView.mapType=.Hybridcase2:mapView.mapType=.Satellitedefault:break}}

Buildandruntheapplication.Changetheselectedsegmentandthemapwillupdateaccordingly.


BronzeChallenge:AnotherTabCreateanewviewcontrollerandaddittothetabbarcontroller.ThisviewcontrollershoulddisplayaWKWebView,whichisaclassusedtodisplaywebcontent.Thewebviewshoulddisplayhttps://www.bignerdranch.comforyoutobookyournextvacation.


https://www.bignerdranch.com

SilverChallenge:User’sLocationAddabuttontotheMapViewControllerthatdisplaysandzoomsinontheuser’scurrentlocation.Youwillneedtousedelegationtoaccomplishthis.RefertothedocumentationforMKMapViewDelegate.


GoldChallenge:DroppingPinsMapviewscandisplaypins,whichareinstancesofMKPinAnnotationView.Addthreepinstothemapview:onewhereyouwereborn,onewhereyouarenow,andoneataninterestinglocationyouhavevisitedinthepast.Addabuttontotheinterfacethatallowsthemaptodisplaythelocationofapin.Subsequenttapsshouldsimplycyclethroughthelistofpins.


FortheMoreCurious:NSAutoresizingMaskLayoutConstraintAswementionedearlier,beforeAutoLayoutiOSapplicationsusedanothersystemformanaginglayout:autoresizingmasks.Eachviewhadanautoresizingmaskthatconstrainedtherelationshipbetweenaviewanditssuperview,butthismaskcouldnotaffectrelationshipsbetweensiblingviews.

Bydefault,viewscreateandaddconstraintsbasedontheirautoresizingmasks.However,thesetranslatedconstraintsoftenconflictwithyourexplicitconstraintsinyourlayout,whichresultsanunsatisfiableconstraintsproblem.

Toseethishappen,commentoutthelineinloadView()thatturnsoffthetranslationofautoresizingmasks.//segmentedControl.translatesAutoresizingMaskIntoConstraints=falseview.addSubview(segmentedControl)

Nowthesegmentedcontrolhasaresizingmaskthatwillbetranslatedintoaconstraint.Buildandruntheapplicationandnavigatetothemapinterface.Youwillnotlikewhatyousee.Theconsolewillreporttheproblemanditssolution.Unabletosimultaneouslysatisfyconstraints.Probablyatleastoneoftheconstraintsinthefollowinglistisoneyoudon'twant.Trythis:(1)lookateachconstraintandtrytofigureoutwhichyoudon'texpect;(2)findthecodethataddedtheunwantedconstraintorconstraintsandfixit.(Note:Ifyou'reseeingNSAutoresizingMaskLayoutConstraintsthatyoudon'tunderstand,refertothedocumentationfortheUIViewpropertytranslatesAutoresizingMaskIntoConstraints)("<NSAutoresizingMaskLayoutConstraint:0x7fb6b8e0ad00h=--&v=--&H:[UISegmentedControl:0x7fb6b9897390(212)]>","<NSLayoutConstraint:0x7fb6b9975350UISegmentedControl:0x7fb6b9897390.leading==UILayoutGuide:0x7fb6b9972640'UIViewLayoutMarginsGuide'.leading>","<NSLayoutConstraint:0x7fb6b9975460UISegmentedControl:0x7fb6b9897390.trailing==UILayoutGuide:0x7fb6b9972640'UIViewLayoutMarginsGuide'.trailing>","<NSLayoutConstraint:0x7fb6b8e0b370'UIView-Encapsulated-Layout-Width'H:[MKMapView:0x7fb6b8d237c0(0)]>","<NSLayoutConstraint:0x7fb6b9972020'UIView-leftMargin-guide-constraint'H:|-(0)-[UILayoutGuide:0x7fb6b9972640'UIViewLayoutMarginsGuide'](LTR)(Names:'|':MKMapView:0x7fb6b8d237c0)>","<NSLayoutConstraint:0x7fb6b9974f50'UIView-rightMargin-guide-constraint'H:[UILayoutGuide:0x7fb6b9972640'UIViewLayoutMarginsGuide']-(0)-|(LTR)(Names:'|':MKMapView:0x7fb6b8d237c0)>")

Willattempttorecoverbybreakingconstraint<NSLayoutConstraint:0x7fb6b9975460UISegmentedControl:0x7fb6b9897390.trailing==UILayoutGuide:0x7fb6b9972640'UIViewLayoutMarginsGuide'.trailing>

MakeasymbolicbreakpointatUIViewAlertForUnsatisfiableConstraintstocatchthisinthedebugger.ThemethodsintheUIConstraintBasedLayoutDebuggingcategoryonUIViewlistedin<UIKit/UIView.h>mayalsobehelpful.

Let’sgooverthisoutput.AutoLayoutisreportingthatitis“Unabletosimultaneouslysatisfyconstraints.”Thishappenswhenaviewhierarchyhasconstraintsthatconflict.

Then,theconsolespitsoutsomehandytipsandalistofallconstraintsthatareinvolved.Eachconstraint’sdescriptionisshownintheconsole.Let’slookattheformatofoneoftheseconstraintsmoreclosely.<NSLayoutConstraint:0x7fb6b9975350UISegmentedControl:0x7fb6b9897390.leading


==UILayoutGuide:0x7fb6b9972640'UIViewLayoutMarginsGuide'.leading>

Thisdescriptionindicatesthattheconstraintlocatedatmemoryaddress0x7fb6b9975350issettingtheleadingedgeoftheUISegmentedControl(at0x7fb6b9897390)equaltotheleadingedgeofthemarginoftheUILayoutGuide(at0x7fb6b9972640).

FiveoftheseconstraintsareinstancesofNSLayoutConstraint.One,however,isaninstanceofNSAutoresizingMaskLayoutConstraint.Thisconstraintistheproductofthetranslationoftheimageview’sautoresizingmask.

Finally,ittellsyouhowitisgoingtosolvetheproblembylistingtheconflictingconstraintthatitwillignore.Unfortunately,itchoosespoorlyandignoresoneofyourexplicitinstancesofNSLayoutConstraintinsteadoftheNSAutoresizingMaskLayoutConstraint.Thisiswhyyourinterfacelookslikeitdoes.

Thenotebeforetheconstraintsarelistedisveryhelpful:theNSAutoresizingMaskLayoutConstraintneedstoberemoved.Betteryet,youcanpreventthisconstraintfrombeingaddedinthefirstplacebyexplicitlydisablingtranslationinloadView()://segmentedControl.translatesAutoresizingMaskIntoConstraints=falseview.addSubview(segmentedControl)


7Localization

TheappealofiOSisglobal–iOSusersliveinmanydifferentcountriesandspeakmanydifferentlanguages.Youcanensurethatyourapplicationisreadyforaglobalaudiencethroughtheprocessesofinternationalizationandlocalization.

Internationalizationismakingsureyournativeculturalinformation(likelanguage,currency,dateformat,numberformat,etc.)isnothardcodedintoyourapplication.Localizationistheprocessofprovidingtheappropriatedatainyourapplicationbasedontheuser’sLanguageandRegionFormatsettings.YoucanfindthesesettingsintheiOSSettingsapplication(Figure7.1).SelecttheGeneralrowandthentheLanguage&Regionrow.

Figure7.1Languageandregionsettings

Applemakestheseprocessesrelativelysimple.AnapplicationthattakesadvantageofthelocalizationAPIsdoesnotevenneedtoberecompiledtobedistributedinotherlanguagesorregions.(Bytheway,since“internationalization”and“localization”arelongwords,youwillsometimesseethemabbreviatedasi18nandL10n,respectively.)


Inthischapter,youwillfirstinternationalizetheWorldTrotterapplicationandthenlocalizeitintoSpanish.


InternationalizationInthisfirstsection,youwillusetheNSNumberFormatterandNSNumberclassestointernationalizetheConversionViewController.

Formatters

InChapter4,youusedaninstanceofNSNumberFormattertosetthetextoftheCelsiuslabelinConversionViewController.NSNumberFormatterhasalocaleproperty,whichissettothedevice’scurrentlocale.WheneveryouuseanNSNumberFormattertocreateanumber,itchecksitslocalepropertyandsetstheformataccordingly.SothetextoftheCelsiuslabelhasbeeninternationalizedfromthestart.

NSLocaleknowshowdifferentregionsdisplaysymbols,dates,anddecimalsandwhethertheyusethemetricsystem.AninstanceofNSLocalerepresentsoneregion’ssettingsforthesevariables.IntheSettingsapplication,theusercanchoosearegion,likeUnitedStatesorUnitedKingdom.(WhydoesAppleuse“region”insteadof“country”?Somecountrieshavemorethanoneregionwithdifferentsettings.ScrollthroughtheoptionsinApplicationRegiontoseeforyourself.)

WhenyoucallthemethodcurrentLocale()onNSLocale,theinstanceofNSLocalethatrepresentstheuser’sregionsettingisreturned.OnceyouhavethatinstanceofNSLocale,youcanaskitquestionslike,“Whatisthecurrencysymbolforthisregion?”or,“Doesthisregionusethemetricsystem?”

Toaskoneofthesequestions,youcallthemethodobjectForKey(_:)ontheNSLocaleinstancewithoneoftheNSLocaleconstantsasanargument.letcurrentLocale=NSLocale.currentLocale()letisMetric=currentLocale.objectForKey(NSLocaleUsesMetricSystem)?.boolValueletcurrencySymbol=currentLocale.objectForKey(NSLocaleCurrencyCode)

EventhoughtheCelsiuslabelisalreadyinternationalized,thereisstillaproblemwithit.ChangethesystemregiontoSpaintosee.Selecttheactiveschemepop-upandselectEditScheme(Figure7.2).

Figure7.2Editscheme

MakesurethatRunisselectedonthelefthandsideandthenselecttheOptionstabatthetop.


FortheApplicationRegionpop-up,selectEuropeandthenSpain(Figure7.3).Finally,Closetheactiveschemewindow.

Figure7.3Selectingadifferentregion

Buildandruntheapplication.OntheConversionViewController,tapthetextfieldandmakesurethesoftwarekeyboardisvisible.Youmayalreadynoticeonedifference:inSpain,thedecimalseparatorisacommainsteadofaperiod(andthethousandsseparatorisaperiodinsteadofacomma).Thenumber123,456.789intheUnitedStateswouldbewritten123.456,789inSpain.

Attempttotypeinmultipledecimalseparators(thecomma)andnoticethattheapplicationhappilyallowsit.Whoops!Yourcodefordisallowingmultipledecimalseparatorschecksforaperiodinsteadofusingalocale-specificdecimalseparator.Let’sfixthat.

OpenConversionViewController.swiftandupdatetextfield(_:shouldChangeCharactersInRange:replacementString:)

tousethelocale-specificdecimalseparator.functextField(textField:UITextField,shouldChangeCharactersInRangerange:NSRange,replacementStringstring:String)->Bool{

letexistingTextHasDecimalSeparator=textField.text?.rangeOfString(".")letreplacementTextHasDecimalSeparator=string.rangeOfString(".")

letcurrentLocale=NSLocale.currentLocale()letdecimalSeparator=currentLocale.objectForKey(NSLocaleDecimalSeparator)as!String

letexistingTextHasDecimalSeparator=textField.text?.rangeOfString(decimalSeparator)letreplacementTextHasDecimalSeparator=string.rangeOfString(decimalSeparator)


ifexistingTextHasDecimalSeparator!=nil&&replacementTextHasDecimalSeparator!=nil{returnfalse}else{returntrue}}

Buildandruntheapplication.Theapplicationnolongerallowsyoutotypeinmultipledecimalseparators,anditdoesthisinawaythatisindependentoftheuser’sregionchoice.

Butthereisstillaproblem.Ifyoutypeinanumberwithadecimalseparatorthatisnotaperiod,theconversiontoCelsiusisnothappening–theCelsiuslabeldisplays“???”.Whatisgoingonhere?InfahrenheitFieldEditingChanged(_:),youareusinganinitializerfortheDoubletypethattakesinastringasitsargument.Thisinitializerdoesnotknowhowtohandleastringthatusessomethingotherthanaperiodforitsdecimalseparator.

Let’sfixthiscodeusingtheNSNumberFormatterclass.InConversionViewController.swift,updatefahrenheitFieldEditingChanged(_:)toconvertthetextfield’sstringintoanumberinalocale-independentway.@IBActionfuncfahrenheitFieldEditingChanged(textField:UITextField){

iflettext=textField.text,value=Double(text){fahrenheitValue=value}else{fahrenheitValue=nil}

iflettext=textField.text,number=numberFormatter.numberFromString(text){fahrenheitValue=number.doubleValue}else{fahrenheitValue=nil}}

Hereyouareusingthenumberformatter’sinstancemethodnumberFromString(_:)toconvertthestringintoanumber.Sincethenumberformatterisawareofthelocale,itisabletoconvertthestringintoanumber.Ifthestringcontainsavalidnumber,themethodreturnsaninstanceofNSNumber.NSNumberisaclassthatcanrepresentavarietyofnumbertypes,includingInt,Float,Double,andmore.YoucanaskaninstanceofNSNumberforitsvaluerepresentedasoneofthosevalues.YouaredoingthatheretogetthedoubleValueofthenumber.

Buildandruntheapplication.Nowthatyouareconvertingthestringinalocale-independentway,thetextfield’svalueisproperlyconvertedtoitsCelsiusvalue.

Baseinternationalization

Wheninternationalizing,youasktheinstanceofNSLocalequestions.ButtheNSLocaleonlyhasafewregion-specificvariables.Thisiswherelocalization–creatingapplication-specificsubstitutionsfordifferentregionandlanguagesettings–comesinto


play.Localizationusuallyinvolveseithergeneratingmultiplecopiesofresources(likeimages,sounds,andinterfacefiles)fordifferentregionsandlanguagesorcreatingandaccessingstringstables(whichyouwillseelaterinthechapter)totranslatetextintodifferentlanguages.

Beforeyougothroughtheprocessoflocalizingresources,youmustunderstandhowaniOSapplicationhandleslocalizedresources.

WhenyoubuildatargetinXcode,anapplicationbundleiscreated.AlloftheresourcesthatyouaddedtothetargetinXcodearecopiedintothisbundle,alongwiththeexecutableitself.ThisbundleisrepresentedatruntimebyaninstanceofNSBundleknownasthemainbundle.ManyclassesworkwiththeNSBundletoloadresources.

Localizingaresourceputsanothercopyoftheresourceintheapplicationbundle.Theseresourcesareorganizedintolanguage-specificdirectories,knownaslprojdirectories.Eachoneofthesedirectoriesisthenameofthelocalizationsuffixedwithlproj.Forexample,theAmericanEnglishlocalizationisen_US,whereenistheEnglishlanguagecodeandUSistheUnitedStatesofAmericaregioncode,sothedirectoryforAmericanEnglishresourcesisen_US.lproj.(Theregioncanbeomittedifyoudonotneedtomakeregionaldistinctionsinyourresourcefiles.)Theselanguageandregioncodesarestandardonallplatforms,notjustiOS.

Whenabundleisaskedforthepathofaresourcefile,itfirstlooksattherootlevelofthebundleforafileofthatname.Ifitdoesnotfindone,itlooksatthelocaleandlanguagesettingsofthedevice,findstheappropriatelprojdirectory,andlooksforthefilethere.Thus,justbylocalizingresourcefiles,yourapplicationwillautomaticallyloadthecorrectfile.

Oneoptionforlocalizingresourcefilesistocreateseparatestoryboardfilesandmanuallyediteachstringineachfile.However,thisapproachdoesnotscalewellifyouareplanningmultiplelocalizations.Whathappenswhenyouaddanewlabelorbuttontoyourlocalizedstoryboard?Youhavetoaddthisviewtothestoryboardforeverylanguage.Notfun.

Tosimplifytheprocessoflocalizinginterfacefiles,Xcodehasafeaturecalledbaseinternationalization.BaseinternationalizationcreatestheBase.lprojdirectory,whichcontainsthemaininterfacefiles.LocalizingindividualinterfacefilescanthenbedonebycreatingjusttheLocalizable.stringsfiles.Itisstillpossibletocreatethefullinterfacefiles,incaselocalizationcannotbedonebychangingstringsalone.However,withthehelpofAutoLayout,stringsreplacementissufficientformostlocalizationneeds.Inthenextsection,youwilluseAutoLayouttoprepareyourlayoutforlocalization.

Preparingforlocalization

OpenMain.storyboard,andthenshowtheassistanteditoreitherbyclickingView→AssistantEditor→ShowAssistantEditororwiththekeyboardshortcutOption-Command-Return.Fromthejumpbardropdown,selectPreview(Figure7.4).Thepreviewassistantallowsyoutoeasilyseehowyourinterfacewilllookacrossscreensizesandorientationsaswellasbetweendifferentlocalizedlanguages.


Figure7.4Openingthepreviewassistant

Inthestoryboard,selecttheConversionViewControllertoseeitspreview(Figure7.5).


Figure7.5Previewassistant

Noticethecontrolsinthelowercornersofthepreviewassistant.The+buttonontheleftsideallowsyoutoaddadditionalscreensizestothepreviewcanvas.Thisallowsyoutoeasilyseehowchangestoyourinterfacepropagateacrossscreensizesandorientationssimultaneously.Thebuttonontherightsideallowsyoutoselectalanguagetopreviewthisinterfacein.

(Ifyourpreviewisforaconfigurationotherthan“iPhone4.7-inch,”usethe+buttontoaddthisconfiguration.ThenclickonwhateverpreviewopenedbydefaultandpresstheDeletekeytoremoveit.)

Youhavenotlocalizedtheapplicationintoanotherlanguageyet,butXcodesuppliesapseudolanguageforyoutouse.Pseudolanguageshelpyouinternationalizeyourapplicationsbeforereceivingtranslationsforallofyourstringsandassets.Thebuilt-in


pseudolanguage,Double-LengthPseudolanguage,mimicslanguagesthataremoreverbosebyrepeatingwhatevertextstringisinthetextelement.So,forexample,“isreally”becomes“isreallyisreally.”

SelecttheLanguagepop-upthatsaysEnglishandchooseDouble-LengthPseudolanguage.Thelabelsallhavetheirtextdoubled(Figure7.6).

Figure7.6Doubledtextstrings

Thedouble-lengthpseudolanguagerevealsaproblemimmediately:thelabelsgooffboththeleftandrightedgesofthescreenandyouareunabletoreadtheentirestrings.Thefixistoconstrainallofthelabelssothattheirleadingandtrailingedgesstaywithinthemarginsoftheirsuperview.Thenyouwillneedtochangethelinecountforthelabelsto0,whichtellsthelabelsthattheirtextshouldwraptomultiplelinesifneeded.Youaregoingtostartbyfixingonelabel,thenrepeatthestepsfortherestofthelabels.


Inthecanvas,selectthe“degreesFahrenheit”label.Youaregoingtoaddconstraintstothislabelinanewway.Control-dragfromthelabeltotheleftsideofthesuperview.Whenyoudo,acontext-sensitivepop-upwillappeargivingyoutheconstraintsthatmakesenseforthisdirection(Figure7.7).SelectLeadingSpacetoContainerMarginfromthelist.

Figure7.7CreatingconstraintsbyControl-dragging

Thedirectionthatyoudraginfluenceswhichpossibleconstraintsaredisplayed.Ahorizontaldragwillshowhorizontalconstraints,andaverticaldragwillshowverticalconstraints.Adiagonaldragwillshowbothhorizontalandverticalconstraints,whichisusefulforsettingupmanyconstraintssimultaneously.

NowControl-dragfromthe“degreesFahrenheit”labeltotherightsideofthesuperviewandselectTrailingSpacetoContainerMargin.

Ontheirown,theseconstraintsarenotverygood.Theymaintaintheexistingfixeddistancebetweentheleadingandtrailingedgesofthelabel,asyoucanseeinthepreviewassistant(Figure7.8).


Figure7.8Previewassistantwithnewconstraints

Whatyoureallywantisforthedistancebetweenthelabelandthemarginstobegreaterthanorequalto0.Youcandothiswithinequalityconstraints.

SelecttheleadingconstraintbyclickingontheI-bartotheleftofthelabel.OpenitsattributesinspectorandchangetheRelationtoGreaterThanorEqualandtheConstantto0(Figure7.9).


Figure7.9Inequalityconstraint

Dothesameforthetrailingconstraint.Takealookatthepreviewassistant;theinterfaceislookingbetter,butthelabelisnowbeingtruncated.

Selectthelabelandopenitsattributesinspector.ChangetheLinescountto0.Nowtakealookatthepreviewassistant;thelabelisnolongerbeingtruncatedandinsteadthetextflowstoasecondline.Sincetheotherlabelsareallrelatedtothelabelabovethem,theyhaveautomaticallybeenmoveddown.

Repeatthestepsabovefortheotherlabels.Youwillneedto:

Addaleadingandtrailingconstrainttoeachlabel.

Settheconstraints’relationtoGreaterThanorEqualandtheconstantto0.(Ashortcutforeditingaconstraintistodouble-clickonit.)

Changethelabel’slinecountto0.

Onceyouaredone,thepreviewassistantwiththedouble-lengthpseudolanguagewilllooklikeFigure7.10.


Figure7.10Previewassistantwithfinalconstraints


LocalizationWorldTrotterisnowinternationalized–itsinterfaceisabletoadapttovariouslanguagesandregions.Nowitistimetolocalizetheapp–thatis,toupdatethestringsandresourcesintheapplicationforanewlanguage.Inthissection,youaregoingtolocalizetheinterfaceofWorldTrotter:theMain.storyboardfile.YouwillcreateEnglishandSpanishlocalizations,whichwillcreatetwolprojdirectoriesinadditiontothebaseone.

Startbylocalizingastoryboardfile.SelectMain.storyboardintheprojectnavigator.

Openthefileinspectorbyclickingthe tabintheinspectorselectororbyusingthekeyboardshortcutOption-Command-1.FindthesectioninthisinspectornamedLocalization.ChecktheEnglishboxandmakesurethatthedropdownsaysLocalizableStrings(Figure7.11).Thiswillcreateastringstablethatyouwilluselatertolocalizetheapplication.

Figure7.11LocalizingintoEnglish

Next,intheprojectnavigator,selecttheWorldTrotterprojectatthetop.ThenselectWorldTrotterundertheProjectsectioninthesidelist,andmakesuretheInfotabisopen.(Ifyoucannotseethesidelist,youcanopenitusingtheShowprojectsandtargetslistbuttonintheupperlefthandcorner(Figure7.12).)

Figure7.12Showingtheprojectsettings

Clickthe+buttonunderLocalizationsandselectSpanish(es).Inthedialog,youcanunchecktheLaunchScreen.storyboardfile;keeptheMain.storyboardfilechecked.


MakesurethatthereferencelanguageisBaseandthefiletypeisLocalizableStrings.ClickFinish.Thiscreatesanes.lprojfolderandgeneratestheMain.stringsfileinitthatcontainsallthestringsfromthebaseinterfacefile.TheLocalizationsconfigurationshouldlooklikeFigure7.13.

Figure7.13Localizations

Lookintheprojectnavigator.ClickthedisclosurebuttonnexttoMain.storyboard(Figure7.14).XcodemovedtheMain.storyboardfiletotheBase.lprojdirectoryandcreatedtheMain.stringsfileinthees.lprojdirectory.

Figure7.14Localizedstoryboardintheprojectnavigator

Intheprojectnavigator,clicktheSpanishversionofMain.strings.Whenthisfileopens,thetextisnotinSpanish.Youhavetotranslatelocalizedfilesyourself;Xcodeisnotthatsmart.


Editthisfileaccordingtothefollowingtext.Thenumbersandordermaybedifferentinyourfile,butyoucanusethetextandtitlefieldsinthecommentstomatchupthetranslations./*Class="UITabBarItem";title="Map";ObjectID="6xh-o5-yRt";*/"6xh-o5-yRt.title"="Map""Mapa";

/*Class="UILabel";text="degreescelsius";ObjectID="7la-u7-mx6";*/"7la-u7-mx6.text"="degreesCelsius""gradosCelsius";

/*Class="UILabel";text="degreesfahrenheit";ObjectID="Dic-rs-P0S";*/"Dic-rs-P0S.text"="degreesFahrenheit""gradosFahrenheit";

/*Class="UILabel";text="100";ObjectID="Eso-Wf-EyH";*/"Eso-Wf-EyH.text"="100";

/*Class="UITextField";placeholder="value";ObjectID="On4-jV-YlY";*/"On4-jV-YlY.placeholder"="value""valor";

/*Class="UILabel";text="isreally";ObjectID="wtF-xR-gbZ";*/"wtF-xR-gbZ.text"="isreally""esrealmente";

/*Class="UITabBarItem";title="Convert";ObjectID="zLY-50-CeX";*/"zLY-50-CeX.title"="Convert""Convertir";

Nowthatyouhavefinishedlocalizingthisstoryboardfile,let’stestitout.First,thereisalittleXcodeglitchtobeawareof:sometimesXcodeignoresaresourcefile’schangeswhenyoubuildanapplication.Toensurethatyourapplicationisbeingbuiltfromscratch,firstdeleteitfromyourdeviceorsimulator.(Pressandholditsiconinthelauncher.Whenitstartstowiggle,tapthedeletebadge.)RelaunchXcode.(Yes,exitandstartitagain.)Then,chooseCleanfromtheProductmenu.Finally,tobeabsolutelysure,pressandholdtheOptionkeywhileopeningtheProductmenuandchooseCleanBuildFolder….Thiswillforcetheapplicationtobeentirelyrecompiled,rebundled,andreinstalled.

Opentheactiveschemepop-upandselectEditScheme.MakesureRunisselectedonthelefthandsideandopentheOptionstab.OpentheApplicationLanguagepop-upandselectSpanish.Finally,confirmthatSpainisstillselectedfromtheApplicationRegionpop-up.Closethewindow.

Buildandruntheapplication.MakesureyouareviewingtheConversionViewController,andyouwillseetheinterfaceinSpanish.Becauseyousettheconstraintsonthelabelstoaccommodatedifferentlengthsoftext,theyresizethemselvesappropriately(Figure7.15).


Figure7.15SpanishConversionViewController

NSLocalizedStringandstringstables

Inmanyplacesinyourapplications,youcreateStringinstancesdynamicallyordisplaystringliteralstotheuser.Todisplaytranslatedversionsofthesestrings,youmustcreateastringstable.Astringstableisafilecontainingalistofkey-valuepairsforallofthestringsthatyourapplicationusesandtheirassociatedtranslations.Itisaresourcefilethatyouaddtoyourapplication,butyoudonotneedtodoalotofworktogetdatafromit.

Youmightuseastringinyourcodelikethis:letgreeting="Hello!"

Tointernationalizethestringinyourcode,youreplaceliteralstringswiththefunctionNSLocalizedString(_:comment:).letgreeting=NSLocalizedString("Hello!",comment:"Thegreetingfortheuser")

Thisfunctiontakestwoarguments:akeyandacommentthatdescribesthestring’suse.Thekeyisthelookupvalueinastringstable.Atruntime,NSLocalizedString(_:comment:)willlookthroughthestringstablesbundledwithyourapplicationforatablethatmatchestheuser’slanguagesettings.Then,inthattable,thefunctiongetsthetranslatedstringthatmatchesthekey.

NowyouaregoingtointernationalizethestringsthattheMapViewControllerdisplaysinitssegmentedcontrol.InMapViewController.swift,locatetheloadView()methodandupdatetheinitializerforthesegmentedcontroltouse


localizedstrings.overridefuncloadView(){

mapView=MKMapView()view=mapView

letsegmentedControl=UISegmentedControl(items:["Standard","Satellite","Hybrid"])

letstandardString=NSLocalizedString("Standard",comment:"Standardmapview")letsatelliteString=NSLocalizedString("Satellite",comment:"Satellitemapview")lethybridString=NSLocalizedString("Hybrid",comment:"Hybridmapview")letsegmentedControl=UISegmentedControl(items:[standardString,satelliteString,hybridString])

OnceyouhavefilesthathavebeeninternationalizedwiththeNSLocalizedString(_:comment:)function,youcangeneratestringstableswithacommand-lineapplication.

OpentheTerminalapp.ThisisaUnixterminal;itisusedtoruncommand-linetools.YouwanttonavigatetothelocationofMapViewController.swift.IfyouhaveneverusedtheTerminalappbefore,hereisahandytrick.InTerminal,typethefollowing:cd

followedbyaspace.(DonotpressEnteryet.)

Next,openFinderandlocateMapViewController.swiftandthefolderthatcontainsit.DragtheiconofthatfolderontotheTerminalwindow.Terminalwillfilloutthepathforyou.Itwilllooksomethinglikethis:cd/Users/cbkeur/iOSDevelopment/WorldTrotter/WorldTrotter/

PressReturn.ThecurrentworkingdirectoryofTerminalisnowthisdirectory.

UsetheterminalcommandlstoprintoutthecontentsoftheworkingdirectoryandconfirmthatMapViewController.swiftisinthatlist.

Togeneratethestringstable,enterthefollowingintoTerminalandpressEnter:genstringsMapViewController.swift

Theresultingfile,Localizable.strings,containsthestringsfromMapViewController.DragthisnewfilefromFinderintotheprojectnavigator(orusetheFile→AddFilesto“WorldTrotter”…menuitem).Whentheapplicationiscompiled,thisresourcewillbecopiedintothemainbundle.

OpenLocalizable.strings.Thefileshouldlooksomethinglikethis:/*Hybridmapview*/"Hybrid"="Hybrid";

/*Satellitemapview*/"Satellite"="Satellite";

/*Standardmapview*/"Standard"="Standard";

NoticethatthecommentaboveyourstringisthesecondargumentyousuppliedtotheNSLocalizedStringfunction.Eventhoughthefunctiondoesnotrequirethecommentargument,includingitwillmakeyourlocalizinglifeeasier.


NowthatyouhavecreatedLocalizable.strings,youneedtolocalizeitinXcode.OpenitsfileinspectorandclicktheLocalize…buttonintheutilityarea.MakesureBaseisselectedfromthepop-upandclickLocalize.AddtheSpanishandEnglishlocalizationbycheckingtheboxnexttoeachlanguage.

Intheprojectnavigator,clickonthedisclosuretrianglethatnowappearsnexttoLocalizable.strings.OpentheSpanishversion.ThestringonthelefthandsideisthekeythatispassedtotheNSLocalizedString(_:comment:)function,andthestringontherighthandsideiswhatisreturned.ChangethetextontherighthandsidetotheSpanishtranslationshownbelow.(Totypeanaccentedcharacter,suchas“é,”pressandholdtheappropriatecharacteronyourkeyboardandthenpresstheappropriatenumberfromthepop-up.)/*Hybridmapview*/"Hybrid"="Hybrid""Híbrido";

/*Satellitemapview*/"Satellite"="Satellite""Satélite";

/*Standardmapview*/"Standard"="Standard""Estándar";

Buildandruntheapplicationagain.Nowallthesestrings,includingthetitlesinthesegmentedcontrol,willappearinSpanish(Figure7.16).Iftheydonot,youmightneedtodeletetheapplication,cleanyourproject,andrebuild.(Orcheckyourschemelanguagesetting.)

Figure7.16SpanishMapViewController

Internationalizationandlocalizationareveryimportantforyourapptoreachthelargestaudience.Additionally,asyousawearlyinthischapter,yourappmightnotworkforsomeusersifyouhavenotproperlyinternationalizedit.Youwillinternationalize(butnotlocalize)yourprojectsintherestofthisbook.

Overthepastfivechapters,youhavebuiltaratherimpressiveapplicationthatallowsthe


usertoconvertbetweenCelsiusandFahrenheitaswellasdisplayamapinafewdifferentways.NotonlydoesthisapplicationscalewellonalliPhonescreensizes,butitisalsolocalizedintoanotherlanguage.Congratulations!


BronzeChallenge:AnotherLocalizationPracticemakesperfect.LocalizeWorldTrotterforanotherlanguage.Useatranslationwebsiteifyouneedhelpwiththelanguage.


FortheMoreCurious:NSBundle’sRoleinInternationalizationTherealworkoftakingadvantageoflocalizationsisdoneforyoubytheclassNSBundle.Abundlerepresentsalocationonthefilesystemthatgroupsthecompiledcodeandresourcestogether.The“mainbundle”isanothernamefortheapplicationbundle,whichcontainsalloftheresourcesandtheexecutablefortheapplication.YouwilllearnmoreabouttheapplicationbundleinChapter15.

Whenanapplicationisbuilt,allofthelprojdirectoriesarecopiedintothemainbundle.Figure7.17showsthemainbundleforWorldTrotterwithsomeadditionalimagesaddedtotheproject.

Figure7.17Applicationbundle

NSBundleknowshowtosearchthroughlocalizationdirectoriesforeverytypeofresourceusingtheinstancemethodURLForResource(_:withExtension:).Whenyouwantapathtoaresourcebundledwithyourapplication,youcallthismethodonthemainbundle.HereisanexampleusingtheresourcefileBoo.png:letpath=NSBundle.mainBundle().URLForResource:("Boo"withExtension:"png")

Whenattemptingtolocatetheresource,thebundlefirstcheckstoseeiftheresourceexistsatthetopleveloftheapplicationbundle.Ifso,itreturnsthefullURLtothatfile.Ifnot,


thebundlegetsthedevice’slanguageandregionsettingsandlooksintheappropriatelprojdirectoriestoconstructtheURL.Ifitstilldoesnotfindit,itlookswithintheBase.lprojdirectory.Finally,ifnofileisfound,itreturnsnil.

IntheapplicationbundleshowninFigure7.17,iftheuser’slanguageissettoSpanish,NSBundlewillfindBoo.pngatthetoplevel,Tom.pngines.lproj,andHat.pnginBase.lproj.

Whenyouaddanewlocalizationtoyourproject,Xcodedoesnotautomaticallyremovetheresourcesfromthetop-leveldirectory.Thisiswhyyoumustdeleteandcleananapplicationwhenyoulocalizeafile–otherwise,thepreviousunlocalizedfilewillstillbeintherootleveloftheapplicationbundle.Eventhoughtherearelprojfoldersintheapplicationbundle,thebundlefindsthetop-levelfilefirstandreturnsitsURL.


FortheMoreCurious:ImportingandExportingasXLIFFTheindustry-standardformatforlocalizationdataistheXLIFFdatatype,whichstandsforXMLLocalisationInterchangeFileFormat.Whenworkingwithatranslator,youwilloftensendthemanXLIFFfilecontainingthedataintheapplicationtolocalize,andtheywillgiveyoubackalocalizedXLIFFfileforyoutoimport.

XcodenativelysupportsimportingandexportinglocalizationdataintheXLIFFformat.Theexportingprocesswilltakecareoffindingandexportingthelocalizedstringswithintheproject,whichyoupreviouslydidmanuallyusingthegenstringstool.

ToexportthelocalizablestringsinXLIFFformat,selecttheproject(WorldTrotter)intheprojectnavigator.ThenselecttheEditormenu,andthenExportForLocalization….Onthenextscreen,youcanchoosewhethertoexportexistingtranslations(whichisprobablyagoodideasothetranslatordoesnotdoredundantwork)andwhichlanguagesyouwouldlikeexported(Figure7.18).

Figure7.18ExportinglocalizationdataasXLIFF

Toimportlocalizations,selecttheproject(WorldTrotter)intheprojectnavigator.ThenselectEditor→ImportLocalizations….Afterchoosingafile,youwillbeabletoconfirmtheupdatesbeforeyouimport.


8ControllingAnimations

Theword“animation”isderivedfromaLatinwordthatmeans“theactofbringingtolife.”Inyourapplications,animationscansmoothlybringinterfaceelementsonscreenorintofocus,theycandrawtheuser’sattentiontoanactionableitem,andtheycangiveclearindicationsofhowyourappisrespondingtotheuser’sactions.Inthischapter,youwillreturntoyourQuizappanduseavarietyofanimationtechniquestobringittolife.

BeforeupdatingQuiz,though,let’stakealookatwhatcanbeanimatedbylookingatthedocumentation.Toopenthedocumentation,openXcode’sHelpmenuandselectDocumentationandAPIReference.Thiswillopenthedocumentationinanewwindow.

Withthedocumentationopen,usethesearchbaratthetoptosearchfortheUIViewClassReference.Double-clickinthesearchresultstoopenitandthenscrolldowntothesectiontitledAnimations.Thedocumentationgivessomeanimationrecommendations(whichwewillfollowinthisbook)andliststhepropertiesonUIViewthatcanbeanimated(Figure8.1).

Figure8.1UIViewanimationdocumentation


BasicAnimationsThedocumentationisalwaysagoodstartingpointforlearningaboutanyiOStechnology.Withthatlittlebitofinformationunderyourbelt,let’sgoaheadandaddsomeanimationstoQuiz.Thefirsttypeofanimationyouaregoingtouseisthebasicanimation.Abasicanimationanimatesbetweenastartvalueandanendvalue(Figure8.2).

Figure8.2Basicanimation

Thefirstanimationyouaregoingtoaddwillanimatethealphavalue(thedegreeoftransparency)ofthequestionlabelassociatedwithViewController.

OpenQuiz.xcodeprojandViewController.swift.Whentheuseradvancestothenextquestion,youwilluseananimationtofadeinthelabel.ThereareclassmethodsonUIViewthatwillallowyoutoaccomplishthis.ThesimplestUIViewanimationmethodis:classfuncanimateWithDuration(duration:NSTimeInterval,animations:()->Void)

Thisclassmethodtakestwoarguments:adurationoftypeNSTimeInterval(whichisanaliasforaDouble)andananimationsvariablethatisaclosure.

Closures

Aclosureisadiscretebundleoffunctionalitythatcanbepassedaroundyourcode.Closuresarealotlikefunctionsandmethods.Infact,functionsandmethodsarejustspecialcasesofclosures.

Closureshavealightweightsyntaxthatallowsthemtobeeasilypassedinasargumentstofunctionsandmethods.Aclosurecanevenbethereturntypeofafunctionormethod.Inthissection,youwilluseaclosuretospecifytheanimationsthatyouwanttooccur.

Thesignatureofaclosureisacomma-separatedlistofargumentswithinparenthesesfollowedbyareturnarrowandthereturntype:(arguments)->returntype

Noticethatthissyntaxissimilartothesyntaxforfunctions:funcfunctionName(arguments)->returntype

Nowtakealookagainattheclosuresignaturethattheanimationsargumentexpects:classfuncanimateWithDuration(duration:NSTimeInterval,animations:()->Void)

Thisclosuretakesinnoargumentsanddoesnotreturnanything.(Youwillalsoseethis


returntypeexpressedas(),whichmeansthesamethingasVoid.)

Theclosuresignatureisprettystraightforwardandfamiliar,buthowdoyoudeclareaclosureincode?Closuresyntaxtakesthefollowingform:{(arguments)->returntypein//code}

Youwriteaclosureexpressioninsidebraces({}).Theclosure’sargumentsarelistedinsideparenthesesimmediatelyaftertheopeningbrace.Aclosure’sreturntypecomesaftertheparametersandusestheregularsyntax.Thekeywordinisusedtoseparatetheclosure’sargumentsandreturntypefromthestatementsinsideofitsbody.

InViewController.swift,addanewmethodtohandletheanimationsanddeclareaclosureconstantthattakesinnoargumentsanddoesnotreturnanything.funcanimateLabelTransitions(){

letanimationClosure={()->Voidin

}}

Nowyouhaveaconstantthatreferencesachunkoffunctionality.Currently,however,thisclosuredoesnotactuallydoanything.AddfunctionalitytotheclosurethatsetsthealphaofthequestionLabelto1.Then,passthisclosureasanargumenttoanimateWithDuration(_:animations:).funcanimateLabelTransitions(){

letanimationClosure={()->Voidinself.questionLabel.alpha=1}

//AnimatethealphaUIView.animateWithDuration(0.5,animations:animationClosure)}

Currently,thequestionLabelalreadyhasanalphaof1whenitcomesonscreen,soyouwillnotseeanythinganimateifyoubuildandrun.Toaddressthis,overrideviewWillAppear(_:)toresetthequestionLabel’salphato0eachtimetheViewController’sviewcomesonscreen.overridefuncviewWillAppear(animated:Bool){super.viewWillAppear(animated)

//Setthelabel'sinitialalphaquestionLabel.alpha=0}

Thecodeaboveworksgreat,butthereareafewmodificationsyoucanmakesoitislessverbose.Updatethecodetomakeitmoreconcise.funcanimateLabelTransitions(){

letanimationClosure={()->Voidinself.questionLabel.alpha=1}

//AnimatethealphaUIView.animateWithDuration(0.5,animations:animationClosure)

UIView.animateWithDuration(0.5,animations:{self.questionLabel.alpha=1


})}

Youhavemadetwochanges:First,youarepassingintheclosureanonymously(i.e.,passingitdirectlyintothemethodinsteadofassigningittoavariableorconstant).Second,youhaveremovedthetypeinformationsincetheclosurecaninferthisfromthecontext.

NowcalltheanimateLabelTransitions()methodwhenevertheusertapstheNextQuestionbutton.@IBActionfuncshowNextQuestion(sender:AnyObject){++currentQuestionIndexifcurrentQuestionIndex==questions.count{currentQuestionIndex=0}

letquestion:String=questions[currentQuestionIndex]questionLabel.text=question

answerLabel.text="???"

animateLabelTransitions()}

Buildandruntheapplication.WhenyoutapontheNextQuestionbutton,thelabelwillfadeintoview.Animationsprovidealessjarringuserexperiencethanhavingviewsjustpopintoexistence.

ThemethodanimateWithDuration(_:animations:)returnsimmediately.Thatis,itstartstheanimation,butitdoesnotwaitfortheanimationtocomplete.


AnotherLabelTheanimationworksgreatthefirsttimetheNextQuestionbuttonispressed,butthereisnovisibleanimationonsubsequentbuttonpressessincethelabelalreadyhasanalphavalueof1.Inthissection,youaregoingtoaddanotherlabeltotheinterface.WhentheNextQuestionbuttonispressed,theexistinglabelwillfadeoutwhilethenewlabel(withthetextofthenextquestion)willfadein.

AtthetopofViewController.swift,replaceyourdeclarationofasinglelabelwithtwolabels.@IBOutletvarquestionLabel:UILabel!@IBOutletvarcurrentQuestionLabel:UILabel!@IBOutletvarnextQuestionLabel:UILabel!

@IBOutletvaranswerLabel:UILabel!

XcodeflagsfourplaceswhereyouneedtoreplacequestionLabelwithoneofyournewlabels.UpdateviewDidLoad()tousecurrentQuestionLabel.UpdateviewWillAppear(_:)andshowNextQuestion(_:)tousenextQuestionLabel.funcviewDidLoad(){super.viewDidLoad()

letquestion=questions[currentQuestionIndex]questionLabel.text=questioncurrentQuestionLabel.text=question}

overridefuncviewWillAppear(animated:Bool){super.viewWillAppear(animated)

//Setthelabel'sinitialalphaquestionLabel.alpha=0nextQuestionLabel.alpha=0}

@IBActionfuncshowNextQuestion(sender:AnyObject){++currentQuestionIndexifcurrentQuestionIndex==questions.count{currentQuestionIndex=0}

letquestion:String=questions[currentQuestionIndex]questionLabel.text=questionnextQuestionLabel.text=question

answerLabel.text="???"

animateLabelTransitions()}

NowupdateanimateLabelTransitions()toanimatethealphaofthetwolabels.YouwillfadeoutthecurrentQuestionLabelandfadeinthenextQuestionLabelsimultaneously.funcanimateLabelTransitions(){

//AnimatethealphaUIView.animateWithDuration(0.5,animations:{self.questionLabel.alpha=1self.currentQuestionLabel.alpha=0self.nextQuestionLabel.alpha=1})


}

OpenMain.storyboard.Nowthatthecodehasbeenupdatedforthesetwolabels,youneedtomaketheconnections.Control-clickontheViewControllertoseealistofconnections.NoticethattheexistingquestionLabelisstillpresentwithayellowwarningsignnexttoit(Figure8.3).Clickonthextoremovethisconnection.

Figure8.3Missingconnection

ConnectthecurrentQuestionLabeloutlettotheexistingquestionlabelbydraggingfromthecirclenexttocurrentQuestionLabeltothelabelonthecanvas.

NowdraganewLabelontotheinterfaceandpositionitnexttotheexistingquestionlabel.ConnectthenextQuestionLabeltothisnewlabel.

Youwantthislabeltobeinthesamepositionastheexistingquestionlabel.Asyouhavelikelyguessed,thebestwaytoachievethisisthroughconstraints.Control-dragfromthenextQuestionLabeltothecurrentQuestionLabelandselectTop.ThenControl-dragupwardfromthenextQuestionLabeltoitssuperviewandselectCenterHorizontallyinContainer.

Atthispoint,nextQuestionLabelismisplaced.Selectthelabel,opentheResolveAutoLayoutIssuesmenu,andselectUpdateFrames.Thelabelswillnowbeontopofoneanother.

Buildandruntheapplication.TaptheNextQuestionbuttonandyouwillseeagracefulfadeforbothofthelabels.

Ifyoutapitagain,however,nofadeoccursbecausethenextQuestionLabelalreadyhasanalphaof1.Tofixthis,youwillswapthereferencestothetwolabels.Whentheanimationcompletes,thecurrentQuestionLabelneedstobesettotheonscreenlabel,andthenextQuestionLabelneedstobesettotheoffscreenlabel.Youwilluseacompletionhandlerontheanimationtoaccomplishthis.


AnimationCompletionItcanoftenbeusefultoknowwhenananimationcompletes.Forinstance,youmightwanttochainanimationstogetherorupdateanotherobjectwhentheanimationcompletes.Toknowwhentheanimationfinishes,passaclosureforthecompletionargument.Youwillusethisopportunitytoswapthetwolabelreferences.

InViewController.swift,updateanimateLabelTransitions()tousetheUIViewanimationmethodthathasthemostparameters,includingonethattakesinacompletionclosure.funcanimateLabelTransitions(){

//AnimatethealphaUIView.animateWithDuration(0.5,animations:{self.currentQuestionLabel.alpha=0self.nextQuestionLabel.alpha=1})UIView.animateWithDuration(0.5,delay:0,options:[],animations:{self.currentQuestionLabel.alpha=0self.nextQuestionLabel.alpha=1},completion:{_inswap(&self.currentQuestionLabel,&self.nextQuestionLabel)})}

Thedelayindicateshowlongthesystemshouldwaitbeforetriggeringtheanimation.Wewilltalkabouttheoptionslaterinthischapter.Fornow,youarepassinginanemptyarrayofnooptions.

Inthecompletionclosure,youneedtotellthesystemthatwhatusedtobethecurrentQuestionLabelisnowthenextQuestionLabelandthatwhatusedtobethenextQuestionLabelisnowthecurrentQuestionLabel.Toaccomplishthis,youusetheswap(_:_:)function,whichacceptstworeferencesandswapsthem.

Buildandruntheapplication.Nowyouareabletotransitionbetweenallofthequestions.


AnimatingConstraintsInthissection,youaregoingtoextendyouranimationtohavethenextQuestionLabelpropertyflyinfromtheleftsideofthescreenandthecurrentQuestionLabelflyouttotherightsideofthescreenwhenevertheuserpressestheNextQuestionbutton.Indoingso,youwilllearnhowtoanimateconstraints.

First,youneedareferencetotheconstraintsthatneedtobemodified.Sofar,allofyour@IBOutletshavebeentoviewobjects.Butoutletsarenotlimitedtoviews–infact,anyobjectinyourinterfacefilecanhaveanoutlet,includingconstraints.

AtthetopofViewController.swift,declaretwooutletsforthetwolabels’centeringconstraints.@IBOutletvarcurrentQuestionLabel:UILabel!@IBOutletvarcurrentQuestionLabelCenterXConstraint:NSLayoutConstraint!

@IBOutletvarnextQuestionLabel:UILabel!@IBOutletvarnextQuestionLabelCenterXConstraint:NSLayoutConstraint!

@IBOutletvaranswerLabel:UILabel!

NowopenMain.storyboard.Youwanttoconnectthesetwooutletstotheirrespectiveconstraints.Theeasiestwaytoaccomplishthisisusingthedocumentoutline.ClickthedisclosuretrianglenexttoConstraintsinthedocumentoutlineandlocateCurrentQuestionLabel.centerX.Control-dragfromtheViewControllertothatconstraint(Figure8.4)andselectthecorrectoutlet.DothesameforNextQuestionLabel.centerX.


Figure8.4Connectingaconstraintoutlet

Currently,theNextQuestionbuttonandtheanswersubviewshavetheircenterXconstrainedtothecenterXofthecurrentQuestionLabel.Whenyouimplementtheanimationforthislabeltoslideoffscreen,theothersubviewswillgowithit.Thisisnotwhatyouwant.

SelecttheconstraintthatcenterstheXvalueoftheNextQuestionbuttontothecurrentQuestionLabelanddeleteit.ThenControl-dragupwardfromtheNextQuestionbuttontoitssuperviewandselectCenterHorizontallyinContainer.

Next,youwantthetwoquestionlabelstobeonescreenwidthapart.ThecenterofnextQuestionLabelwillbehalfofthescreenwidthtotheleftoftheview.ThecenterofthecurrentQuestionLabelwillbeatitscurrentposition,centeredinthescreen.

Whentheanimationistriggered,bothlabelswillmoveafullscreenwidthtotheright,placingthenextQuestionLabelatthecenterofthescreenandthecurrentQuestionLabelhalfascreenwidthtotherightofthescreen(Figure8.5).


Figure8.5Slidingthelabels

Toaccomplishthis,whentheviewofViewControllerisloadedyouneedtomovethenextQuestionLabeltoitsoffscreenposition.

InViewController.swift,addanewmethodandcallitfromviewDidLoad().funcviewDidLoad(){super.viewDidLoad()

letquestion=questions[currentQuestionIndex]currentQuestionLabel.text=question

updateOffScreenLabel()}

funcupdateOffScreenLabel(){letscreenWidth=view.frame.widthnextQuestionLabelCenterXConstraint.constant=-screenWidth}

Nowyouwanttoanimatethelabelstogofromlefttoright.Animatingconstraintsisabitdifferentthananimatingotherproperties.Ifyoumodifytheconstantofaconstraintwithinananimationblock,noanimationwilloccur.Why?Afteraconstraintismodified,thesystemneedstorecalculatetheframesforalloftherelatedviewsinthehierarchytoaccommodatethischange.Itwouldbeexpensiveforanyconstraintchangetotriggerthisautomatically.(Imagineifyouupdatedquiteafewconstraints–youwouldnotwantittorecalculatetheframesaftereachchange.)Soyoumustaskthesystemtorecalculatetheframeswhenyouaredone.Todothis,youcallthemethodlayoutIfNeeded()onaview.Thiswillforcetheviewtolayoutitssubviewsbasedonthelatestconstraints.

InViewController.swift,updateanimateLabelTransitions()tochangetheconstraintconstantsandthenforcethelayoutoftheviews.funcanimateLabelTransitions(){

//Animatethealpha//andthecenterXconstraintsletscreenWidth=view.frame.widthself.nextQuestionLabelCenterXConstraint.constant=0self.currentQuestionLabelCenterXConstraint.constant+=screenWidth

UIView.animateWithDuration(0.5,delay:0,options:[],


animations:{self.currentQuestionLabel.alpha=0self.nextQuestionLabel.alpha=1

self.view.layoutIfNeeded()},completion:{_inswap(&self.currentQuestionLabel,&self.nextQuestionLabel)})}

Finally,inthecompletionhandler,youneedtoswapthetwoconstraintoutletsandresetthenextQuestionLabeltobeontheleftsideofthescreen.funcanimateLabelTransitions(){


UIView.animateWithDuration(0.5,delay:0,options:[],animations:{self.currentQuestionLabel.alpha=0self.nextQuestionLabel.alpha=1

self.view.layoutIfNeeded()},completion:{_inswap(&self.currentQuestionLabel,&self.nextQuestionLabel)swap(&self.currentQuestionLabelCenterXConstraint,&self.nextQuestionLabelCenterXConstraint)

self.updateOffScreenLabel()})}

Buildandruntheapplication.Theanimationworksalmostperfectly.Thelabelsslideonandoffthescreen,andthealphavalueanimatesappropriatelyaswell.

Thereisonesmallproblemtofix,butitcanbeabitdifficulttosee.Toseeitmoreeasily,turnonSlowAnimationsfromtheDebugmenuinthesimulator(Command-T).Thewidthofallofthelabelsgetsanimated(toseethisontheanswerLabel,youneedtoclicktheShowAnswerbutton).Thisisbecausetheintrinsiccontentsizechangeswhenthetextchanges.Thefixistoforcetheviewtolayoutitssubviewsbeforetheanimationbegins.Thiswillupdatetheframesofallthreelabelstoaccommodatethenexttextbeforethealphaandslidinganimationsstart.

UpdateanimateLabelTransitions()toforcetheviewtolayoutitssubviewsbeforetheanimationbegins.funcanimateLabelTransitions(){

//Forceanyoutstandinglayoutchangestooccurview.layoutIfNeeded()


UIView.animateWithDuration(0.5,delay:0,


options:[],animations:{self.currentQuestionLabel.alpha=0self.nextQuestionLabel.alpha=1


self.updateOffScreenLabel()})}

Buildandruntheapplicationandcyclethroughsomequestionsandanswers.Theminoranimationissueisnowresolved.


TimingFunctionsTheaccelerationoftheanimationiscontrolledbyitstimingfunction.Bydefault,animationsuseanease-in/ease-outtimingfunction.Touseadrivinganalogy,thiswouldmeanthedriveracceleratessmoothlyfromresttoaconstantspeedandthengraduallyslowsdownattheend,comingtorest.

Othertimingfunctionsincludelinear(aconstantspeedfrombeginningtoend),ease-in(acceleratingtoaconstantspeedandthenendingabruptly),andease-out(beginningatfullspeedandthenslowingdownattheend).

InViewController.swift,updatetheanimationinanimateLabelTransitions()tousealineartimingfunction.UIView.animateWithDuration(0.5,delay:0,options:[.CurveLinear],animations:{self.currentQuestionLabel.alpha=0self.nextQuestionLabel.alpha=1


self.updateOffScreenLabel()})

Now,asopposedtousingthedefaultease-in/ease-outanimationcurve,theanimationwillhavealinearanimationcurve.TheoptionsparametertakesinaUIViewAnimationOptionsargument.Whyisthisargumentinsquarebrackets?Therearemanyoptionsforananimationinadditiontothetimingfunction.Becauseofthis,youneedawayofspecifyingmorethanoneoption–anarray.UIViewAnimationOptionsconformstotheOptionSetTypeprotocol,whichallowsyoutogroupmultiplevaluesusinganarray.

Herearesomeofthepossibleanimationoptionsthatyoucanpassintotheoptionsparameter.

Animationcurveoptions

Thesecontroltheaccelerationoftheanimation.Possiblevaluesare

UIViewAnimationOptions.CurveEaseInOut

UIViewAnimationOptions.CurveEaseIn

UIViewAnimationOptions.CurveEaseOut

UIViewAnimationOptions.CurveLinear

UIViewAnimationOptions.AllowUserInteraction

Bydefault,viewscannotbeinteractedwithwhenanimating.Specifyingthis


optionoverridesthedefault.Thiscanbeusefulforrepeatinganimations,suchasapulsingview.

UIViewAnimationOptions.Repeat

Repeatstheanimationindefinitely;oftenpairedwiththeUIViewAnimationOptions.Autoreverseoption.

UIViewAnimationOptions.Autoreverse

Runstheanimationforwardandthenbackward,returningtheviewtoitsinitialstate.

BesuretocheckouttheConstantssectionoftheUIViewClassReferencetoseeallofthepossibleoptions.


BronzeChallenge:SpringAnimationsiOShasapowerfulphysicsenginebuiltin.Aneasywaytoharnessthispowerisbyusingaspringanimation.//UIView

classfuncanimateWithDuration(duration:NSTimeInterval,delay:NSTimeInterval,usingSpringWithDampingdampingRatio:CGFloat,initialSpringVelocityvelocity:CGFloat,options:UIViewAnimationOptions,animations:()->Void,completion:((Bool)->Void)?)

Usethismethodtohavethetwolabelsanimateonandoffthescreeninaspring-likefashion.RefertotheUIViewdocumentationtounderstandeachofthearguments.


SilverChallenge:LayoutGuidesIfyourotateintolandscape,thenextQuestionLabelbecomesvisible.Insteadofhardcodingthespacingconstraint’sconstant,useaninstanceofUILayoutGuidetospacethetwolabelsapart.ThislayoutguideshouldhaveawidthconstraintequaltotheViewController’sviewtoensurethatthenextQuestionLabelremainsoffscreenwhennotanimating.


9UITableViewand

UITableViewControllerManyiOSapplicationsshowtheuseralistofitemsandallowtheusertoselect,delete,orreorderitemsonthelist.Whetheranapplicationdisplaysalistofpeopleintheuser’saddressbookoralistofbest-sellingitemsontheAppStore,itisaUITableViewdoingthework.

AUITableViewdisplaysasinglecolumnofdatawithavariablenumberofrows.Figure9.1showssomeexamplesofUITableView.

Figure9.1ExamplesofUITableView


BeginningtheHomepwnerApplicationInthischapter,youaregoingtostartanapplicationcalledHomepwnerthatkeepsaninventoryofallyourpossessions.Inthecaseofafireorothercatastrophe,youwillhavearecordforyourinsurancecompany.(“Homepwner,”bytheway,isnotatypo.Ifyouneedadefinitionfortheword“pwn,”pleasevisithttp://www.urbandictionary.com.)

Sofar,youriOSprojectshavebeensmall,butHomepwnerwillgrowintoarealisticallycomplexapplicationoverthecourseofeightchapters.Bytheendofthischapter,HomepwnerwillpresentalistofIteminstancesinaUITableView,asshowninFigure9.2.

Figure9.2Homepwner:phase1

CreateanewiOSSingleViewApplicationprojectandconfigureitasshowninFigure9.3.


http://www.urbandictionary.com

Figure9.3ConfiguringHomepwner


UITableViewControllerAUITableViewisaviewobject.RecallthatintheModel-View-Controllerdesignpattern,whichiOSdevelopersdotheirbesttofollow,eachclassfallsintoexactlyoneofthefollowingcategories:

model:holdsdataandknowsnothingabouttheuserinterface

view:isvisibletotheuserandknowsnothingaboutthemodelobjects

controller:keepstheuserinterfaceandthemodelobjectsinsyncandcontrolstheflowoftheapplication

Thus,aUITableView,aviewobject,doesnothandleapplicationlogicordata.WhenusingaUITableView,youmustconsiderwhatelseisnecessarytogetthetableworkinginyourapplication:

AUITableViewtypicallyneedsaviewcontrollertohandleitsappearanceonthescreen.

AUITableViewneedsadatasource.AUITableViewasksitsdatasourceforthenumberofrowstodisplay,thedatatobeshowninthoserows,andothertidbitsthatmakeaUITableViewausefuluserinterface.Withoutadatasource,atableviewisjustanemptycontainer.ThedataSourceforaUITableViewcanbeanytypeofobjectaslongasitconformstotheUITableViewDataSourceprotocol.

AUITableViewtypicallyneedsadelegatethatcaninformotherobjectsofeventsinvolvingtheUITableView.ThedelegatecanbeanyobjectaslongasitconformstotheUITableViewDelegateprotocol.

AninstanceoftheclassUITableViewControllercanfillallthreeroles:viewcontroller,datasource,anddelegate.

UITableViewControllerisasubclassofUIViewController,soaUITableViewControllerhasaview.AUITableViewController’sviewisalwaysaninstanceofUITableView,andtheUITableViewControllerhandlesthepreparationandpresentationoftheUITableView.

WhenaUITableViewControllercreatesitsview,thedataSourceanddelegatepropertiesoftheUITableViewareautomaticallysettopointattheUITableViewController(Figure9.4).


Figure9.4UITableViewController-UITableViewrelationship

SubclassingUITableViewController

NowyouaregoingtoimplementasubclassofUITableViewControllerforHomepwner.CreateanewSwiftfilenamedItemsViewController.InItemsViewController.swift,defineaUITableViewControllersubclassnamedItemsViewController.importFoundationimportUIKit

classItemsViewController:UITableViewController{

}

NowopenMain.storyboard.Youwanttheinitialviewcontrollertobeatableviewcontroller.SelecttheexistingViewControlleronthecanvasandpressDelete.ThendragaTableViewControllerfromtheobjectlibraryontothecanvas.WiththeTableViewControllerselected,openitsidentityinspectorandchangetheclasstoItemsViewController.Finally,opentheattributesinspectorforItemsViewControllerandchecktheboxforIsInitialViewController.

Buildandrunyourapplication.Youshouldseeanemptytableview,asshowninFigure9.5.AsasubclassofUIViewController,aUITableViewControllerinheritstheviewproperty.Whenthispropertyisaccessedforthefirsttime,theloadView()methodiscalled,whichcreatesandloadsaviewobject.AUITableViewController’sviewisalwaysaninstanceofUITableView,soaskingfortheviewofaUITableViewControllergetsyouabright,shiny,andemptytableview.


Figure9.5EmptyUITableView

YounolongerneedtheViewController.swiftfilethatthetemplatecreatedforyou.SelectthisfileintheprojectnavigatorandpressDelete.


CreatingtheItemClassYourtableviewneedsomerowstodisplay.Eachrowinthetableviewwilldisplayanitemwhichwillhaveinformationsuchasaname,serialnumber,andvalueindollars.

CreateanewSwiftfilenamedItem.InItem.swift,definetheItemclassandgiveitfourproperties.importFoundationimportUIKit

classItem:NSObject{varname:StringvarvalueInDollars:IntvarserialNumber:String?letdateCreated:NSDate}

IteminheritsfromNSObject.NSObjectisthebaseclassthatmostObjective-Cclassesinheritfrom.AlloftheUIKitclassesthatyouhaveworkedwith–UIView,UITextField,andUIViewController,tonameafew–inheriteitherdirectlyorindirectlyfromNSObject.YourownclasseswilloftenneedtoinheritfromNSObjectwhentheyneedtointerfacewiththeruntimesystem.

NoticethatserialNumberisanoptionalString,necessarybecauseanitemmaynothaveaserialnumber.Also,noticethatnoneofthepropertieshaveadefaultvalue.Youwillneedtogivethemvaluesinadesignatedinitializer.

Custominitializers

YoulearnedaboutstructinitializersinChapter2.Initializersonstructsarefairlystraightforwardbecausestructsdonotsupportinheritance.Classes,ontheotherhand,havesomerulesforinitializerstosupportinheritance.

Classescanhavetwokindsofinitializers:designatedinitializersandconvenienceinitializers.

Adesignatedinitializerisaprimaryinitializerfortheclass.Everyclasshasatleastonedesignatedinitializer.Adesignatedinitializerensuresthatallpropertiesintheclasshaveavalue.Onceitensuresthat,adesignatedinitializercallsadesignatedinitializeronitssuperclass(ifithasone).

ImplementanewdesignatedinitializerontheItemclassthatsetstheinitialvaluesforalloftheproperties.importUIKit

classItem:NSObject{varname:StringvarvalueInDollars:IntvarserialNumber:String?letdateCreated:NSDate

init(name:String,serialNumber:String?,valueInDollars:Int){self.name=nameself.valueInDollars=valueInDollarsself.serialNumber=serialNumber


self.dateCreated=NSDate()

super.init()}}

Thisinitializertakesinargumentsforthename,serialNumber,andvalueInDollars.Sincetheargumentnamesandthepropertynamesarethesame,youmustuseselftodistinguishthepropertyfromtheargument.

Nowthatyouhaveimplementedyourowncustominitializer,youlosethefreeinitializer–init()–thatclasseshave.Thefreeinitializerisusefulwhenallofyourclass’spropertieshavedefaultvaluesandyoudonotneedtodoadditionalworktocreatethenewinstance.TheItemclassdoesnotsatisfythiscriteria,soyouhavedeclaredacustominitializerfortheclass.

Everyclassmusthaveatleastonedesignatedinitializer,butconvenienceinitializersareoptional.Youcanthinkofconvenienceinitializersashelpers.Aconvenienceinitializeralwayscallsanotherinitializeronthesameclass.Convenienceinitializersareindicatedbytheconveniencekeywordbeforetheinitializername.

AddaconvenienceinitializertoItemthatcreatesarandomlygenerateditem.convenienceinit(random:Bool=false){ifrandom{letadjectives=["Fluffy","Rusty","Shiny"]letnouns=["Bear","Spork","Mac"]

varidx=arc4random_uniform(UInt32(adjectives.count))letrandomAdjective=adjectives[Int(idx)]

idx=arc4random_uniform(UInt32(nouns.count))letrandomNoun=nouns[Int(idx)]

letrandomName="\(randomAdjective)\(randomNoun)"letrandomValue=Int(arc4random_uniform(100))letrandomSerialNumber=NSUUID().UUIDString.componentsSeparatedByString("-").first!

self.init(name:randomName,serialNumber:randomSerialNumber,valueInDollars:randomValue)}else{self.init(name:"",serialNumber:nil,valueInDollars:0)}}

Ifrandomistrue,theinstanceisconfiguredwitharandomname,serialnumber,andvalue.(Thearc4random_uniformfunctionreturnsarandomvaluebetween0,inclusive,andthevaluepassedinastheargument,exclusive.)Noticethatattheendofbothbranchesoftheconditional,youarecallingthroughtothedesignatedinitializerforItem.Convenienceinitializersmustcallanotherinitializeronthesametype,whereasdesignatedinitializersmustcalladesignatedinitializeronitssuperclass.

TheItemclassisreadyforwork.InthenextsectionyouwilldisplayanarrayofIteminstancesinatableview.


UITableView’sDataSourceTheprocessofprovidingrowstoaUITableViewinCocoaTouch(thecollectionofframeworksusedtobuildiOSapps)isdifferentfromthetypicalproceduralprogrammingtask.Inaproceduraldesign,youtellthetableviewwhatitshoulddisplay.InCocoaTouch,thetableviewasksanotherobject–itsdataSource–whatitshoulddisplay.Inthiscase,theItemsViewControlleristhedatasource,soitneedsawaytostoreitemdata.

YouaregoingtouseanarraytostoretheIteminstances,butwithatwist.ThearraythatholdstheIteminstanceswillbeabstractedintoanotherobject–anItemStore(Figure9.6).

Figure9.6Homepwnerobjectdiagram

Ifanobjectwantstoseealloftheitems,itwillasktheItemStoreforthearraythatcontainsthem.Infuturechapters,thestorewillberesponsibleforperformingoperationsonthearray,likereordering,adding,andremovingitems.Itwillalsoberesponsibleforsavingandloadingtheitemsfromdisk.

CreateanewSwiftfilenamedItemStore.InItemStore.swift,definetheItemStoreclassanddeclareapropertytostorethelistofItems.importFoundationimportUIKit


classItemStore{

varallItems=[Item]()

}

ItemStoreisaSwiftbaseclass–itdoesnotinheritfromanyotherclass.UnliketheItemclassthatyoudefinedearlier,ItemStoredoesnotrequireanyofthebehaviorthatNSObjectaffords.

TheItemsViewControllerwillcallamethodonItemStorewhenitwantsanewItemtobecreated.TheItemStorewilloblige,creatingtheobjectandaddingittoanarrayofinstancesofItem.

InItemStore.swift,implementcreateItem()tocreateandreturnanewItem.funccreateItem()->Item{letnewItem=Item(random:true)

allItems.append(newItem)

returnnewItem}

Givingthecontrolleraccesstothestore

InItemsViewController.swift,addapropertyforanItemStore.classItemsViewController:UITableViewController{

varitemStore:ItemStore!

Now,whereshouldyousetthispropertyontheItemsViewControllerinstance?Whentheapplicationfirstlaunches,theAppDelegate’sapplication(_:didFinishLaunchingWithOptions:)methodiscalled.TheAppDelegateisdeclaredinAppDelegate.swiftand,asthenameimplies,servesasthedelegatefortheapplicationitself.Itisresponsibleforhandlingthechangesinstatethattheapplicationgoesthrough.YouwilllearnmoreabouttheAppDelegateandthestatesthattheapplicationgoesthroughinChapter15.

OpenAppDelegate.swift.AccesstheItemsViewController(whichwillbetherootViewControllerofthewindow)andsetitsitemStorepropertytobeanewinstanceofItemStore.funcapplication(application:UIApplication,didFinishLaunchingWithOptionslaunchOptions:[NSObject:AnyObject]?)->Bool{//Overridepointforcustomizationafterapplicationlaunch.

//CreateanItemStoreletitemStore=ItemStore()

//AccesstheItemsViewControllerandsetitsitemstoreletitemsController=window!.rootViewControlleras!ItemsViewControlleritemsController.itemStore=itemStore

returntrue}

Finally,inItemStore.swift,implementthedesignatedinitializertoaddfiverandomitems.


init(){for_in0..<5{createItem()}}

AtthispointyoumaybewonderingwhyitemStorewassetexternallyontheItemsViewController.Whydidn’ttheItemsViewControllerinstanceitselfjustcreateaninstanceofthestore?Thereasonforthisapproachisbasedonafairlycomplextopiccalledthedependencyinversionprinciple.Theessentialgoalofthisprincipleistodecoupleobjectsinanapplicationbyinvertingcertaindependenciesbetweenthem.Thisresultsinmorerobustandmaintainablecode.

Thedependencyinversionprinciplestatesthat:

1. High-levelobjectsshouldnotdependonlow-levelobjects.Bothshoulddependonabstractions.

2. Abstractionsshouldnotdependondetails.Detailsshoulddependonabstractions.

TheabstractionrequiredbythedependencyinversionprincipleinHomepwneristheconceptofa“store.”Astoreisalower-levelobjectthatretrievesandsavesIteminstancesthroughdetailsthatareonlyknowntothatclass.ItemsViewControllerisahigher-levelobjectthatonlyknowsthatitwillbeprovidedwithautilityobject(thestore)fromwhichitcanobtainalistofIteminstancesandtowhichitcanpassneworupdatedIteminstancestobestoredpersistently.ThisresultsinadecouplingbecauseItemsViewControllerisnotdependentonItemStore.Infact,aslongasthestoreabstractionisrespected,ItemStorecouldbereplacedbyanotherobjectthatfetchesIteminstancesdifferently(suchasbyusingawebservice)withoutanychangestoItemsViewController.

Acommonpatternusedwhenimplementingthedependencyinversionprincipleisdependencyinjection.Initssimplestform,higher-levelobjectsdonotassumewhichlower-levelobjectstheyneedtouse.Instead,thosearepassedtothemthroughaninitializerorproperty.InyourimplementationofItemsViewController,youusedinjectionthroughapropertytogiveitastore.

Implementingdatasourcemethods

Nowthattherearesomeitemsinthestore,youneedtoteachItemsViewControllerhowtoturnthoseitemsintorowsthatitsUITableViewcandisplay.WhenaUITableViewwantstoknowwhattodisplay,itcallsmethodsfromthesetofmethodsdeclaredintheUITableViewDataSourceprotocol.

OpenthedocumentationandsearchfortheUITableViewDataSourceprotocolreference.SelectTasksfromthelefthandpane(Figure9.7).


Figure9.7UITableViewDataSourceprotocoldocumentation

IntheConfiguringaTableViewtask,noticethattwoofthemethodsaremarkedRequired.ForItemsViewControllertoconformtoUITableViewDataSource,itmustimplementtableView(_:numberOfRowsInSection:)andtableView(_:cellForRowAtIndexPath:).Thesemethodstellthetableviewhowmanyrowsitshoulddisplayandwhatcontenttodisplayineachrow.

WheneveraUITableViewneedstodisplayitself,itcallsaseriesofmethods(therequiredmethodsplusanyoptionalonesthathavebeenimplemented)onitsdataSource.TherequiredmethodtableView(_:numberOfRowsInSection:)returnsanintegervalueforthenumberofrowsthattheUITableViewshoulddisplay.InthetableviewforHomepwner,thereshouldbearowforeachentryinthestore.

InItemsViewController.swift,implementtableView(_:numberOfRowsInSection:).overridefunctableView(tableView:UITableView,numberOfRowsInSectionsection:Int)->Int{returnitemStore.allItems.count}

Wonderingaboutthesectionthatthismethodrefersto?Tableviewscanbebrokenupintosections,andeachsectionhasitsownsetofrows.Forexample,intheaddressbook,allnamesbeginningwith“C”aregroupedtogetherinasection.Bydefault,atableviewhasonesection,andinthischapter,youwillworkwithonlyone.Onceyouunderstandhowatableviewworks,itisnothardtousemultiplesections.Infact,usingsectionsisthefirstchallengeattheendofthischapter.


ThesecondrequiredmethodintheUITableViewDataSourceprotocolistableView(_:cellForRowAtIndexPath:).Toimplementthismethod,youneedtolearnaboutanotherclass–UITableViewCell.


UITableViewCellsEachrowofatableviewisaview.TheseviewsareinstancesofUITableViewCell.Inthissection,youwillcreatetheinstancesofUITableViewCelltofillthetableview.

Acellitselfhasonesubview–itscontentView(Figure9.8).ThecontentViewisthesuperviewforthecontentofthecell.Thecellmayalsohaveanaccessoryview.

Figure9.8UITableViewCelllayout

Theaccessoryviewshowsanaction-orientedicon,suchasacheckmark,adisclosureicon,oraninformationbutton.Theseiconsareaccessedthroughpredefinedconstantsfortheappearanceoftheaccessoryview.ThedefaultisUITableViewCellAccessoryType.None,andthatiswhatyouaregoingtouseinthischapter.ButyouwillseetheaccessoryviewagaininChapter22.(Curiousnow?SeethedocumentationforUITableViewCellformoredetails.)

TherealmeatofaUITableViewCellisthecontentView,whichhasthreesubviewsofitsown(Figure9.9).TwoofthosesubviewsareUILabelinstancesthatarepropertiesofUITableViewCellnamedtextLabelanddetailTextLabel.ThethirdsubviewisaUIImageViewcalledimageView.Inthischapter,youwillusetextLabelanddetailTextLabel.


Figure9.9UITableViewCellhierarchy

EachcellalsohasaUITableViewCellStylethatdetermineswhichsubviewsareusedandtheirpositionwithinthecontentView.ExamplesofthesestylesandtheirconstantsareshowninFigure9.10.

Figure9.10UITableViewCellStyle:stylesandconstants

CreatingandretrievingUITableViewCells

Fornow,eachcellwilldisplaythenameofItemasitstextLabelandtheserialNumberofItemasitsdetailTextLabel.Tomakethishappen,youneed


toimplementthesecondrequiredmethodfromtheUITableViewDataSourceprotocol,tableView(_:cellForRowAtIndexPath:).Thismethodwillcreateacell,setitstextLabeltothenameoftheItem,setitsdetailTextLabeltothevalueInDollarsoftheItem,andreturnittotheUITableView(Figure9.11).

Figure9.11UITableViewCellretrieval

HowdoyoudecidewhichcellanItemcorrespondsto?OneoftheparameterssenttotableView(_:cellForRowAtIndexPath:)isanNSIndexPath,whichhastwoproperties:sectionandrow.Whenthismethodiscalledonadatasource,thetableviewisasking,“CanIhaveacelltodisplayinsectionX,rowY?”Becausethereisonlyonesectioninthisexercise,yourimplementationwillonlybeconcernedwiththeindexpath’srow.

InItemsViewController.swift,implementtableView(_:cellForRowAtIndexPath:)sothatthenthrowdisplaysthenthentryintheallItemsarray.overridefunctableView(tableView:UITableView,cellForRowAtIndexPathindexPath:NSIndexPath)->UITableViewCell{//CreateaninstanceofUITableViewCell,withdefaultappearanceletcell=UITableViewCell(style:.Value1,reuseIdentifier:"UITableViewCell")

//Setthetextonthecellwiththedescriptionoftheitem//thatisatthenthindexofitems,wheren=rowthiscell//willappearinonthetableviewletitem=itemStore.allItems[indexPath.row]

cell.textLabel?.text=item.namecell.detailTextLabel?.text="$\(item.valueInDollars)"

returncell}

BuildandruntheapplicationnowandyouwillseeaUITableViewpopulatedwithalistWOW! eBook

www.wowebook.org

ofrandomitems.

ReusingUITableViewCells

iOSdeviceshavealimitedamountofmemory.IfyouweredisplayingalistwiththousandsofentriesinaUITableView,youwouldhavethousandsofinstancesofUITableViewCell.Mostofthesecellswouldtakeupmemoryneedlessly.Afterall,iftheusercannotcurrentlyseeacellonscreen,thenthereisnoreasonforthatcelltohaveaclaimonmemory.

Toconservememoryandimproveperformance,youcanreusetableviewcells.Whentheuserscrollsthetable,somecellsmoveoffscreen.Offscreencellsareputintoapoolofcellsavailableforreuse.Then,insteadofcreatingabrandnewcellforeveryrequest,thedatasourcefirstchecksthepool.Ifthereisanunusedcell,thedatasourceconfiguresitwithnewdataandreturnsittothetableview(Figure9.12).

Figure9.12ReusableinstancesofUITableViewCell

Thereisoneproblem:sometimesaUITableViewhasdifferenttypesofcells.Occasionally,yousubclassUITableViewCelltocreateaspeciallookorbehavior.However,differentsubclassesfloatingaroundthepoolofreusablecellscreatethepossibilityofgettingbackacellofthewrongtype.Youmustbesureofthetypeofthecellreturnedsothatyoucanbesureofwhatpropertiesandmethodsithas.

Notethatyoudonotcareaboutgettinganyspecificcelloutofthepoolbecauseyouaregoingtochangethecellcontentanyway.Whatyouneedisacellofaspecifictype.The


goodnewsisthateverycellhasareuseIdentifierpropertyoftypeString.Whenadatasourceasksthetableviewforareusablecell,itpassesastringandsays,“Ineedacellwiththisreuseidentifier.”Byconvention,thereuseidentifieristypicallythenameofthecellclass.

Toreusecells,youneedtoregistereitheraprototypecelloraclasswiththetableviewforaspecificreuseidentifier.YouaregoingtoregisterthedefaultUITableViewCellclass.Youtellthetableview,“Hey,anytimeIaskforacellwiththisreuseidentifier,givemebackacellthatisthisspecificclass.”Thetableviewwilleithergiveyouacellfromthereusepoolorinstantiateanewcelliftherearenocellsofthattypeinthereusepool.

OpenMain.storyboard.NoticeinthetableviewthatthereisasectionforPrototypeCells(Figure9.13).Inthisarea,youcanconfigurethedifferentkindsofcellsthatyouneedfortheassociatedtableview.Ifyouarecreatingcustomcells,thisiswhereyouwillsetuptheinterfaceforthecells.ItemsViewControlleronlyneedsonekindofcell,andusingoneofthebuilt-instyleswillworkgreatfornow,soyouwillonlyneedtoconfiguresomeattributesonthecellthatisalreadyonthecanvas.

Figure9.13Prototypecells

Selecttheprototypecellandopenitsattributesinspector.ChangetheStyletoRightDetail(whichcorrespondstoUITableViewCellStyle.Value1)andgiveitanIdentifierofUITableViewCell(Figure9.14).


Figure9.14Tableviewcellattributes

Next,inItemsViewController.swift,updatetableView(_:cellForRowAtIndexPath:)toreusecells.overridefunctableView(tableView:UITableView,cellForRowAtIndexPathindexPath:NSIndexPath)->UITableViewCell{//CreateaninstanceofUITableViewCell,withdefaultappearanceletcell=UITableViewCell(style:.Value1,reuseIdentifier:"UITableViewCell")

//Getaneworrecycledcellletcell=tableView.dequeueReusableCellWithIdentifier("UITableViewCell",forIndexPath:indexPath)

...}

ThemethoddequeueReusableCellWithIdentifier(_:forIndexPath:)willcheckthepool,orqueue,ofcellstoseewhetheracellwiththecorrectreuseidentifieralreadyexists.Ifso,itwill“dequeue”thatcell.Ifthereisnotanexistingcell,anewcellwillbecreatedandreturned.

Buildandruntheapplication.Thebehavioroftheapplicationshouldremainthesame.Reusingcellsmeansthatyouonlyhavetocreateahandfulofcells,whichputsfewerdemandsonmemory.Yourapplication’susers(andtheirdevices)willthankyou.


ContentInsetsAsyouhavebeenrunningtheapplicationthroughoutthischapter,youmighthavenoticedthatthefirsttableviewcellunderlapsthestatusbar(Figure9.15).Theinterfacesfortheapplicationsyoucreatefilluptheentirewindowofthedevice.Thestatusbar,ifvisible,isplacedontopoftheinterface,soyourinterfacesmustaccountfortheplacementofthestatusbar.

Figure9.15Tableviewcellunderlappingstatusbar

Tohavethetableviewcellsnotunderlapthestatusbar,youwilladdsomepaddingtothetopofthetableview.AUITableViewisasubclassofUIScrollView,fromwhichitinheritsthecontentInsetproperty.Youcanthinkofthecontentinsetasapaddingforallfoursidesofthescrollview.

InItemsViewController.swift,overrideviewDidLoad()toupdatethetableviewcontentinset.overridefuncviewDidLoad(){super.viewDidLoad()

//GettheheightofthestatusbarletstatusBarHeight=UIApplication.sharedApplication().statusBarFrame.height

letinsets=UIEdgeInsets(top:statusBarHeight,left:0,bottom:0,right:0)tableView.contentInset=insetstableView.scrollIndicatorInsets=insets}

Thetopofthetableviewisgivenacontentinsetequaltotheheightofthestatusbar.Thiswillmakethecontentappearbelowthestatusbarwhenthetableviewisscrolledtothetop.Thescrollindicatorswillalsounderlapthestatusbar,soyougivethemthesameinsetstohavethemappearjustbelowthestatusbar.

NoticethatyouaccessthetableViewpropertyontheItemsViewControllertogetatthetableview.ThispropertyisinheritedfromUITableViewControllerandreturnsthecontroller’stableview.WhileyoucangetthesameobjectbyaccessingtheviewofaUITableViewController,usingtableViewtellsthecompilerthatthe


returnedobjectwillbeaninstanceofUITableView.Thus,callingamethodoraccessingapropertythatisspecifictoUITableViewwillnotgenerateanerror.

Buildandruntheapplication.Thetableviewcellcontentnolongerunderlapsthestatusbarwhenthetableviewisscrolledtothetop(Figure9.16).

Figure9.16Tableviewwithadjustedcontentinset


BronzeChallenge:SectionsHavetheUITableViewdisplaytwosections–oneforitemsworthmorethan$50andonefortherest.Beforeyoustartthischallenge,copythefoldercontainingtheprojectandallofitssourcefilesinFinder.Thentacklethechallengeinthecopiedproject;youwillneedtheoriginaltobuildoninthefollowingchapters.


SilverChallenge:ConstantRowsMakeitsothelastrowoftheUITableViewalwayshasthetextNomoreitems!.Makesurethisrowappearsregardlessofthenumberofitemsinthestore(including0items).


GoldChallenge:CustomizingtheTableMakeeachrow’sheight60points,exceptforthelastrowfromthesilverchallenge,whichshouldremain44points.Then,changethefontsizeofeveryrowexceptthelastto20points.Finally,makethebackgroundoftheUITableViewdisplayanimage.(Tomakethispixel-perfect,youwillneedanimageofthecorrectsizedependingonyourdevice.RefertothechartinChapter1.)


10EditingUITableView

Inthelastchapter,youcreatedanapplicationthatdisplaysalistofIteminstancesinaUITableView.Thenextstepisallowingtheusertointeractwiththetable–toadd,delete,andmoverows.Figure10.1showswhatHomepwnerwilllooklikebytheendofthischapter.

Figure10.1Homepwnerineditingmode


EditingModeUITableViewhasaneditingproperty,andwhenthispropertyissettotrue,theUITableViewenterseditingmode.Oncethetableviewisineditingmode,therowsofthetablecanbemanipulatedbytheuser.Dependingonhowthetableviewisconfigured,theusercanchangetheorderoftherows,addrows,orremoverows.Editingmodedoesnotallowtheusertoeditthecontentofarow.

Butfirst,theuserneedsawaytoputtheUITableViewineditingmode.Fornow,youaregoingtoincludeabuttonintheheaderviewofthetable.Aheaderviewappearsatthetopofatableandisusefulforaddingsection-wideortable-widetitlesandcontrols.ItcanbeanyUIViewinstance.

Notethatthetableviewusestheword“header”intwodifferentways:therecanbeatableheaderandtherecanbesectionheaders.Likewise,therecanbeatablefooterandsectionfooters(Figure10.2).

Figure10.2Sectionheadersandfooters

Youarecreatingatableheaderview.Itwillhavetwosubviewsthatareinstancesof


UIButton:onetotoggleeditingmodeandtheothertoaddanewItemtothetable.Youcouldcreatethisviewprogrammatically,butinthiscaseyouwillcreatetheviewanditssubviewsinthestoryboardfile.

First,let’ssetupthenecessarycode.ReopenHomepwner.xcodeproj.InItemsViewController.swift,stubouttwomethodsintheimplementation.classItemsViewController:UITableViewController{

varitemStore:ItemStore!

@IBActionfuncaddNewItem(sender:AnyObject){

}

@IBActionfunctoggleEditingMode(sender:AnyObject){

}

NowopenMain.storyboard.Fromtheobjectlibrary,dragaViewtotheverytopofthetableview.Thiswilladdtheviewasaheaderviewforthetableview.Resizetheheightofthisviewtobeabout60points.(Youcanusethesizeinspectorifyouwanttomakeitexact.)

NowdragtwoButtonsfromtheobjectlibrarytotheheaderview.ChangetheirtextandpositionthemasshowninFigure10.3.Youdonotneedtobeexact–youwilladdconstraintssoontopositionthebuttons.

Figure10.3Addingbuttonstotheheaderview

SelectbothofthebuttonsandopentheAutoLayoutAlignmenu.SelectVerticallyinContainerwithaconstantof0.MakesureUpdateFramesissettoNone,andthenclickAdd2Constraints(Figure10.4).


Figure10.4Alignmenuconstraints

OpenthePinmenuandconfigureitexactlylikeFigure10.5.Makesurethevaluesfortheleadingandtrailingconstraintssaveafteryouhavetypedthem;sometimesthevaluesdonotsave,soitcanbeabittricky.Whenyouhavedonethat,clickAdd4Constraints.


Figure10.5Pinmenuconstraints

Finally,connecttheactionsforthetwobuttonsasshowninFigure10.6.

Figure10.6Connectingthetwoactions

Buildandruntheapplicationtoseetheinterface.

Nowlet’simplementthetoggleEditingMode(_:)method.YoucouldtoggletheeditingpropertyofUITableViewdirectly.However,UIViewControlleralso


hasaneditingproperty.AUITableViewControllerinstanceautomaticallysetstheeditingpropertyofitstableviewtomatchitsowneditingproperty.Bysettingtheeditingpropertyontheviewcontrolleritself,itcanensurethatotheraspectsoftheinterfacealsoenterandleaveeditingmode.YouwillseeanexampleofthisinChapter13withUIViewController’seditButtonItem().

Tosettheeditingpropertyforaviewcontroller,youcallthemethodsetEditing(_:animated:).InItemsViewController.swift,implementtoggleEditingMode(_:).@IBActionfunctoggleEditingMode(sender:AnyObject){//Ifyouarecurrentlyineditingmode...ifediting{//Changetextofbuttontoinformuserofstatesender.setTitle("Edit",forState:.Normal)

//TurnoffeditingmodesetEditing(false,animated:true)}else{//Changetextofbuttontoinformuserofstatesender.setTitle("Done",forState:.Normal)

//EntereditingmodesetEditing(true,animated:true)}}

Buildandrunyourapplication.TaptheEditbuttonandtheUITableViewwillentereditingmode(Figure10.7).

Figure10.7UITableViewineditingmode


AddingRowsTherearetwocommoninterfacesforaddingrowstoatableviewatruntime.

Abuttonabovethecellsofthetableview:usuallyforaddingarecordforwhichthereisadetailview.Forexample,intheContactsapp,youtapabuttonwhenyoumeetanewpersonandwanttotakedownhisorherinformation.

Acellwithagreenplussign:usuallyforaddinganewfieldtoarecord,suchaswhenyouwanttoaddabirthdaytoaperson’srecordintheContactsapp.Ineditingmode,youtapthegreenplussignnextto“addbirthday.”

Inthisexercise,youwillusethefirstoptionandcreateaNewbuttonintheheaderview.Whenthisbuttonistapped,anewrowwillbeaddedtotheUITableView.

InItemsViewController.swift,implementaddNewItem(_:).@IBActionfuncaddNewItem(sender:AnyObject){//Makeanewindexpathforthe0thsection,lastrowletlastRow=tableView.numberOfRowsInSection(0)letindexPath=NSIndexPath(forRow:lastRow,inSection:0)

//InsertthisnewrowintothetabletableView.insertRowsAtIndexPaths([indexPath],withRowAnimation:.Automatic)}

Buildandruntheapplication.TaptheAddbuttonand…theapplicationcrashes.Theconsoletellsyouthatthetableviewhasaninternalinconsistencyexception.

Rememberthat,ultimately,itisthedataSourceoftheUITableViewthatdeterminesthenumberofrowsthetableviewshoulddisplay.Afterinsertinganewrow,thetableviewhassixrows(theoriginalfiveplusthenewone).WhentheUITableViewasksitsdataSourceforthenumberofrows,theItemsViewControllerconsultsthestoreandreturnsthatthereshouldbefiverows.TheUITableViewcannotresolvethisinconsistencyandthrowsanexception.

YoumustmakesurethattheUITableViewanditsdataSourceagreeonthenumberofrows.Thus,youmustaddanewItemtotheItemStorebeforeyouinsertthenewrow.

InItemsViewController.swift,updateaddNewItem(_:).@IBActionfuncaddNewItem(sender:AnyObject){//Makeanewindexpathforthe0thsection,lastrowletlastRow=tableView.numberOfRowsInSection(0)letindexPath=NSIndexPath(forRow:lastRow,inSection:0)

//InsertthisnewrowintothetabletableView.insertRowsAtIndexPaths([indexPath],withRowAnimation:.Automatic)

//CreateanewitemandaddittothestoreletnewItem=itemStore.createItem()

//Figureoutwherethatitemisinthearrayifletindex=itemStore.allItems.indexOf(newItem){letindexPath=NSIndexPath(forRow:index,inSection:0)

//InsertthisnewrowintothetabletableView.insertRowsAtIndexPaths([indexPath],withRowAnimation:.Automatic)}


}

Buildandruntheapplication.TaptheAddbutton,andthenewrowwillslideintothebottompositionofthetable.Rememberthattheroleofaviewobjectistopresentmodelobjectstotheuser;updatingviewswithoutupdatingthemodelobjectsisnotveryuseful.

Nowthatyouhavetheabilitytoaddrowsanditems,younolongerneedthecodethatputsfiverandomitemsintothestore.

OpenItemStore.swiftandremovetheinitializercode.init(){for_in0..<5{createItem()}}

Buildandruntheapplication.Therewillnolongerbeanyrowswhenyoufirstlaunchtheapplication,butyoucanaddsomebytappingtheAddbutton.


DeletingRowsIneditingmode,theredcircleswiththeminussign(showninFigure10.7)aredeletioncontrols,andtappingoneshoulddeletethatrow.However,atthispoint,youcannotactuallydeletetherow.(Tryitandsee.)Beforethetableviewwilldeletearow,itcallsamethodonitsdatasourceabouttheproposeddeletionandwaitsforconfirmation.

Whendeletingacell,youmustdotwothings:removetherowfromtheUITableViewandremovetheItemassociatedwithitfromtheItemStore.Topullthisoff,theItemStoremustknowhowtoremoveobjectsfromitself.

InItemStore.swift,implementanewmethodtoremoveaspecificitem.funcremoveItem(item:Item){ifletindex=allItems.indexOf(item){allItems.removeAtIndex(index)}}

NowyouwillimplementtableView(_:commitEditingStyle:forRowAtIndexPath:),amethodfromtheUITableViewDataSourceprotocol.(ThismethodiscalledontheItemsViewController.KeepinmindthatwhiletheItemStoreisthewherethedataiskept,theItemsViewControlleristhetableview’sdataSource.)

WhentableView(_:commitEditingStyle:forRowAtIndexPath:)iscalledonthedatasource,twoextraargumentsarepassedalongwithit.ThefirstistheUITableViewCellEditingStyle,which,inthiscase,is.Delete.TheotherargumentistheNSIndexPathoftherowinthetable.

InItemsViewController.swift,implementthismethodtohavetheItemStoreremovetherightobjectandconfirmtherowdeletionbycallingthemethoddeleteRowsAtIndexPaths(_:withRowAnimation:)onthetableview.overridefunctableView(tableView:UITableView,commitEditingStyleeditingStyle:UITableViewCellEditingStyle,forRowAtIndexPathindexPath:NSIndexPath){//Ifthetableviewisaskingtocommitadeletecommand...ifeditingStyle==.Delete{letitem=itemStore.allItems[indexPath.row]//RemovetheitemfromthestoreitemStore.removeItem(item)

//AlsoremovethatrowfromthetableviewwithananimationtableView.deleteRowsAtIndexPaths([indexPath],withRowAnimation:.Automatic)}}

Buildandrunyourapplication,createsomerows,andthendeletearow.Itwilldisappear.Noticethatswipe-to-deleteworksalso.


MovingRowsTochangetheorderofrowsinaUITableView,youwilluseanothermethodfromtheUITableViewDataSourceprotocol–tableView(_:moveRowAtIndexPath:toIndexPath:).

Todeletearow,youhadtocallthemethoddeleteRowsAtIndexPaths(_:withRowAnimation:)ontheUITableViewtoconfirmthedeletion.Movingarow,however,doesnotrequireconfirmation:ThetableviewmovestherowonitsownauthorityandreportsthemovetoitsdatasourcebycallingthemethodtableView(_:moveRowAtIndexPath:toIndexPath:).Youimplementthismethodtoupdateyourdatasourcetomatchtheneworder.

Butbeforeyoucanimplementthismethod,youneedtogivetheItemStoreamethodtochangetheorderofitemsinitsallItemsarray.

InItemStore.swift,implementthisnewmethod.funcmoveItemAtIndex(fromIndex:Int,toIndex:Int){iffromIndex==toIndex{return}

//GetreferencetoobjectbeingmovedsoyoucanreinsertitletmovedItem=allItems[fromIndex]

//RemoveitemfromarrayallItems.removeAtIndex(fromIndex)

//InsertiteminarrayatnewlocationallItems.insert(movedItem,atIndex:toIndex)}

InItemsViewController.swift,implementtableView(_:moveRowAtIndexPath:toIndexPath:)toupdatethestore.overridefunctableView(tableView:UITableView,moveRowAtIndexPathsourceIndexPath:NSIndexPath,toIndexPathdestinationIndexPath:NSIndexPath){//UpdatethemodelitemStore.moveItemAtIndex(sourceIndexPath.row,toIndex:destinationIndexPath.row)}

Buildandrunyourapplication.TapEditandcheckoutthenewreorderingcontrols(thethreehorizontallines)onthesideofeachrow.Touchandholdareorderingcontrolandmovetherowtoanewposition(Figure10.8).


Figure10.8Movingarow

NotethatsimplyimplementingtableView(_:moveRowAtIndexPath:toIndexPath:)causedthereorderingcontrolstoappear.TheUITableViewcanaskitsdatasourceatruntimewhetheritimplementstableView(_:moveRowAtIndexPath:toIndexPath:).Ifitdoes,thenthetableviewaddsthereorderingcontrolswheneverthetableviewenterseditingmode.


DisplayingUserAlertsInthissection,youaregoingtolearnaboutuseralertsandthedifferentwaysofconfiguringanddisplayingthem.Useralertscanprovideyourapplicationwithabetteruserexperience,soyouwillusethemfairlyoften.

Alertsareoftenusedtowarnusersthatanimportantactionisabouttohappenandperhapsgivethemtheopportunitytocancelthataction.Whenyouwanttodisplayanalert,youcreateaninstanceofUIAlertControllerwithapreferredstyle.ThetwoavailablestylesareUIAlertControllerStyle.ActionSheetandUIAlertControllerStyle.Alert(Figure10.9).The.ActionSheetstyleisusedtopresenttheuserwithalistofactionsfromwhichtochoose.The.Alerttypeisusedtodisplaycriticalinformationtorequiretheusertodecidehowtoproceed.Thedistinctionmayseemsubtle,butiftheusercanbackoutofadecisionoriftheactionisnotcritical,thenan.ActionSheetisprobablythebestchoice.

Figure10.9UIAlertControllerstyles

YouaregoingtouseaUIAlertControllertoconfirmthedeletionofitems.Youwillusethe.ActionSheetstylesincethepurposeofthealertistoconfirmorcancelapossiblydestructiveaction.


OpenItemsViewController.swiftandmodifytableView(_:commitEditingStyle:forRowAtIndexPath:)toasktheusertoconfirmorcancelthedeletionofanitem.overridefunctableView(tableView:UITableView,commitEditingStyleeditingStyle:UITableViewCellEditingStyle,forRowAtIndexPathindexPath:NSIndexPath){//Ifthetableviewisaskingtocommitadeletecommand...ifeditingStyle==.Delete{letitem=itemStore.allItems[indexPath.row]

lettitle="Delete\(item.name)?"letmessage="Areyousureyouwanttodeletethisitem?"

letac=UIAlertController(title:title,message:message,preferredStyle:.ActionSheet)

//RemovetheitemfromthestoreitemStore.removeItem(item)

//AlsoremovethatrowfromthetableviewwithananimationtableView.deleteRowsAtIndexPaths([indexPath],withRowAnimation:.Automatic)}}

Afterdeterminingthattheuserwantstodeleteanitem,youcreateaninstanceofUIAlertControllerwithanappropriatetitleandmessagedescribingwhatactionisabouttotakeplace.Also,youspecifythe.ActionSheetstyleforthealert.

TheactionsthattheusercanchoosefromwhenshownanalertareinstancesofUIAlertAction,andyoucanaddmultipleonesregardlessofthealert’sstyle.ActionsareaddedtotheUIAlertControllerusingtheaddAction(_:)method.

AddthenecessaryactionstotheactionsheetintableView(_:commitEditingStyle:forRowAtIndexPath:)....


letcancelAction=UIAlertAction(title:"Cancel",style:.Cancel,handler:nil)ac.addAction(cancelAction)

letdeleteAction=UIAlertAction(title:"Delete",style:.Destructive,handler:{(action)->Voidin//Removetheitemfromthestoreself.itemStore.removeItem(item)

//Alsoremovethatrowfromthetableviewwithananimationself.tableView.deleteRowsAtIndexPaths([indexPath],withRowAnimation:.Automatic)})ac.addAction(deleteAction)

...

Thefirstactionhasatitleof“Cancel”andiscreatedusingthe.Cancelstyle.The.Cancelstyleresultsintextinastandardbluefont.ThisactionwillallowtheusertobackoutofdeletinganItem.Thehandlerparameterallowsaclosuretobeexecutedwhenthatactionoccurs.Sincenootheractionisneeded,nilispassedastheargument.

Thesecondactionhasatitleof“Delete”andiscreatedusingthe.Destructivestyle.Sincedestructiveactionsshouldbeclearlymarkedandnoticed,the.Destructive


styleresultsinbrightredtext.Iftheuserselectsthisaction,thentheitemandthetableviewcellneedtoberemoved.Thisisalldonewithinthehandlerclosurethatispassedtotheaction’sinitializer.

Nowthattheactionshavebeenadded,thealertcontrollercanbedisplayedtotheuser.SinceUIAlertControllerisasubclassofUIViewController,youcanpresentittotheusermodally.Amodalviewcontrollertakesovertheentirescreenuntilithasfinisheditswork.

Topresentaviewcontrollermodally,youcallpresentViewController(_:animated:completion:)ontheviewcontrollerwhoseviewisonthescreen.Theviewcontrollertobepresentedispassedtoit,andthisviewcontroller’sviewtakesoverthescreen....



//PresentthealertcontrollerpresentViewController(ac,animated:true,completion:nil)

...

Buildandruntheapplicationanddeleteanitem.Anactionsheetwillbepresentedforyoutoconfirmthedeletion(Figure10.10).


Figure10.10Deletinganitem


DesignPatternsAdesignpatternsolvesacommonsoftwareengineeringproblem.Designpatternsarenotactualsnippetsofcode,butinsteadareabstractideasorapproachesthatyoucanuseinyourapplications.Gooddesignpatternsarevaluableandpowerfultoolsforanydeveloper.

Theconsistentuseofdesignpatternsthroughoutthedevelopmentprocessreducesthementaloverheadinsolvingaproblemsoyoucancreatecomplexapplicationsmoreeasilyandrapidly.Herearesomeofthedesignpatternsthatyouhavealreadyused:

Delegation:Oneobjectdelegatescertainresponsibilitiestoanotherobject.YouuseddelegationwiththeUITextFieldtobeinformedwhenthecontentsofthetextfieldchange.

Datasource:Adatasourceissimilartoadelegate,butinsteadofreactingtoanotherobject,adatasourceisresponsibleforprovidingdatatoanotherobjectwhenrequested.Youusedthedatasourcepatternwithtableviews:Eachtableviewhasadatasourcethatisresponsiblefor,ataminimum,tellingthetableviewhowmanyrowstodisplayandwhichcellitshoulddisplayateachindexpath.

Model-View-Controller:Eachobjectinyourapplicationsfulfillsoneofthreeroles.Modelobjectsarethedata.Viewsdisplaytheuserinterface.Controllersprovidethegluethattiesthemodelsandviewstogether.

Target-actionpairs:Oneobjectcallsamethodonanotherobjectwhenaspecificeventoccurs.Thetargetistheobjectthathasamethodcalledonit,andtheactionisthemethodbeingcalled.Forexample,youusedtarget-actionpairswithbuttons:whenatoucheventoccurs,amethodwillbecalledonanotherobject(oftenaviewcontroller).

Appleisveryconsistentinitsuseofthesedesignpatterns,andsoitisimportanttounderstandandrecognizethem.Keepaneyeoutforthesepatternsasyoucontinuethroughthisbook!Recognizingthemwillhelpyoulearnnewclassesandframeworksmuchmoreeasily.


BronzeChallenge:RenamingtheDeleteButtonWhendeletingarow,aconfirmationbuttonappearslabeledDelete.ChangethelabelofthisbuttontoRemove.


SilverChallenge:PreventingReorderingMakeitsothetableviewalwaysshowsafinalrowthatsaysNomoreitems!.(Thispartofthechallengeisthesameasachallengefromthelastchapter.Ifyouhavealreadydoneit,youcancopyyourcodefrombefore.)Now,makeitsothatthefinalrowcannotbemoved.


GoldChallenge:ReallyPreventingReorderingAftercompletingthesilverchallenge,youmaynoticethateventhoughyoucannotmovetheNomoreitems!rowitself,youcanstilldragotherrowsunderneathit.Makeitsothatnomatterwhat,theNomoreitems!rowcanneverbeknockedoutofthelastposition.Finally,makeitundeletable.


11SubclassingUITableViewCell

AUITableViewdisplaysalistofUITableViewCellobjects.Formanyapplications,thebasiccellwithitstextLabel,detailTextLabel,andimageViewissufficient.However,whenyouneedacellwithmoredetailoradifferentlayout,yousubclassUITableViewCell.

Inthischapter,youwillcreateasubclassofUITableViewCellnamedItemCellthatwilldisplayIteminstancesmoreeffectively.EachofthesecellswillshowanItem’sname,itsvalueindollars,anditsserialnumber(Figure11.1).

Figure11.1Homepwnerwithsubclassedtableviewcells

YoucustomizetheappearanceofUITableViewCellsubclassesbyaddingsubviewstoitscontentView.AddingsubviewstothecontentViewinsteadofdirectlytothecellitselfisimportantbecausethecellwillresizeitscontentViewatcertaintimes.Forexample,whenatableviewenterseditingmode,thecontentViewresizesitselftomakeroomfortheeditingcontrols(Figure11.2).IfyouaddedsubviewsdirectlytotheUITableViewCell,theseeditingcontrolswouldobscurethesubviews.Thecellcannotadjustitssizewhenenteringeditmode(itmustremainthewidthofthetableview),butthecontentViewcanresize,anditdoes.


Figure11.2Tableviewcelllayoutinstandardandeditingmode


CreatingItemCellCreateanewSwiftfilenamedItemCell.InItemCell.swift,defineItemCellasaUITableViewCellsubclass.importFoundationimportUIKit

classItemCell:UITableViewCell{

}

TheeasiestwaytoconfigureaUITableViewCellsubclassisthroughastoryboard.InChapter9,yousawthatstoryboardsfortableviewcontrollershaveaPrototypeCellssection.ThisiswhereyouwilllayoutthecontentfortheItemCell.

OpenMain.storyboardandselecttheUITableViewCellinthedocumentoutline.Openitsattributesinspector,changetheStyletoCustom,andchangetheIdentifiertoItemCell.

Nowopenitsidentityinspector(the tab).IntheClassfield,enterItemCell(Figure11.3).

Figure11.3Changingthecellclass

Changetheheightoftheprototypecelltobeabout65pointstall.YoucanchangeiteitheronthecanvasorbyselectingthetableviewcellandchangingtheRowHeightfromitssizeinspector.

AnItemCellwilldisplaythreetextelements,sodragthreeUILabelobjectsontothecell.ConfigurethemasshowninFigure11.4.Makethetextofthebottomlabelaslightlysmallerfontinalightshadeofgray.

Figure11.4ItemCell’slayout

Addconstraintstothesethreelabelsasfollows.

1. Selectthetop-leftlabelandopentheAutoLayoutPinmenu.SelectthetopandleftstrutandthenclickAdd2Constraints.


2. Youwantthebottom-leftlabeltoalwaysbealignedwiththetop-leftlabel.Control-dragfromthebottom-leftlabeltothetop-leftlabelandselectLeading.

3. Withthebottom-leftlabelstillselected,openthePinmenu,selectthebottomstrut,andthenclickAdd1Constraint.

4. SelecttherightlabelandControl-dragfromthislabeltoitssuperviewonitsrightside.SelectbothTrailingSpacetoContainerMarginandCenterVerticallyinContainer.

5. Selectthebottom-leftlabelandopenitssizeinspector.FindtheVerticalContentHuggingPriorityandloweritto250.LowertheVerticalContentCompressionResistancePriorityto749.YouwilllearnwhattheseAutoLayoutpropertiesdoinChapter12.

6. Yourframesmightbemisplaced,soopentheResolveAutoLayoutIssuesmenuandupdatetheframesforthethreelabels.


ExposingthePropertiesofItemCellForItemsViewControllertoconfigurethecontentofanItemCellintableView(_:cellForRowAtIndexPath:),thecellmusthavepropertiesthatexposethethreelabels.ThesepropertieswillbesetthroughoutletconnectionsinMain.storyboard.

Thenextstep,then,istocreateandconnectoutletsonItemCellforeachofitssubviews.

OpenItemCell.swiftandaddthreepropertiesfortheoutlets.importUIKit

classItemCell:UITableViewCell{

@IBOutletvarnameLabel:UILabel!@IBOutletvarserialNumberLabel:UILabel!@IBOutletvarvalueLabel:UILabel!

}

YouaregoingtoconnecttheoutletsforthethreeviewstotheItemCell.Whenconnectingoutletsearlierinthebook,youControl-draggedfromviewcontrollerinthestoryboardtotheappropriateview.ButtheoutletsforItemCellarenotoutletsonacontroller.Theyareoutletsonaview:thecustomUITableViewCellsubclass.

Therefore,toconnecttheoutletsforItemCell,youwillconnectthemtotheItemCell.

OpenMain.storyboard.Control-clickontheItemViewCellinthedocumentoutlineandmakethethreeoutletconnectionsshowninFigure11.5.

Figure11.5Connectingtheoutlets


UsingItemCellLet’sgetyourcustomcellsonscreen.InItemsViewController’stableView(_:cellForRowAtIndexPath:)method,youwilldequeueaninstanceofItemCellforeveryrowinthetable.

NowthatyouareusingacustomUITableViewCellsubclass,thetableviewneedstoknowhowtalleachrowis.Thereareafewwaystoaccomplishthis,butthesimplestwayistosettherowHeightpropertyofthetableviewtoaconstantvalue.Youwillseeanotherwaylaterinthischapter.

OpenItemsViewController.swiftandupdateviewDidLoad()tosettheheightofthetableviewcells.overridefuncviewDidLoad(){super.viewDidLoad()


letinsets=UIEdgeInsets(top:statusBarHeight,left:0,bottom:0,right:0)tableView.contentInset=insetstableView.scrollIndicatorInsets=insets

tableView.rowHeight=65}

NowthatyouhaveregisteredtheItemCellwiththetableview(usingtheprototypecellsinthestoryboard),youcanaskthetableviewtodequeueacellwiththeidentifier“ItemCell.”

InItemsViewController.swift,modifytableView(_:cellForRowAtIndexPath:).overridefunctableView(tableView:UITableView,cellForRowAtIndexPathindexPath:NSIndexPath)->UITableViewCell{//Getaneworrecycledcellletcell=tableView.dequeueReusableCellWithIdentifier("UITableViewCell",forIndexPath:indexPath)

letcell=tableView.dequeueReusableCellWithIdentifier("ItemCell",forIndexPath:indexPath)as!ItemCell

//Setthetextonthecellwiththedescriptionoftheitem//thatisatthenthindexofitems,wheren=rowthiscell//willappearinonthetableviewletitem=itemStore.allItems[indexPath.row]

cell.textLabel?.text=item.namecell.detailTextLabel?.text="$\(item.valueInDollars)"

//ConfigurethecellwiththeItemcell.nameLabel.text=item.namecell.serialNumberLabel.text=item.serialNumbercell.valueLabel.text="$\(item.valueInDollars)"

returncell}

First,thereuseidentifierisupdatedtoreflectyournewsubclass.Thecodeattheendofthismethodisfairlyobvious–foreachlabelonthecell,setitstexttosomepropertyfromtheappropriateItem.


Buildandruntheapplication.ThenewcellsnowloadwiththeirlabelspopulatedwiththevaluesfromeachItem.


DynamicCellHeightsCurrently,thecellshaveafixedheightof65points.Itismuchbettertoallowthecontentofthecelltodriveitsheight.Thatway,ifthecontenteverchanges,thetableviewcell’sheightcanchangeautomatically.

Youcanachievethisgoal,asyouhaveprobablyguessed,withAutoLayout.TheUITableViewCellneedstohaveverticalconstraintsthatwillexactlydeterminetheheightofthecell.Currently,ItemCelldoesnothavesufficientconstraintsforthis.Youneedtoaddaconstraintbetweenthetwoleftlabelsthatfixestheverticalspacingbetweenthem.

OpenMain.storyboard.Control-dragfromthenameLabeltotheserialNumberLabelandselectVerticalSpacing.

NowopenItemsViewController.swiftandupdateviewDidLoad(_:)totellthetableviewthatitshouldcomputethecellheightsbasedontheconstraints.overridefuncviewDidLoad(){super.viewDidLoad()



tableView.rowHeight=65tableView.rowHeight=UITableViewAutomaticDimensiontableView.estimatedRowHeight=65}

UITableViewAutomaticDimensionisthedefaultvalueforrowHeight,sowhileitisnotnecessarytoadd,itisusefulforunderstandingwhatisgoingon.SettingtheestimatedRowHeightpropertyonthetableviewcanimproveperformance.Insteadofaskingeachcellforitsheightwhenthetableviewloads,settingthispropertyallowssomeofthatperformancecosttobedeferreduntiltheuserstartsscrolling.

Buildandruntheapplication.Theapplicationwilllookthesameasitdidbefore.Inthenextsection,youwilllearnaboutatechnologycalledDynamicTypethatwilltakeadvantageoftheautomaticallyresizingtableviewcells.


DynamicTypeCreatinganinterfacethatappealstoeveryonecanbedaunting.Somepeopleprefermorecompactinterfacessotheycanseemoreinformationatatime.Othersmightwanttobeabletoeasilyseeinformationataglance,orperhapstheyhavepooreyesight.Inshort:peoplehavedifferentneeds.Gooddevelopersstrivetomakeappsthatmeetthoseneeds.

DynamicTypeisatechnologythathelpsrealizethisgoalbyprovidingspecificallydesignedtextstylesthatareoptimizedforlegibility.UserscanselectoneofsevenpreferredtextsizesfromwithinApple’sSettingsapplication(plusafewadditionallargersizesfromwithintheAccessibilitysection),andappsthatsupportDynamicTypewillhavetheirfontsscaledappropriately.Inthissection,youwillupdateItemCelltosupportDynamicType.Figure11.6showstheapplicationrenderedatthesmallestandlargestuser-selectableDynamicTypesizes.

Figure11.6ItemCellwithDynamicTypesupported

TheDynamicTypesystemiscenteredaroundtextstyles.Whenafontisrequestedforagiventextstyle,thesystemwillconsidertheuser’spreferredtextsizeinassociationwiththetextstyletoreturnanappropriatelyconfiguredfont.Figure11.7showsthesixdifferenttextstyles.


Figure11.7Differenttextstyles

OpenMain.storyboard.Let’supdatethelabelstousethetextstylesinsteadoffixedfonts.SelectthenameLabelandvalueLabelandopentheattributesinspector.ClickonthetexticontotherightofFont.ForFont,chooseTextStyles-Body(Figure11.8).RepeatthesamestepsfortheserialNumberLabel,choosingtheCaption1textstyle.

Figure11.8Changingthetextstyle

Nowlet’schangethepreferredfontsize.YoudothisthroughtheSettingsapplication.

Buildandruntheapplication.PresstheHomebutton(oruseHomefromtheHardware


menu)andopenApple’sSettingsapplication.UnderGeneral,selectAccessibilityandthenLargerText.(Onanactualdevice,thismenuisaccessedinSettingsunderDisplay&BrightnessandthenTextSize.)Dragthesliderallthewaytothelefttosetthefontsizetothesmallestvalue(Figure11.9).PresstheHomebuttonagaintosavethesechanges.

Figure11.9Textsizesettings

Buildandruntheapplication.(Ifyouswitchbacktotheapplication,eitherusingthetaskswitcherorthroughtheHomescreen,youwillnotseethechanges.Youwillfixthatinthenextsection.)Addsomeitemstothetableviewandyouwillseethenewsmallerfontsizesinaction.

Respondingtouserchanges

Whentheuserchangesthepreferredtextsizeandreturnstotheapplication,thetableviewwillgetreloaded.Unfortunately,thelabelswillnotknowaboutthenewpreferredtext


size.Tofixthis,youneedtoupdatethelabelsmanually.

OpenItemCell.swiftandaddanewmethodthatupdatesallthreelabels.funcupdateLabels(){letbodyFont=UIFont.preferredFontForTextStyle(UIFontTextStyleBody)nameLabel.font=bodyFontvalueLabel.font=bodyFont

letcaption1Font=UIFont.preferredFontForTextStyle(UIFontTextStyleCaption1)serialNumberLabel.font=caption1Font}

NowopenItemsViewController.swiftandcallthismethodintableView(_:cellForRowAtIndexPath:).overridefunctableView(tableView:UITableView,cellForRowAtIndexPathindexPath:NSIndexPath)->UITableViewCell{//Getaneworrecycledcellletcell=tableView.dequeueReusableCellWithIdentifier("UITableViewCell",forIndexPath:indexPath)as!ItemCell

//Updatethelabelsforthenewpreferredtextsizecell.updateLabels()

letitem=itemStore.allItems[indexPath.row]

cell.nameLabel.text=item.namecell.serialNumberLabel.text=item.serialNumbercell.valueLabel.text="$\(item.valueInDollars)"

returncell}

Buildandruntheapplication.GointoSettingsandchangethepreferredreadingsizetothelargestsize.Unlikebefore,youcannowswitchbacktoHomepwner,eitherbyopeningthetaskswitcherorthroughtheHomescreen,andthetableviewwillupdatetoreflectthenewpreferredtextsize.


BronzeChallenge:CellColorsUpdatetheItemCelltodisplaythevalueInDollarsingreenifthevalueislessthan50andredifthevalueisgreaterthanorequalto50.


12StackViews

YouhavebeenusingAutoLayoutthroughoutthisbooktocreateflexibleinterfacesthatscaleacrossdevicetypesandsizes.AutoLayoutisaverypowerfultechnology,butwiththatpowercomescomplexity.Layingoutaninterfacewelloftenneedsalotofconstraints,anditcanbedifficulttocreatedynamicinterfacesduetotheneedtoconstantlyaddandremoveconstraints.

Often,aninterface(orasubsectionoftheinterface)canbelaidoutinalinearfashion.Let’sthinkaboutsomeoftheotherapplicationsyouhavewritteninthisbook.TheQuizapplicationthatyouwroteinChapter1consistedoffoursubviewsthatwerelaidoutvertically.ThesameistruefortheWorldTrotterapplicationthatyouwrote:theConversionViewControllerhadaverticalinterfaceconsistingofatextfieldandafewlabels.

Interfacesthathavealinearlayoutaregreatcandidatesforusingastackview.AstackviewisaninstanceofUIStackViewthatallowsyoutocreateaverticalorhorizontallayoutthatiseasytolayoutandmanagesmostoftheconstraintsthatyouwouldtypicallyhavetomanageyourself.Perhapsbestofall,youareabletoneststackviewswithinotherstackviews,whichallowsyoutocreatetrulyamazinginterfacesinafractionofthetime.

Inthischapter,youaregoingtocontinueworkingonHomepwnertocreateaninterfacefordisplayingthedetailsofaspecificItem.Theinterfacethatyoucreatewillconsistofmultiplenestedstackviews,bothverticalandhorizontal(Figure12.1).


Figure12.1Homepwnerwithstackviews


UsingUIStackViewYouaregoingtocreateaninterfaceforeditingthedetailsforanItem.Youwillgetthebasicinterfaceworkinginthischapter,andthenyouwillfinishimplementingthedetailsinChapter13.

Atthetoplevel,youwillhaveaverticalstackviewwithfourelementsdisplayingtheitem’sname,serialnumber,value,anddatecreated(Figure12.2).

Figure12.2Verticalstackviewlayout

OpenyourHomepwnerprojectandthenopenMain.storyboard.DraganewViewControllerfromtheobjectlibraryontothecanvas.DragaVerticalStackViewfromtheobjectlibraryontotheviewfortheViewController.Addconstraintstothestackviewtopinittotheleadingandtrailingmargins,andpinthetopandbottomedgestobe8pointsfromthetopandbottomlayoutguides.

NowdragfourinstancesofUILabelfromtheobjectlibraryontothestackview.Fromtoptobottom,givetheselabelsthetext“Name,”“Serial,”“Value,”and“DateCreated.”(Figure12.3).


Figure12.3Labelsaddedtothestackview

Youcanseeaproblemrightaway:thelabelsallhavearedborder(indicatinganAutoLayoutproblem)andthereisawarningthatsomeviewsareverticallyambiguous.Therearetwowaysyoucanfixthisissue:byusingAutoLayout,orbyusingapropertyonthestackview.Let’sworkthroughtheAutoLayoutsolutionfirstbecauseithighlightsanimportantaspectofAutoLayout.

Implicitconstraints

YoulearnedinChapter3thateveryviewhasanintrinsiccontentsize.Youalsolearnedthatifyoudonotspecifyconstraintsthatexplicitlydeterminethewidthorheight,theviewwillderiveitswidthorheightfromitsintrinsiccontentsize.Howdoesthiswork?

Itdoesthisusingimplicitconstraintsderivedfromaview’scontenthuggingprioritiesanditscontentcompressionresistancepriorities.Aviewhasoneoftheseprioritiesforeachaxis:

horizontalcontenthuggingpriority


verticalcontenthuggingpriority

horizontalcontentcompressionresistancepriority

verticalcontentcompressionresistancepriority

Contenthuggingpriorities

Thecontenthuggingpriorityislikearubberbandthatisplacedaroundaview.Therubberbandmakestheviewnotwanttobebiggerthanitsintrinsiccontentsizeinthatdimension.Eachpriorityisassociatedwithavaluefrom0to1000.Avalueof1000meansthataviewcannotgetbiggerthanitsintrinsiccontentsizeonthatdimension.

Let’slookatanexamplewithjustthehorizontaldimension.Sayyouhavetwolabelsnexttooneanotherwithconstraintsbothbetweenthetwoviewsandbetweeneachviewanditssuperview,asshowninFigure12.4.

Figure12.4Twolabelssidebyside

Thisworksgreatuntilthesuperviewbecomeswider.Atthatpoint,whichlabelshouldbecomewider?Thefirstlabel,thesecondlabel,orboth?AsFigure12.5shows,theinterfaceiscurrentlyambiguous.

Figure12.5Ambiguouslayout

Thisiswherethecontenthuggingprioritybecomesrelevant.Theviewwiththehighercontenthuggingpriorityistheonethatdoesnotstretch.Youcanthinkaboutthepriorityvalueasthe“strength”oftherubberband.Thehigherthepriorityvalue,thestrongerthe


rubberband,andthemoreitwantstohugtoitsintrinsiccontentsize.

Contentcompressionresistancepriorities

Thecontentcompressionresistanceprioritiesdeterminehowmuchaviewresistsgettingsmallerthanitsintrinsiccontentsize.ConsiderthesametwolabelsfromFigure12.4.Whatwouldhappenifthesuperview’swidthdecreased?Oneofthelabelswouldneedtotruncateitstext(Figure12.6).Butwhichone?

Figure12.6Compressedambiguouslayout

Theviewwiththegreatercontentcompressionresistancepriorityistheonethatwillresistcompressionand,therefore,nottruncateitstext.

Withthisknowledge,youcannowfixtheproblemwiththestackview.

SelecttheDateCreatedlabelandopenitssizeinspector.FindtheVerticalContentHuggingPriorityandloweritto249.Nowtheotherthreelabelshaveahighercontenthuggingpriority,sotheywillallhugtotheirintrinsiccontentheight.TheDateCreatedlabelwillstretchtofillintheremainingspace.

Stackviewdistribution

Let’stakealookatanotherwayofsolvingtheproblem.Stackviewshaveanumberofpropertiesthatdeterminehowtheircontentislaidout.

Selectthestackview,eitheronthecanvasorusingthedocumentoutline.OpenitsattributesinspectorandfindthesectionatthetoplabeledStackView.OneofthepropertiesthatdetermineshowthecontentislaidoutistheDistributionproperty.CurrentlyitissettoFill,whichletstheviewslayouttheircontentbasedontheirintrinsiccontentsize.ChangethevaluetoFillEqually.Thiswillresizethelabelssothattheyallhavethesameheight,ignoringtheintrinsiccontentsize(Figure12.7).Besuretoreadthedocumentationfortheotherdistributionvaluesthatastackviewcanhave.


Figure12.7Stackviewsettofillequally

ChangetheDistributionofthestackviewbacktoFill;thisisthevalueyouwillwantgoingforwardinthischapter.

Nestedstackviews

Oneofthemostpowerfulfeaturesofstackviewsisthattheycanbenestedwithinoneanother.Youwillusethistonesthorizontalstackviewswithinthelargerverticalstackview.ThetopthreelabelswillhaveatextfieldnexttothemthatdisplaysthecorrespondingvaluefortheItemandwillalsoallowtheusertoeditthatvalue.

SelecttheNamelabelonthecanvas.Clicktheleft-mosticonintheAutoLayoutconstraintsmenu: .Thiswillembedtheselectedviewinastackview.

Selectthenewstackviewandopenitsattributesinspector.Thestackviewiscurrentlyaverticalstackview,butyouwantittobeahorizontalstackview.ChangetheAxistoHorizontal.

NowdragaTextFieldfromtheobjectlibrarytotherightoftheNamelabel.Sincelabels,by


default,haveagreatercontenthuggingprioritythantextfields,thelabelhugstoitsintrinsiccontentwidthandthetextfieldstretches.Thelabelandthetextfieldcurrentlyhavethesamecontentcompressionresistancepriorities,whichwouldresultinanambiguouslayoutifthetextfield’stextwastoolong.OpenthesizeinspectorforthetextfieldandsetitsHorizontalContentCompressionResistancePriorityto749.

Stackviewspacing

Thelabelandtextfieldlookalittlesquishedsincethereisnospacingbetweenthem.Stackviewsallowyoutocustomizethespacingbetweenitems.

Selectthehorizontalstackviewandopenitsattributesinspector.ChangetheSpacingtobe8points.Noticethatthetextfieldshrinkstoaccommodatethespacing,becauseitislessresistanttocompressionthanthelabel.

RepeatthesesamestepsfortheSerialandValuelabels.

1. Selectthelabelandclickthe icon.

2. Changethestackviewtobeahorizontalstackview.

3. Dragatextfieldontothehorizontalstackviewandchangeitshorizontalcontentcompressionresistanceprioritytobe749.

4. Updatethestackviewtohaveaspacingof8points.

Thereareacoupleofothertweaksyouwillwanttomaketotheinterface.Theverticalstackviewneedssomespacing.TheDateCreatedlabelshouldhaveacentertextalignment.AndtheName,Serial,andValuelabelsshouldbethesamewidth.

Selecttheverticalstack,openitsattributesinspector,andupdatetheSpacingtobe8points.ThenselecttheDateCreatedlabel,openitsattributesinspector,andchangetheAlignmenttobecentered.Thatsolvesthefirsttwoissues.

Althoughstackviewssubstantiallyreducethenumberofconstraintsthatyouneedtoaddtoyourinterface,someconstraintsarestillimportant.Withtheinterfaceasis,thetextfieldsdonotalignontheirleadingedgeduetothedifferenceinthewidthsofthelabels.(ThedifferenceisnotverynoticeableinEnglish,butitbecomesmorepronouncedwhenlocalizedintootherlanguages.)Tosolvethis,youwilladdleadingedgeconstraintsbetweenthethreetextfields.

Control-dragfromtheNametextfieldtotheSerialtextfieldandselectLeading.ThendothesamefortheSerialtextfieldandtheValuetextfield.ThecompletedinterfacewilllooklikeFigure12.8.


Figure12.8Finalstackviewinterface

Stackviewsallowyoutocreateveryrichinterfacesinafractionofthetimeitwouldtaketoconfigurethemmanuallyusingconstraints.Constraintsarestilladded,buttheyarebeingmanagedbythestackviewitselfinsteadofbyyou.Stackviewsallowyoutohaveverydynamicinterfacesatruntime.YoucanaddandremoveviewsfromstackviewsbyusingaddArrangedSubview(_:),insertArrangedSubview(_:atIndex:),andremoveArrangedSubview(_:).Youcanalsotogglethehiddenpropertyonaviewinastackview.Thestackviewwillautomaticallylayoutitscontenttoreflectthatvalue.


SeguesMostiOSapplicationshaveanumberofviewcontrollersthatusersnavigatebetween.Storyboardsallowyoutosetuptheseinteractionsassegueswithouthavingtowritecode.

Aseguemovesanotherviewcontroller’sviewontothescreenandisrepresentedbyaninstanceofUIStoryboardSegue.Eachseguehasastyle,anactionitem,andanidentifier.Thestyleofaseguedetermineshowtheviewcontrollerwillbepresented.Theactionitemistheviewobjectinthestoryboardfilethattriggersthesegue,likeabutton,atableviewcell,orsomeotherUIControl.Theidentifierisusedtoprogrammaticallyaccessthesegue.Thisisusefulwhenyouwanttotriggeraseguethatdoesnotcomefromanactionitem,likeashakeorsomeotherinterfaceelementthatcannotbesetupinthestoryboardfile.

Let’sstartwithashowsegue.Ashowseguedisplaysaviewcontrollerdependingonthecontextinwhichitisdisplayed.Theseguewillbebetweenthetableviewcontrollerandthenewviewcontroller.Theactionitemswillbethetableview’scells;tappingacellwillshowtheviewcontrollermodally.

InMain.storyboard,selecttheItemCellprototypecellontheItemsViewController.Control-dragfromthecelltothenewviewcontrollerthatyousetupintheprevioussection.(MakesureyouareControl-draggingfromthecellandnotthetableview!)Ablackpanelwillappearthatliststhepossiblestylesforthissegue.SelectShowfromtheSelectionSeguesection(Figure12.9).

Figure12.9Settingupashowsegue

Noticethearrowthatgoesfromthetableviewcontrollertothenewviewcontroller.Thisisasegue.Theiconinthecircletellsyouthatthissegueisashowsegue–eachseguehas


auniqueicon.

Buildandruntheapplication.Tapacellandthenewviewcontrollerwillslideupfromthebottomofthescreen.(Slidingupfromthebottomisthedefaultbehaviorwhenpresentingaviewcontrollermodally.Thiscanbecustomized,however,asyousawinChapter10whenyoupresentedtheUIAlertControllermodally.)

Sofar,sogood!Buttherearetwoproblemsatthemoment:theviewcontrollerisnotdisplayingtheinformationfortheItemthatwasselected,andthereisnowaytodismisstheviewcontrollertoreturntotheItemsViewController.Youwillfixthefirstissueinthenextsection,andyouwillfixthesecondissueinChapter13.


HookingUptheContentTodisplaytheinformationfortheselectedItem,youwillneedtocreateanewUIViewControllersubclass.

CreateanewSwiftfileandnameitDetailViewController.OpenDetailViewController.swiftanddeclareanewUIViewControllersubclassnamedDetailViewController.importFoundationimportUIKit

classDetailViewController:UIViewController{

}

Becauseyouneedtobeabletoaccessthesesubviewsduringruntime,DetailViewControllerneedsoutletsforthem.TheplanistoaddfournewoutletstoDetailViewControllerandthenmaketheconnections.Inpreviousexercises,youdidthisintwodistinctsteps:first,youaddedtheoutletsintheSwiftfile,andsecond,youmadeconnectionsinthestoryboardfile.Youcandobothatonceusingtheassistanteditor.

WithDetailViewController.swiftopen,Option-clickonMain.storyboardintheprojectnavigator.ThiswillopenthefileintheassistanteditorrightnexttoDetailViewController.swift.(YoucantoggletheassistanteditorbyclickingthemiddlebuttonfromtheEditorcontrolatthetopoftheworkspace.TheshortcuttodisplaytheassistanteditorisCommand-Option-Return,andtheshortcuttoreturntothestandardeditorisCommand-Return.)

Yourwindowhasbecomealittlecluttered.Let’smakesometemporaryspace.HidethenavigatorareabyclickingtheleftbuttonintheViewcontrolatthetopoftheworkspace(theshortcutforthisisCommand-0).Then,hidethedocumentoutlineinInterfaceBuilderbyclickingthetogglebuttoninthelowerleftcorneroftheeditor.YourworkspaceshouldnowlooklikeFigure12.10.


Figure12.10Layingouttheworkspace

Beforeyouconnecttheoutlets,youneedtotellthedetailinterfacethatitshouldbeassociatedwiththeDetailViewController.SelecttheViewControlleronthecanvasandopenitsidentityinspector.ChangetheClasstobeDetailViewController(Figure12.11).

Figure12.11Settingtheviewcontrollerclass

ThethreeinstancesofUITextFieldandbottominstanceofUILabelwillbeoutletsinDetailViewController.Control-dragfromtheUITextFieldnexttotheNamelabeltothetopofDetailViewController.swift,asshowninFigure12.12.


Figure12.12Draggingfromstoryboardtosourcefile

Letgoandapop-upwindowwillappear.EnternameFieldintotheNamefield,selectStrongfromtheStoragepop-upmenu,andclickConnect(Figure12.13).

Figure12.13Autogeneratinganoutletandmakingaconnection

Thiswillcreatean@IBOutletpropertyoftypeUITextFieldnamednameFieldinDetailViewController.

Inaddition,thisUITextFieldisnowconnectedtothenameFieldoutletoftheDetailViewController.YoucanverifythisbyControl-clickingontheDetailViewControllertoseetheconnections.AlsonoticethathoveringyourmouseabovethenameFieldconnectioninthepanelthatappearswillrevealtheUITextFieldthatyouconnected.Twobirds,onestone.

CreatetheotherthreeoutletsthesamewayandnamethemasshowninFigure12.14.


Figure12.14Connectiondiagram

Aftermakingtheconnections,DetailViewController.swiftshouldlooklikethis:importUIKit

classDetailViewController:UIViewController{

@IBOutletvarnameField:UITextField!@IBOutletvarserialNumberField:UITextField!@IBOutletvarvalueField:UITextField!@IBOutletvardateLabel:UILabel!

}

Ifyourfilelooksdifferent,thenyouroutletsarenotconnectedcorrectly.Fixanydisparitiesbetweenyourfileandthecodeshownaboveinthreesteps:First,gothroughtheControl-dragprocessandmakeconnectionsagainuntilyouhavethefourlinesshownaboveinyourDetailViewController.swift.Second,removeanywrongcode(likenon-propertymethoddeclarationsorproperties)thatgotcreated.Finally,checkforanybadconnectionsinthestoryboardfile.InMain.storyboard,Control-clickontheDetailViewController.Ifthereareyellowwarningsignsnexttoanyconnection,clickthexiconnexttothoseconnectionstodisconnectthem.

Itisimportanttoensurethattherearenobadconnectionsinaninterfacefile.Abadconnectiontypicallyhappenswhenyouchangethenameofapropertybutdonotupdatetheconnectionintheinterfacefile.Or,youcompletelyremoveapropertybutdonotremoveitfromtheinterfacefile.Eitherway,abadconnectionwillcauseyourapplicationtocrashwhentheinterfacefileisloaded.


Withtheconnectionsmade,youcanclosetheassistanteditorandreturntoviewingjustDetailViewController.swift.

DetailViewControllerwillholdontoareferencetotheItemthatisbeingdisplayed.Whenitsviewisloaded,youwillsetthetextoneachtextfieldtotheappropriatevaluefromtheIteminstance.

InDetailViewController.swift,addapropertyforanIteminstanceandoverrideviewWillAppear(_:)tosetuptheinterface.classDetailViewController:UIViewController{

@IBOutletvarnameField:UITextField!@IBOutletvarserialNumberField:UITextField!@IBOutletvarvalueField:UITextField!@IBOutletvardateLabel:UILabel!

varitem:Item!


nameField.text=item.nameserialNumberField.text=item.serialNumbervalueField.text="\(item.valueInDollars)"dateLabel.text="\(item.dateCreated)"}}

InsteadofusingstringinterpolationtoprintoutthevalueInDollarsanddateCreated,itwouldbebettertouseaformatter.YouusedaninstanceofNSNumberFormatterinChapter4.Youwilluseanotheronehere,aswellasaninstanceofNSDateFormattertoformatthedateCreated.

AddaninstanceofNSNumberFormatterandNSDateFormattertotheDetailViewController.UsetheseformattersinviewWillAppear(_:)toformatthevalueInDollarsanddateCreated.varitem:Item!

letnumberFormatter:NSNumberFormatter={letformatter=NSNumberFormatter()formatter.numberStyle=.DecimalStyleformatter.minimumFractionDigits=2formatter.maximumFractionDigits=2returnformatter}()

letdateFormatter:NSDateFormatter={letformatter=NSDateFormatter()formatter.dateStyle=.MediumStyleformatter.timeStyle=.NoStylereturnformatter}()


nameField.text=item.nameserialNumberField.text=item.serialNumbervalueField.text="\(item.valueInDollars)"dateLabel.text="\(item.dateCreated)"valueField.text=numberFormatter.stringFromNumber(item.valueInDollars)dateLabel.text=dateFormatter.stringFromDate(item.dateCreated)}


PassingDataAroundWhenarowinthetableviewistapped,youneedawayoftellingtheDetailViewControllerwhichitemwasselected.Wheneverasegueistriggered,theprepareForSegue(_:sender:)methodiscalledontheviewcontrollerinitiatingthesegue.Thismethodhastwoarguments:theUIStoryboardSegue,whichgivesyouinformationaboutwhichsegueishappening,andthesender,whichistheobjectthattriggeredthesegue(aUITableViewCelloraUIButton,forexample).

TheUIStoryboardSeguegivesyouthreepiecesofinformation:thesourceviewcontroller(wherethesegueoriginates),thedestinationviewcontroller(wherethesegueends),andtheidentifierofthesegue.Theidentifierletsyoudifferentiatesegues.Let’sgivethesegueausefulidentifier.

OpenMain.storyboardagain.Selecttheshowseguebyclickingonthearrowbetweenthetwoviewcontrollersandopentheattributesinspector.Fortheidentifier,enterShowItem(Figure12.15).

Figure12.15Segueidentifier

Withyoursegueidentified,youcannowpassyourIteminstancesaround.OpenItemsViewController.swiftandimplementprepareForSegue(_:sender:).overridefuncprepareForSegue(segue:UIStoryboardSegue,sender:AnyObject?){//Ifthetriggeredsegueisthe"ShowItem"segueifsegue.identifier=="ShowItem"{

//Figureoutwhichrowwasjusttappedifletrow=tableView.indexPathForSelectedRow?.row{

//Gettheitemassociatedwiththisrowandpassitalongletitem=itemStore.allItems[row]letdetailViewController=segue.destinationViewControlleras!DetailViewControllerdetailViewController.item=item}}}

Buildandruntheapplication.TaponarowandtheDetailViewControllerwill


slideonscreen,displayingthedetailsforthatitem.YouwillfixtheinabilitytogobacktotheItemsViewControllerinChapter13.

ManyprogrammersnewtoiOSstrugglewithhowdataispassedbetweenviewcontrollers.HavingallofthedataintherootviewcontrollerandpassingsubsetsofthatdatatothenextUIViewController(likeyoudid)isacleanandefficientwayofperformingthistask.


BronzeChallenge:MoreStackViewsQuizandWorldTrotteraregoodcandidatesforusingstackviews.UpdatebothoftheseapplicationstouseUIStackView.


13UINavigationController

InChapter5,youlearnedaboutUITabBarControllerandhowitallowsausertoaccessdifferentscreens.Atabbarcontrollerisgreatforscreensthatareindependentofeachother,butwhatifyouhavescreensthatproviderelatedinformation?

Forexample,theSettingsapplicationhasmultiplerelatedscreensofinformation:alistofsettings(likeSounds),adetailedpageforeachsetting,andaselectionpageforeachdetail(Figure13.1).Thistypeofinterfaceiscalledadrill-downinterface.

Figure13.1Drill-downinterfaceinSettings

Inthischapter,youwilluseaUINavigationControllertoaddadrill-downinterfacetoHomepwnerthatletstheuserseeandeditthedetailsofanItem.ThesedetailswillbepresentedbytheDetailViewControllerthatyoucreatedinChapter12(Figure13.2).


Figure13.2HomepwnerwithUINavigationController


UINavigationControllerAUINavigationControllermaintainsanarrayofviewcontrollerspresentingrelatedinformationinastack.WhenaUIViewControllerisontopofthestack,itsviewisvisible.

WhenyouinitializeaninstanceofUINavigationController,yougiveitaUIViewController.ThisUIViewControllerisaddedtothenavigationcontroller’sviewControllersarrayandbecomesthenavigationcontroller’srootviewcontroller.Therootviewcontrollerisalwaysonthebottomofthestack.(Notethatwhilethisviewcontrollerisreferredtoasthenavigationcontroller’s“rootviewcontroller,”UINavigationControllerdoesnothavearootViewControllerproperty.)

MoreviewcontrollerscanbepushedontopoftheUINavigationController’sstackwhiletheapplicationisrunning.TheseviewcontrollersareaddedtotheendoftheviewControllersarraythatcorrespondstothetopofthestack.UINavigationController’stopViewControllerpropertykeepsareferencetotheviewcontrolleratthetopofthestack.

Whenaviewcontrollerispushedontothestack,itsviewslidesontothescreenfromtheright.Whenthestackispopped(i.e.,thelastitemisremoved),thetopviewcontrollerisremovedfromthestackanditsviewslidesofftotheright,exposingtheviewofnextviewcontrolleronthestack,whichbecomesthetopviewcontroller.Figure13.3showsanavigationcontrollerwithtwoviewcontrollers.TheviewofthetopViewControlleriswhattheusersees.

Figure13.3UINavigationController’sstack


UINavigationControllerisasubclassofUIViewController,soithasaviewofitsown.Itsviewalwayshastwosubviews:aUINavigationBarandtheviewoftopViewController(Figure13.4).

Figure13.4AUINavigationController’sview

Inthischapter,youwilladdaUINavigationControllertotheHomepwnerapplicationandmaketheItemsViewControllertheUINavigationController’srootviewcontroller.TheDetailViewControllerwillbepushedontotheUINavigationController’sstackwhenanItemisselected.ThisviewcontrollerwillallowtheusertoviewandeditthepropertiesofanItemselectedfromthetableviewofItemsViewController.TheobjectdiagramfortheupdatedHomepwnerapplicationisshowninFigure13.5.


Figure13.5Homepwnerobjectdiagram

Thisapplicationisgettingfairlylarge,asyoucansee.Fortunately,viewcontrollersandUINavigationControllerknowhowtodealwiththistypeofcomplicatedobjectdiagram.WhenwritingiOSapplications,itisimportanttotreateachUIViewControllerasitsownlittleworld.ThestuffthathasalreadybeenimplementedinCocoaTouchwilldotheheavylifting.

BeginbygivingHomepwneranavigationcontroller.ReopentheHomepwnerproject.TheonlyrequirementsforusingaUINavigationControllerarethatyougiveitarootviewcontrollerandadditsviewtothewindow.

OpenMain.storyboardandselecttheItemsViewController.Then,fromtheEditormenu,chooseEmbedIn→NavigationController.ThiswillsettheItemsViewControllertobetherootviewcontrollerofaUINavigationController.ItwillalsoupdatethestoryboardtosettheNavigationControllerastheinitialviewcontroller.

SelecttheDetailViewController,andyouwillseethatitsinterfacehasmisplacedviewsnow.Thestackviewhasaconstrainttothetoplayoutguide,butwhenaviewcontrollerisembeddedwithinanavigationcontroller,thebottomofthetoplayoutguidecorrespondstothebottomofthenavigationbar(notthebottomofthestatusbar).WiththeDetailView


Controllerselected,opentheResolveAutoLayoutIssuespop-up.SelectUpdateFramesfromthesectionAllViewsinDetailViewControllerandthestackviewwillrepositionitself.

Buildandruntheapplicationand…theapplicationcrashes.Whatishappening?YoupreviouslycreatedacontractwiththeAppDelegatethataninstanceofItemsViewControllerwouldbetherootViewControllerofthewindow:letitemsController=window!.rootViewControlleras!ItemsViewController

YouhavenowbrokenthiscontractbyembeddingtheItemsViewControllerwithinaUINavigationController.Youneedtoupdatethecontract.

OpenAppDelegate.swiftandupdateapplication(_:didFinishLaunchingWithOptions:)toreflectthenewviewcontrollerhierarchy.funcapplication(application:UIApplication,didFinishLaunchingWithOptionslaunchOptions:[NSObject:AnyObject]?)->Bool{//Overridepointforcustomizationafterapplicationlaunch

letitemStore=ItemStore()

//AccesstheItemsViewControllerandsetitsitemstoreletitemsController=window!.rootViewControlleras!ItemsViewControllerletnavController=window!.rootViewControlleras!UINavigationControllerletitemsController=navController.topViewControlleras!ItemsViewControlleritemsController.itemStore=itemStore

returntrue}

Buildandruntheapplicationagain.Homepwnerworksagainandhasaverynice,iftotallyempty,UINavigationBaratthetopofthescreen(Figure13.6).NoticehowthescreenadjustedtofitItemsViewController’sviewaswellasthenewnavigationbar.UINavigationControllerdidthisforyou.

Figure13.6Homepwnerwithanemptynavigationbar


NavigatingwithUINavigationControllerWiththeapplicationstillrunning,createanewitemandselectthatrowfromtheUITableView.NotonlyareyoutakentoDetailViewController’sview,butyoualsogetafreeanimationandaBackbuttonintheUINavigationBar.TapthisbuttontogetbacktoItemsViewController.

NoticethatyoudidnothavetochangetheshowseguethatyoucreatedinChapter12togetthisbehavior.Asmentionedinthatchapter,theshowseguepresentsthedestinationviewcontrollerinawaythatmakessensegiventhesurroundingcontext.Whenashowsegueistriggeredfromaviewcontrollerembeddedwithinanavigationcontroller,thedestinationviewcontrollerispushedontothenavigationcontroller’sviewcontrollerstack.

SincetheUINavigationController’sstackisanarray,itwilltakeownershipofanyviewcontrolleraddedtoit.Thus,theDetailViewControllerisownedonlybytheUINavigationControlleraftertheseguefinishes.Whenthestackispopped,theDetailViewControllerisdestroyed.Thenexttimearowistapped,anewinstanceofDetailViewControlleriscreated.

Havingaviewcontrollerpushthenextviewcontrollerisacommonpattern.Therootviewcontrollertypicallycreatesthenextviewcontroller,andthenextviewcontrollercreatestheoneafterthat,andsoon.Someapplicationsmayhaveviewcontrollersthatcanpushdifferentviewcontrollersdependingonuserinput.Forexample,thePhotosapppushesavideoviewcontrolleroranimageviewcontrollerontothenavigationstackdependingonwhattypeofmediaisselected.

Buildandrunyourapplication.CreateanewitemandselectthatrowintheUITableView.TheviewthatappearswillcontaintheinformationfortheselectedItem.Whileyoucaneditthisdata,theUITableViewwillnotreflectthosechangeswhenyoureturntoit.Tofixthisproblem,youneedtoimplementcodetoupdatethepropertiesoftheItembeingedited.Inthenextsection,youwillseewhentodothis.


AppearingandDisappearingViewsWheneveraUINavigationControllerisabouttoswapviews,itcallstwomethods:viewWillDisappear(_:)andviewWillAppear(_:).TheUIViewControllerthatisabouttobepoppedoffthestackhasviewWillDisappear(_:)called.TheUIViewControllerthatwillthenbeontopofthestackhasviewWillAppear(_:)calledonit.

Toholdontochangesinthedata,whenaDetailViewControllerispoppedoffthestackyouwillsetthepropertiesofitsitemtothecontentsofthetextfields.Whenimplementingthesemethodsforviewsappearinganddisappearing,itisimportanttocallthesuperclass’simplementation–itmighthavesomeworktodoandneedstobegiventhechancetodoit.InDetailViewController.swift,implementviewWillDisappear(_:).overridefuncviewWillDisappear(animated:Bool){super.viewWillDisappear(animated)

//"Save"changestoitemitem.name=nameField.text??""item.serialNumber=serialNumberField.text

ifletvalueText=valueField.text,value=numberFormatter.numberFromString(valueText){item.valueInDollars=value.integerValue}else{item.valueInDollars=0}}

NowthevaluesoftheItemwillbeupdatedwhentheusertapstheBackbuttonontheUINavigationBar.WhenItemsViewControllerappearsbackonthescreen,themethodviewWillAppear(_:)iscalled.TakethisopportunitytoreloadtheUITableViewsotheusercanimmediatelyseethechanges.

InItemsViewController.swift,overrideviewWillAppear(_:)toreloadthetableview.overridefuncviewWillAppear(animated:Bool){super.viewWillAppear(animated)

tableView.reloadData()}

Buildandrunyourapplicationonceagain.Nowyoucanmovebackandforthbetweentheviewcontrollersthatyoucreatedandchangethedatawithease.


DismissingtheKeyboardRuntheapplication,addanitem,andtouchthetextfieldwiththeitem’sname.Whenyoutouchthetextfield,akeyboardappearsonscreen,asyousawinyourWorldTrotterappinChapter4(Figure13.7).(Ifyouareusingthesimulatorandthekeyboarddoesnotappear,rememberthatyoucanpressCommand-Ktotogglethedevicekeyboard.)

Figure13.7Keyboardappearswhenatextfieldistouched

TheappearanceofthekeyboardinresponsetoatouchisbuiltintotheUITextFieldclassaswellasUITextView,soyoudonothavetodoanythingextraforthekeyboardtoappear.However,attimesyouwillwanttomakesurethekeyboardbehavesasyouwantitto.

Forexample,thekeyboardcoversmorethanathirdofthescreen.Rightnow,itdoesnotobscureanything,butsoonyouwilladdmoredetailsthatextendtothebottomofthe


screen,anduserswillwantawaytohidethekeyboardwhenitisnotneeded.Inthissection,youaregoingtogivetheusertwowaystodismissthekeyboard:pressingthekeyboard’sReturnkeyandtappinganywhereelseonthedetailviewcontroller’sview.Butfirst,let’slookatthecombinationofeventsthatmaketexteditingpossible.

Eventhandlingbasics

Whenyoutouchaview,aneventiscreated.Thisevent(knownasa“touchevent”)istiedtoaspecificlocationintheviewcontroller’sview.Thatlocationdetermineswhichviewinthehierarchythetoucheventisdeliveredto.

Forexample,whenyoutapaUIButtonwithinitsbounds,itwillreceivethetoucheventandrespondinbutton-likefashion–bycallingtheactionmethodonitstarget.Itisperfectlyreasonabletoexpectthatwhenaviewinyourapplicationistouched,thatviewreceivesatouchevent,anditmaychoosetoreacttothateventorignoreit.However,viewsinyourapplicationcanalsorespondtoeventswithoutbeingtouched.Agoodexampleofthisisashake.Ifyoushakethedevicewithyourapplicationrunning,oneofyourviewsonthescreencanrespond.Butwhichone?Anotherinterestingcaseisrespondingtothekeyboard.DetailViewController’sviewcontainsthreeUITextFields.Whichonewillreceivethetextwhentheusertypes?

Forboththeshakeandkeyboardevents,thereisnoeventlocationwithinyourviewhierarchytodeterminewhichviewwillreceivetheevent,soanothermechanismmustbeused.Thismechanismisthefirstresponderstatus.Manyviewsandcontrolscanbeafirstresponderwithinyourviewhierarchy–butonlyoneatatime.Thinkofitasaflagthatcanbepassedamongviews.Whicheverviewholdstheflagwillreceivetheshakeorkeyboardevent.

InstancesofUITextFieldandUITextViewhaveanuncommonresponsetotouchevents.Whentouched,atextfieldoratextviewbecomesthefirstresponder,whichinturntriggersthesystemtoputthekeyboardonscreenandsendthekeyboardeventstothetextfieldorview.Thekeyboardandthetextfieldorviewhavenodirectconnection,buttheyworktogetherthroughthefirstresponderstatus.

Thisisaneatwaytoensurethatthekeyboardinputisdeliveredtothecorrecttextfield.TheconceptofafirstresponderispartofthebroadertopicofeventhandlinginCocoaTouchprogramming,andthisincludestheUIResponderclassandtheresponderchain.YouwilllearnmoreaboutthemwhenyouhandletoucheventsinChapter17,andyoucanalsovisitApple’sEventHandlingGuideforiOSformoreinformation.

DismissingbypressingtheReturnkey

Nowlet’sgetbacktoallowinguserstodismissthekeyboard.Ifyoutouchanothertextfieldintheapplication,thattextfieldwillbecomethefirstresponder,andthekeyboardwillstayonscreen.Thekeyboardwillonlygiveupandgoawaywhennotextfield(ortextview)isthefirstresponder.Todismissthekeyboardthen,youcallresignFirstResponder()onthetextfieldthatisthefirstresponder.


TohavethetextfieldresigninresponsetotheReturnkeybeingpressed,youaregoingtoimplementtheUITextFieldDelegatemethodtextFieldShouldReturn(_:).ThismethodiscalledwhenevertheReturnkeyispressed.

First,inDetailViewController.swift,haveDetailViewControllerconformtotheUITextFieldDelegateprotocol.classDetailViewController:UIViewController,UITextFieldDelegate{

Next,implementtextFieldShouldReturn(_:)tocallresignFirstResponder()onthetextfieldthatispassedin.functextFieldShouldReturn(textField:UITextField)->Bool{textField.resignFirstResponder()returntrue}

Finally,openMain.storyboardandconnectthedelegatepropertyofeachtextfieldtotheDetailViewController(Figure13.8).(Control-dragfromeachUITextFieldtotheDetailViewControllerandchoosedelegate.)

Figure13.8Connectingthedelegatepropertyofatextfield

Buildandruntheapplication.TapatextfieldandthenpresstheReturnkeyonthekeyboard.Thekeyboardwilldisappear.Togetthekeyboardback,touchanytextfield.

Dismissingbytappingelsewhere

ItwouldbestylishtoalsodismissthekeyboardiftheusertapsanywhereelseonDetailViewController’sview.Todothis,youaregoingtouseagesturerecognizerwhentheviewistapped,justasyoudidintheWorldTrotterapp.Intheactionmethod,youwillcallresignFirstResponder()onthetextfield.

OpenMain.storyboardandfindTapGestureRecognizerintheobjectlibrary.DragthisobjectontothebackgroundviewfortheDetailViewController.Youwillseeareferencetothisgesturerecognizerinthescenedock.


Intheprojectnavigator,Option-clickDetailViewController.swifttoopenitintheassistanteditor.Control-dragfromthetapgesturerecognizerinthestoryboardtotheimplementationofDetailViewController.

Inthepop-upthatappears,selectActionfromtheConnectionmenu.NametheactionbackgroundTapped.FortheType,chooseUITapGestureRecognizer(Figure13.9).

Figure13.9ConfiguringaUITapGestureRecognizeraction

ClickConnectandthestubfortheactionmethodwillappearinDetailViewController.swift.UpdatethemethodtocallendEditing(_:)ontheviewofDetailViewController.@IBActionfuncbackgroundTapped(sender:AnyObject){view.endEditing(true)}

CallingendEditing(_:)isaconvenientwaytodismissthekeyboardwithouthavingtoknow(orcare)whichtextfieldisthefirstresponder.Whentheviewgetsthiscall,itcheckstoseeifanytextfieldinitshierarchyisthefirstresponder.Ifso,thenresignFirstResponder()iscalledonthatparticularview.

Buildandrunyourapplication.Taponatextfieldtoshowthekeyboard.Tapontheviewoutsideofatextfieldandthekeyboardwilldisappear.

Thereisonefinalcasewhereyouneedtodismissthekeyboard.WhentheusertapstheBackbutton,viewWillDisappear(_:)iscalledontheDetailViewControllerbeforeitispoppedoffthestack.InDetailViewController.swift,updatetheimplementationofviewWillDisappear(_:)tocallendEditing(_:).overridefuncviewWillDisappear(animated:Bool){super.viewWillDisappear(animated)

//Clearfirstresponderview.endEditing(true)

//"Save"changestoitemitem.name=nameField.text??""item.serialNumber=serialNumberField.text

ifletvalueText=valueField.text,letvalue=numberFormatter.numberFromString(valueText){item.valueInDollars=value.integerValue}else{item.valueInDollars=0}}


UINavigationBarInthissection,youaregoingtogivetheUINavigationBaradescriptivetitlefortheUIViewControllerthatiscurrentlyontopoftheUINavigationController’sstack.

EveryUIViewControllerhasanavigationItempropertyoftypeUINavigationItem.However,unlikeUINavigationBar,UINavigationItemisnotasubclassofUIView,soitcannotappearonthescreen.Instead,thenavigationitemsuppliesthenavigationbarwiththecontentitneedstodraw.WhenaUIViewControllercomestothetopofaUINavigationController’sstack,theUINavigationBarusestheUIViewController’snavigationItemtoconfigureitself,asshowninFigure13.10.

Figure13.10UINavigationItem

Bydefault,aUINavigationItemisempty.Atthemostbasiclevel,aUINavigationItemhasasimpletitlestring.WhenaUIViewControllerismovedtothetopofthenavigationstackanditsnavigationItemhasavalidstringforitstitleproperty,thenavigationbarwilldisplaythatstring(Figure13.11).

Figure13.11UINavigationItemwithtitle

ThetitlefortheItemsViewControllerwillalwaysremainthesame,soyoucansetthetitleofitsnavigationitemwithinthestoryboarditself.


OpenMain.storyboard.Double-clickonthecenterofthenavigationbarabovetheItemsViewControllertoedititstitle.Giveitatitleof“Homepwner”(Figure13.12).

Figure13.12Settingthetitleinastoryboard

Buildandruntheapplication.NoticethestringHomepwneronthenavigationbar.Createandtaponarowandnoticethatthenavigationbarnolongerhasatitle.ItwouldbenicetohavetheDetailViewController’snavigationitemtitlebethenameoftheItemitisdisplaying.SincethetitlewilldependontheItemthatisbeingdisplayed,youneedtosetthetitleofthenavigationItemdynamicallyincode.

InDetailViewController.swift,addapropertyobservertotheitempropertythatupdatesthetitleofthenavigationItem.varitem:Item!{didSet{navigationItem.title=item.name}}

Buildandruntheapplication.CreateandtaparowandyouwillseethatthetitleofthenavigationbaristhenameoftheItemyouselected.

Anavigationitemcanholdmorethanjustatitlestring,asshowninFigure13.13.TherearethreecustomizableareasforeachUINavigationItem:aleftBarButtonItem,arightBarButtonItem,andatitleView.TheleftandrightbarbuttonitemsarereferencestoinstancesofUIBarButtonItem,whichcontaintheinformationforabuttonthatcanonlybedisplayedonaUINavigationBaroraUIToolbar.

Figure13.13UINavigationItemwitheverything

RecallthatUINavigationItemisnotasubclassofUIView.Instead,


UINavigationItemencapsulatesinformationthatUINavigationBarusestoconfigureitself.Similarly,UIBarButtonItemisnotaview,butholdstheinformationabouthowasinglebuttonontheUINavigationBarshouldbedisplayed.(AUIToolbaralsousesinstancesofUIBarButtonItemtoconfigureitself.)

ThethirdcustomizableareaofaUINavigationItemisitstitleView.YoucaneitheruseabasicstringasthetitleorhaveasubclassofUIViewsitinthecenterofthenavigationitem.Youcannothaveboth.Ifitsuitsthecontextofaspecificviewcontrollertohaveacustomview(likeasegmentedcontroloratextfield,forexample),youwouldsetthetitleViewofthenavigationitemtothatcustomview.Figure13.13showsanexamplefromthebuilt-inMapsapplicationofaUINavigationItemwithacustomviewasitstitleView.Typically,however,atitlestringissufficient.

Addingbuttonstothenavigationbar

Inthissection,youaregoingtoreplacethetwobuttonsthatareinthetable’sheaderviewwithtwobarbuttonitemsthatwillappearintheUINavigationBarwhentheItemsViewControllerisontopofthestack.

First,let’sworkonabarbuttonitemforaddingnewitems.ThisbuttonwillsitontherightsideofthenavigationbarwhentheItemsViewControllerisontopofthestack.Whentapped,itwilladdanewItem.

Abarbuttonitemhasatarget-actionpairthatworkslikeUIControl’starget-actionmechanism:whentapped,itsendstheactionmessagetothetarget.

InMain.storyboard,opentheobjectlibraryanddragaBarButtonItemtotherightsideofItemsViewController’snavigationbar.Selectthisbarbuttonitemandopenitsattributesinspector.ChangetheSystemItemtoAdd(Figure13.14).

Figure13.14Systembarbuttonitem

Control-dragfromthisbarbuttonitemtotheItemsViewControllerandselectaddNewItem:WOW! eBook

www.wowebook.org

(Figure13.15).

Figure13.15ConnectingtheaddNewItem:action

Buildandruntheapplication.Tapthe+buttonandanewrowwillappearinthetable.

Nowlet’sreplacetheEditbutton.Viewcontrollersexposeabarbuttonitemthatwillautomaticallytoggletheireditingmode.ThereisnowaytoaccessthisthroughInterfaceBuilder,soyouwillneedtoaddthisbarbuttonitemprogrammatically.

InItemsViewController.swift,overridetheinit(coder:)methodtosettheleftbarbuttonitem.requiredinit?(coderaDecoder:NSCoder){super.init(coder:aDecoder)

navigationItem.leftBarButtonItem=editButtonItem()}

Buildandruntheapplication,addsomeitems,andtaptheEditbutton.TheUITableViewenterseditingmode!TheeditButtonItem()methodcreatesaUIBarButtonItemwiththetitleEdit.Evenbetter,thisbuttoncomeswithatarget-actionpair:itcallsthemethodsetEditing(_:animated:)toitsUIViewControllerwhentapped.

OpenMain.storyboard.NowthatHomepwnerhasafullyfunctionalnavigationbar,youcangetridoftheheaderviewandtheassociatedcode.SelecttheheaderviewonthetableviewandpressDelete.

Also,theUINavigationControllerwillhandleupdatingtheinsetsforthetableview.InItemsViewController.swift,deletethefollowingcode.overridefuncviewDidLoad(){super.viewDidLoad()



tableView.rowHeight=UITableViewAutomaticDimension


tableView.estimatedRowHeight=65}

Finally,removethetoggleEditingMode(_:)method.@IBActionfunctoggleEditingMode(sender:AnyObject){ifediting{//Changetextofbuttontoinformuserofstatesender.setTitle("Edit",forState:.Normal)

//TurnoffeditingmodesetEditing(false,animated:true)}else{//Changetextofbuttontoinformuserofstatesender.setTitle("Done",forState:.Normal)

//EntereditingmodesetEditing(true,animated:true)}}

Buildandrunagain.TheoldEditandAddbuttonsaregone,leavingyouwithalovelyUINavigationBar(Figure13.16).

Figure13.16Homepwnerwithnavigationbar


BronzeChallenge:DisplayingaNumberPadThekeyboardfortheUITextFieldthatdisplaysaItem’svalueInDollarsisaQWERTYkeyboard.Itwouldbebetterifitwereanumberpad.ChangetheKeyboardTypeofthatUITextFieldtotheNumberPad.(Hint:youcandothisinthestoryboardfileusingtheattributesinspector.)


SilverChallenge:ACustomUITextFieldMakeasubclassofUITextFieldandoverridethebecomeFirstResponder()andresignFirstResponder()methods(inheritedfromUIResponder)sothatitsborderstylechangeswhenitisthefirstresponder.YoucanusetheborderStylepropertyofUITextFieldtoaccomplishthis.UseyoursubclassforthetextfieldsinDetailViewController.


GoldChallenge:PushingMoreViewControllersCurrently,instancesofItemcannothavetheirdateCreatedpropertychanged.ChangeItemsothattheycan,andthenaddabuttonunderneaththedateLabelinDetailViewControllerwiththetitleChangeDate.Whenthisbuttonistapped,pushanotherviewcontrollerinstanceontothenavigationstack.ThisviewcontrollershouldhaveaUIDatePickerinstancethatmodifiesthedateCreatedpropertyoftheselectedItem.


14Camera

Inthischapter,youaregoingtoaddphotostotheHomepwnerapplication.YouwillpresentaUIImagePickerControllersothattheusercantakeandsaveapictureofeachitem.TheimagewillthenbeassociatedwithanIteminstanceandviewableintheitem’sdetailview(Figure14.1).

Figure14.1Homepwnerwithcameraaddition

Imagestendtobeverylarge,soitisagoodideatostoreimagesseparatelyfromotherdata.Thus,youaregoingtocreateasecondstoreforimages.ImageStorewillfetchandcacheimagesastheyareneeded.


DisplayingImagesandUIImageViewYourfirststepistohavetheDetailViewControllergetanddisplayanimage.AneasywaytodisplayanimageistoputaninstanceofUIImageViewonthescreen.

OpenHomepwner.xcodeprojandMain.storyboard.ThendraganinstanceofUIImageViewontotheviewatthebottomofthestackview(Figure14.2).Selecttheimageviewandopenitssizeinspector.Youwanttheverticalcontenthuggingandcontentcompressionresistanceprioritiesfortheimageviewtobelowerthanthoseoftheotherviews.ChangetheVerticalContentHuggingPrioritytobe248andtheVerticalContentCompressionResistancePrioritytobe749.

Figure14.2UIImageViewonDetailViewController’sview

AUIImageViewdisplaysanimageaccordingtotheimageview’scontentModeproperty.Thispropertydetermineswheretopositionandhowtoresizethecontentwithintheimageview’sframe.UIImageView’sdefaultvalueforcontentModeisUIViewContentMode.ScaleToFill,whichadjuststheimagetoexactlymatchtheboundsoftheimageview.Ifyoukeepthedefault,animagetakenbythecamerawillbe


scaledtofitintothesquareUIImageView.Tomaintaintheimage’saspectratio,youhavetoupdatecontentMode.

WiththeUIImageViewselected,opentheattributesinspector.FindtheModeattributeandchangeittoAspectFit(Figure14.3).Youwillnotseeachangeonthestoryboard,butnowimageswillberesizedtofitwithintheboundsoftheUIImageView.

Figure14.3ChangeUIImageView’smodetoAspectFit

Next,Option-clickDetailViewController.swiftintheprojectnavigatortoopenitintheassistanteditor.Control-dragfromtheUIImageViewtothetopofDetailViewController.swift.NametheoutletimageViewandchooseStrongasthestoragetype.ClickConnect(Figure14.4).

Figure14.4CreatingtheimageViewoutlet

ThetopofDetailViewController.swiftshouldnowlooklikethis:classDetailViewController:UIViewController{

@IBOutletvarnameField:UITextField!@IBOutletvarserialNumberField:UITextField!@IBOutletvarvalueField:UITextField!@IBOutletvardateLabel:UILabel!@IBOutletvarimageView:UIImageView!

AddingacamerabuttonWOW! eBook

www.wowebook.org

Nowyouneedabuttontoinitiatethephoto-takingprocess.YouwillcreateaninstanceofUIToolbarandplaceitatthebottomofDetailViewController’sview.

InMain.storyboard,pressCommand-Returntoclosetheassistanteditorandgiveyourselfmoreroomtoworkinthestoryboard.Youaregoingtoneedtotemporarilybreakyourinterfaceinordertoaddthetoolbartotheinterface.

SelectthebottomconstraintforthestackviewandpressDeletetoremoveit.Youneedtomakeroomforthetoolbaronthebottom.AsofXcode7.1,itisdifficulttoresizethestackview.Soinstead,dragthestackviewupabit(Figure14.5).Theviewwillbemisplacedfornow,butyouwillfixthisshortly.

Figure14.5Movingthestackviewoutoftheway

Nowdragatoolbarfromtheobjectlibraryontothebottomoftheview.SelectthetoolbarandopentheAutoLayoutPinmenu.ConfiguretheconstraintsexactlyasshowninFigure14.6andthenclickAdd5Constraints.Sinceyouchosetheoptiontoupdateframes,thestackviewisrepositionedtoitscorrectlocation.


Figure14.6Toolbarconstraints

AUIToolbarworksalotlikeaUINavigationBar–youcanaddinstancesofUIBarButtonItemtoit.However,whereanavigationbarhastwoslotsforbarbuttonitems,atoolbarhasanarrayofbarbuttonitems.Youcanplaceasmanybarbuttonitemsinatoolbarascanfitonthescreen.

Bydefault,anewinstanceofUIToolbarthatiscreatedinaninterfacefilecomeswithoneUIBarButtonItem.Selectthisbarbuttonitemandopentheattributesinspector.ChangetheSystemItemtoCamera,andtheitemwillshowacameraicon(Figure14.7).


Figure14.7UIToolbarwithbarbuttonitem

Buildandruntheapplicationandnavigatetoanitem’sdetailstoseethetoolbarwithitscamerabarbuttonitem.Youhavenotconnectedthecamerabuttontoanactionyet,sotappingonitwillnotdoanything.

Thecamerabuttonneedsatargetandanaction.WithMain.storyboardstillopen,Option-clickDetailViewController.swiftintheprojectnavigatortoreopenitintheassistanteditor.

InMain.storyboard,selectthecamerabuttonbyfirstclickingonthetoolbarandthenthebuttonitself.Control-dragfromtheselectedbuttontoDetailViewController.swift.

IntheConnectionpop-upmenu,selectActionastheconnectiontype,nameittakePicture,selectUIBarButtonItemasthetype,andclickConnect(Figure14.8).

Figure14.8Creatinganaction


Ifyoumadeanymistakeswhilemakingthisconnection,youwillneedtoopenMain.storyboardanddisconnectanybadconnections.(Lookforyellowwarningsignsintheconnectionsinspector.)


TakingPicturesandUIImagePickerControllerInthetakePicture(_:)method,youwillinstantiateaUIImagePickerControllerandpresentitonthescreen.WhencreatinganinstanceofUIImagePickerController,youmustsetitssourceTypepropertyandassignitadelegate.Sincethereissetupworkneededfortheimagepickercontroller,youneedtocreateandpresentitprogrammaticallyinsteadofthroughthestoryboard.

Settingtheimagepicker’ssourceType

ThesourceTypeconstanttellstheimagepickerwheretogetimages.Ithasthreepossiblevalues(Figure14.9):

UIImagePickerControllerSourceType.Camera

Theuserwilltakeanewpicture.

UIImagePickerControllerSourceType.PhotoLibrary

Theuserwillbepromptedtoselectanalbumandthenaphotofromthatalbum.

UIImagePickerControllerSourceType.SavedPhotosAlbum

Theuserpicksfromthemostrecentlytakenphotos.

Figure14.9Examplesofthreesourcetypes

Thefirstsourcetype,.Camera,willnotworkonadevicethatdoesnothaveacamera.Sobeforeusingthistype,youhavetocheckforacamerabycallingthemethod


isSourceTypeAvailable(_:)ontheUIImagePickerControllerclass:classfuncisSourceTypeAvailable(type:UIImagePickerControllerSourceType)->Bool

CallingthismethodreturnsaBooleanvalueforwhetherthedevicesupportsthepassed-insourcetype.

InDetailViewController.swift,findthestubfortakePicture(_:).AddthefollowingcodetocreatetheimagepickerandsetitssourceType.@IBActionfunctakePicture(sender:UIBarButtonItem){

letimagePicker=UIImagePickerController()

//Ifthedevicehasacamera,takeapicture;otherwise,//justpickfromphotolibraryifUIImagePickerController.isSourceTypeAvailable(.Camera){imagePicker.sourceType=.Camera}else{imagePicker.sourceType=.PhotoLibrary}}

Settingtheimagepicker’sdelegate

Inadditiontoasourcetype,theUIImagePickerControllerinstanceneedsadelegate.WhentheuserselectsanimagefromtheUIImagePickerController’sinterface,thedelegateissentthemessageimagePickerController(_:didFinishPickingMediaWithInfo:).(Iftheusertapsthecancelbutton,thenthedelegatereceivesthemessageimagePickerControllerDidCancel(_:).)

Theimagepicker’sdelegatewillbetheinstanceofDetailViewController.AtthetopofDetailViewController.swift,declarethatDetailViewControllerconformstotheUINavigationControllerDelegateandtheUIImagePickerControllerDelegateprotocols.classDetailViewController:UIViewController,UITextFieldDelegate,UINavigationControllerDelegate,UIImagePickerControllerDelegate{

WhyUINavigationControllerDelegate?UIImagePickerController’sdelegatepropertyisactuallyinheritedfromitssuperclass,UINavigationController,andwhileUIImagePickerControllerhasitsowndelegateprotocol,itsinheriteddelegatepropertyisdeclaredtoreferenceanobjectthatconformstoUINavigationControllerDelegate.

InDetailViewController.swift,addthefollowingcodetotakePicture(_:)tosettheinstanceofDetailViewControllertobetheimagepicker’sdelegate.@IBActionfunctakePicture(sender:AnyObject){

letimagePicker=UIImagePickerController()

//Ifthedevicehasacamera,takeapicture;otherwise,//justpickfromphotolibraryifUIImagePickerController.isSourceTypeAvailable(.Camera){imagePicker.sourceType=.Camera


}else{imagePicker.sourceType=.PhotoLibrary}

imagePicker.delegate=self}

Presentingtheimagepickermodally

OncetheUIImagePickerControllerhasasourcetypeandadelegate,youcandisplayitbypresentingtheviewcontrollermodally.

InDetailViewController.swift,addcodetotheendoftakePicture(_:)topresenttheUIImagePickerController.imagePicker.delegate=self

//PlaceimagepickeronthescreenpresentViewController(imagePicker,animated:true,completion:nil)}

Buildandruntheapplication.SelectanItemtoseeitsdetailsandthentapthecamerabuttonontheUIToolbar.UIImagePickerController’sinterfacewillappearonthescreen(Figure14.10),andyoucantakeapictureorchooseanexistingimageifyourdevicedoesnothaveacamera.

(Ifyouareworkingonthesimulator,therearesomedefaultimagesalreadyinthephotolibrary.Ifyouwouldliketoaddyourown,youcandraganimagefromyourcomputerontothesimulator,anditwillbeaddedtothesimulator’sphotolibrary.Alternatively,youcanopenSafariinthesimulatorandnavigatetoapagewithanimage.ClickandholdtheimageandchooseSaveImagetosaveitinthesimulator’sphotolibrary.)


Figure14.10UIImagePickerController’spreviewinterface

Savingtheimage

SelectinganimagedismissestheUIImagePickerControllerandreturnsyoutothedetailview.However,youdonothaveareferencetothephotooncetheimagepickerisdismissed.Tofixthis,youaregoingtoimplementthedelegatemethodimagePickerController(_:didFinishPickingMediaWithInfo:).Thismethodiscalledontheimagepicker’sdelegatewhenaphotohasbeenselected.

InDetailViewController.swift,implementthismethodtoputtheimageintotheUIImageViewandthencallthemethodtodismisstheimagepicker.funcimagePickerController(picker:UIImagePickerController,didFinishPickingMediaWithInfoinfo:[String:AnyObject]){

//Getpickedimagefrominfodictionaryletimage=info[UIImagePickerControllerOriginalImage]as!UIImage


//PutthatimageonthescreenintheimageviewimageView.image=image

//Takeimagepickeroffthescreen-//youmustcallthisdismissmethoddismissViewControllerAnimated(true,completion:nil)}

Buildandruntheapplicationagain.Take(orselect)aphoto.Theimagepickerisdismissed,andyouarereturnedtotheDetailViewController’sview,whereyouwillseetheselectedphoto.

Homepwner’suserscouldhavehundredsofitemstocatalog,andeachonecouldhavealargeimageassociatedwithit.KeepinghundredsofinstancesofIteminmemoryisnotabigdeal.Keepinghundredsofimagesinmemorywouldbebad:First,youwillgetalowmemorywarning.Then,ifyourapp’smemoryfootprintcontinuestogrow,theOSwillterminateit.Thesolution,whichyouaregoingtoimplementinthenextsection,istostoreimagestodiskandonlyfetchthemintoRAMwhentheyareneeded.Thisfetchingwillbedonebyanewclass,ImageStore.Whentheapplicationreceivesalow-memorynotification,theImageStore’scachewillbeflushedtofreethememorythatthefetchedimageswereoccupying.


CreatingImageStoreInChapter15,youwillhaveinstancesofItemwriteouttheirpropertiestoafile,whichwillthenbereadinwhentheapplicationstarts.However,becauseimagestendtobeverylarge,itisagoodideatokeepthemseparatefromotherdata.YouaregoingtostorethepicturestheusertakesinaninstanceofaclassnamedImageStore.Theimagestorewillfetchandcachetheimagesastheyareneeded.Itwillalsobeabletoflushthecacheifthedevicerunslowonmemory.

CreateanewSwiftfilenamedImageStore.InImageStore.swift,definetheImageStoreclassandaddapropertythatisaninstanceofNSCache.importFoundationimportUIKit

classImageStore{

letcache=NSCache()

}

Thecacheworksverymuchlikeadictionary(whichyousawinChapter2).Youareabletoadd,remove,andupdatevaluesassociatedwithagivenkey.Unlikeadictionary,thecachewillautomaticallyremoveobjectsifthesystemgetslowonmemory.Whilethiscouldbeaprobleminthischapter(sinceimageswillonlyexistwithinthecache),youwillfixtheprobleminChapter15whenyouwillalsowritetheimagestothefilesystem.

Nowimplementthreemethodsforadding,retrieving,anddeletinganimagefromthedictionary.classImageStore:NSObject{

letcache=NSCache()

funcsetImage(image:UIImage,forKeykey:String){cache.setObject(image,forKey:key)}

funcimageForKey(key:String)->UIImage?{returncache.objectForKey(key)as?UIImage}

funcdeleteImageForKey(key:String){cache.removeObjectForKey(key)}

}


GivingViewControllersAccesstotheImageStoreTheDetailViewControllerneedsaninstanceofImageStoretofetchandstoreimages.YouwillinjectthisdependencyintotheDetailViewController’sdesignatedinitializer,justasyoudidforItemsViewControllerandItemStoreinChapter9.

InDetailViewController.swift,addapropertyforanImageStore.varitem:Item!{didSet{navigationItem.title=item.name}}varimageStore:ImageStore!

NowdothesameinItemsViewController.swift.letitemStore:ItemStorevarimageStore:ImageStore!

Next,stillinItemsViewController.swift,updateprepareForSegue(_:sender:)tosettheimageStorepropertyonDetailViewController.overridefuncprepareForSegue(segue:UIStoryboardSegue,sender:AnyObject?){ifsegue.identifier=="ShowItem"{ifletrow=tableView.indexPathForSelectedRow?.row{letitem=itemStore.allItems[row]letdetailViewController=segue.destinationViewControlleras!DetailViewControllerdetailViewController.item=itemdetailViewController.imageStore=imageStore}}}

Finally,updateAppDelegate.swifttocreateandinjecttheImageStore.funcapplication(application:UIApplication,didFinishLaunchingWithOptionslaunchOptions:[NSObject:AnyObject]?)->Bool{//Overridepointforcustomizationafterapplicationlaunch.


//CreateanImageStoreletimageStore=ImageStore()

//GettheItemsViewControllerletnavController=window!.rootViewControlleras!UINavigationControllerletitemsController=navController.topViewControlleras!ItemsViewController

//AccesstheItemsViewControllerandsetitsitemstoreandimagestoreitemsController.itemStore=itemStoreitemsController.imageStore=imageStore


CreatingandUsingKeysWhenanimageisaddedtothestore,itwillbeputintothecacheunderauniquekey,andtheassociatedItemobjectwillbegiventhatkey.WhentheDetailViewControllerwantsanimagefromthestore,itwillaskitsitemforthekeyandsearchthecachefortheimage.

AddapropertytoItem.swifttostorethekey.letdateCreated:NSDateletitemKey:String

Theimagekeysneedtobeuniqueforyourcachetowork.Whiletherearemanywaystohacktogetherauniquestring,youaregoingtousetheCocoaTouchmechanismforcreatinguniversallyuniqueidentifiers(UUIDs),alsoknownasgloballyuniqueidentifiers(GUIDs).ObjectsoftypeNSUUIDrepresentaUUIDandaregeneratedusingthetime,acounter,andahardwareidentifier,whichisusuallytheMACaddressoftheWiFicard.Whenrepresentedasastring,UUIDslooksomethinglikethis:4A73B5D2-A6F4-4B40-9F82-EA1E34C1DC04

InItem.swift,generateaUUIDandsetitastheitemKey.init(name:String,serialNumber:String?,valueInDollars:Int){

self.name=nameself.serialNumber=serialNumberself.valueInDollars=valueInDollarsself.dateCreated=NSDate()self.itemKey=NSUUID().UUIDString

super.init()}

Then,inDetailViewController.swift,updateimagePickerController(_:didFinishPickingMediaWithInfo:)tostoretheimageintheImageStore.funcimagePickerController(picker:UIImagePickerController,didFinishPickingMediaWithInfoinfo:[String:AnyObject]){

//Getpickedimagefrominfodictionaryletimage=info[UIImagePickerControllerOriginalImage]as!UIImage

//StoretheimageintheImageStorefortheitem'skeyimageStore.setImage(image,forKey:item.itemKey)

//PutthatimageonthescreenintheimageviewimageView.image=image

//Takeimagepickeroffthescreen-//youmustcallthisdismissmethoddismissViewControllerAnimated(true,completion:nil)}

Eachtimeanimageiscaptured,itwillbeaddedtothestore.BoththeImageStoreandtheItemwillknowthekeyfortheimage,sobothwillbeabletoaccessitasneeded(Figure14.11).


Figure14.11Accessingimagesfromthecache

Similarly,whenanitemisdeleted,youneedtodeleteitsimagefromtheimagestore.InItemsViewController.swift,updatetableView(_:commitEditingStyle:forRowAtIndexPath:)toremovetheitem’simagefromtheimagestore.overridefunctableView(tableView:UITableView,commitEditingStyleeditingStyle:UITableViewCellEditingStyle,forRowAtIndexPathindexPath:NSIndexPath){

//Ifthetableviewisaskingtocommitadeletecommand...ifeditingStyle==.Delete{letitem=itemStore.allItems[indexPath.row]

lettitle="Delete\(item.name)?"letmessage="Areyousureyouwanttodeletethisitem?"



//Removetheitem'simagefromtheimagestoreself.imageStore.deleteImageForKey(item.itemKey)


//PresentthealertcontrollerpresentViewController(ac,animated:true,completion:nil)


}}


WrappingUpImageStoreNowthattheImageStorecanstoreimagesandinstancesofItemhaveakeytogetanimage(Figure14.11),youneedtoteachDetailViewControllerhowtograbtheimagefortheselectedItemandplaceitinitsimageView.

TheDetailViewController’sviewwillappearwhentheusertapsarowinItemsViewControllerandwhentheUIImagePickerControllerisdismissed.Inbothofthesesituations,theimageViewshouldbepopulatedwiththeimageoftheItembeingdisplayed.

InDetailViewController.swift,addcodetoviewWillAppear(_:)todothis.overridefuncviewWillAppear(animated:Bool){super.viewWillAppear(animated)

nameField.text=item.nameserialNumberField.text=item.serialNumbervalueField.text=currencyFormatter.stringFromNumber(item.valueInDollars)dateLabel.text=dateFormatter.stringFromDate(item.dateCreated)

//Gettheitemkeyletkey=item.itemKey

//Ifthereisanassociatedimagewiththeitem//displayitontheimageviewletimageToDisplay=imageStore.imageForKey(key)imageView.image=imageToDisplay}

Buildandruntheapplication.CreateanItemandselectitfromtheUITableView.Then,tapthecamerabuttonandtakeapicture.Theimagewillappearasitshould.


BronzeChallenge:EditinganImageUIImagePickerControllerhasabuilt-ininterfaceforeditinganimageonceithasbeenselected.AllowtheusertoedittheimageandusetheeditedimageinsteadoftheoriginalimageinDetailViewController.


SilverChallenge:RemovinganImageAddabuttonthatclearstheimageforanitem.


GoldChallenge:CameraOverlayAUIImagePickerControllerhasacameraOverlayViewproperty.MakeitsothatpresentingtheUIImagePickerControllershowsacrosshairinthemiddleoftheimagecapturearea.


FortheMoreCurious:NavigatingImplementationFilesBothofyourviewcontrollershavequiteafewmethodsintheirimplementationfiles.TobeaneffectiveiOSdeveloper,youmustbeabletogotothecodeyouarelookingforquicklyandeasily.ThesourceeditorjumpbarinXcodeisonetoolatyourdisposal(Figure14.12).

Figure14.12Sourceeditorjumpbar

Thejumpbarshowsyouwhereexactlyyouarewithintheproject(andalsowherethecursoriswithinagivenfile).Figure14.13breaksdownthejumpbardetails.

Figure14.13Jumpbardetails

Thebreadcrumbtrailnavigationofthejumpbarmirrorstheprojectnavigationhierarchy.Ifyouclickonanyofthesections,youwillbepresentedwithapopoverofthatsectionintheprojecthierarchy.Fromthere,youcaneasilynavigatetootherpartsoftheproject.

Figure14.14showsthefilepopoverfortheHomepwnerfolder.

Figure14.14Filepopover


Perhapsmostusefulistheabilitytonavigateeasilywithinanimplementationfile.Ifyouclickonthelastelementinthebreadcrumbtrail,youwillgetapopoverwiththecontentsofthefile,includingallofthemethodsimplementedwithinthatfile.

Whilethepopoverisstillvisible,youcanstarttypingtofiltertheitemsinthelist.Atanypoint,youcanusetheupanddownarrowkeysandthenpresstheReturnkeytojumptothatmethodinthecode.Figure14.15showswhatyougetwhenyousearchfor“indexpath”inItemsViewController.swift.

Figure14.15Filepopoverwith“indexpath”search

//MARK:

Asyourclassesgetlonger,itcangetmoredifficulttofindamethodburiedinalonglistofmethods.Agoodwaytoorganizeyourmethodsistouse//MARK:comments.

Twouseful//MARK:commentsarethedividerandthelabel://Thisisadivider//MARK:-

//Thisisalabel//MARK:MyAwesomeMethods

Thedividerandlabelcanbecombined://MARK:-ViewlifecycleoverridefuncviewDidLoad(){...}overridefuncviewWillAppear(animated:Bool){...}

//MARK:-ActionsfuncaddNewItem(sender:AnyObject){...}

Adding//MARK:commentstoyourcodedoesnotchangethecodeitself;itjusttellsXcodehowtovisuallyorganizeyourmethods.Youcanseetheresultsbyopeningthecurrentfileiteminthejumpbar.Figure14.16presentsawell-organizedItemsViewController.swift.


Figure14.16Filepopoverwith//MARK:s

Ifyoumakeahabitofusing//MARK:comments,youwillforceyourselftoorganizeyourcode.Ifdonethoughtfully,thiswillmakeyourcodemorereadableandeasiertoworkwith.


15Saving,Loading,andApplication

StatesTherearemanywaystosaveandloaddatainaniOSapplication.ThischapterwilltakeyouthroughsomeofthemostcommonmechanismsaswellastheconceptsyouneedforwritingtoorreadingfromthefilesysteminiOS.Alongtheway,youwillbeupdatingHomepwnersothatitsdatapersistsbetweenruns(Figure15.1).

Figure15.1Homepwnerinthetaskswitcher


ArchivingMostiOSapplicationsare,atbase,doingthesamething:providinganinterfacefortheusertomanipulatedata.Everyobjectinanapplicationhasaroleinthisprocess.Modelobjectsareresponsibleforholdingontothedatathattheusermanipulates.Viewobjectsreflectthatdata,andcontrollersareresponsibleforkeepingtheviewsandthemodelobjectsinsync.Therefore,savingandloading“data”almostalwaysmeanssavingandloadingmodelobjects.

InHomepwner,themodelobjectsthatausermanipulatesareinstancesofItem.ForHomepwnertobeausefulapplication,instancesofItemmustpersistbetweenrunsoftheapplication.YouwillbeusingarchivingtosaveandloadItemobjects.

ArchivingisoneofthemostcommonwaysofpersistingmodelobjectsiniOS.Archivinganobjectinvolvesrecordingallofitspropertiesandsavingthemtothefilesystem.Unarchivingrecreatestheobjectfromthatdata.

ClasseswhoseinstancesneedtobearchivedandunarchivedmustconformtotheNSCodingprotocolandimplementitstworequiredmethods,encodeWithCoder(_:)andinit(coder:).protocolNSCoding{funcencodeWithCoder(aCoder:NSCoder)init?(coderaDecoder:NSCoder)}

Whenobjectsareaddedtoaninterfacefile,suchasastoryboardfile,theyarearchived.Atruntime,theobjectsareloadedintomemorybybeingunarchivedfromtheinterfacefile.UIViewandUIViewControllerbothconformtotheNSCodingprotocol,sobothcanbearchivedandunarchivedwithoutanyextraeffortfromyou.

YourItemclass,ontheotherhand,doesnotcurrentlyconformtoNSCoding.OpenHomepwner.xcodeprojandaddthisprotocoldeclarationinItem.swift.classItem:NSObject,NSCoding{

Thenextstepistoimplementtherequiredmethods.Let’sstartwithencodeWithCoder(_:).WhenanItemissentthemessageencodeWithCoder(_:),itwillencodeallofitspropertiesintotheNSCoderobjectthatispassedasanargument.Whilesaving,youwilluseNSCodertowriteoutastreamofdata.Thatstreamwillbestoredonthefilesystemandisorganizedaskey-valuepairs.

InItem.swift,implementencodeWithCoder(_:)toaddthenamesandvaluesoftheitem’spropertiestothestream.funcencodeWithCoder(aCoder:NSCoder){aCoder.encodeObject(name,forKey:"name")aCoder.encodeObject(dateCreated,forKey:"dateCreated")aCoder.encodeObject(itemKey,forKey:"itemKey")aCoder.encodeObject(serialNumber,forKey:"serialNumber")

aCoder.encodeInteger(valueInDollars,forKey:"valueInDollars")}

NoticethatreferencestoobjectsareencodedwithencodeObject(_:forKey:),but


valueInDollarsisencodedwithencodeInteger(_:forKey:).BecausevalueInDollarsisanIntandnotanobject,youcannotencodeitwithencodeObject(_:forKey:).

Why,then,canyouuseencodeObject(_:forKey:)forname,whichisaString?Stringisnotaclass,andthusinstancesofStringarenotobjects.WhenyoupassaStringtoamethodthatexpectsanobject,likeencodeObject(_:forKey:),SwiftconvertsittoanNSString–thestringclassfromtheFoundationframework–thankstothespecialrelationshipbetweenStringandNSString.

TofindoutwhichencodingmethodstouseforotherSwifttypes,youcancheckthedocumentationforNSCoder.Regardlessofthetypeoftheencodedvalue,thereisalwaysakey,whichisastringthatidentifieswhichpropertyisbeingencoded.Byconvention,thiskeyisthenameofthepropertybeingencoded.

Encodingisarecursiveprocess.Whenanobjectisencoded(thatis,whenitisthefirstargumentinencodeObject(_:forKey:)),thatobjectissentencodeWithCoder(_:).DuringtheexecutionofitsencodeWithCoder(_:)method,itencodesitsobject(orString)propertiesusingencodeObject(_:forKey:)(Figure15.2).Thus,eachobjectencodesanyobjectsthatitreferences,whichencodeanyobjectsthattheyreference,andsoon.

Figure15.2Encodinganobject

Tobeencoded,theseobjectsmustalsoconformtoNSCoding.Let’sconfirmthisforNSDate.Theprotocolsthatatypeconformstoarelistedinitsreference.Insteadofopeningupthedocumentationbrowserasyouhavedonebefore,youcantakeashortcuttogettothereferencedirectlyfromyourcode.

InItem.swift,holddowntheOptionkey,mouseoveranoccurrenceofNSDateinyourcode,andclick.Apop-upwindowwillappearwithabriefdescriptionofthetypeand


linkstoitsreference,asshowninFigure15.3.

Figure15.3Option-clickingNSDate

Atthetop,noticetheDeclarationlinewheretheclassdeclarationforNSDateisgiven.HereyoucanseethatNSDatedoesconformtotheNSCodingprotocol.

YoucanalsoclicktheReferencelinkatthebottomtobetakentotheNSDatereference.Atthetopofthereferenceyouwillseealistofprotocolsthattheclassconformsto.

(YouarenotdoingthesamecheckforStringbecauseStringdoesnotconformtoNSCoding.RecallthatwhenyoupassaStringtoencodeObject(_:forKey:),thestringisconvertedtoanNSString,andNSStringdoesconformtoNSCoding.)

Option-clickingisnotjustforclassesandtypes.Youcanusethesameshortcutformethods,protocols,andmore.Keepthisinmindasyourunacrossitemsinyourcodethatyouwanttoknowmoreabout.

Nowbacktokeysandencoding.ThepurposeofthekeyistoretrievetheencodedvaluewhenthisItemisloadedfromthefilesystemlater.Objectsbeingloadedfromanarchivearesentthemessageinit(coder:).ThismethodshouldgraballoftheobjectsthatwereencodedinencodeWithCoder(_:)andassignthemtotheappropriateproperty.

InItem.swift,implementinit(coder:).requiredinit(coderaDecoder:NSCoder){name=aDecoder.decodeObjectForKey("name")as!StringdateCreated=aDecoder.decodeObjectForKey("dateCreated")as!NSDateitemKey=aDecoder.decodeObjectForKey("itemKey")as!StringserialNumber=aDecoder.decodeObjectForKey("serialNumber")as!String?

valueInDollars=aDecoder.decodeIntegerForKey("valueInDollars")

super.init()}

NoticethatthismethodhasanNSCoderargument,too.Ininit(coder:),theNSCoderisfullofdatatobeconsumedbytheItembeinginitialized.AlsonoticethatyoucalldecodeObjectForKey:tothecontainertogetobjects(includinginstancesofStringasNSStringobjects)anddecodeIntegerForKey(_:)togetthe


valueInDollars.

InChapter9,wetalkedabouttheinitializerchainanddesignatedinitializers.Theinit(coder:)methodisnotpartofthisdesignpattern.YouwillkeepItem’sdesignatedinitializerthesame,andinit(coder:)willnotcallit.

InstancesofItemarenowNSCoding-compliantandcanbesavedtoandloadedfromthefilesystemusingarchiving.Youcanbuildtheapplicationtomakesuretherearenosyntaxerrors,butyoustillneedawaytokickoffthesavingandloading.Youalsoneedaplaceonthefilesystemtostorethesaveditems.


ApplicationSandboxEveryiOSapplicationhasitsownapplicationsandbox.Anapplicationsandboxisadirectoryonthefilesystemthatisbarricadedfromtherestofthefilesystem.Yourapplicationmuststayinitssandbox,andnootherapplicationcanaccessyoursandbox.Figure15.4showstheapplicationsandboxforiTunes/iCloud.

Figure15.4Applicationsandbox

Theapplicationsandboxcontainsanumberofdirectories:

Documents/

Thisdirectoryiswhereyouwritedatathattheapplicationgeneratesduringruntimeandthatyouwanttopersistbetweenrunsoftheapplication.ItisbackedupwhenthedeviceissynchronizedwithiTunesoriCloud.Ifsomethinggoeswrongwiththedevice,filesinthisdirectorycanberestoredfromiTunesoriCloud.InHomepwner,thefilethatholdsthedataforallyouritemswillbestoredhere.

Library/Caches/

Thisdirectoryiswhereyouwritedatathattheapplicationgeneratesduringruntimeandthatyouwanttopersistbetweenrunsoftheapplication.However,unliketheDocumentsdirectory,itdoesnotgetbackedupwhenthedeviceissynchronizedwithiTunesoriCloud.Amajorreasonfornotbackingupcacheddataisthatthedatacanbeverylargeandextendthetimeittakestosynchronizeyourdevice.Datastoredsomewhereelse–likeawebserver–canbeplacedinthisdirectory.Iftheuserneedstorestorethedevice,thisdatacanbedownloadedfromthewebserveragain.Ifthedeviceisverylowondiskspace,thesystemmaydeletethecontentsofthisdirectory.

Library/Preferences/

ThisdirectoryiswhereanypreferencesarestoredandwheretheSettingsapplicationlooksforapplicationpreferences.Library/PreferencesishandledautomaticallybytheclassNSUserDefaultsandisbackedupwhen


thedeviceissynchronizedwithiTunesoriCloud.

tmp/

Thisdirectoryiswhereyouwritedatathatyouwillusetemporarilyduringanapplication’sruntime.TheOSmaypurgefilesinthisdirectorywhenyourapplicationisnotrunning.However,tobetidyyoushouldstillexplicitlyremovefilesfromthisdirectorywhenyounolongerneedthem.ThisdirectorydoesnotgetbackedupwhenthedeviceissynchronizedwithiTunesoriCloud.

ConstructingafileURL

TheinstancesofItemfromHomepwnerwillbesavedtoasinglefileintheDocumentsdirectory.TheItemStorewillhandlewritingtoandreadingfromthatfile.Todothis,theItemStoreneedstoconstructaURLtothisfile.

ImplementanewpropertyinItemStore.swifttostorethisURL.varallItems=[Item]()letitemArchiveURL:NSURL={letdocumentsDirectories=NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory,inDomains:.UserDomainMask)letdocumentDirectory=documentsDirectories.first!returndocumentDirectory.URLByAppendingPathComponent("items.archive")}()

Insteadofassigningavaluetothepropertydirectly,thevalueisbeingsetusingaclosure.YoumayrecallthatyoudidthiswiththenumberFormatterpropertyinChapter4.Noticethattheclosureherehasasignatureof()->NSURL,meaningitdoesnottakeinanyargumentsanditreturnsaninstanceofNSURL.WhentheItemStoreclassisinstantiated,thisclosurewillberunandthereturnvaluewillbeassignedtotheitemArchiveURLproperty.Usingaclosurelikethisallowsyoutosetthevalueforavariableorconstantthatrequiresmultiplelinesofcode,whichcanbeveryusefulwhenconfiguringobjects.Thismakesyourcodemoremaintainablebecauseitkeepsthepropertyandthecodeneededtogeneratethepropertytogether.

ThemethodURLsForDirectory(_:inDomains:)searchesthefilesystemforaURLthatmeetsthecriteriagivenbythearguments.(Double-checkthatyourfirstargumentis.DocumentDirectoryandnot.DocumentationDirectory.Autocomplete’sfirstsuggestionis.DocumentationDirectory,soitiseasytointroducethiserrorandendupwiththewrongURL.)

IniOS,thelastargumentisalwaysthesame.(ThismethodisborrowedfromOSX,wheretherearesignificantlymoreoptions.)ThefirstargumentisanNSSearchPathDirectoryenumerationthatspecifiesthedirectoryinthesandboxyouwanttheURLto.Forexample,searchingfor.CachesDirectorywillreturntheCachesdirectoryintheapplication’ssandbox.

YoucansearchthedocumentationforNSSearchPathDirectorytolocatetheotheroptions.RememberthattheseenumerationvaluesaresharedbyiOSandOSX,sonotallofthemwillworkoniOS.


ThereturnvalueofURLsForDirectory(_:inDomains:)isanarrayofURLs.Itisanarraybecause,inOSX,theremaybemultipleURLsthatmeetthesearchcriteria.IniOS,however,therewillonlybeone(ifthedirectoryyousearchedforisanappropriatesandboxdirectory).Therefore,thenameofthearchivefileisappendedtothefirstandonlyURLinthearray.ThiswillbewherethearchiveofIteminstanceswilllive.


NSKeyedArchiverandNSKeyedUnarchiverYounowhaveaplacetosavedataonthefilesystemandamodelobjectthatcanbesavedtothefilesystem.Thefinaltwoquestionsare:howdoyoukickoffthesavingandloadingprocesses,andwhendoyoudoit?TosaveinstancesofItem,youwillusetheclassNSKeyedArchiverwhentheapplication“exits.”

InItemStore.swift,implementanewmethodthatcallsarchiveRootObject(_:toFile:)ontheNSKeyedArchiverclass.funcsaveChanges()->Bool{print("Savingitemsto:\(itemArchiveURL.path!)")returnNSKeyedArchiver.archiveRootObject(allItems,toFile:itemArchiveURL.path!)}

ThearchiveRootObject(_:toFile:)methodtakescareofsavingeverysingleIteminallItemstotheitemArchiveURL.Yes,itisthatsimple.HereishowarchiverRootObject(_:toFile:)works:

ThemethodbeginsbycreatinganinstanceofNSKeyedArchiver.(NSKeyedArchiverisaconcretesubclassoftheabstractclassNSCoder.)

ThemethodencodeWithCoder:iscalledonallItemsandispassedtheinstanceofNSKeyedArchiverasanargument.

TheallItemsarraythencallsencodeWithCoder(_:)toalloftheobjectsitcontains,passingthesameNSKeyedArchiver.Thus,allyourinstancesofItemencodetheirinstancevariablesintotheverysameNSKeyedArchiver(Figure15.5).

TheNSKeyedArchiverwritesthedataitcollectedtothepath.

Figure15.5ArchivingtheallItemsarray


WhentheuserpressestheHomebuttononthedevice,themessageapplicationDidEnterBackground(_:)issenttotheAppDelegate.ThatiswhenyouwanttosendsaveChangestotheItemStore.

OpenAppDelegate.swiftandaddapropertytotheclasstostoretheItemStoreinstance.YouwillneedapropertyinordertoreferencetheinstanceinapplicationDidEnterBackground(_:).classAppDelegate:UIResponder,UIApplicationDelegate{

varwindow:UIWindow?letitemStore=ItemStore()

Thenupdateapplication(_:didFinishLaunchingWithOptions:)tousethispropertyinsteadofthelocalconstant.funcapplication(application:UIApplication,didFinishLaunchingWithOptionslaunchOptions:[NSObject:AnyObject]?)->Bool{//Overridepointforcustomizationafterapplicationlaunch.


//CreateanImageStoreletimageStore=ImageStore()

//GettheItemsViewControllerletnavController=window!.rootViewControlleras!UINavigationControllerletitemsController=navController.topViewControlleras!ItemsViewController

//SettheitemstoreaswellastheimagestoreitemsController.itemStore=itemStoreitemsController.imageStore=imageStore

returntrue}

Sincethepropertyandthelocalconstantwerenamedthesame,youonlyneededtoremovethecodethatcreatedthelocalconstant.

InAppDelegate.swift,implementapplicationDidEnterBackground(_:)tokickoffsavingtheIteminstances.(Thismethodmayhavealreadybeenimplementedbythetemplate.Ifso,makesuretoaddcodetotheexistingmethodinsteadofwritingabrandnewone.)funcapplicationDidEnterBackground(application:UIApplication){letsuccess=itemStore.saveChanges()if(success){print("SavedalloftheItems")}else{print("CouldnotsaveanyoftheItems")}}

Buildandruntheapplicationonthesimulator.CreateafewinstancesofItem,thenpresstheHomebuttontoleavetheapplication.Checktheconsoleandyoushouldseealogstatementindicatingthattheitemsweresaved.

WhileyoucannotyetloadtheseinstancesofItembackintotheapplication,youcanstillverifythatsomethingwassaved.

Intheconsole’slogstatements,findonethatlogsouttheitemArchiveURLlocationandanotherthatindicateswhethersavingwassuccessful.Ifsavingwasnotsuccessful,


confirmthatyouritemArchiveURLisbeingcreatedcorrectly.Iftheitemsweresavedsuccessfully,copythepaththatisprintedtotheconsole.

OpenFinderandpressCommand-Shift-G.PastethefilepaththatyoucopiedfromtheconsoleandpressReturn.Youwillbetakentothedirectorythatcontainstheitems.archivefile.PressCommand-Uptonavigatetotheparentdirectoryofitems.archive.Thisistheapplication’ssandboxdirectory.Here,youcanseetheDocuments,Library,andtmpdirectoriesalongsidetheapplicationitself(Figure15.6).

Figure15.6Homepwner’ssandbox

Thelocationofthesandboxdirectorycanchangebetweenrunsoftheapplication;however,thecontentsofthatsandboxwillremainunchanged.Duetothis,youmayneedtocopyandpastethedirectoryintoFinderfrequentlywhileworkingonanapplication.

Loadingfiles

Nowlet’sturntoloadingthesefiles.ToloadinstancesofItemwhentheapplicationlaunches,youwillusetheclassNSKeyedUnarchiverwhentheItemStoreiscreated.

InItemStore.swift,overrideinit()toaddthefollowingcode.init(){ifletarchivedItems=NSKeyedUnarchiver.unarchiveObjectWithFile(itemArchiveURL.path!)as?[Item]{allItems+=archivedItems}}

TheunarchiveObjectWithFile(_:)methodwillcreateaninstanceofNSKeyedUnarchiverandloadthearchivelocatedattheitemArchiveURLintothatinstance.TheNSKeyedUnarchiverwilltheninspectthetypeoftherootobjectinthearchiveandcreateaninstanceofthattype.Inthiscase,thetypewillbeanarrayofItemsbecauseyoucreatedthisarchivewitharootobjectoftype[Item].(IftherootobjectwasaninstanceofIteminstead,thenunarchiveObjectWithFile(_:)wouldreturnanItem.)

Thenewlycreatedarrayisthensentinit(coder:)and,asyoumayhaveguessed,theNSKeyedUnarchiverispassedastheargument.Thearraystartsdecodingitscontents(instancesofItem)fromtheNSKeyedUnarchiverandsendseachoftheseobjectsthemessageinit(coder:),passingthesameNSKeyedUnarchiver.

Buildandruntheapplication.Youritemswillbeavailableuntilyouexplicitlydelete


them.Onethingtonoteabouttestingyoursavingandloadingcode:ifyoukillHomepwnerfromXcode,themethodapplicationDidEnterBackground(_:)willnotgetachancetobecalledandtheitemarraywillnotbesaved.YoumustpresstheHomebuttonfirstandthenkillitfromXcodebyclickingtheStopbutton.


ApplicationStatesandTransitionsInHomepwner,theitemsarearchivedwhentheapplicationentersthebackgroundstate.Itisusefultounderstandthestatesanapplicationcanbein,whatcausesapplicationstotransitionbetweenstates,andhowyourcodecanbenotifiedofthesetransitions.ThisinformationissummarizedinFigure15.7.

Figure15.7Statesofatypicalapplication

Whenanapplicationisnotrunning,itisinthenotrunningstateanditdoesnotexecuteanycodeorhaveanymemoryreservedinRAM.

Aftertheuserlaunchesanapplication,itenterstheactivestate.Whenintheactivestate,anapplication’sinterfaceisonthescreen,itisacceptingevents,anditscodeishandlingthoseevents.

Whileintheactivestate,anapplicationcanbetemporarilyinterruptedbyasystemeventlikeanSMSmessage,pushnotification,phonecall,oralarm.Anoverlaywillappearontopofyourapplicationtohandlethisevent,andtheapplicationenterstheinactivestate.Intheinactivestate,anapplicationisvisiblebehindtheoverlayandisexecutingcode,butitisnotreceivingevents.Applicationstypicallyspendverylittletimeintheinactivestate.YoucanforceanactiveapplicationintotheinactivestatebypressingtheLockbuttonatthetopofthedevice.Theapplicationwillstayinactiveuntilthedeviceisunlocked.

WhentheuserpressestheHomebuttonorswitchestoanotherapplicationinsomeother


way,theapplicationentersthebackgroundstate.(Actually,itspendsabriefmomentintheinactivestatebeforetransitioningtothebackgroundstate.)Inthebackgroundstate,anapplication’sinterfaceisnotvisibleorreceivingevents,butitcanstillexecutecode.Bydefault,anapplicationthatentersthebackgroundstatehasabout10secondsbeforeitentersthesuspendedstate.Yourapplicationshouldnotrelyonthisnumber;instead,itshouldsaveuserdataandreleaseanysharedresourcesasquicklyaspossible.

Anapplicationinthesuspendedstatecannotexecutecode.Youcannotseeitsinterface,andanyresourcesitdoesnotneedwhilesuspendedaredestroyed.Asuspendedapplicationisessentiallyflash-frozenandcanbequicklythawedwhentheuserrelaunchesit.Table15.1summarizesthecharacteristicsofthedifferentapplicationstates.

Table15.1Applicationstates

State Visible ReceivesEvents ExecutesCode

NotRunning No No No

Active Yes Yes Yes

Inactive Mostly No Yes

Background No No Yes

Suspended No No No

Youcanseewhatapplicationsareinthebackgroundorsuspendedbydouble-tappingtheHomebuttontogettothetaskswitcher(Figure15.8).YoucandothisinthesimulatorbypressingCommand-Shift-Htwice.(Recentlyrunapplicationsthathavebeenterminatedmayalsoappearinthisdisplay.)


Figure15.8Backgroundandsuspendedapplicationsinthetaskswitcher

Anapplicationinthesuspendedstatewillremaininthatstateaslongasthereisadequatesystemmemory.WhentheOSdecidesmemoryisgettinglow,itterminatessuspendedapplicationsasneeded.Asuspendedapplicationgetsnoindicationthatitisabouttobeterminated.Itissimplyremovedfrommemory.(Anapplicationmayremaininthetaskswitcherafterithasbeenterminated,butitwillhavetorelaunchwhentapped.)

Whenanapplicationchangesitsstate,amethodiscalledontheapplicationdelegate.HerearesomeofthemethodsfromtheUIApplicationDelegateprotocolthatannounceapplicationstatetransitions.(ThesearealsoshowninFigure15.7.)optionalfuncapplication(application:UIApplication,didFinishLaunchingWithOptionslaunchOptions:[NSObject:AnyObject]?)->BooloptionalfuncapplicationDidBecomeActive(application:UIApplication)optionalfuncapplicationWillResignActive(application:UIApplication)optionalfuncapplicationDidEnterBackground(application:UIApplication)optionalfuncapplicationWillEnterForeground(application:UIApplication)

Youcanimplementthesemethodstotaketheappropriateactionsforyourapplication.WOW! eBook

www.wowebook.org

Transitioningtothebackgroundstateisagoodplacetosaveanyoutstandingchangesbecauseitisthelasttimeyourapplicationcanexecutecodebeforeitentersthesuspendedstate.Onceinthesuspendedstate,anapplicationcanbeterminatedatthewhimoftheOS.


WritingtotheFilesystemwithNSDataYourarchivinginHomepwnersavesandloadstheitemKeyforeachItem,butwhatabouttheimages?Atthemoment,theyarelostwhentheappentersthebackgroundstate.Inthissection,youwillextendtheimagestoretosaveimagesastheyareaddedandfetchthemastheyareneeded.

TheimagesforIteminstancesshouldalsobestoredintheDocumentsdirectory.Youcanusetheimagekeygeneratedwhentheusertakesapicturetonametheimageinthefilesystem.

ImplementanewmethodinImageStore.swiftnamedimageURLForKey(_:)tocreateaURLinthedocumentsdirectoryusingagivenkey.funcimageURLForKey(key:String)->NSURL{

letdocumentsDirectories=NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory,inDomains:.UserDomainMask)letdocumentDirectory=documentsDirectories.first!

returndocumentDirectory.URLByAppendingPathComponent(key)}

Tosaveandloadanimage,youaregoingtocopytheJPEGrepresentationoftheimageintoabufferinmemory.Insteadofjustcreatingabuffer,Swiftprogrammershaveahandyclasstocreate,maintain,anddestroythesesortsofbuffers–NSData.AnNSDatainstanceholdssomenumberofbytesofbinarydata,andyouwilluseNSDatatostoreimagedata.

InImageStore.swift,modifysetImage(_:forKey:)togetaURLandsavetheimage.funcsetImage(image:UIImage,forKeykey:String){cache.setObject(image,forKey:key)

//CreatefullURLforimageletimageURL=imageURLForKey(key)

//TurnimageintoJPEGdataifletdata=UIImageJPEGRepresentation(image,0.5){//WriteittofullURLdata.writeToURL(imageURL,atomically:true)}}

Let’sexaminethiscodemoreclosely.ThefunctionUIImageJPEGRepresentationtakestwoparameters:aUIImageandacompressionquality.ThecompressionqualityisaFloatfrom0to1,where1isthehighestquality(leastcompression).ThefunctionreturnsaninstanceofNSDataifthecompressionsucceedsandnilifitdoesnot.

ThisNSDatainstancecanbewrittentothefilesystembycallingwriteToURL(_:atomically:).ThebytesheldinthisNSDataarethenwrittentotheURLspecifiedbythefirstparameter.Thesecondparameter,atomically,isaBooleanvalue.Ifitistrue,thefileiswrittentoatemporaryplaceonthefilesystem,and,oncethewritingoperationiscomplete,thatfileisrenamedtotheURLofthefirstparameter,replacinganypreviouslyexistingfile.Writingatomicallypreventsdata


corruptionshouldyourapplicationcrashduringthewriteprocedure.

Itisworthnotingthatthiswayofwritingdatatothefilesystemisnotarchiving.WhileNSDatainstancescanbearchived,usingthemethodwriteToURL(_:atomically:)copiesthebytesintheNSDatadirectlytothefilesystem.

Nowthattheimageisstoredinthefilesystem,theImageStorewillneedtoloadthatimagewhenitisrequested.TheclassmethodimageWithContentsOfFile(_:)ofUIImagewillreadinanimagefromafile,givenaURL.

InImageStore.swift,updatethemethodimageForKey(_:)sothattheImageStorewillloadtheimagefromthefilesystemifitdoesnotalreadyhaveit.funcimageForKey(key:String)->UIImage?{

returncache.objectForKey(key)as?UIImage

ifletexistingImage=cache.objectForKey(key)as?UIImage{returnexistingImage}

letimageURL=imageURLForKey(key)guardletimageFromDisk=UIImage(contentsOfFile:imageURL.path!)else{returnnil}

cache.setObject(imageFromDisk,forKey:key)returnimageFromDisk}

Whatisthatguardstatement?guardisaconditionalstatement,likeanifstatement.Thecompilerwillonlycontinuepasttheguardstatementiftheconditionwithintheguardistrue.Here,theconditioniswhethertheUIImageinitializationissuccessful.Iftheinitializationfails,theelseblockisexecuted,whichallowsyoutohaveanearlyreturn.Iftheinitializationsucceeds,anyvariablesorconstantsboundintheguardstatement(here,imageFromDisk)areusableaftertheguardstatement.

Thecodeaboveisfunctionallyequivalenttothefollowingcode:ifletimageFromDisk=UIImage(contentsOfFile:imageURL.path!){cache.setObject(imageFromDisk,forKey:key)returnimageFromDisk}

returnnil

Whileyoucoulddothis,guardprovidesbothacleaner–and,moreimportantly,asafer–waytoensurethatyouexitifdonothavewhatyouneed.

Usingguardalsoforcesthefailurecasetobedirectlytiedtotheconditionbeingchecked.Thismakesthecodemorereadableandeasiertoreasonabout.

Youareabletosaveanimagetodiskandretrieveanimagefromdisk,sothelastthingyouneedtodoisaddfunctionalitytoremoveanimagefromdisk.

InImageStore.swift,makesurethatwhenanimageisdeletedfromthestore,itisalsodeletedfromthefilesystem.(Youwillseeanerrorwhenyoutypeinthiscode,whichwewilldiscussnext.)funcdeleteImageForKey(key:String){


cache.removeObjectForKey(key)

letimageURL=imageURLForKey(key)NSFileManager.defaultManager().removeItemAtURL(imageURL)}

Let’stakealookattheerrormessagethatthiscodegenerated,showninFigure15.9.

Figure15.9Errorwhenremovingtheimagefromdisk

ThiserrormessageislettingyouknowthatthemethodremoveItemAtURL(_:)canfail,butyouarenothandlingtheerror.Let’sfixthis.


ErrorHandlingItisoftenusefultohaveawayofrepresentingthepossibilityoffailurewhencreatingmethods.Youhaveseenonewayofrepresentingfailurethroughoutthisbookwiththeuseofoptionals.Optionalsprovideasimplewaytorepresentfailurewhenyoudonotcareaboutthereasonforfailure.ConsiderthecreationofanIntfromaString.lettheMeaningOfLife="42"letnumberFromString=Int(theMeaningOfLife)

ThisinitializeronInttakesaStringparameterandreturnsanoptionalInt(anInt?).ThisisbecausethestringmaynotbeabletoberepresentedasanInt.ThecodeabovewillsuccessfullycreateanInt,butthefollowingcodewillnot:letpi="ApplePie"letnumberFromString=Int(pi)

Thestring“ApplePie”cannotberepresentedasanInt,sonumberFromStringwillcontainnil.Anoptionalworkswellforrepresentingfailureherebecauseyoudonotcarewhyitfailed.Youjustwanttoknowwhetheritwassuccessful.

Whenyouneedtoknowwhysomethingfailed,anoptionalwillnotprovideenoughinformation.Thinkaboutremovingtheimagefromthefilesystem–whymightitfail?PerhapsthereisnofileatthespecifiedURL,ortheURLismalformed,oryoudonothavepermissiontoremovethatfile.Thereareanumberofreasonsthismethodcouldfail,andyoumightwanttohandleeachcasedifferently.

Swiftprovidesaricherrorhandlingsystemwithcompilersupporttoensurethatyourecognizewhensomethingbadcouldhappen.YoualreadysawthiswhentheSwiftcompilertoldyouthatyouwerenothandlingapossibleerrorwhenremovingthefilefromdisk.

Ifamethodcouldgenerateanerror,itsmethodsignatureneedstoindicatethisusingthethrowskeyword.HereisthemethoddefinitionforremoveItemAtURL(_:):funcremoveItemAtURL(URL:NSURL)throws

Thethrowskeywordindicatesthatthismethodcouldthrowanerror.(Ifyouarefamiliarwiththrowingexceptioninotherlanguages,Swift’serrorhandlingisnotthesameasthrowingexception.)Byusingthiskeyword,thecompilerensuresthatanyonewhousesthismethodknowsthatthismethodcanthrowanerror–and,moreimportantly,thatthecalleralsohandlesanypotentialerrors.Thisishowthecompilerwasabletoletyouknowthatyouarenothandlingerrorswhenattemptingtoremoveafilefromdisk.

Tocallamethodthatcanthrow,youuseado-catchstatement.Withinthedoblock,youannotateanymethodsthatmightthrowanerrorusingthetrykeywordtoreinforcetheideathatthecallmightfail.

InImageStore.swift,updatedeleteImageForKey(_:)tocallremoveItemAtURL(_:)usingado-catchstatement.funcdeleteImageForKey(key:String){cache.removeObjectForKey(key)

letimageURL=imageURLForKey(key)


NSFileManager.defaultManager().removeItemAtURL(imageURL)do{tryNSFileManager.defaultManager().removeItemAtURL(imageURL)}catch{

}}

Ifamethoddoesthrowanerror,thentheprogramimmediatelyexitsthedoblock;nofurthercodeinthedoblockisexecuted.Atthatpoint,theerrorispassedtothecatchblockforittobehandledinsomeway.

UpdatedeleteImageForKey(_:)toprintouttheerrortotheconsole.funcdeleteImageForKey(key:String){cache.removeObjectForKey(key)

letimageURL=imageURLForKey(key)do{tryNSFileManager.defaultManager().removeItemAtURL(imageURL)}catch{print("Errorremovingtheimagefromdisk:\(error)")}}

Withinthecatchblock,thereisanimpliciterrorconstantthatcontainsinformationdescribingtheerror.Youcanoptionallygivethisconstantanexplicitname.

UpdatedeleteImageForKey(_:)touseanexplicitnamefortheerrorbeingcaught.funcdeleteImageForKey(key:String){cache.removeObjectForKey(key)

letimageURL=imageURLForKey(key)do{tryNSFileManager.defaultManager().removeItemAtURL(imageURL)}catch{print("Errorremovingtheimagefromdisk:\(error)")}catchletdeleteError{print("Errorremovingtheimagefromdisk:\(deleteError)")}}

Thereisalotmorethatyoucandowitherrorhandling,butthisisthebasicknowledgethatyouneedfornow.Wewillcovermoredetailsasyouprogressthroughthisbook.

BuildandruntheapplicationnowthattheImageStoreiscomplete.TakeaphotoforanitemandexittheapplicationtotheHomescreen(onthesimulator,selectHardware→HomeorpressShift-Command-H;onahardwaredevicesimplypresstheHomebutton).Launchtheapplicationagain.Selectingthatsameitemwillshowallitssaveddetails–includingthephotoyoujusttook.

Noticethattheimagesaresavedimmediatelyafterbeingtaken,whiletheinstancesofItemaresavedonlywhentheapplicationentersthebackground.Yousavetheimagesrightawaybecausetheyarejusttoobigtokeepinmemoryforlong.


BronzeChallenge:PNGInsteadofsavingeachimageasaJPEG,saveitasaPNG.


FortheMoreCurious:ApplicationStateTransitionsLet’swritesomequickcodetogetabetterunderstandingofthedifferentapplicationstatetransitions.

InAppDelegate.swift,implementtheapplicationstatetransitiondelegatemethodssothattheyprintoutthenameofthemethod.Youwillneedtoaddfourmoremethods.(Checktomakesurethetemplatehasnotalreadycreatedthesemethodsbeforewritingbrandnewones.)Ratherthanhardcodingthenameofthemethodintheprint,usethe__FUNCTION__expression.Atcompiletime,the__FUNCTION__expressionwillevaluatetoaStringrepresentingthenameofthemethod.funcapplicationWillResignActive(application:UIApplication){print(__FUNCTION__)}

funcapplicationDidEnterBackground(application:UIApplication){print(__FUNCTION__)

letsuccess=itemStore.saveChanges()if(success){print("SavedalloftheItems")}else{print("CouldnotsaveanyoftheItems")}}

funcapplicationWillEnterForeground(application:UIApplication){print(__FUNCTION__)}

funcapplicationDidBecomeActive(application:UIApplication){print(__FUNCTION__)}

funcapplicationWillTerminate(application:UIApplication){print(__FUNCTION__)}

Now,addthesameprint()statementtothetopofapplication(_:didFinishLaunchingWithOptions:).funcapplication(application:UIApplication,didFinishLaunchingWithOptionslaunchOptions:[NSObject:AnyObject]?)->Bool{print(__FUNCTION__)...}

Buildandruntheapplication.Youwillseethattheapplicationgetssentapplication(_:didFinishLaunchingWithOptions:)andthenapplicationDidBecomeActive(_:).Playaroundtoseewhatactionscausewhattransitions.

PresstheHomebuttonandtheconsolewillreportthattheapplicationbrieflyinactivatedandthenwentintothebackgroundstate.RelaunchtheapplicationbytappingitsiconontheHomescreenorinthetaskswitcher.Theconsolewillreportthattheapplicationenteredtheforegroundandthenbecameactive.

PresstheHomebuttontoexittheapplicationagain.Then,double-presstheHomebuttontoopenthetaskswitcher.SwipetheHomepwnerapplicationupandoffthisdisplaytoquit


theapplication.Notethatnomethodiscalledonyourapplicationdelegateatthispoint–itissimplyterminated.


FortheMoreCurious:ReadingandWritingtotheFilesystemInadditiontoarchivingandNSData’sbinaryreadandwritemethods,thereareafewmoremethodsfortransferringdatatoandfromthefilesystem.Oneofthem,CoreData,iscomingupinChapter21.Acoupleothersareworthmentioninghere.

UsingNSDataworkswellforbinarydata.Fortextdata,Stringhastwoinstancemethods:writeToURL(_:atomically:encoding:)andinit(contentsOfURL:encoding:).Theyareusedasfollows://SavesomeStringtothefilesystemdo{trysomeString.writeToURL(someURL,atomically:true,encoding:NSUTF8StringEncoding)}catch{print("ErrorwritingtoURL:\(error)")}

//LoadsomeStringfromthefilesystemdo{letmyEssay=tryString(contentsOfURL:someURL,encoding:NSUTF8StringEncoding)print(myEssay)}catch{print("ErrorreadingfromURL:\(error)")}

Notethatinmanylanguages,anythingunexpectedresultsinanexceptionbeingthrown.InSwift,exceptionsarenearlyalwaysusedtoindicateprogrammererror.Whenanexceptionisthrown,theinformationaboutwhatwentwrongisinanNSExceptionobject.Thatinformationisusuallyjustahinttotheprogrammer,like,“Youtriedtoaccessthe7thobjectinthisarray,butthereareonlytwo.”Thesymbolsforthecallstack(asitappearedwhentheexceptionwasthrown)arealsointheNSException.

Whendoyouuseexceptions,andwhendoyouuseerrorhandling?Ifyouarewritingamethodthatshouldonlybecalledwithanoddnumberasanargument,throwanexceptionifitiscalledwithanevennumber–thecallerismakinganerrorandyouwanttohelpthatprogrammerfindtheerror.Ifyouarewritingamethodthatwantstoreadthecontentsofaparticulardirectorybutdoesnothavethenecessaryprivileges,useSwift’serrorhandlingandthrowanerrortothecallertoindicatewhyyouwereunabletofulfillthisveryreasonablerequest.

Propertylistserializabletypescanalsobewrittentothefilesystem.TheonlytypesthatarepropertylistserializableareString,NSNumber(includingprimitiveslikeInt,Double,andBool),NSDate,NSData,Array<T>,andDictionary<K,V>.WhenanArray<T>orDictionary<K,V>iswrittentothefilesystemwiththesemethods,anXMLpropertylistiscreated.AnXMLpropertylistisacollectionoftaggedvalues:<?xmlversion="1.0"encoding="UTF-8"?><!DOCTYPEplistPUBLIC"-//Apple//DTDPLIST1.0//EN""http://www.apple.com/DTDs/PropertyList-1.0.dtd"><plistversion="1.0"><array><dict>


<key>firstName</key><string>Christian</string><key>lastName</key><string>Keur</string></dict><dict><key>firstName</key><string>Aaron</string><key>lastName</key><string>Hillegass</string></dict></array></plist>

XMLpropertylistsareaconvenientwaytostoredatabecausetheycanbereadonnearlyanysystem.Manywebserviceapplicationsusepropertylistsasinputandoutput.Thecodeforwritingandreadingapropertylistlookslikethis:letauthors:AnyObject=[["firstName":"Christian","lastName":"Keur"],["firstName":"Aaron","lastName":"Hillegass"]]

//WritearraytodiskifNSPropertyListSerialization.propertyList(authors,isValidForFormat:.XMLFormat_v1_0){do{letdata=tryNSPropertyListSerialization.dataWithPropertyList(authors,format:.XMLFormat_v1_0,options:0)data.writeToURL(url,atomically:true)}catch{print("Errorwritingplist:\(error)")}}

//Readarrayfromdiskdo{letdata=tryNSData(contentsOfURL:url,options:[])letauthors=tryNSPropertyListSerialization.propertyListWithData(data,options:[],format:nil)print("Readinauthors:\(authors)")}catch{print("Errorreadingplist:\(error)")}


FortheMoreCurious:TheApplicationBundleWhenyoubuildaniOSapplicationprojectinXcode,youcreateanapplicationbundle.Theapplicationbundlecontainstheapplicationexecutableandanyresourcesyouhavebundledwithyourapplication.Resourcesarethingslikestoryboardfiles,images,andaudiofiles–anyfilesthatwillbeusedatruntime.Whenyouaddaresourcefiletoaproject,Xcodeissmartenoughtorealizethatitshouldbebundledwithyourapplication.

Howcanyoutellwhichfilesarebeingbundledwithyourapplication?SelecttheHomepwnerprojectfromtheprojectnavigator.CheckouttheBuildPhasespaneintheHomepwnertarget.EverythingunderCopyBundleResourceswillbeaddedtotheapplicationbundlewhenitisbuilt.

EachitemintheHomepwnertargetgroupisoneofthephasesthatoccurswhenyoubuildaproject.TheCopyBundleResourcesphaseiswherealloftheresourcesinyourprojectgetcopiedintotheapplicationbundle.

Youcancheckoutwhatanapplicationbundlelookslikeonthefilesystemafteryouinstallanapplicationonthesimulator.Printtheapplicationbundlepathtotheconsoleandthennavigatetothatdirectory.print(NSBundle.mainBundle().bundlePath)

Control-clicktheapplicationbundleandchooseShowPackageContentsfromthecontextualmenu(Figure15.10).

Figure15.10Viewinganapplicationbundle

AFinderwindowwillappearshowingyouthecontentsoftheapplicationbundle


(Figure15.11).WhenusersdownloadyourapplicationfromtheAppStore,thesefilesarecopiedtotheirdevices.

Figure15.11Theapplicationbundle

Youcanloadfilesfromtheapplication’sbundleatruntime.TogetthefullURLforfilesintheapplicationbundle,youneedtogetareferencetotheapplicationbundleandthenaskitfortheURLofaresource.//GetareferencetotheapplicationbundleletapplicationBundle=NSBundle.mainBundle()

//AskfortheURLtoaresourcenamedmyImage.pnginthebundleifleturl=applicationBundle.URLForResource("myImage",ofType:"png"){//DosomethingwithURL}

IfyouaskfortheURLtoafilethatisnotintheapplication’sbundle,thismethodwillreturnnil.Ifthefiledoesexist,thenthefullURLisreturned,andyoucanusethisURLtoloadthefilewiththeappropriateclass.

Bearinmindthatfileswithintheapplicationbundleareread-only.Youcannotmodifythem,norcanyoudynamicallyaddfilestotheapplicationbundleatruntime.Filesintheapplicationbundlearetypicallythingslikebuttonimages,interfacesoundeffects,ortheinitialstateofadatabaseyoushipwithyourapplication.Youwillusethismethodinlaterchapterstoloadthesetypesofresourcesatruntime.


16SizeClasses

Often,youwantanapplication’sinterfacetohaveadifferentlayoutdependingonthedimensionsandorientationofthescreen.Inthischapter,youwillmodifytheinterfaceforDetailViewControllerinHomepwnersothatwhenitappearsonascreenthathasarelativelysmallheight,thesetoftextfieldsandtheimageviewwillbesidebysideinsteadofstackedontopofoneanother(Figure16.1).

Figure16.1TwopossiblelayoutsforHomepwner’sDetailViewController

Therelativesizesofscreensaredefinedinsizeclasses.Asizeclassrepresentsarelativeamountofscreenspaceinagivendimension.Eachdimension(widthandheight)caneitherbecompactorregular,sotherearefourpossiblecombinationsofsizeclasses:

CompactWidth|CompactHeight

iPhoneswith3.5-,4-,or4.7-inchscreensinlandscapeorientationCompactWidth|RegularHeight

iPhonesofallsizesinportraitorientationRegularWidth|CompactHeight

iPhoneswith5.5-inchscreensinlandscapeorientation


RegularWidth|RegularHeight

iPadsofallsizesinallorientations

Noticethatthesizeclassescoverbothscreensizesandorientations.Insteadofthinkingaboutinterfacesintermsoforientationordevice,itisbettertothinkintermsofsizeclasses.

Ifyouwantalayouttocovermultiplesizeclasses,thenyoucanusethewildcardAny.AnyWidth|AnyHeightisthedefaultandthemostgeneric.WhenyoueditintheAnyWidth|AnyHeightconfiguration,youareprovidinganinterfacethatiscommonacrossallsizeclasses.ForDetailViewController,youwantalayoutspecifictoalliPhonesinlandscapeorientation,soyouaregoingtodefineitforAnyWidth|CompactHeight.

OpenHomepwner.xcodeprojandMain.storyboard.Selectthe tabtoopenthefileinspector(Command-Option-1)andconfirmthatthecheckboxforUseSizeClassesischecked(Figure16.2).

Figure16.2Sizeclassesenabled


AnotherSizeClassWheneditingtheinterfaceforaspecificsizeclasscombination,youareabletochange:

propertiesformanyviews

whetheraspecificsubviewisinstalled

whetheraspecificconstraintisinstalled

theconstantofaconstraint

thefontforsubviewsthatdisplaytext

InHomepwner,youaregoingtofocusonthefirstiteminthatlist–adjustingviewpropertiesdependingonthecurrentsizeclassconfiguration.Thegoalistohavetheimageviewbeontherightsideofthelabelsandtextfieldsinacompactheightenvironment.Inaregularheightenvironment,theimageviewwillbebelowthelabelsandtextfields(asitcurrentlyis).Stackviewswillmakethisremarkablyeasy.

Tobegin,youaregoingtoembedtheexistingverticalstackviewwithinanotherstackview.Thiswillmakeiteasytoaddanimageviewtotherightsideofthelabelsandtextfields.Selecttheverticalstackviewandclickthe icontoembedthisstackviewwithinanotherstackview.Withthisnewstackviewselected,opentheAutoLayoutPinmenuandconfigureitasseeninFigure16.3.

Figure16.3Stackviewconstraints

Next,openthenewstackview’sattributesinspector.IncreasetheSpacingtobe8.

Nowyouaregoingtomovetheimageviewfromtheinnerstackviewtotheouterstack


viewthatyoujustcreated.Thisishowyouwillbeabletohavetheimageviewontherightsideoftherestoftheinterface:inacompactheightenvironment,thestackviewwillsettobehorizontalandtheimageviewwilltakeuptherightsideoftheinterface.

Movingtheimageviewfromonestackviewtotheothercanbealittletricky,soyouaregoingtodoitinafewsteps.

OpenthedocumentoutlineandexpandthesectionfortheDetailViewControllerScene.ExpandtheoutertwostackviewsasseeninFigure16.4.

Figure16.4Expandingthedocumentoutline

DragtheImageViewrightabovethestackviewthatitiscurrentlycontainedwithin(Figure16.5).Thiswillmoveitfromtheinnerstackviewtotheouterstackview.


Figure16.5Movingtheimageviewtotheouterstackview

Finally,collapsetheinnerstackviewanddragtheImageViewtobebelowitinthestack(Figure16.6).MakesuretheImageViewisindentedatthesamelevelastheinnerstackview.Youmayneedtoupdateframesatthispointtogetridofanywarnings.

Figure16.6Movingtheimageviewbelowtheinnerstackview

Buildandruntheapplication.Confirmthatthebehaviorofthestackviewisunchanged.

Atthispoint,youhaveupdatedeverythingthatiscommontoallsizeclasses.Nextyouwillmodifyspecificsizeclassestochangethelayoutofthecontent.

AtthebottomofInterfaceBuilder,clickonwAnyhAny.Thepop-upthatappearsallowsyoutoselectasizeclass.Theinterfacecanbeconfusing.Thetrickisthatthebottom-rightsquarecoveredbytheblueareatellsyouthesizeclass.Currently,theblueareastretchesrightanddowntodeadcenter,andthelabelatthetopreadsAny(Figure16.7).


Figure16.7Choosingasizeclasstoedit

Dragtoresizetheblueareatoseetheothersizeclasses.FindandselectCompactHeight(thetop-centerboxinthegrid).Noticethattheviewcontrollerisshorter(Figure16.8).Additionally,thebottomofInterfaceBuilderhasabluebartoremindyouthatyouarenolongereditingthelayoutforallsizeclasses.


Figure16.8DetailViewControllerintheAny|Compactenvironment

Whatyouwillnowdoisupdatethepropertiesfortheouterstackviewfortheimageviewtobeontherightsideoftherestoftheinterface.

Selecttheouterstackviewandopenitsattributesinspector.UndertheStackViewheading,findtheAxispropertyandclickthe+buttononitsleftside.Fromthepop-upmenu,chooseAnyWidth,thenCompactHeight(Figure16.9).Thiswillallowyoutocustomizethispropertyforjustthecurrentsizeclassconfiguration.

Figure16.9Addingasize-class-specificoption

Forthenewoption(wAnyhC),chooseHorizontal(Figure16.10).Now,whenevertheinterfacehasacompactheight,theouterstackviewwillhaveahorizontalconfiguration.Whentheinterfacehasaregularheight,theouterstackviewwillhaveaverticalconfiguration.


Figure16.10Customizingtheaxis

Thelastchangeyouwanttomakeisfortheinnerstackviewandtheimageviewtofilltheouterstackviewequally.Todothis,youwillcustomizetheouterstackview’sdistribution.

Withtheattributesinspectorstillopenfortheouterstackview,clickonthe+nexttoDistributionandselectAnyWidth|CompactHeight(current)fromthepop-upmenu.ChangethedistributionforthissizeclasstobeFillEqually(Figure16.11).

Figure16.11Customizingthedistribution

BuildandruntheapplicationoniPhone.Createanitemanddrilldowntoitsdetailstoaddaphoto.Rotatebetweenportraitandlandscapeandnoticehowtheinterfaceislaidoutasyouspecifiedforbothregularandcompactheight.

Withthat,yourHomepwnerapplicationiscomplete.Youhavebuiltanappwithaflexibleinterfacethatcantakephotosandstoredata,andwehopeyouareproudofyouraccomplishment!Takesometimetocelebrate.


BronzeChallenge:StackedTextFieldandLabelsInacompactheightenvironment,makeitsothetextfieldsandlabelsarestackedverticallyinsteadofhorizontally(Figure16.12).

Figure16.12Textfieldsandlabelsstacked


17TouchEventsandUIResponder

Inthenexttwochapters,youwillcreateTouchTracker,anappthatletstheuserdrawbytouchingthescreen.Inthischapter,youwillcreateaviewthatdrawslinesinresponsetotheuserdraggingacrossit(Figure17.1).Usingmultitouch,theuserwillbeabletodrawmorethanonelineatatime.

Figure17.1TouchTracker


TouchEventsAsasubclassofUIResponder,aUIViewcanoverridefourmethodstohandlethefourdistincttouchevents:

afingerorfingerstouchesthescreenfunctouchesBegan(touches:Set<UITouch>,withEventevent:UIEvent?)

afingerorfingersmovesacrossthescreen(thismessageissentrepeatedlyasafingermoves)functouchesMoved(touches:Set<UITouch>,withEventevent:UIEvent?)

afingerorfingersisremovedfromthescreenfunctouchesEnded(touches:Set<UITouch>,withEventevent:UIEvent?)

asystemevent,likeanincomingphonecall,interruptsatouchbeforeitendsfunctouchesCancelled(touches:Set<UITouch>?,withEventevent:UIEvent?)

Let’swalkthroughthetypicallifecycleofatouch.Whentheuser’sfingertouchesthescreen,aninstanceofUITouchiscreated.ThetouchesBegan(_:withEvent:)methodiscalledontheUIViewthatthefingertouched,andtheUITouchispassedinthroughtheSetoftouches.

Asthefingermovesaroundthescreen,thetouchobjectisupdatedtocontainthecurrentlocationofthefingeronthescreen.Then,thesameUIViewthatthetouchbeganonissentthemessagetouchesMoved(_:withEvent:).TheSetthatispassedasanargumenttothismethodcontainsthesameUITouchthatoriginallywascreatedwhenthefingeritrepresentstouchedthescreen.

Whenthefingerisremovedfromthescreen,thetouchobjectisupdatedonelasttimetocontainthecurrentlocationofthefingerandtheviewthatthetouchbeganonissentthemessagetouchesEnded(_:withEvent:).Afterthatmethodfinishesexecuting,theUITouchobjectisdestroyed.

Fromthisinformation,youcandrawafewconclusionsabouthowtouchobjectswork:

OneUITouchcorrespondstoonefingeronthescreen.Thistouchobjectlivesaslongasthefingerisonthescreenandalwayscontainsthecurrentpositionofthefingeronthescreen.

Theviewthatthefingerstartedonwillreceiveeverytoucheventmessageforthatfinger.EvenifthefingermovesbeyondtheframeoftheUIViewthatthetouchbeganon,thetouchesMoved(_:withEvent:)andtouchesEnded(_:withEvent:)methodswillstillbecalledonthatview.Thus,ifatouchbeginsonaview,thenthatviewownsthetouchforthelifeofthetouch.

Youdonothaveto–norshouldyouever–keepareferencetoaUITouchobject.Theapplicationwillgiveyouaccesstoatouchobjectviathe


UIRespondermethodscalledatthedistinctpointsinthetouch’slifecycle.

Everytimeatouchdoessomething–likebegins,moves,orends–atoucheventisaddedtoaqueueofeventsthattheUIApplicationobjectmanages.Inpractice,thequeuerarelyfillsup,andeventsaredeliveredimmediately.ThedeliveryofthesetoucheventsinvolvessendingoneoftheUIRespondermessagestotheviewthatownsthetouch.

Whataboutmultipletouches?Ifmultiplefingersdothesamethingattheexactsametimetothesameview,allofthesetoucheventsaredeliveredatonce.Eachtouchobject–oneforeachfinger–isincludedintheSetpassedasanargumentintheUIRespondermessages.However,thewindowofopportunityforthe“exactsametime”isfairlyshort.So,insteadofonerespondermessagewithallofthetouches,thereareusuallymultiplerespondermessageswithoneormoreofthetouches.


CreatingtheTouchTrackerApplicationNowlet’sgetstartedwithyourapplication.InXcode,createanewSingleViewUniversalprojectandnameitTouchTracker(Figure17.2).

Figure17.2CreatingTouchTracker

InbuildingTouchTracker,youaregoingtousethedefaultviewcontrollerandthestoryboardthatthetemplatecreated.Foritsviewandmodellayers,youaregoingtocreateacustomviewclassandacustomstructure.Figure17.3showsthemajorpiecesofTouchTracker.


Figure17.3ObjectdiagramforTouchTracker

Let’sbeginwithyourcustomstruct.


CreatingtheLineStructYouaregoingtocreatethecustomLinetype.Sofar,allofthetypesthatyouhavecreatedhavebeenclasses.Infact,theyhavebeenCocoaTouchsubclasses;forexample,youhavecreatedsubclassesofNSObject,UIViewController,andUIView.

Linewillbeastruct.Youhaveusedstructsthroughoutthisbook–CGRect,CGSize,andCGPointareallstructs.SotooareString,Int,Array,andDictionary.Nowyouaregoingtocreateoneofyourown.

CreateanewSwiftfilenamedLine.

InLine.swift,importCoreGraphicsanddeclaretheLinestruct.DeclaretwoCGPointpropertiesthatwilldeterminethebeginningandendingpointfortheline.importFoundationimportCoreGraphics

structLine{

varbegin=CGPoint.zerovarend=CGPoint.zero}

Structs

Structsdifferfromclassesinanumberofways:

Structsdonotsupportinheritance.

Structsgetamember-wiseinitializerifnootherinitializersaredeclared.Themember-wiseinitializertakesinanargumentforeachpropertywithinthetype.TheLinestruct,forexample,hasthemember-wiseinitializerinit(begin:CGPoint,end:CGPoint).

Ifallpropertieshavedefaultvaluesandnootherinitializersaredeclared,structsalsogainanemptyinitializer(init())thatcreatesaninstanceandsetsallofthepropertiestotheirdefaultvalue.

Perhapsmostimportantly,structs(andenums)arevaluetypes–asopposedtoclasses,whicharereferencetypes.

Valuetypesvs.referencetypes

Valuetypesaretypeswhosevaluesarecopiedwhentheyareassignedtoanotherinstanceorpassedintheargumentofafunction.Thismeansthatassigninganinstanceofavaluetypetoanotheractuallyassignsacopyofthefirstinstancetothesecondinstance.ValuetypesplayanimportantroleinSwift.Forexample,arraysanddictionariesarebothvaluetypes.Allenumsandstructsyouwritearevaluetypesaswell.

Referencetypesarenotcopiedwhentheyareassignedtoaninstanceorpassedintoanargumentofafunction.Instead,areferencetothesameinstanceispassed.Classesand


closuresarereferencetypes.

Sowhichdoyouchoose?Ingeneral,wesuggeststartingoutwithavaluetypeunlessyouabsolutelyknowyouneedthebenefitsofareferencetype.Valuetypesareeasiertoreasonaboutbecauseyoudonotneedtoworryaboutwhathappenstoaninstancewhenyouchangevaluesonacopy.Ifyouwouldlikeadeeperdiscussiononthistopic,youshouldcheckoutSwiftProgramming:TheBigNerdRanchGuide.


CreatingDrawViewInadditiontoacustomstruct,TouchTrackerneedsacustomview.

CreateanewSwiftfilenamedDrawView.InDrawView.swift,definetheDrawViewclass.Addtwoproperties:anoptionalLinetokeeptrackofalinethatispossiblybeingdrawnandanarrayofLinestokeeptrackoflinesthathavebeendrawn.importFoundationimportUIKit

classDrawView:UIView{

varcurrentLine:Line?varfinishedLines=[Line]()

}

AninstanceofDrawViewwillbetheviewoftheapplication’srootViewController,thedefaultViewControllerincludedintheproject.TheviewcontrollerneedstoknowthatitsviewwillbeaninstanceofDrawView.

OpenMain.storyboard.SelecttheViewandopentheidentityinspector(Command-Option-3).UnderCustomClass,changetheClasstoDrawView(Figure17.4).

Figure17.4Changingtheviewclass


DrawingwithDrawViewAninstanceofDrawViewneedstobeabledrawlines.YouaregoingtowriteamethodthatusesUIBezierPathtocreateandstrokeapathbasedonthepropertiesofagivenLine.ThenyouwilloverridedrawRect(_:)todrawthelinesinthearrayoffinishedlinesaswellasthecurrentline,ifany.

InDrawView.swift,implementthemethodforstrokinglinesandoverridedrawRect(_:).varcurrentLine:Line?varfinishedLines=[Line]()

funcstrokeLine(line:Line){letpath=UIBezierPath()path.lineWidth=10path.lineCapStyle=CGLineCap.Round

path.moveToPoint(line.begin)path.addLineToPoint(line.end)path.stroke()}

overridefuncdrawRect(rect:CGRect){//DrawfinishedlinesinblackUIColor.blackColor().setStroke()forlineinfinishedLines{strokeLine(line)}

ifletline=currentLine{//Ifthereisalinecurrentlybeingdrawn,doitinredUIColor.redColor().setStroke()strokeLine(line)}}


TurningTouchesintoLinesAlineisdefinedbytwopoints.YourLinestoresthesepointsaspropertiesnamedbeginandend.Whenatouchbegins,youwillcreateaLineandsetbothofitspropertiestothepointwherethetouchbegan.Whenthetouchmoves,youwillupdatetheLine’send.Whenthetouchends,youwillhaveyourcompleteLine.

InDrawView.swift,implementtouchesBegan(_:withEvent:)tocreateanewline.overridefunctouchesBegan(touches:Set<UITouch>,withEventevent:UIEvent?){lettouch=touches.first!

//Getlocationofthetouchinview'scoordinatesystemletlocation=touch.locationInView(self)

currentLine=Line(begin:location,end:location)

setNeedsDisplay()}

Thiscodefirstfiguresoutthelocationofthetouchwithintheview’scoordinatesystem.ThenitcallssetNeedsDisplay(),whichflagstheviewtoberedrawnattheendoftherunloop.

Next,stillinDrawView.swift,implementtouchesMoved(_:withEvent:)sothatitupdatestheendofthecurrentLine.overridefunctouchesMoved(touches:Set<UITouch>,withEventevent:UIEvent?){lettouch=touches.first!letlocation=touch.locationInView(self)

currentLine?.end=location

setNeedsDisplay()}

Finally,inDrawView.swift,updatetheendlocationofthecurrentLineandaddittothefinishedLinesarraywhenthetouchends.overridefunctouchesEnded(touches:Set<UITouch>,withEventevent:UIEvent?){ifvarline=currentLine{lettouch=touches.first!letlocation=touch.locationInView(self)line.end=location

finishedLines.append(line)}currentLine=nil

setNeedsDisplay()}

Buildandruntheapplicationanddrawsomelinesonthescreen.Whileyouaredrawing,thelineswillappearinred.Oncefinished,theywillappearinblack.

Handlingmultipletouches

Whendrawinglines,youmayhavenoticedthathavingmorethanonefingeronthescreendoesnotdoanything–thatis,youcanonlydrawonelineatatime.Let’supdate


DrawViewsothatyoucandrawasmanylinesasyoucanfitfingersonthescreen.

Bydefault,aviewwillonlyacceptonetouchatatime.IfonefingerhasalreadytriggeredtouchesBegan(_:withEvent:)buthasnotfinished–andthereforehasnottriggeredtouchesEnded(_:withEvent:)–subsequenttouchesareignored.Inthiscontext,“ignored”meansthatneithertouchesBegan(_:withEvent:)noranyotherUIRespondermethodrelatedtotheextratoucheswillbecalledontheDrawView.

InMain.storyboard,selecttheDrawViewandopentheattributesinspector.ChecktheboxlabeledMultipleTouch(Figure17.5),whichwillsettheDrawViewinstance’smultipleTouchesEnabledpropertytotrue.

Figure17.5MultipleTouchenabled

NowthatDrawViewwillacceptmultipletouches,eachtimeafingertouchesthescreen,moves,orisremovedfromthescreen,theappropriateUIResponderwillbecalledontheview.However,younowhaveaproblem:yourUIRespondercodeassumestherewillonlybeonetouchactiveandonelinebeingdrawnatatime.

Forexample,eachtouch-handlingmethodasksforthefirstelementinthesetoftouchesitreceives.Inasingle-touchview,therewillonlyeverbeoneobjectintheset,soaskingforanyobjectalwaysreturnsthetouchthattriggeredtheevent.Inamultiple-touchview,thesetcancontainmorethanonetouch.Also,DrawViewhasonlyoneproperty(currentLine)thathangsontoalineinprogress.Obviously,youwillneedtoholdasmanylinesastherearetouchescurrentlyonthescreen.Whileyoucouldcreateafewmoreproperties,likecurrentLine1andcurrentLine2,itwouldbeahassletomanagewhichpropertycorrespondstowhichtouch.

Insteadofaddingmoreproperties,youaregoingreplacethesingleLinewithadictionarycontaininginstancesofLine.InDrawView.swift,addanewpropertytoreplacecurrentLine.classDrawView:UIView{

varcurrentLine:Line?varcurrentLines=[NSValue:Line]()


ThekeytostorethelineinthedictionarywillbederivedfromtheUITouchobjectthatthelinecorrespondsto.Asmoretoucheventsoccur,youcanusethesamealgorithmtoderivethekeyfromtheUITouchthattriggeredtheeventanduseittolookuptheappropriateLineinthedictionary.

NowyouneedtoupdatetheUIRespondermethodstoaddlinesthatarecurrentlybeingdrawntothisdictionary.InDrawView.swift,updatethecodeintouchesBegan(_:withEvent:).overridefunctouchesBegan(touches:Set<UITouch>,withEventevent:UIEvent?){

lettouch=touches.first!

//Getlocationofthetouchinview'scoordinatesystemletlocation=touch.locationInView(self)currentLine=Line(begin:location,end:location)

//Let'sputinalogstatementtoseetheorderofeventsprint(__FUNCTION__)

fortouchintouches{letlocation=touch.locationInView(self)

letnewLine=Line(begin:location,end:location)

letkey=NSValue(nonretainedObject:touch)currentLines[key]=newLine}

setNeedsDisplay()}

Inthiscode,youfirstprintoutthenameofthemethod,whichisrepresentedbythe__FUNCTION__expression.Atcompiletime,thecompilerwillevaluatethisexpressionandreturnaStringthatrepresentsthenameofthemethod.

Second,youenumerateoverallofthetouchesthatbegan,becauseitispossiblethatmorethanonetouchcanbeginatthesametime.(Typically,touchesbeginatdifferenttimesandtouchesBegan(_:withEvent:)getscalledmultipletimesontheDrawViewforeachtouch.Butyouhavetopreparefortheimprobable,ifnottheimpossible.)

Next,noticetheuseofNSValue(nonretainedObject:)toderivethekeytostoretheLine.ThismethodcreatesanNSValueinstancethatholdsontotheaddressoftheUITouchobjectthatwillbeassociatedwiththisline.SinceaUITouchiscreatedwhenatouchbegins,updatedthroughoutitslifetime,anddestroyedwhenthetouchends,theaddressofthatobjectwillbeconstantthrougheachtouch-event-handlingmethod.Figure17.6showsthenewstateofaffairs.

Youmaybewondering:WhynotusetheUITouchitselfasthekey?WhygothroughthehoopofcreatinganNSValue?ThedocumentationforUITouchsaysthatyoushouldneverretain(inotherwords,keepastrongreferenceto)aUITouchobject.Therefore,youwrapthememoryaddressoftheUITouchinaninstanceofNSValue,usingitsinit(nonretainedObject:)initializer.Thedocumentationforthismethodstates:“Thismethodisusefulifyouwanttoaddanobjecttoacollectionbutdon’twantthecollectiontocreateastrongreferencetoit,”whichisexactlywhatyouwant.SincethesameUITouchobjectisreusedfortheentiretyofthattouch’slifecycle,youcanrecreate


thesameNSValueusingthesameUITouch.

Figure17.6ObjectdiagramformultitouchTouchTracker

UpdatetouchesMoved(_:withEvent:)inDrawView.swiftsothatitcanlookuptherightLine.overridefunctouchesMoved(touches:Set<NSObject>,withEventevent:UIEvent){

lettouch=touches.first!letlocation=touch.locationInView(self)

currentLine?.end=location


fortouchintouches{letkey=NSValue(nonretainedObject:touch)currentLines[key]?.end=touch.locationInView(self)}

setNeedsDisplay()}

Now,updatetouchesEnded(_:withEvent:)tomoveanyfinishedlinesintothefinishedLinesarray.overridefunctouchesEnded(touches:Set<NSObject>,withEventevent:UIEvent){

ifvarline=currentLine{lettouch=touches.first!letlocation=touch.locationInView(self)line.end=location

finishedLines.append(line)}currentLine=nil


fortouchintouches{letkey=NSValue(nonretainedObject:touch)ifvarline=currentLines[key]{


line.end=touch.locationInView(self)

finishedLines.append(line)currentLines.removeValueForKey(key)}}

setNeedsDisplay()}

Finally,updatedrawRect(_:)todraweachlineincurrentLines.overridefuncdrawRect(rect:CGRect){

//DrawfinishedlinesinblackUIColor.blackColor().setStroke()forlineinfinishedLines{strokeLine(line)}

ifletline=currentLine{//Ifthereisalinecurrentlybeingdrawn,doitinredUIColor.redColor().setStroke()strokeLine(line)}

//DrawcurrentlinesinredUIColor.redColor().setStroke()for(_,line)incurrentLines{strokeLine(line)}}

Buildandruntheapplicationandstartdrawinglineswithmultiplefingers.(Onthesimulator,holddowntheOptionkeywhileyoudragtosimulatemultiplefingers.)Noticetheorderingofthelogmessagesintheconsole.

YoushouldknowthatwhenaUIRespondermethodliketouchesMoved(_:withEvent:)iscalledonaview,onlythetouchesthathavemovedwillbeinthesetoftouches.Thus,itispossibleforthreetouchestobeonaview,butonlyonetouchtobeinthesetoftouchespassedintooneofthesemethods.Additionally,onceaUITouchbeginsonaview,alltoucheventmethodsarecalledonthatsameviewoverthetouch’slifetime,evenifthattouchmovesoffoftheviewitbeganon.

ThelastthingleftforthebasicsofTouchTrackeristohandlewhathappenswhenatouchiscanceled.AtouchcanbecanceledwhentheapplicationisinterruptedbytheOS(forexample,whenaphonecallcomesin)whileatouchiscurrentlyonthescreen.Whenatouchiscanceled,anystateitsetupshouldbereverted.Inthiscase,youshouldremoveanylinesinprogress.

InDrawView.swift,implementtouchesCancelled(_:withEvent:).overridefunctouchesCancelled(touches:Set<UITouch>?,withEventevent:UIEvent?){


currentLines.removeAll()

setNeedsDisplay()}


@IBInspectableWhenworkinginInterfaceBuilder,youareabletomodifyattributesfortheviewsthatyouaddtothecanvas.Forexample,youcansetthebackgroundcoloronaview,thetextonalabel,andthecurrentprogressonaslider.YoucanaddthissamebehaviortoyourowncustomUIViewsubclassesforcertaintypes.Let’saddintheabilityfortheDrawView’scurrentlinecolor,finishedlinecolor,andlinethicknesstobecustomizedthroughInterfaceBuilder.

InDrawView.swift,declarethreepropertiestoreferencethesevalues.Givethemdefaultvaluesandhavetheviewflagitselfforredrawingwheneverthesepropertieschange.varcurrentLines=[NSValue:Line]()varfinishedLines=[Line]()

@IBInspectablevarfinishedLineColor:UIColor=UIColor.blackColor(){didSet{setNeedsDisplay()}}

@IBInspectablevarcurrentLineColor:UIColor=UIColor.redColor(){didSet{setNeedsDisplay()}}

@IBInspectablevarlineThickness:CGFloat=10{didSet{setNeedsDisplay()}}

The@IBInspectablekeywordletsInterfaceBuilderknowthatthisisapropertythatyouwanttocustomizethroughtheattributesinspector.Manyofthecommontypesaresupportedby@IBInspectable:Booleans,strings,numbers,CGPoint,CGSize,CGRect,UIColor,UIImage,andafewmoreareallcandidates.

NowupdatestrokeLine(_:)anddrawView(_:)tousethesenewproperties.funcstrokeLine(line:Line){letpath=UIBezierPath()path.lineWidth=10path.lineWidth=lineThicknesspath.lineCapStyle=CGLineCap.Round

path.moveToPoint(line.begin)path.addLineToPoint(line.end)path.stroke()}

overridefuncdrawRect(rect:CGRect){//DrawfinishedlinesinblackUIColor.blackColor().setStroke()finishedLineColor.setStroke()forlineinfinishedLines{strokeLine(line)}

//DrawcurrentlinesinredUIColor.redColor().setStroke()currentLineColor.setStroke()for(_,line)incurrentLines{strokeLine(line)


}}

NowwhenyouaddaDrawViewtothecanvasinInterfaceBuilder,youcancustomizethesethreepropertiesintheattributesinspectortobedifferentfordifferentinstances(Figure17.7).

Figure17.7CustomizingDrawView


SilverChallenge:ColorsMakeitsotheangleatwhichalineisdrawndictatesitscoloronceithasbeenaddedtocurrentLines.


GoldChallenge:CirclesUsetwofingerstodrawcircles.Tryhavingeachfingerrepresentonecorneroftheboundingboxaroundthecircle.RecallthatyoucansimulatetwofingersonthesimulatorbyholdingdowntheOptionkey.(Hint:thisismucheasierifyoutracktouchesthatareworkingonacircleinaseparatedictionary.)


FortheMoreCurious:TheResponderChainInChapter13,wetalkedbrieflyaboutthefirstresponder.AUIRespondercanbeafirstresponderandreceivetouchevents.UIViewisoneexampleofaUIRespondersubclass,buttherearemanyothers,includingUIViewController,UIApplication,andUIWindow.Youareprobablythinking,“Butyoucan’ttouchaUIViewController.It’snotanonscreenobject.”Youareright–youcannotsendatoucheventdirectlytoaUIViewController,butviewcontrollerscanreceiveeventsthroughtheresponderchain.

EveryUIResponderhasamethodcallednextResponder(),andtogethertheseobjectsmakeuptheresponderchain(Figure17.8).Atoucheventstartsattheviewthatwastouched.ThenextResponderofaviewistypicallyitsUIViewController(ifithasone)oritssuperview(ifitdoesnot).ThenextResponder()ofaviewcontrolleristypicallyitsview’ssuperview.Thetop-mostsuperviewisthewindow.Thewindow’snextResponder()isthesingletoninstanceofUIApplication.

Figure17.8Responderchain

HowdoesaUIRespondernothandleanevent?ItforwardsthesamemessagetoitsnextResponder().ThatiswhatthedefaultimplementationofmethodsliketouchesBegan(_:withEvent:)do.Soifamethodisnotoverridden,itsnextresponderwillattempttohandlethetouchevent.Iftheapplication(thelastobjectintheresponderchain)doesnothandletheevent,thenitisdiscarded.

Youcanexplicitlysendamessagetoanextresponder,too.Let’ssaythereisaviewthattrackstouches,butifadouble-tapoccurs,itsnextrespondershouldhandleit.Thecodewouldlooklikethis:overridefunctouchesBegan(touches:Set<UITouch>,withEventevent:UIEvent?){lettouch=touches.first!iftouch.tapCount==2{nextResponder()?.touchesBegan(touches,withEvent:event)}


else{//Goontohandletouchesthatarenotdouble-taps}}


FortheMoreCurious:UIControlTheclassUIControlisthesuperclassforseveralclassesinCocoaTouch,includingUIButtonandUISlider.Youhaveseenhowtosetthetargetsandactionsforthesecontrols.NowwecantakeacloserlookathowUIControloverridesthesameUIRespondermethodsyouimplementedinthischapter.

InUIControl,eachpossiblecontroleventisassociatedwithaconstant.Buttons,forexample,typicallysendactionmessagesontheUIControlEvents.TouchUpInsidecontrolevent.Atargetregisteredforthiscontroleventwillonlyreceiveitsactionmessageiftheusertouchesthecontrolandthenliftsthefingeroffthescreeninsidetheframeofthecontrol.

Forabutton,however,youcanhaveactionsonothereventtypes.Forexample,youmighttriggeramethodiftheuserremovesthefingerinsideoroutsidetheframe.Assigningthetargetandactionprogrammaticallywouldlooklikethis:button.addTarget(self,action:"resetTemperature:",forControlEvents:[.TouchUpInside,.TouchUpOutside])

NowconsiderhowUIControlhandlesUIControlEvents.TouchUpInside.//Nottheexactcode.Thereisabitmoregoingon!overridefunctouchesEnded(touches:Set<UITouch>,withEventevent:UIEvent?){

//Referencetothetouchthatisendinglettouch=touches.first!

//Locationofthatpointinthiscontrol'scoordinatesystemlettouchLocation=touch.locationInView(self)

//Isthatpointstillinmyviewingbounds?ifself.bounds.contains(touchLocation){//Sendoutactionmessagestoalltargetsregisteredforthisevent!sendActionsForControlEvents(.TouchUpInside)}else{//Thetouchendedoutsidethebounds:differentcontroleventsendActionsForControlEvents(.TouchUpOutside)}}

Sohowdotheseactionsgetsenttotherighttarget?AttheendoftheUIRespondermethodimplementations,thecontrolsendsthemessagesendActionsForControlEvents(_:)toitself.Thismethodlooksatallofthetarget-actionpairsthecontrolhas.Ifanyofthemareregisteredforthecontroleventpassedastheargument,thosetargetsaresentanactionmessage.

However,acontrolneversendsamessagedirectlytoitstargets.Instead,itroutesthesemessagesthroughtheUIApplicationobject.Whynothavecontrolssendtheactionmessagesdirectlytothetargets?Controlscanalsohavenil-targetedactions.IfaUIControl’stargetisnil,theUIApplicationfindsthefirstresponderofitsUIWindowandsendstheactionmessagetoit.


18UIGestureRecognizerand

UIMenuControllerInChapter17,youhandledrawtouchesbyimplementingmethodsfromUIResponder.Sometimesyouwanttodetectaspecificpatternoftouchesthatmakeagesture,likeapinchoraswipe.Insteadofwritingcodetodetectcommongesturesyourself,youcanuseinstancesofUIGestureRecognizer.

AUIGestureRecognizerinterceptstouchesthatareontheirwaytobeinghandledbyaview.Whenitrecognizesaparticulargesture,itcallsamethodontheobjectofyourchoice.ThereareseveraltypesofgesturerecognizersbuiltintotheSDK.Inthischapter,youwillusethreeofthemtoallowTouchTrackeruserstoselect,move,anddeletelines(Figure18.1).YouwillalsoseehowtouseanotherinterestingiOSclass,UIMenuController.


Figure18.1TouchTrackerbytheendofthechapter


UIGestureRecognizerSubclassesYoudonotinstantiateUIGestureRecognizeritself.Instead,thereareanumberofsubclassesofUIGestureRecognizer,andeachoneisresponsibleforrecognizingaparticulargesture.

TouseaninstanceofaUIGestureRecognizersubclass,yougiveitatarget-actionpairandattachittoaview.Wheneverthegesturerecognizerrecognizesitsgestureontheview,itwillsendtheactionmessagetoitstarget.AllUIGestureRecognizeractionmessageshavethesameform:funcaction(gestureRecognizer:UIGestureRecognizer){}

Whenrecognizingagesture,thegesturerecognizerinterceptsthetouchesdestinedfortheview(Figure18.2).Thus,thetypicalUIRespondermethodsliketouchesBegan(_:withEvent:)maynotbecalledonaviewwithgesturerecognizers.

Figure18.2Gesturerecognizersintercepttouches


DetectingTapswithUITapGestureRecognizerThefirstUIGestureRecognizersubclassyouwilluseisUITapGestureRecognizer.Whentheusertapsthescreentwice,allofthelinesonthescreenwillbecleared.

OpenTouchTracker.xcodeprojfromChapter17andDrawView.swift.Addaninit?(coder:)methodandinstantiateaUITapGestureRecognizerthatrequirestwotapstofireandcalltheactionmethodonitstarget.requiredinit?(coderaDecoder:NSCoder){super.init(coder:aDecoder)

letdoubleTapRecognizer=UITapGestureRecognizer(target:self,action:"doubleTap:")doubleTapRecognizer.numberOfTapsRequired=2addGestureRecognizer(doubleTapRecognizer)}

Nowwhenadouble-tapoccursonaninstanceofDrawView,themethoddoubleTap(_:)willbecalledonthatinstance.ImplementthismethodinDrawView.swift.funcdoubleTap(gestureRecognizer:UIGestureRecognizer){print("Recognizedadoubletap")

currentLines.removeAll(keepCapacity:false)finishedLines.removeAll(keepCapacity:false)setNeedsDisplay()}

NoticethattheargumenttotheactionmethodforagesturerecognizeristheinstanceofUIGestureRecognizerthatcalledthemethod.Inthecaseofadouble-tap,youdonotneedanyinformationfromtherecognizer,butyouwillneedinformationfromtheotherrecognizersyouinstalllaterinthechapter.

Buildandruntheapplication,drawafewlines,anddouble-tapthescreentoclearthem.

Youmayhavenoticed(especiallyonthesimulator)thatthefirsttapofadouble-tapresultsinasmallreddotbeingdrawn.ThisdotappearsbecausetouchesBegan(_:withEvent:)iscalledontheDrawViewonthefirsttap,creatingaveryshortline.Checktheconsoleandyouwillseethefollowingsequenceofevents:touchesBegan(_:withEvent:)RecognizedadoubletaptouchesCancelled(_:withEvent:)

Gesturerecognizersworkbyinspectingtoucheventstodetermineiftheirparticulargesturehasoccurred.Beforeagestureisrecognized,thegesturerecognizerinterceptsalltheUIRespondermethodcalls.Ifithasnotrecognizeditsgesture,eachcallisforwardedontotheview.

Recognizingataprequiresthatatouchbeginandend.ThismeansthattheUITapGestureRecognizercannotknowwhetherthetouchisatapwhentouchesBegan(_:withEvent:)isoriginallycalled,sothemethodiscalledontheviewaswell.Whenthetouchends,thetapisrecognizedandthegesturerecognizerclaims


thetouchforitself.ItdoessobycallingtouchesCancelled(_:withEvent:)ontheview.Afterthat,nomoreUIRespondermethodswillbecalledontheviewforthatparticulartouch.

Topreventthisreddotfromappearingtemporarily,youmustpreventtouchesBegan(_:withEvent:)frombeingcalledontheview.YoucantellaUIGestureRecognizertodelaycallingtouchesBegan(_:withEvent:)onitsviewifitisstillpossiblethatitsgesturemightberecognizedforthattouch.

InDrawView.swift,modifyinit?(coder:)todojustthis.requiredinit?(coderaDecoder:NSCoder){super.init(coder:aDecoder)

letdoubleTapRecognizer=UITapGestureRecognizer(target:self,action:"doubleTap:")doubleTapRecognizer.numberOfTapsRequired=2doubleTapRecognizer.delaysTouchesBegan=trueaddGestureRecognizer(doubleTapRecognizer)}

Buildandruntheapplication,drawsomelines,andthendouble-taptoclearthem.Youwillnolongerseethereddotwhiledouble-tapping.


MultipleGestureRecognizersThenextstepistoaddanothergesturerecognizerthatallowstheusertoselectaline.(Later,auserwillbeabletodeletetheselectedline.)YouwillinstallanotherUITapGestureRecognizerontheDrawViewthatonlyrequiresonetap.

InDrawView.swift,modifyinit?(coder:)toaddthisgesturerecognizer.requiredinit?(coderaDecoder:NSCoder){super.init(coder:aDecoder)

letdoubleTapRecognizer=UITapGestureRecognizer(target:self,action:"doubleTap:")doubleTapRecognizer.numberOfTapsRequired=2doubleTapRecognizer.delaysTouchesBegan=trueaddGestureRecognizer(doubleTapRecognizer)

lettapRecognizer=UITapGestureRecognizer(target:self,action:"tap:")tapRecognizer.delaysTouchesBegan=trueaddGestureRecognizer(tapRecognizer)}

Now,implementtap(_:)inDrawView.swifttologthetaptotheconsole.functap(gestureRecognizer:UIGestureRecognizer){print("Recognizedatap")}

Buildandruntheapplication.Trytappinganddouble-tapping.Tappingoncelogstheappropriatemessagetotheconsole.Double-tapping,however,triggersbothtap(_:)anddoubleTap(_:).

Insituationswhereyouhavemultiplegesturerecognizers,itisnotuncommontohaveonegesturerecognizerfireandclaimatouchwhenyoureallywantedanothergesturerecognizertohandleit.Inthesecases,yousetupdependenciesbetweenrecognizersthatsay,“Waitamomentbeforeyoufire,becausethistouchmightbemine!”

Ininit?(coder:),makeitsothetapRecognizerwaitsuntilthedoubleTapRecognizerfailstorecognizeadouble-tapbeforeclaimingthesingletapforitself.requiredinit?(coderaDecoder:NSCoder){super.init(coder:aDecoder)

letdoubleTapRecognizer=UITapGestureRecognizer(target:self,action:"doubleTap:")doubleTapRecognizer.numberOfTapsRequired=2doubleTapRecognizer.delaysTouchesBegan=trueaddGestureRecognizer(doubleTapRecognizer)

lettapRecognizer=UITapGestureRecognizer(target:self,action:"tap:")tapRecognizer.delaysTouchesBegan=truetapRecognizer.requireGestureRecognizerToFail(doubleTapRecognizer)addGestureRecognizer(tapRecognizer)}

Buildandruntheapplicationagainandtryoutsometaps.Asingletapnowtakesasmallamountoftimetofireafterthetapoccurs,butdouble-tappingnolongertriggersthetap(_:)message.

Next,let’sbuildontheDrawViewsothattheusercanselectalinewhenitistapped.First,addapropertyatthetopofDrawView.swifttoholdontotheindexofaselected


line.classDrawView:UIView{

varcurrentLines=[NSValue:Line]()varfinishedLines=[Line]()varselectedLineIndex:Int?

NowmodifydrawRect(_:)todrawtheselectedlineingreen.overridefuncdrawRect(rect:CGRect){finishedLineColor.setStroke()forlineinfinishedLines{strokeLine(line)}

currentLineColor.setStroke()for(_,line)incurrentLines{strokeLine(line)}

ifletindex=selectedLineIndex{UIColor.greenColor().setStroke()letselectedLine=finishedLines[index]strokeLine(selectedLine)}}

InDrawView.swift,addanindexOfLineAtPoint(_:)methodthatreturnstheindexoftheLineclosesttoagivenpoint.funcindexOfLineAtPoint(point:CGPoint)->Int?{

//Findalineclosetopointfor(index,line)infinishedLines.enumerate(){letbegin=line.beginletend=line.end

//CheckafewpointsonthelinefortinCGFloat(0).stride(to:1.0,by:0.05){letx=begin.x+((end.x-begin.x)*t)lety=begin.y+((end.y-begin.y)*t)

//Ifthetappedpointiswithin20points,let'sreturnthislineifhypot(x-point.x,y-point.y)<20.0{returnindex}}}

//Ifnothingiscloseenoughtothetappedpoint,thenwedidnotselectalinereturnnil}

Thestridemethodwillallowttostartat0andgoupto(butnotreach)thetovalue,incrementingthevalueoftbythebyvalue.Itisanalogoustothefollowingcode:forvart:CGFloat=0;t<1.0;t+=0.05{...}

Thereareother,betterwaystodeterminetheclosestlinetoapoint,butthissimpleimplementationwillworkforyourpurposes.

Thepointtobepassedinisthepointwherethetapoccurred.Youcaneasilygetthisinformation.EveryUIGestureRecognizerhasalocationInView(_:)method.Callingthismethodonthegesturerecognizerwillgiveyouthecoordinatewherethegestureoccurredinthecoordinatesystemoftheviewthatispassedastheargument.


InDrawView.swift,updatetap(_:)tocalllocationInView(_:)onthegesturerecognizer,passtheresulttoindexOfLineAtPoint(_:),andmakethereturnedindextheselectedLineIndex.functap(gestureRecognizer:UIGestureRecognizer){print("Recognizedatap")

letpoint=gestureRecognizer.locationInView(self)selectedLineIndex=indexOfLineAtPoint(point)

setNeedsDisplay()}

Iftheuserdouble-tapstoclearalllineswhilealineisselected,theapplicationwilltrap.Toaddressthis,updatedoubleTap(_:)tosettheselectedLineIndextonil.funcdoubleTap(gestureRecognizer:UIGestureRecognizer){print("Recognizedadoubletap")

selectedLineIndex=nilcurrentLines.removeAll(keepCapacity:false)finishedLines.removeAll(keepCapacity:false)setNeedsDisplay()}

Buildandruntheapplication.Drawafewlinesandthentapone.Thetappedlineshouldappearingreen,butrememberthatittakesamomentbeforethetapisknownnottobeadouble-tap.


UIMenuControllerNextyouaregoingtomakeitsothatwhentheuserselectsaline,amenuwiththeoptiontodeletethatlineappearswheretheusertapped.Thereisabuilt-inclassforprovidingthissortofmenucalledUIMenuController(Figure18.3).AmenucontrollerhasalistofUIMenuItemobjectsandispresentedinanexistingview.Eachitemhasatitle(whatshowsupinthemenu)andanaction(themessageitsendsthefirstresponderofthewindow).

Figure18.3AUIMenuController

ThereisonlyoneUIMenuControllerperapplication.Whenyouwishtopresentthisinstance,youfillitwithmenuitems,giveitarectangletopresentfrom,andsetittobevisible.

DothisinDrawView.swift’stap(_:)methodiftheuserhastappedonaline.Iftheusertappedsomewherethatisnotnearaline,thecurrentlyselectedlinewillbedeselectedandthemenucontrollerwillhide.functap(gestureRecognizer:UIGestureRecognizer){print("Recognizedatap")

letpoint=gestureRecognizer.locationInView(self)selectedLineIndex=indexOfLineAtPoint(point)

//Grabthemenucontrollerletmenu=UIMenuController.sharedMenuController()

ifselectedLineIndex!=nil{

//MakeDrawViewthetargetofmenuitemactionmessagesbecomeFirstResponder()

//Createanew"Delete"UIMenuItemletdeleteItem=UIMenuItem(title:"Delete",action:"deleteLine:")menu.menuItems=[deleteItem]

//Tellthemenuwhereitshouldcomefromandshowitmenu.setTargetRect(CGRect(x:point.x,y:point.y,width:2,height:2),inView:self)menu.setMenuVisible(true,animated:true)}else{//Hidethemenuifnolineisselectedmenu.setMenuVisible(false,animated:true)}

setNeedsDisplay()}

Foramenucontrollertoappear,aviewthatrespondstoatleastoneactionmessageintheUIMenuController’smenuitemsmustbethefirstresponderofthewindow–thisiswhyyoucalledthemethodbecomeFirstResponder()ontheDrawViewbeforesettingupthemenucontroller.


Ifyouhaveacustomviewclassthatneedstobecomethefirstresponder,youmustalsooverridecanBecomeFirstResponder().InDrawView.swift,overridethismethodtoreturntrue.overridefunccanBecomeFirstResponder()->Bool{returntrue}

Youcanbuildandruntheapplicationnow,butwhenyouselectaline,themenuwillnotappear.Whenbeingpresented,themenucontrollergoesthrougheachmenuitemandasksthefirstresponderifitimplementstheactionmessageforthatitem.Ifthefirstresponderdoesnotimplementthatmethod,thenthemenucontrollerwillnotshowtheassociatedmenuitem.Ifnomenuitemshavetheiractionmessagesimplementedbythefirstresponder,themenuisnotshownatall.

TogettheDeletemenuitem(andthemenuitself)toappear,implementdeleteLine(_:)inDrawView.swift.funcdeleteLine(sender:AnyObject){//RemovetheselectedlinefromthelistoffinishedLinesifletindex=selectedLineIndex{finishedLines.removeAtIndex(index)selectedLineIndex=nil

//RedraweverythingsetNeedsDisplay()}}

Buildandruntheapplication.Drawaline,taponit,andthenselectDeletefromthemenuitem.

Ifyouselectalineandthendouble-taptoclearalllines,themenucontrollerwillstillbevisible.IftheselectedLineIndexeverbecomesnil,themenucontrollershouldnotbevisible.

AddapropertyobservertoselectedLineIndexthatsetsthemenucontrollertobenotvisibleiftheindexissettonil.varselectedLineIndex:Int?{didSet{ifselectedLineIndex==nil{letmenu=UIMenuController.sharedMenuController()menu.setMenuVisible(false,animated:true)}}}

Buildandruntheapplication.Drawaline,selectit,andthendouble-tapthebackground.Thelinesandthemenucontrollerwillnolongerbevisible.


MoreGestureRecognizersInthissection,youaregoingtoaddtheabilityforausertoselectalinebypressingandholding(alongpress)andthenmovetheselectedlinebydraggingthefinger(apan).ThiswillrequiretwomoresubclassesofUIGestureRecognizer:UILongPressGestureRecognizerandUIPanGestureRecognizer.

UILongPressGestureRecognizer

InDrawView.swift,instantiateaUILongPressGestureRecognizerininit?(coder:)andaddittotheDrawView....

addGestureRecognizer(tapRecognizer)

letlongPressRecognizer=UILongPressGestureRecognizer(target:self,action:"longPress:")addGestureRecognizer(longPressRecognizer)}

NowwhentheuserholdsdownontheDrawView,themethodlongPress(_:)willbecalledonit.Bydefault,atouchmustbeheld0.5secondstobecomealongpress,butyoucanchangetheminimumPressDurationofthegesturerecognizerifyoulike.

Sofar,youhaveworkedwithtapgestures.Atapisadiscretegesture.Bythetimeitisrecognized,thegestureisover,andtheactionmessagehasbeendelivered.Alongpress,ontheotherhand,isacontinuousgesture.Continuousgesturesoccurovertime.Tokeeptrackofwhatisgoingonwithacontinuousgesture,youcancheckarecognizer’sstateproperty.

Forexample,consideratypicallongpress:

Whentheusertouchesaview,thelongpressrecognizernoticesapossiblelongpressbutmustwaittoseewhetherthetouchisheldlongenoughtobecomealongpressgesture.Therecognizer’sstateisUIGestureRecognizerState.Possible.

Oncetheuserholdsthetouchlongenough,thelongpressisrecognizedandthegesturehasbegun.Therecognizer’sstateisUIGestureRecognizerState.Began.

Whentheuserremovesthefinger,thegesturehasended.Therecognizer’sstateisUIGestureRecognizerState.Ended.

Whenthelong-pressgesturerecognizertransitionsfrompossibletobeganandfrombegantoended,itsendsitsactionmessagetoitstarget.Todeterminewhichtransitiontriggeredtheaction,youcheckthegesturerecognizer’sstate.

Rememberthatthelongpressispartofalargerfeature.Inthenextsection,youwillenabletheusertomovetheselectedlinebydraggingitwiththesamefingerthatbeganthelongpress.SohereistheplanforimplementingthelongPress(_:)actionmethod:


Whentherecognizerisinthebeganstate,youwillselecttheclosestlinetowherethegestureoccurred.Whentherecognizerisintheendedstate,youwilldeselecttheline.

InDrawView.swift,implementlongPress(_:).funclongPress(gestureRecognizer:UIGestureRecognizer){print("Recognizedalongpress")

ifgestureRecognizer.state==.Began{letpoint=gestureRecognizer.locationInView(self)selectedLineIndex=indexOfLineAtPoint(point)

ifselectedLineIndex!=nil{currentLines.removeAll(keepCapacity:false)}}elseifgestureRecognizer.state==.Ended{selectedLineIndex=nil}

setNeedsDisplay()}

Buildandruntheapplication.Drawalineandthenpressandholdit;thelinewillturngreenandbecometheselectedline.Whenyouletgo,thelinewillreverttoitsformercolorandwillnolongerbetheselectedline.

UIPanGestureRecognizerandsimultaneousrecognizers

InDrawView.swift,declareaUIPanGestureRecognizerasapropertysothatyouhaveaccesstoitinallofyourmethods.classDrawView:UIView{

varcurrentLines=[NSValue:Line]()varfinishedLines=[Line]()varselectedLineIndex:Int?varmoveRecognizer:UIPanGestureRecognizer!

Next,inDrawView.swift,addcodetoinit?(coder:)toinstantiateaUIPanGestureRecognizer,setoneofitsproperties,andaddittotheDrawView.letlongPressRecognizer=UILongPressGestureRecognizer(target:self,action:"longPress:")addGestureRecognizer(longPressRecognizer)

moveRecognizer=UIPanGestureRecognizer(target:self,action:"moveLine:")moveRecognizer.cancelsTouchesInView=falseaddGestureRecognizer(moveRecognizer)}

WhatiscancelsTouchesInView?EveryUIGestureRecognizerhasthisproperty,whichdefaultstotrue.WhencancelsTouchesInViewistrue,thegesturerecognizerwill“eat”anytouchitrecognizes,andtheviewwillnotgetachancetohandlethetouchviathetraditionalUIRespondermethods,liketouchesBegan(_:withEvent:).

Usually,thisiswhatyouwant,butnotalways.Inthiscase,ifthepangesturerecognizerweretoeatitstouches,thenuserswouldnotbeabletodrawlines.WhenyousetcancelsTouchesInViewtofalse,youensurethatanytouchrecognizedbythegesturerecognizerwillalsobedeliveredtotheviewviatheUIRespondermethods.


InDrawView.swift,addasimpleimplementationfortheactionmethod:funcmoveLine(gestureRecognizer:UIPanGestureRecognizer){print("Recognizedapan")}

Buildandruntheappanddrawsomelines.BecausecancelsTouchesInViewisfalse,thepangestureisrecognized,butlinescanstillbedrawn.YoucancommentoutthelinethatsetscancelsTouchesInViewandrunagaintoseethedifference.

Soon,youwillupdatemoveLine(_:)toredrawtheselectedlineastheuser’sfingermovesacrossthescreen.Butfirstyouneedtwogesturerecognizerstobeabletohandlethesametouch.Normally,whenagesturerecognizerrecognizesitsgesture,iteatsitandnootherrecognizergetsachancetohandlethattouch.Tryit:runtheapp,drawaline,pressandholdtoselecttheline,andthenmoveyourfingeraround.Theconsolereportsthelongpressbutnotthepan.

Inyourcase,thedefaultbehaviorisproblematic:youruserswillpressandholdtoselectalineandthenpantomovetheline-withoutliftingthefingerinbetween.Thus,thetwogestureswilloccursimultaneously,andthepangesturerecognizermustbeallowedtorecognizeapaneventhoughthelongpressgesturehasalreadyrecognizedalongpress.

Toallowagesturerecognizertorecognizeitsgesturesimultaneouslywithanothergesturerecognizer,youimplementamethodfromtheUIGestureRecognizerDelegateprotocol:optionalfuncgestureRecognizer(_gestureRecognizer:UIGestureRecognizer,shouldRecognizeSimultaneouslyWithGestureRecognizerotherGestureRecognizer:UIGestureRecognizer)->Bool

Thefirstparameteristhegesturerecognizerthatisaskingforguidance.Itsaystoitsdelegate,“Sothere’smeandthisotherrecognizer,andoneofusjustrecognizedagesture.Shouldtheonewhodidnotrecognizeitstayinthepossiblestateandcontinuetotrackthistouch?”

Notethatthecallitselfdoesnottellyouwhichofthetworecognizershasrecognizeditsgesture-and,thus,whichofthemwillpotentiallybedeprivedofthechancetorecognizeitsgesture.

Bydefault,themethodreturnsfalse,andthegesturerecognizerstillinthepossiblestateleavesthetouchinthehandsofthegesturealreadyintherecognizedstate.Youcanimplementthemethodtoreturntruetoallowbothrecognizerstorecognizetheirgesturesinthesametouch.(Ifyouneedtodeterminewhichofthetworecognizershasrecognizeditsgesture,youcanchecktherecognizers’stateproperties.)

Toenablepanningwhilelongpressing,youaregoingtogivethepangesturerecognizeradelegate(theDrawView).Then,whenthelongpressrecognizerrecognizesitsgesture,thepangesturerecognizerwillcallthesimultaneousrecognitionmethodonitsdelegate.YouwillimplementthismethodinDrawViewtoreturntrue.Thiswillallowthepangesturerecognizertorecognizeanypanningthatoccurswhilealongpressisinprogress.

First,inDrawView.swift,declarethatDrawViewconformstotheUIGestureRecognizerDelegateprotocol.classDrawView:UIView,UIGestureRecognizerDelegate{


varcurrentLines=[NSValue:Line]()varfinishedLines=[Line]()varselectedLineIndex:Int?varmoveRecognizer:UIPanGestureRecognizer!

Next,ininit?(coder:),settheDrawViewtobethedelegateoftheUIPanGestureRecognizer.letlongPressRecognizer=UILongPressGestureRecognizer(target:self,action:"longPress:")addGestureRecognizer(longPressRecognizer)

moveRecognizer=UIPanGestureRecognizer(target:self,action:"moveLine:")moveRecognizer.delegate=selfmoveRecognizer.cancelsTouchesInView=falseaddGestureRecognizer(moveRecognizer)}

Finally,inDrawView.swift,implementthedelegatemethodtoreturntrue.funcgestureRecognizer(gestureRecognizer:UIGestureRecognizer,shouldRecognizeSimultaneouslyWithGestureRecognizerotherGestureRecognizer:UIGestureRecognizer)->Bool{returntrue}

Forthissituation,whereonlyyourpangesturerecognizerhasadelegate,thereisnoneedtodomorethanreturntrue.Inmorecomplicatedscenarios,youwouldusethepassed-ingesturerecognizerstomorecarefullycontrolsimultaneousrecognition.

Now,whenalongpressbegins,theUIPanGestureRecognizerwillcontinuetokeeptrackofthetouch,andiftheuser’sfingerbeginstomove,thepanrecognizerwillrecognizethepan.Toseethedifference,runtheapp,drawaline,selectit,andthenpan.Theconsolewillreportbothgestures.

TheUIGestureRecognizerDelegateprotocolincludesothermethodstohelpyoutweakthebehaviorofyourgesturerecognizers.Visittheprotocolreferencepageformoreinformation.

Inadditiontothestatesyouhavealreadyseen,apangesturerecognizersupportsthechangedstate.Whenafingerstartstomove,thepanrecognizerentersthebeganstateandcallsamethodonitstarget.Whilethefingermovesaroundthescreen,therecognizertransitionstothechangedstateandcallstheactionmethodonitstargetrepeatedly.Whenthefingerleavesthescreen,therecognizer’sstateissettoended,andthemethodiscalledonthetargetforthefinaltime.

ThenextstepistoimplementthemoveLine(_:)methodthatthepanrecognizercallsonitstarget.Inthisimplementation,youwillcallthemethodtranslationInView(_:)onthepanrecognizer.ThisUIPanGestureRecognizermethodreturnshowfarthepanhasmovedasaCGPointinthecoordinatesystemoftheviewpassedastheargument.Whenthepangesturebegins,thispropertyissettothezeropoint(wherebothxandyequalzero).Asthepanmoves,thisvalueisupdated–ifthepangoesveryfartotheright,ithasahighxvalue;ifthepanreturnstowhereitbegan,itstranslationgoesbacktothezeropoint.

InDrawView.swift,implementmoveLine(_:).NoticethatbecauseyouwillsendthegesturerecognizeramethodfromtheUIPanGestureRecognizerclass,the


parameterofthismethodmustbeareferencetoaninstanceofUIPanGestureRecognizerratherthanUIGestureRecognizer.funcmoveLine(gestureRecognizer:UIPanGestureRecognizer){print("Recognizedapan")

//Ifalineisselected...ifletindex=selectedLineIndex{//Whenthepanrecognizerchangesitsposition...ifgestureRecognizer.state==.Changed{//Howfarhasthepanmoved?lettranslation=gestureRecognizer.translationInView(self)

//Addthetranslationtothecurrentbeginningandendpointsoftheline//Makesuretherearenocopyandpastetypos!finishedLines[index].begin.x+=translation.xfinishedLines[index].begin.y+=translation.yfinishedLines[index].end.x+=translation.xfinishedLines[index].end.y+=translation.y

//RedrawthescreensetNeedsDisplay()}}else{//Ifnolineisselected,donotdoanythingreturn}}

Buildandruntheapplication.Touchandholdonalineandbegindragging–andyouwillimmediatelynoticethatthelineandyourfingerarewayoutofsync.Thismakessensebecauseyouareaddingthecurrenttranslationoverandoveragaintotheline’soriginalendpoints.Youreallyneedthegesturerecognizertoreportthechangeintranslationsincethelasttimethismethodwascalledinstead.Fortunately,youcandothis.Youcansetthetranslationofapangesturerecognizerbacktothezeropointeverytimeitreportsachange.Then,thenexttimeitreportsachange,itwillhavethetranslationsincethelastevent.

NearthebottomofmoveLine(_:)inDrawView.swift,addthefollowinglineofcode.finishedLines[index].end.x+=translation.xfinishedLines[index].end.y+=translation.y

gestureRecognizer.setTranslation(CGPoint.zero,inView:self)

//RedrawthescreensetNeedsDisplay()

Buildandruntheapplicationandmovealinearound.Worksgreat!


MoreonUIGestureRecognizerYouhaveonlyscratchedthesurfaceofUIGestureRecognizer.Therearemoresubclasses,moreproperties,andmoredelegatemethods,andyoucanevencreaterecognizersofyourown.ThissectionwillgiveyouanideaofwhatUIGestureRecognizeriscapableof.Youcanstudythedocumentationtolearnevenmore.

Whenagesturerecognizerisonaview,itisreallyhandlingalloftheUIRespondermethods,liketouchesBegan(_:withEvent:),foryou.Gesturerecognizersareprettygreedy,sotheytypicallydonotletaviewreceivetoucheventsortheyatleastdelaythedeliveryofthoseevents.Youcansetpropertiesontherecognizer,likedelaysTouchesBegan,delaysTouchesEnded,andcancelsTouchesInView,tochangethisbehavior.Ifyouneedfinercontrolthanthisall-or-nothingapproach,youcanimplementdelegatemethodsfortherecognizer.

Attimes,youmayhavetwogesturerecognizerslookingforverysimilargestures.YoucanchainrecognizerstogethersothatoneisrequiredtofailforthenextonetostartusingthemethodrequireGestureRecognizerToFail(_:).

Onethingyoumustunderstandtomastergesturerecognizersishowtheyinterprettheirstate.Overall,therearesevenstatesarecognizercanenter:

UIGestureRecognizerState.Possible

UIGestureRecognizerState.Failed

UIGestureRecognizerState.Began

UIGestureRecognizerState.Cancelled

UIGestureRecognizerState.Changed

UIGestureRecognizerState.Recognized

UIGestureRecognizerState.Ended

Thepossiblestateiswhererecognizersspendmostoftheirtime.Whenagesturetransitionstoanystateotherthanthepossiblestateorthefailedstate,theactionmessageoftherecognizerissentanditsstatepropertycanbecheckedtoseewhy.

Thefailedstateisusedbyrecognizerswatchingforamultitouchgesture.Atsomepoint,theuser’sfingersmayachieveapositionfromwhichtheycannolongermakethatrecognizer’sgesture.Atthatpoint,thegesturerecognizerfails.Arecognizerentersthecanceledstatewhenitisinterrupted,suchasbyanincomingphonecall.

Ifagestureiscontinuous,likeapan,thegesturerecognizerwillenterthebeganstateandthengointothechangedstateuntilthegestureends.Whenthegestureends(oriscanceled),therecognizerenterstheended(orcanceled)stateandsendsitsactionmessageafinaltimebeforereturningtothepossiblestate.

Forgesturerecognizersthatpickuponadiscretegesturelikeatap,youwillonlyseetheWOW! eBook

www.wowebook.org

recognizedstate(whichhasthesamevalueastheendedstate).

Thefourbuilt-inrecognizersthatyoudidnotimplementinthischapterareUIPinchGestureRecognizer,UISwipeGestureRecognizer,UIScreenEdgePanGestureRecognizer,andUIRotationGestureRecognizer.Eachhaspropertiesthatallowyoutofine-tuneitsbehavior.Thedocumentationwillshowyouhow.

Finally,ifthereisagesturethatyouwanttorecognizethatisnotimplementedbythebuilt-insubclassesofUIGestureRecognizer,youcansubclassUIGestureRecognizeryourself.Thisisanintenseundertakingandoutsidethescopeofthisbook.YoucanreadtheMethodsforSubclassingsectionoftheUIGestureRecognizerdocumentationtolearnwhatisrequired.


SilverChallenge:MysteriousLinesThereisabugintheapplication.Ifyoutaponalineandthenstartdrawinganewonewhilethemenuisvisible,youwilldragtheselectedlineanddrawanewlineatthesametime.Fixthisbug.


GoldChallenge:SpeedandSizePiggy-backoffofthepangesturerecognizertorecordthevelocityofthepanwhenyouaredrawingaline.Adjustthethicknessofthelinebeingdrawnbasedonthisspeed.Makenoassumptionsabouthowsmallorlargethevelocityvalueofthepanrecognizercanbe.(Inotherwords,logavarietyofvelocitiestotheconsolefirst.)


PlatinumChallenge:ColorsHaveathree-fingerswipeupwardbringupapanelthatshowscolors.Selectingoneofthosecolorsshouldmakeanylinesyoudrawafterwardappearinthatcolor.Noextralinesshouldbedrawnbyputtingupthatpanel–oranylinesdrawnshouldbeimmediatelydeletedwhentheapplicationrealizesthatitisdealingwithathree-fingerswipe.


FortheMoreCurious:UIMenuControllerandUIResponderStandardEditActionsTheUIMenuControlleristypicallyresponsibleforshowingtheuseran“edit”menuwhenitisdisplayed.(Thinkofatextfieldortextviewwhenyoupressandhold.)Therefore,anunmodifiedmenucontroller(onethatyoudonotsetthemenuitemsfor)alreadyhasdefaultmenuitemsthatitpresents,likeCut,Copy,andotherfamiliaroptions.Eachitemhasanactionmessagewiredup.Forexample,cut:issenttotheviewpresentingthemenucontrollerwhentheCutmenuitemistapped.

AllinstancesofUIResponderimplementthesemethods,but,bydefault,thesemethodsdonotdoanything.SubclasseslikeUITextFieldoverridethesemethodstodosomethingappropriatefortheircontext,likecutthecurrentlyselectedtext.ThemethodsarealldeclaredintheUIResponderStandardEditActionsprotocol.

IfyouoverrideamethodfromUIResponderStandardEditActionsinaview,itsmenuitemwillautomaticallyappearinanymenuyoushowforthatview.ThisworksbecausethemenucontrollercallsthemethodcanPerformAction(_:withSender:)onitsview,whichreturnstrueorfalsedependingonwhethertheviewimplementsthismethod.

Ifyouwanttoimplementoneofthesemethodsbutdonotwantittoappearinthemenu,youcanoverridecanPerformAction(_:withSender:)toreturnfalse.overridefunccanPerformAction(action:Selector,withSendersender:AnyObject?)->Bool{

ifaction=="copy:"{returnfalse}else{//Elsereturnthedefaultbehaviorreturnsuper.canPerformAction(action,withSender:sender)}}


19WebServices

Inthenextfourchapters,youwillcreateanapplicationnamedPhotoramathatreadsinalistofrecentphotosfromFlickr.Thischapterwilllaythefoundationandfocusonimplementingthewebservicerequestsresponsibleforfetchingthemetadataforrecentphotosaswellasdownloadingtheimagedataforaspecificphoto.InChapter20,youwilldisplayalloftherecentphotosinagridlayout.Figure19.1showsPhotoramaattheendofthischapter.

Figure19.1Photorama

YourwebbrowserusestheHTTPprotocoltocommunicatewithawebserver.Inthesimplestinteraction,thebrowsersendsarequesttotheserverspecifyingaURL.Theserverrespondsbysendingbacktherequestedpage(typicallyHTMLandimages),whichthebrowserformatsanddisplays.

Inmorecomplexinteractions,browserrequestsincludeotherparameters,suchasformdata.Theserverprocessestheseparametersandreturnsacustomized,ordynamic,webpage.

Webbrowsersarewidelyusedandhavebeenaroundforalongtime,sothetechnologiessurroundingHTTParestableandwelldeveloped:HTTPtrafficpassesneatlythroughmostfirewalls,webserversareverysecureandhavegreatperformance,andwebapplicationdevelopmenttoolshavebecomeeasytouse.

YoucanwriteaclientapplicationforiOSthatleveragestheHTTPinfrastructuretotalktoaweb-enabledserver.Theserversideofthisapplicationisawebservice.YourclientapplicationandthewebservicecanexchangerequestsandresponsesviaHTTP.


BecausetheHTTPprotocoldoesnotcarewhatdataittransports,theseexchangescancontaincomplexdata.ThisdataistypicallyinJSON(JavaScriptObjectNotation)orXML(ExtensibleMarkupLanguage)format.Ifyoucontrolthewebserveraswellastheclient,youcanuseanyformatyoulike.Ifnot,youhavetobuildyourapplicationtousewhatevertheserversupports.

PhotoramawillmakeawebservicerequesttogetrecentphotosfromFlickr.Thewebserviceishostedathttps://api.flickr.com/services/rest.ThedatathatisreturnedwillbeJSONthatdescribesthephotos.


StartingthePhotoramaApplicationCreateanewSingleViewApplicationfortheUniversaldevicefamily.NamethisapplicationPhotorama,asshowninFigure19.2.

Figure19.2Creatingasingleviewapplication

Let’sknockoutthebasicUIbeforefocusingonwebservices.CreateanewSwiftfilenamedPhotosViewController.InPhotosViewController.swift,definethePhotosViewControllerclassandgiveitanimageViewproperty.importFoundationimportUIKit

classPhotosViewController:UIViewController{

@IBOutletvarimageView:UIImageView!

}

Intheprojectnavigator,deletetheexistingViewController.swift.

OpenMain.storyboardandselecttheViewController.OpenitsidentityinspectorandchangetheClasstoPhotosViewController.WiththePhotosViewControllerstillselected,selecttheEditormenuandchooseEmbedIn→NavigationController.

SelecttheNavigationControllerandopenitsattributesinspector.UndertheViewControllerheading,makesuretheboxforIsInitialViewControllerischecked.

DraganImageViewontothecanvasforPhotosViewControllerandaddconstraintstopinittoalledgesofthesuperview.ConnecttheimageviewtotheimageViewoutletonPhotosViewController.OpentheattributesinspectorfortheimageviewandchangetheModetoAspectFill.

Finally,double-clickonthecenterofthenavigationbarforthePhotosViewControllerand


giveitatitleofPhotorama.YourinterfacewilllooklikeFigure19.3.

Figure19.3InitialPhotoramainterface

Buildandruntheapplicationtomakesuretherearenoerrors.


BuildingtheURLCommunicationwithserversisdoneviarequests.Arequestencapsulatesinformationabouttheinteractionbetweentheapplicationandtheserver,anditsmostimportantpieceofinformationisthedestinationURL.

Inthissection,youwillbuilduptheURLforretrievingrecentphotosfromtheFlickrwebservice.Thearchitectureoftheapplicationwillreflectbestpractices.Forexample,eachtypethatyoucreatewillencapsulateasingleresponsibility.Thiswillmakeyourtypesrobustandflexibleandyourapplicationeasiertoreasonabout.TobeagoodiOSdeveloper,younotonlyneedtogetthejobdone,youalsoneedtogetitdonethoughtfullyandwithforesight.

FormattingURLsandrequests

Theformatofawebservicerequestvariesdependingontheserverthattherequestisreachingoutto.Therearenoset-in-stoneruleswhenitcomestowebservices.Youwillneedtofindthedocumentationforthewebservicetoknowhowtoformatarequest.Aslongasaclientapplicationsendstheserverwhatitwants,youhaveaworkingexchange.

Flickr’srecentphotoswebservicewantsaURLthatlookslikethis:https://api.flickr.com/services/rest/?method=flickr.photos.getRecent&api_key=a6d819499131071f158fd740860a5a88&extras=url_h,date_taken&format=json&nojsoncallback=1

Webservicerequestscomeinallsortsofformats,dependingonwhatthecreatorofthatwebserviceistryingtoaccomplish.Therecentphotoswebservice,wherepiecesofinformationarebrokenupintokey-valuepairs,isprettycommon.

Thekey-valuepairsthataresuppliedaspartoftheURLarecalledqueryitems.EachofthequeryitemsfortherecentphotosrequestisdefinedbyandisuniquetotheFlickrAPI.

ThemethoddetermineswhichendpointyouwanttohitontheFlickrAPI.Fortherecentphotos,thisisthestring“flickr.photos.getRecent”.

Theapi_keyisakeythatFlickrgeneratestoauthorizeanapplicationtousetheFlickrAPI.

Theextrasareattributespassedintocustomizetheresponse.Here,theurl_h,date_takenvaluetellstheFlickrserverthatyouwantthephotoURLstoalsocomebackintheresponsealongwiththedatethephotowastaken.

TheformatitemspecifiesthatyouwantthepayloadcomingbacktobeJSON.

ThenojsoncallbackitemspecifiesthatyouwantJSONbackinitsrawformat.

NSURLComponents

Youwillcreatetwotypestodealwithallofthewebserviceinformation.The


FlickrAPIstructwillberesponsibleforknowingandhandlingallFlickr-relatedinformation.ThisincludesknowinghowtogeneratetheURLsthattheFlickrAPIexpectsaswellasknowingtheformatoftheincomingJSONandhowtoparsethatJSONintotherelevantmodelobjects.ThePhotoStoreclasswillhandletheactualwebservicecalls.Let’sstartbycreatingtheFlickrAPIstruct.

CreateanewSwiftfilenamedFlickrAPI.ThisfilewilldefineaFlickrAPIstructthatwillcontainalloftheknowledgethatisspecifictotheFlickrAPI.

InFlickrAPI.swift,declaretheFlickrAPIstruct.importFoundation

structFlickrAPI{

}

AnenumerationwillbeusedtospecifywhichendpointontheFlickrservertohit.Forthisapplication,youwillonlybeworkingwiththeendpointtogetrecentphotos.However,FlickrsupportsmanyadditionalAPIs,suchassearchingforimagesbasedonastring.Usinganenumnowwillmakeiteasiertoaddendpointsinthefuture.

InFlickrAPI.swift,createtheMethodenumeration.EachcaseofMethodhasarawvaluethatmatchesthecorrespondingFlickrendpoint.Also,defineaconstantforthebaseURLstring.importFoundation

enumMethod:String{caseRecentPhotos="flickr.photos.getRecent"}

structFlickrAPI{

}

InChapter2,youlearnedthatenumerationscanhaverawvaluesassociatedwiththem.AlthoughtherawvaluesareoftenInts,youcanseehereagreatuseofStringastherawvaluefortheMethodenumeration.

Nowdeclareatype-levelpropertytoreferencethebaseURLstringforthewebservicerequests.enumMethod:String{caseRecentPhotos="flickr.photos.getRecent"}

structFlickrAPI{

staticletbaseURLString="https://api.flickr.com/services/rest"}

Atype-levelproperty(ormethod)isonethatisaccessedonthetypeitself–inthiscase,theFlickrAPItype.Forstructs,typepropertiesandmethodsaredeclaredwiththestatickeyword;classesusetheclasskeyword.YouusedatypemethodontheUIViewclassinChapter8whenyoucalledtheanimateWithDuration(_:animations:)method.YoualsousedatypemethodonUIImagePickerControllerinChapter14whenyoucalledtheisSourceTypeAvailable(_:)method.Here,youaredeclaringatype-levelpropertyonFlickrAPI.


ThebaseURLStringisanimplementationdetailoftheFlickrAPItype,andnootherfileneedstoknowaboutit.Instead,theywillaskforacompletedURLfromFlickrAPI.TokeepotherfilesfrombeingabletoaccessbaseURLString,youwillmarkthepropertyasprivate.structFlickrAPI{

privatestaticletbaseURLString="https://api.flickr.com/services/rest"}

Thisiscalledaccesscontrol.Youcancontrolwhatcanaccessthepropertiesandmethodsonyourowntypes.Therearethreelevelsofaccesscontrolthatcanbeappliedtotypes,properties,andmethods:

public–Anythingcanaccessthistype,property,ormethod.Thisincludeseverythinginyourproject(whichisonemodule)aswellascodefromothermodules(suchasthird-partyframeworks).

internal–Thisisthedefault.Anythinginthecurrentmodulecanaccessthistype,property,ormethod.Foranapp,onlyfileswithinyourprojectcanaccessthese.Ifyouwriteathird-partylibrary,thenonlyfileswithinthatthird-partylibrarycanaccessthem–appsthatuseyourthird-partylibrarycannot.

private–Anythinginthesamesourcefilecanseethistype,property,ormethod.Thisisdifferentthanotherlanguageswhereonlythetypecanseeit.InSwift,thevisibilityforthingsmarkedasprivateisthesourcefile(the.swiftfile).

NowyouaregoingtocreateatypemethodthatbuildsuptheFlickrURLforaspecificendpoint.Thismethodwillaccepttwoarguments:thefirstwillspecifywhichendpointtohit,usingtheMethodenumeration,andthesecondwillbeanoptionaldictionaryofqueryitemparametersassociatedwiththerequest.

ImplementthismethodinyourFlickrAPIstructinFlickrAPI.swift.Fornow,thismethodwillreturnanemptyURL.privatestaticfuncflickrURL(methodmethod:Method,parameters:[String:String]?)->NSURL{

returnNSURL()}

NoticethattheflickrURL(method:parameters:)methodisprivate.ItisanimplementationdetailoftheFlickrAPIstruct.AninternaltypemethodwillbeexposedtotherestoftheprojectforeachofthespecificendpointURLs(whichiscurrentlyjusttherecentphotosendpoint).TheseinternaltypemethodswillcallthroughtotheflickrURL(method:parameters:)method.

InFlickrAPI.swift,defineandimplementtherecentPhotosURL()method.staticfuncrecentPhotosURL()->NSURL{returnflickrURL(method:.RecentPhotos,parameters:["extras":"url_h,date_taken"])}

TimetoconstructthefullURL.YouhavethebaseURLdefinedasaconstant,andthe


queryitemsarebeingpassedintotheflickrURL(method:parameters:)methodviatheparametersargument.YouwillbuilduptheURLusingtheNSURLComponentsclass,whichisdesignedtotakeinthesevariouscomponentsandconstructanNSURLfromthem.

UpdatetheflickrURL(method:parameters:)methodtoconstructaninstanceofNSURLComponentsfromthebaseURL.Then,loopovertheincomingparametersandcreatetheassociatedNSURLQueryIteminstances.privatestaticfuncflickrURL(methodmethod:Method,parameters:[String:String]?)->NSURL?{

returnNSURL()

letcomponents=NSURLComponents(string:baseURLString)!

varqueryItems=[NSURLQueryItem]()

ifletadditionalParams=parameters{for(key,value)inadditionalParams{letitem=NSURLQueryItem(name:key,value:value)queryItems.append(item)}}components.queryItems=queryItems

returncomponents.URL!}

ThelaststepinsettinguptheURListopassintheparametersthatarecommontoallrequests:method,api_key,format,andnojsoncallback.

TheAPIkeyisatokengeneratedbyFlickrtoidentifyyourapplicationandauthenticateitwiththewebservice.WehavegeneratedanAPIkeyforthisapplicationbycreatingaFlickraccountandregisteringthisapplication.(IfyouwouldlikeyourownAPIkey,youwillneedtoregisteranapplicationathttps://www.flickr.com/services/apps/create/.)

InFlickrAPI.swift,createaconstantthatreferencesthistoken.structFlickrAPI{

privatestaticletbaseURLString="https://api.flickr.com/services/rest"privatestaticletAPIKey="a6d819499131071f158fd740860a5a88"

Double-checktomakesureyouhavetypedintheAPIkeyexactlyaspresentedhere.Ithastomatchexactlyortheserverwillrejectyourrequests.IfyourAPIkeyisnotworkingorifyouhaveanyproblemswiththerequests,checkouttheforumsathttp://forums.bignerdranch.comforhelp.

FinishimplementingflickrURL(method:parameters:)toaddthecommonqueryitemstotheNSURLComponents.privatestaticfuncflickrURL(methodmethod:Method,parameters:[String:String]?)->NSURL{

letcomponents=NSURLComponents(string:baseURLString)!

varqueryItems=[NSURLQueryItem]()

letbaseParams=["method":method.rawValue,"format":"json",


https://www.flickr.com/services/apps/create/

"nojsoncallback":"1","api_key":APIKey]

for(key,value)inbaseParams{letitem=NSURLQueryItem(name:key,value:value)queryItems.append(item)}

ifletadditionalParams=parameters{for(key,value)inadditionalParams{letitem=NSURLQueryItem(name:key,value:value)queryItems.append(item)}}components.queryItems=queryItems

returncomponents.URL!}


SendingtheRequestAURLrequestencapsulatesinformationaboutthecommunicationfromtheapplicationtotheserver.Mostimportantly,itspecifiestheURLoftheserverfortherequest,butitalsohasatimeoutinterval,acachepolicy,andothermetadataabouttherequest.ArequestisrepresentedbytheNSURLRequestclass.CheckouttheFortheMoreCurioussectionattheendofthischapterformoreinformation.

TheNSURLSessionAPIisacollectionofclassesthatusearequesttocommunicatewithaserverinanumberofways.TheNSURLSessionTaskclassisresponsibleforcommunicatingwithaserver.TheNSURLSessionclassisresponsibleforcreatingtasksthatmatchagivenconfiguration.

Anewclass,PhotoStore,willberesponsibleforinitiatingthewebservicerequests.ItwillusetheNSURLSessionAPIandtheFlickrAPIstructtofetchalistofrecentphotosanddownloadtheimagedataforeachphoto.

CreateanewSwiftfilenamedPhotoStore.

OpenPhotoStore.swiftanddeclarethePhotoStoreclass.importFoundation

classPhotoStore{

}

NSURLSession

Let’slookatafewofthepropertiesonNSURLRequest:

allHTTPHeaderFields–ThisisadictionaryofmetadataabouttheHTTPtransaction,includingcharacterencodingandhowtheservershouldhandlecaching.

allowsCellularAccess–ThisBooleanrepresentswhetherarequestisallowedtousecellulardata.

cachePolicy–Thisdeterminesifandhowthelocalcacheshouldbeused.

HTTPMethod–Thisistherequestmethod.ThedefaultisGET;othervaluesarePOST,PUT,andDELETE.

timeoutInterval–Thisisthemaximumdurationaconnectiontotheserverwillbeattemptedfor.

TheclassthatcommunicateswiththewebserviceisaninstanceofNSURLSessionTask.Therearethreekindsoftasks:datatasks,downloadtasks,anduploadtasks.NSURLSessionDataTaskretrievesdatafromtheserverandreturnsitasNSDatainmemory.NSURLSessionDownloadTaskretrievesdatafromtheserverandreturnsitasafilesavedtothefilesystem.NSURLSessionUploadTasksendsdatatotheserver.


Often,youwillhaveagroupofrequeststhathavemanypropertiesincommon.Forexample,maybesomedownloadsshouldneverhappenovercellulardata,ormaybecertainrequestsshouldbecacheddifferentlythanothers.Itcanbecometedioustoconfigurerelatedrequeststhesameway.

ThisiswhereNSURLSessioncomesinhandy.NSURLSessionactsasafactoryforNSURLSessionTaskinstances.Thesessioniscreatedwithaconfigurationthatspecifiespropertiesthatarecommonacrossallofthetasksthatitcreates.AlthoughmanyapplicationsmightonlyneedtouseasingleinstanceofNSURLSession,havingthepowerandflexibilityofmultiplesessionsisagreattooltohaveatyourdisposal.

InPhotoStore.swift,addapropertytoholdontoaninstanceofNSURLSession.classPhotoStore{

letsession:NSURLSession={letconfig=NSURLSessionConfiguration.defaultSessionConfiguration()returnNSURLSession(configuration:config)}()

}

InPhotoStore.swift,implementthefetchRecentPhotos()methodtocreateanNSURLRequestthatconnectstoapi.flickr.comandasksforthelistofrecentphotos.Then,usetheNSURLSessiontocreateanNSURLSessionDataTaskthattransfersthisrequesttotheserver.funcfetchRecentPhotos(){

leturl=FlickrAPI.recentPhotosURL()letrequest=NSURLRequest(URL:url)lettask=session.dataTaskWithRequest(request){(data,response,error)->Voidin

ifletjsonData=data{ifletjsonString=NSString(data:jsonData,encoding:NSUTF8StringEncoding){print(jsonString)}}elseifletrequestError=error{print("Errorfetchingrecentphotos:\(requestError)")}else{print("Unexpectederrorwiththerequest")}}task.resume()}

CreatingtheNSURLRequestisfairlystraightforward:youcreateanNSURLinstanceusingtheFlickrAPIstructandinstantiatearequestobjectwithit.

Bygivingthesessionarequestandacompletionclosuretocallwhentherequestfinishes,thesessionwillreturnaninstanceofNSURLSessionTask.SincePhotoramaisrequestingdatafromawebservice,thetypeoftaskwillbeaninstanceofNSURLSessionDataTask.Tasksarealwayscreatedinthesuspendedstate,socallingresume()onthetaskwillstartthewebservicerequest.Fornow,thecompletionblockwilljustprintouttheJSONdatareturnedfromtherequest.

Tomakearequest,PhotosViewControllerwillcalltheappropriatemethodson


PhotoStore.Todothis,PhotosViewControllerneedsareferencetoaninstanceofPhotoStore.

AtthetopofPhotosViewController.swift,addapropertytohangontoaninstanceofPhotoStore.classPhotosViewController:UIViewController{

@IBOutletvarimageView:UIImageView!varstore:PhotoStore!

ThestoreisadependencyofthePhotosViewController.YouwillusepropertyinjectiontogivethePhotosViewControlleritsstoredependency,justasyoudidwiththeviewcontrollersinHomepwner.

OpenAppDelegate.swiftandusepropertyinjectiontogivethePhotosViewControlleraninstanceofPhotoStore.funcapplication(application:UIApplication,didFinishLaunchingWithOptionslaunchOptions:[NSObject:AnyObject]?)->Bool{//Overridepointforcustomizationafterapplicationlaunch.

letrootViewController=window!.rootViewControlleras!UINavigationControllerletphotosViewController=rootViewController.topViewControlleras!PhotosViewControllerphotosViewController.store=PhotoStore()

returntrue}

NowthatthePhotosViewControllercaninteractwiththePhotoStore,kickoffthewebserviceexchangewhentheviewcontrolleriscomingonscreenforthefirsttime.

InPhotosViewController.swift,overrideviewDidLoad()andfetchtherecentphotos.overridefuncviewDidLoad(){super.viewDidLoad()

store.fetchRecentPhotos()}

Buildandruntheapplication.AstringrepresentationoftheJSONdatacomingbackfromthewebservicewillprinttotheconsole.(Ifyoudonotseeanythingprinttotheconsole,makesureyoutypedtheURLandAPIkeycorrectly.)

TheresponsewilllooksomethinglikeFigure19.4.


Figure19.4Webserviceconsoleoutput


ModelingthePhotoNext,youwillcreateaPhotoclasstorepresenteachphotothatisreturnedfromthewebservicerequest.Therelevantpiecesofinformationthatyouwillneedforthisapplicationaretheid,thetitle,theurl_h,andthedatetaken.

CreateanewSwiftfileandnameitPhoto.OpenthisfileanddeclarethePhotoclasswithpropertiesforthephotoID,thetitle,andtheremoteURL.Finally,addadesignatedinitializerthatsetsuptheinstance.importFoundation

classPhoto{

lettitle:StringletremoteURL:NSURLletphotoID:StringletdateTaken:NSDate

init(title:String,photoID:String,remoteURL:NSURL,dateTaken:NSDate){self.title=titleself.photoID=photoIDself.remoteURL=remoteURLself.dateTaken=dateTaken}}

YouwillusethisclassshortlyonceyouareparsingtheJSONdata.


JSONDataJSONdata,especiallywhenitiscondensedlikeitisinyourconsole,mayseemdaunting.However,itisactuallyaverysimplesyntax.JSONcancontainthemostbasictypesusedtorepresentmodelobjects:arrays,dictionaries,strings,andnumbers.AJSONdictionarycontainsoneormorekey-valuepairs,wherethekeyisastringandthevaluecanbeanotherdictionaryorastring,number,orarray.Anarraycanconsistofstrings,numbers,dictionaries,andotherarrays.Thus,aJSONdocumentisanestedsetofthesetypesofvalues.

HereisanexampleofsomereallysimpleJSON:{"name":"Christian","friends":["Stacy","Mikey"],"job":{"company":"BigNerdRanch","title":"SeniorNerd"}}

ThisJSONdocumentbeginsandendswithcurlybraces({and}),whichinJSONdelimitadictionary.Withinthecurlybracesarethekey-valuepairsthatbelongtothedictionary.Thisdictionarycontainsthreekey-valuepairs(name,friends,andjob).

Astringisrepresentedbytextwithinquotationmarks.Stringsareusedasthekeyswithinadictionaryandcanbeusedasvalues,too.Thus,thevalueassociatedwiththenamekeyinthetop-leveldictionaryisthestringChristian.

Arraysarerepresentedwithsquarebrackets([and]).AnarraycancontainanyotherJSONinformation.Inthiscase,thefriendskeyholdsanarrayofstrings(StacyandMikey).

Adictionarycancontainotherdictionaries,andthefinalkeyinthetop-leveldictionary,job,isassociatedwithadictionarythathastwokey-valuepairs(companyandtitle).

PhotoramawillparseouttheusefulinformationfromtheJSONdataandstoreitinaPhotoinstance.

NSJSONSerialization

Applehasabuilt-inclassforparsingJSONdata,NSJSONSerialization.YoucanhandthisclassabunchofJSONdata,anditwillcreateadictionaryforeveryJSONdictionary(theJSONspecificationcallsthese“objects”),anarrayforeveryJSONarray,aStringforeveryJSONstring,andanNSNumberforeveryJSONnumber.Let’sseehowthisclasshelpsyou.

OpenPhotoStore.swiftandupdatefetchRecentPhotos()toprinttheJSONobjecttotheconsole.funcfetchRecentPhotos(){

leturl=FlickrAPI.recentPhotosURL()letrequest=NSURLRequest(URL:url)


lettask=session.dataTaskWithRequest(request){(data,response,error)->Voidin

ifletjsonData=data{ifletjsonString=NSString(data:jsonData,encoding:NSUTF8StringEncoding){print(jsonString)}do{letjsonObject:AnyObject=tryNSJSONSerialization.JSONObjectWithData(jsonData,options:[])print(jsonObject)}catchleterror{print("ErrorcreatingJSONobject:\(error)")}}elseifletrequestError=error{print("Errorfetchingrecentphotos:\(requestError)")}else{print("Unexpectederrorwiththerequest")}}task.resume()}

Buildandrun,thenchecktheconsole.YouwillseetheJSONdataagain,butnowitwillbeformatteddifferentlybecauseprint()doesagoodjobformattingdictionariesandarrays.

TheformatoftheJSONdataisdictatedbytheAPI,soyouwilladdthecodetoparsetheJSONtotheFlickrAPIstruct.

Parsingthedatathatcomesbackfromtheservercouldgowronginanumberofways:ThedatamightnotcontainJSON.Thedatacouldbecorrupt.ThedatamightcontainJSONbutnotmatchtheformatthatyouexpect.Tomanagethepossibilityoffailure,youwilluseanenumerationwithassociatedvaluestorepresentthesuccessorfailureoftheparsing.

Enumerationsandassociatedvalues

YoulearnedaboutthebasicsofenumerationsinChapter2,andyouhavebeenusingthemthroughoutthisbook–includingtheMethodenumusedearlierinthischapter.Associatedvaluesareausefulfeatureofenumerations.Let’stakeamomenttolookatasimpleexamplebeforeyouusethisfeatureinPhotorama.

Enumerationsareaconvenientwayofdefiningandrestrictingthepossiblevaluesforavariable.Forexample,let’ssayyouareworkingonahomeautomationapp.Youcoulddefineanenumerationtospecifytheovenstate,likethis:enumOvenState{caseOncaseOff}

Iftheovenison,youalsoneedtoknowwhattemperatureitissetto.Associatedvaluesareaperfectsolutiontothissituation.enumOvenState{caseOn(Double)


caseOff}

varovenState=OvenState.On(450)

Eachcaseofanenumerationcanhavedataofanytypeassociatedwithit.ForOvenState,its.OncasehasanassociatedDoublethatrepresentstheoven’stemperature.Noticethatnotallcasesneedtohaveassociatedvalues.

Retrievingtheassociatedvaluefromanenumisoftendoneusingaswitchstatement.switchovenState{caselet.On(temperature):print("Theovenisonandsetto\(temperature)degrees.")case.Off:print("Theovenisoff.")}

Notethatthe.Oncaseusesaletkeywordtostoretheassociatedvalueinthetemperatureconstant,whichcanbeusedwithinthecaseclause.(Itcanusethevarkeywordinsteadiftemperatureneedstobeavariable.)ConsideringthevaluegiventoovenState,theswitchstatementabovewouldresultintheline“Theovenisonandsetto450degrees.”printedtotheconsole.

Inthenextsection,youwilluseanenumerationwithassociatedvaluestotieasuccessfulresultstatusofarequesttotheFlickrwebservicewiththedatacontainingrecentphotosortieafailureresultstatuswitherrorinformation.

ParsingJSONdata

InFlickrAPI.swift,addanenumerationnamedPhotosResulttothetopofthefilethathasacaseforbothsuccessandfailure.enumMethod:String{caseRecentPhotos="flickr.photos.getRecent"}

enumPhotosResult{caseSuccess([Photo])caseFailure(ErrorType)}

IfthedataisvalidJSONandcontainsanarrayofphotos,thosephotoswillbeassociatedwiththeSuccesscase.Ifthereareanyerrorsduringtheparsingprocess,therelevantErrorTypewillbepassedalongwiththeFailurecase.

ErrorTypeisaprotocolthatallerrorsconformto.NSErroristheerrorthatmanyiOSframeworksthrow,anditconformstoErrorType.YouwillcreateyourownErrorTypeshortly.

InFlickrAPI.swift,implementamethodthattakesinaninstanceofNSDataandusestheNSJSONSerializationclasstoconvertthedataintothebasicfoundationobjects.staticfuncphotosFromJSONData(data:NSData)->PhotosResult{do{letjsonObject:AnyObject=tryNSJSONSerialization.JSONObjectWithData(data,options:[])

varfinalPhotos=[Photo]()


return.Success(finalPhotos)}catchleterror{return.Failure(error)}}

(Thiscodewillgeneratesomewarnings.Youwillresolvethemshortly.)

IftheincomingdataisvalidJSONdata,thenthejsonObjectinstancewillreferencetheappropriatemodelobject.Ifnot,thentherewasaproblemwiththedataandyoupassalongtheerror.YounowneedtogetthephotoinformationoutoftheJSONobjectandintoinstancesofPhoto.

WhentheNSURLSessionDataTaskfinishes,youwilluseNSJSONSerializationtoconverttheJSONdataintoadictionary.Figure19.5showshowthedatawillbestructured.

AtthetopleveloftheincomingJSONdataisadictionary.Thevalueassociatedwiththe“photos”keycontainstheimportantinformation,andthemostimportantisthearrayofdictionaries.

Figure19.5JSONobjects


Asyoucansee,youhavetodigprettydeeplytogettheinformationthatyouneed.

IfthestructureoftheJSONdoesnotmatchyourexpectation,youwillreturnacustomerror.

AtthetopofFlickrAPI.swift,declareacustomenumtorepresentpossibleerrorsfortheFlickrAPI.enumPhotosResult{caseSuccess([Photo])caseFailure(ErrorType)}

enumFlickrError:ErrorType{caseInvalidJSONData}

InphotosFromJSONData(_:),digdownthroughtheJSONtogettothearrayofdictionariesrepresentingtheindividualphotos.staticfuncphotosFromJSONData(data:NSData)->PhotosResult{do{letjsonObject:AnyObject=tryNSJSONSerialization.JSONObjectWithData(data,options:[])

guardletjsonDictionary=jsonObjectas?[NSObject:AnyObject],photos=jsonDictionary["photos"]as?[String:AnyObject],photosArray=photos["photo"]as?[[String:AnyObject]]else{

//TheJSONstructuredoesn'tmatchourexpectationsreturn.Failure(FlickrError.InvalidJSONData)}

varfinalPhotos=[Photo]()return.Success(finalPhotos)}catchleterror{return.Failure(error)}}

ThenextstepistogetthephotoinformationoutofthedictionaryandintoPhotomodelobjects.

YouwillneedaninstanceofNSDateFormattertoconvertthedatetakenstringintoaninstanceofNSDate.

InFlickrAPI.swift,addaconstantinstanceofNSDateFormatter.privatestaticletbaseURLString="https://api.flickr.com/services/rest"privatestaticletAPIKey="a6d819499131071f158fd740860a5a88"

privatestaticletdateFormatter:NSDateFormatter={letformatter=NSDateFormatter()formatter.dateFormat="yyyy-MM-ddHH:mm:ss"returnformatter}()

StillinFlickrAPI.swift,writeanewmethodtoparseaJSONdictionaryintoaPhotoinstance.privatestaticfuncphotoFromJSONObject(json:[String:AnyObject])->Photo?{guardletphotoID=json["id"]as?String,title=json["title"]as?String,dateString=json["datetaken"]as?String,photoURLString=json["url_h"]as?String,url=NSURL(string:photoURLString),


dateTaken=dateFormatter.dateFromString(dateString)else{

//Don'thaveenoughinformationtoconstructaPhotoreturnnil}

returnPhoto(title:title,photoID:photoID,remoteURL:url,dateTaken:dateTaken)}

NowupdatephotosFromJSONData(_:)toparsethedictionariesintoPhotoobjectsandthenreturntheseaspartoftheSuccessenumerator.AlsohandlethepossibilitythattheJSONformathaschanged,sonophotoswereabletobefound.staticfuncphotosFromJSONData(data:NSData)->PhotosResult{

do{letjsonObject:AnyObject=tryNSJSONSerialization.JSONObjectWithData(data,options:[])



varfinalPhotos=[Photo]()forphotoJSONinphotosArray{ifletphoto=photoFromJSONObject(photoJSON){finalPhotos.append(photo)}}

iffinalPhotos.count==0&&photosArray.count>0{//Weweren'tabletoparseanyofthephotos//MaybetheJSONformatforphotoshaschangedreturn.Failure(FlickrError.InvalidJSONData)}return.Success(finalPhotos)}catchleterror{return.Failure(error)}}

Next,inPhotoStore.swift,writeanewmethodthatwillprocesstheJSONdatathatisreturnedfromthewebservicerequest.funcprocessRecentPhotosRequest(datadata:NSData?,error:NSError?)->PhotosResult{guardletjsonData=dataelse{return.Failure(error!)}

returnFlickrAPI.photosFromJSONData(jsonData)}

NowupdatefetchRecentPhotos()tousethemethodyoujustcreated.funcfetchRecentPhotos(){leturl=FlickrAPI.recentPhotosURL()letrequest=NSURLRequest(URL:url)lettask=session.dataTaskWithRequest(request){(data,response,error)->Voidin

ifletjsonData=data{do{letjsonObject:AnyObject=tryNSJSONSerialization.JSONObjectWithData(jsonData,options:[])print(jsonObject)}


catchleterror{print("ErrorcreatingJSONobject:\(error)")}}elseifletrequestError=error{print("Errorfetchingrecentphotos:\(requestError)")}else{print("Unexpectederrorwiththerequest")}

letresult=self.processRecentPhotosRequest(data:data,error:error)}task.resume()}

NowupdatethemethodsignatureforfetchRecentPhotos()totakeinacompletionclosurethatwillbecalledoncethewebservicerequestiscompleted.funcfetchRecentPhotos(){funcfetchRecentPhotos(completioncompletion:(PhotosResult)->Void){leturl=FlickrAPI.recentPhotosURL()letrequest=NSURLRequest(URL:url)lettask=session.dataTaskWithRequest(request){(data,response,error)->Voidin

letresult=self.processRecentPhotosRequest(data:data,error:error)completion(result)}task.resume()}

Fetchingdatafromawebserviceisanasynchronousprocess.Oncetherequeststarts,itmaytakeanontrivialamountoftimeforaresponsetocomebackfromtheserver.Becauseofthis,thefetchRecentPhotos(_:)methodcannotdirectlyreturnaninstanceofPhotosResult.Instead,thecallerofthismethodwillsupplyacompletionclosureforthePhotoStoretocalloncetherequestiscomplete.

ThisfollowsthesamepatternthatNSURLSessionTaskuseswithitscompletionhandler:thetaskiscreatedwithaclosureforittocalloncethewebservicerequestcompletes.Figure19.6describestheflowofdatawiththewebservicerequest.


Figure19.6Webservicerequestdataflow

InPhotosViewController.swift,updatetheimplementationoftheviewDidLoad()toprintouttheresultofthewebservicerequest.overridefuncviewDidLoad()super.viewDidLoad()

store.fetchRecentPhotos(){(photosResult)->Voidin

switchphotosResult{caselet.Success(photos):print("Successfullyfound\(photos.count)recentphotos.")caselet.Failure(error):print("Errorfetchingrecentphotos:\(error)")}

}}

Buildandruntheapplication.Oncethewebservicerequestcompletes,youshouldseethenumberofphotosfoundprintedtotheconsole.


DownloadingandDisplayingtheImageDataYouhavedonealotalreadyinthischapter:youhavesuccessfullyinteractedwiththeFlickrAPIviaawebservicerequest,andyouhaveparsedtheincomingJSONdataintoPhotomodelobjects.Unfortunately,youhavenothingtoshowforitexceptsomelogmessagesintheconsole.

Inthissection,youwillusetheURLreturnedfromthewebservicerequesttodownloadtheimagedata.ThenyouwillcreateaninstanceofUIImagefromthatdataand,finally,youwilldisplaythefirstimagereturnedfromtherequestinaUIImageView.(Inthenextchapter,youwilldisplayalloftheimagesthatarereturnedinagridlayoutdrivenbyaUICollectionView.)

Thefirststepisdownloadingtheimagedata.Thisprocesswillbeverysimilartothewebservicerequesttodownloadthephotos’JSONdata.

OpenPhoto.swiftandgiveitanoptionalUIImageproperty.ImporttheUIKitframeworkinsteadofFoundation.importFoundationimportUIKit

classPhoto{

lettitle:StringletremoteURL:NSURLletphotoID:StringletdateTaken:NSDatevarimage:UIImage?

OpenPhotoStore.swift,importUIKit,andaddanenumerationtothetopofthefilethatrepresentstheresultofdownloadingtheimage.ThisenumerationwillfollowthesamepatternasthePhotosResultenumeration,takingadvantageofassociatedvalues.YouwillalsocreateanErrorTypetorepresentphotoerrors.importFoundationimportUIKit

enumImageResult{caseSuccess(UIImage)caseFailure(ErrorType)}

enumPhotoError:ErrorType{caseImageCreationError}

Ifthedownloadissuccessful,theSuccesscasewillhavetheUIImageassociatedwithit.Ifthereisanerror,theFailurecasewillhavetheErrorTypeassociatedwithit.

Now,inthesamefile,implementamethodtodownloadtheimagedata.LikethefetchRecentPhotos(_:)method,thisnewmethodwilltakeinacompletionclosurethatwillreturnaninstanceofImageResult.funcfetchImageForPhoto(photo:Photo,completion:(ImageResult)->Void){

letphotoURL=photo.remoteURLletrequest=NSURLRequest(URL:photoURL)

lettask=session.dataTaskWithRequest(request){


(data,response,error)->Voidin

}task.resume()}

Nowimplementamethodthatprocessesthedatafromthewebservicerequestintoanimage,ifpossible.funcprocessImageRequest(datadata:NSData?,error:NSError?)->ImageResult{

guardletimageData=data,image=UIImage(data:imageData)else{

//Couldn'tcreateanimageifdata==nil{return.Failure(error!)}else{return.Failure(PhotoError.ImageCreationError)}}

return.Success(image)}

StillinPhotoStore.swift,updatefetchImageForPhoto(_:completion:)tousethisnewmethod.funcfetchImageForPhoto(photo:Photo,completion:(ImageResult)->Void){


lettask=session.dataTaskWithRequest(request){(data,response,error)->Voidin

letresult=self.processImageRequest(data:data,error:error)

ifcaselet.Success(image)=result{photo.image=image}

completion(result)}task.resume()}

Sinceyouonlyneedtohandlethe.Successcase,youuseanifcasestatementtocheckwhetherresulthasavalueof.Success.Thisisidenticaltoonlyhandlingthatcaseinaswitchstatement://Thiscodeifcaselet.Success(image)=result{photo.image=image}

//Behavesjustlikethiscodeswitchresult{caselet.Success(image):photo.image=imagecase.Failure(_):break}

Totestthiscode,youwilldownloadtheimagedataforthefirstphotothatisreturnedfromtherecentphotosrequestanddisplayitontheimageview.

OpenPhotosViewController.swiftandupdateviewDidLoad().


overridefuncviewDidLoad()super.viewDidLoad()


switchphotosResult{caselet.Success(photos):print("Successfullyfound\(photos.count)recentphotos.")

ifletfirstPhoto=photos.first{self.store.fetchImageForPhoto(firstPhoto){(imageResult)->Voidin

switchimageResult{caselet.Success(image):self.imageView.image=imagecaselet.Failure(error):print("Errordownloadingimage:\(error)")}}}caselet.Failure(error):print("Errorfetchingrecentphotos:\(error)")}}}

Althoughyoucouldbuildandruntheapplicationatthispoint,theimagemayormaynotappearintheimageviewwhenthewebservicerequestfinishes.Why?Thecodethatupdatestheimageviewisnotbeingrunonthemainthread.


TheMainThreadModerniOSdeviceshavemulticoreprocessorsthatenablethemtorunmultiplechunksofcodesimultaneously.Thesecomputationsproceedinparallel,sothisisreferredtoasparallelcomputing.Whendifferentcomputationsareinflightatthesametime,thisisknownasconcurrency,andthecomputationsaresaidtobehappeningconcurrently.Acommonwaytoexpressthisisbyrepresentingeachcomputationwithadifferentthreadofcontrol.

Sofarinthisbook,allofyourcodehasbeenrunningonthemainthread.ThemainthreadissometimesreferredtoastheUI(userinterface)thread,becauseanycodethatmodifiestheUImustrunonthemainthread.

Whenthewebservicecompletes,youwantittoupdatetheimageview.Butbydefault,NSURLSessionDataTaskrunsthecompletionhandleronabackgroundthread.Youneedawaytoforcecodetorunonthemainthreadinordertoupdatetheimageview.YoucandothateasilyusingtheNSOperationQueueclass.

InPhotosViewController.swift,updateviewDidLoad()tocallthecompletionclosureonthemainthread.overridefuncviewDidLoad()super.viewDidLoad()


switchphotosResult{caselet.Success(photos):print("Successfullyfound\(photos.count)recentphotos.")

ifletfirstPhoto=photos.first{self.store.fetchImageForPhoto(firstPhoto){(imageResult)->Voidin

switchimageResult{caselet.Success(image):self.imageView.image=imageNSOperationQueue.mainQueue().addOperationWithBlock{self.imageView.image=image}caselet.Failure(error):print("Errordownloadingimage:\(error)")}}}caselet.Failure(error):print("Errorfetchingrecentphotos:\(error)")}}}

Buildandruntheapplication.Nowthattheimageviewisbeingupdatedonthemainthread,youwillhavesomethingtoshowforallyourhardwork:theimagewillappearwhenthewebservicerequestfinishes.(Itmighttakealittletimetoshowtheimageifthewebservicerequesttakesawhiletofinish.)


BronzeChallenge:PrintingtheResponseInformationThecompletionhandlerfordataTaskWithRequest(_:completionHandler:)providesaninstanceofNSURLResponse.WhenmakingHTTPrequests,thisresponseisoftypeNSURLHTTPResponse(asubclassofNSURLResponse).PrintthestatusCodeandheaderFieldstotheconsole.Thesepropertiesareveryusefulwhendebuggingwebservicecalls.


FortheMoreCurious:HTTPWhenNSURLSessionTaskinteractswithawebserver,itdoessoaccordingtotherulesoutlinedintheHTTPspecification.Thespecificationisveryclearabouttheexactformatoftherequest/responseexchangebetweentheclientandtheserver.AnexampleofasimpleHTTPrequestisshowninFigure19.7.

Figure19.7HTTPrequestformat

AnHTTPrequesthasthreeparts:arequestline,requestheaders,andanoptionalrequestbody.Therequestlineisthefirstlineoftherequestandtellstheserverwhattheclientistryingtodo.Inthisrequest,theclientistryingtoGETtheresourceat/index.html.(ItalsospecifiestheHTTPversionthattherequestwillbeconformingto.)

ThewordGETisanHTTPmethod.WhilethereareanumberofsupportedHTTPmethods,youwillseeGETandPOSTmostoften.ThedefaultofNSURLRequest,GET,indicatesthattheclientwantsaresourcefromtheserver.Theresourcerequestedmightbeanactualfileonthewebserver’sfilesystem,oritcouldbegenerateddynamicallyatthemomenttherequestisreceived.Asaclient,youshouldnotcareaboutthisdetail,butmorethanlikelytheJSONresourcesyourequestedinthischapterwerecreateddynamically.

Inadditiontogettingthingsfromaserver,youcansenditinformation.Forexample,manywebserversallowyoutouploadphotos.AclientapplicationwouldpasstheimagedatatotheserverthroughanHTTPrequest.Inthissituation,youwouldusetheHTTPmethodPOST,andyouwouldincludearequestbody.Thebodyofarequestisthepayloadyouaresendingtotheserver–typicallyJSON,XML,orbinarydata.

Whentherequesthasabody,itmustalsohavetheContent-Lengthheader.Handily,NSURLRequestwillcomputethesizeofthebodyandaddthisheaderforyou.

HereisanexampleofhowtoPOSTanimagetoanimaginarysiteusinganNSMutableURLRequest.ifletsomeURL=NSURL(string:"http://www.photos.example.com/upload"){letimage=profileImage()


letdata=UIImagePNGRepresentation(image)

letreq=NSMutableURLRequest(URL:someURL)

//ThisaddstheHTTPbodydataandautomaticallysetsthecontent-lengthheaderreq.HTTPBody=data

//ThischangestheHTTPmethodintherequestlinereq.HTTPMethod="POST"

//Ifyouwantedtosetarequestheader,suchastheAcceptheaderreq.setValue("text/json",forHTTPHeaderField:"Accept")}

Figure19.8showswhatasimpleHTTPresponsemightlooklike.WhileyouwillnotbemodifyingthecorrespondingNSHTTPURLResponseinstance,itisnicetounderstandwhatitismodeling.

Figure19.8HTTPresponseformat

Asyoucansee,theformatoftheresponseisnottoodifferentfromtherequest.Itincludesastatusline,responseheaders,and,ofcourse,theresponsebody.Yes,thisiswherethatpesky“404NotFound”comesfrom!


20CollectionViews

Inthischapter,youwillcontinueworkingonthePhotoramaapplicationbydisplayingtherecentFlickrphotosinagridusingtheUICollectionViewclass.Thischapterwillalsoreinforcethedatasourcedesignpatternthatyouusedinpreviouschapters.Figure20.1showsyouwhattheapplicationwilllooklikeattheendofthischapter.

Figure20.1Photoramawithacollectionview

InChapter9,youworkedwithUITableView.Tableviewsareagreatwaytodisplayandeditacolumnofinformationinahierarchicallist.Likeatableview,acollectionviewalsodisplaysanorderedcollectionofitems,butinsteadofdisplayingtheinformationinahierarchicallist,thecollectionviewhasalayoutobjectthatdrivesthedisplayofinformation.Youwilluseabuilt-inlayoutobject,theUICollectionViewFlowLayout,topresenttherecentphotosinascrollablegrid.


DisplayingtheGridLet’stackletheinterfacefirst.YouaregoingtochangetheuserinterfaceforPhotosViewControllertodisplayacollectionviewinsteadofdisplayingtheimageview.

OpenMain.storyboardandlocatethePhotoramaimageview.DeletetheimageviewfromthecanvasanddragaCollectionViewontothecanvas.Selectboththecollectionviewanditssuperview.(Theeasiestwaytodothisisusingthedocumentoutline.)OpentheAutoLayoutAlignmenu,configureitlikeFigure20.2,andclickAdd4Constraints.

Figure20.2Collectionviewconstraints

BecauseyouusedtheAlignmenutopintheedges,thecollectionviewwillbepinnedtothetopoftheentireviewinsteadoftothetoplayoutguide.Thisisusefulforscrollviews(andtheirsubclasses,likeUITableViewandUICollectionView)sothatthecontentwillscrollunderneaththenavigationbar.Thescrollviewwillautomaticallyupdateitsinsetstomakethecontentvisible,asyousawinChapter9.ThecanvaswillnowlooklikeFigure20.3.


Figure20.3Storyboardcanvas

Currently,thecollectionviewandthecollectionviewcellsbothhaveaclearbackgroundcolor.Opentheattributesinspectorforthecollectionviewandchangeitsbackgroundcolortowhite.Thenselectthecollectionviewcell–thesmallrectangleintheupper-leftcornerofthecollectionview–andgiveitablackbackgroundcolor.

Selecttheblackcollectionviewcellandopenitsattributesinspector.SettheIdentifiertoUICollectionViewCell(Figure20.4).

Figure20.4Settingthereuseidentifier

Thecollectionviewisnowonthecanvas,butyouneedawaytopopulatethecellswithdata.Todothis,youwillcreateanewclasstoactasthedatasourceofthecollectionview.


CollectionViewDataSourceApplicationsareconstantlychanging,sopartofbeingagoodiOSdeveloperisbuildingapplicationsinawaythatallowsthemtoadapttochangingrequirements.

ThePhotoramaapplicationwilldisplayasinglecollectionviewofphotos.YoucoulddosomethingsimilartowhatyoudidinHomepwnerandmakethePhotosViewControllerbethedatasourceofthecollectionview.Theviewcontrollerwouldimplementtherequireddatasourcemethods,andeverythingwouldworkjustfine.

Atleast,itwouldworkfornow.Whatif,sometimeinthefuture,youdecidedtohaveadifferentscreenthatalsodisplayedacollectionviewofphotos?Maybeinsteadofdisplayingtherecentphotos,itwoulduseadifferentwebservicetodisplayallthephotosmatchingasearchterm.Inthiscase,youwouldneedtoreimplementthesamedatasourcemethodswithinthenewviewcontrollerwithessentiallythesamecode.Thatwouldnotbeideal.

Instead,youwillabstractoutthecollectionviewdatasourcecodeintoanewclass.Thisclasswillberesponsibleforrespondingtodatasourcequestions–anditwillbereusableasnecessary.

CreateanewSwiftfilenamedPhotoDataSource.OpenthisfileanddeclarethePhotoDataSourceclass.importFoundationimportUIKit

classPhotoDataSource:NSObject,UICollectionViewDataSource{

varphotos=[Photo]()

}

ToconformtotheUICollectionViewDataSourceprotocol,atypealsoneedstoconformtotheNSObjectProtocol.TheeasiestandmostcommonwaytoconformtothisprotocolistosubclassfromNSObject,asyoudidabove.

TheUICollectionViewDataSourceprotocoldeclarestworequiredmethodstoimplement:funccollectionView(collectionView:UICollectionView,numberOfItemsInSectionsection:Int)->IntfunccollectionView(collectionView:UICollectionView,cellForItemAtIndexPathindexPath:NSIndexPath)->UICollectionViewCell

YoumightnoticethatthesetwomethodslookverysimilartothetworequiredmethodsofUITableViewDataSourcethatyousawinChapter9.Thefirstdatasourcecallbackaskshowmanycellstodisplay,andthesecondasksfortheUICollectionViewCelltodisplayforagivenindexpath.

ImplementthesetwomethodsinPhotoDataSource.swift.classPhotoDataSource:NSObject,UICollectionViewDataSource{

varphotos=[Photo]()


funccollectionView(collectionView:UICollectionView,numberOfItemsInSectionsection:Int)->Int{returnphotos.count}

funccollectionView(collectionView:UICollectionView,cellForItemAtIndexPathindexPath:NSIndexPath)->UICollectionViewCell{

letidentifier="UICollectionViewCell"letcell=collectionView.dequeueReusableCellWithReuseIdentifier(identifier,forIndexPath:indexPath)

returncell}}

Next,thecollectionviewneedstoknowthataninstanceofPhotoDataSourceisthedatasourceobject.

InPhotosViewController.swift,addapropertytoreferenceaninstanceofPhotoDataSourceandanoutletforaUICollectionViewinstance.Also,youwillnotneedtheimageViewanymore,sodeleteit.classPhotosViewController:UIViewController{

@IBOutletvarimageView:UIImageView!@IBOutletvarcollectionView:UICollectionView!

varstore:PhotoStore!letphotoDataSource=PhotoDataSource()

UpdateviewDidLoad()tosetthedatasourceonthecollectionview.overridefuncviewDidLoad(){super.viewDidLoad()

collectionView.dataSource=photoDataSource

...

Finally,updatethephotoDataSourceobjectwiththeresultofthewebservicerequestandreloadthecollectionview.overridefuncviewDidLoad()super.viewDidLoad()

collectionView.dataSource=photoDataSource


switchphotosResult{caselet.Success(photos):print("Successfullyfound\(photos.count)recentphotos.")ifletfirstPhoto=photos.first{self.store.fetchImageForPhoto(firstPhoto){(imageResult)->VoidinswitchimageResult{caselet.Success(image):NSOperationQueue.mainQueue().addOperationWithBlock(){self.imageView.image=image}caselet.Failure(error):print("Errordownloadingimage:\(error)")}}}caselet.Failure(error):print("Errorfetchingrecentphotos:\(error)")}


NSOperationQueue.mainQueue().addOperationWithBlock(){switchphotosResult{caselet.Success(photos):print("Successfullyfound\(photos.count)recentphotos.")self.photoDataSource.photos=photoscaselet.Failure(error):self.photoDataSource.photos.removeAll()print("Errorfetchingrecentphotos:\(error)")}self.collectionView.reloadSections(NSIndexSet(index:0))}}}

ThelastthingyouneedtodoismakethecollectionViewoutletconnection.

OpenMain.storyboardandnavigatetothecollectionview.Control-dragfromthePhotosViewControllertothecollectionviewandconnectittothecollectionViewoutlet.

Buildandruntheapplication.Afterthewebservicerequestcompletes,checktheconsoletoconfirmthatphotoswerefound.OntheiOSdevice,therewillbeagridofblacksquarescorrespondingtothenumberofphotosfound(Figure20.5).Thesecellsarearrangedinaflowlayout.Aflowlayoutfitsasmanycellsonarowaspossiblebeforeflowingdowntothenextrow.IfyourotatetheiOSdevice,youwillseethecellsfillthegivenarea.

Figure20.5Initialflowlayout


CustomizingtheLayoutThedisplayofcellsisnotdrivenbythecollectionviewitselfbutbythecollectionview’slayout.Thelayoutobjectisresponsiblefortheplacementofcellsonscreen.Layouts,inturn,aredrivenbyasubclassofUICollectionViewLayout.

TheflowlayoutthatPhotoramaiscurrentlyusingisUICollectionViewFlowLayout,whichistheonlyconcreteUICollectionViewLayoutsubclassprovidedbytheUIKitframework.

SomeofthepropertiesyoucancustomizeonUICollectionViewFlowLayoutare:

scrollDirection–Doyouwanttoscrollverticallyorhorizontally?

minimumLineSpacing–Whatistheminimumspacingbetweenlines?

minimumInteritemSpacing–Whatistheminimumspacingbetweenitemsinarow(orcolumn,ifscrollinghorizontally)?

itemSize–Whatisthesizeofeachitem?

sectionInset–Whatarethemarginsusedtolayoutcontentforeachsection?

Figure20.6showshowthesepropertiesaffectthepresentationofcellsusingUICollectionViewFlowLayout.


Figure20.6UICollectionViewFlowLayoutproperties

OpenMain.storyboardandselectthecollectionview.OpenthesizeinspectorandconfiguretheCellSize,MinimumSpacing,andSectionInsetsasshowninFigure20.7.


Figure20.7Collectionviewsizeinspector

Buildandruntheapplicationtoseehowthelayouthaschanged.


CreatingaCustomUICollectionViewCellNextyouaregoingtocreateacustomUICollectionViewCellsubclasstodisplaythephotos.Whiletheimagedataisdownloading,thecollectionviewcellwilldisplayaspinningactivityindicatorusingtheUIActivityIndicatorViewclass.

CreateanewSwiftfilenamedPhotoCollectionViewCell.InPhotoCollectionViewCell.swift,definePhotoCollectionViewCellasasubclassofUICollectionViewCell.Thenaddoutletstoreferencetheimageviewandtheactivityindicatorview.importFoundationimportUIKit

classPhotoCollectionViewCell:UICollectionViewCell{

@IBOutletvarimageView:UIImageView!@IBOutletvarspinner:UIActivityIndicatorView!

}

Theactivityindicatorviewshouldonlyspinwhenthecellisnotdisplayinganimage.InsteadofalwaysupdatingthespinnerwhentheimageViewisupdated,orviceversa,youwillwriteahelpermethodtotakecareofitforyou.

CreatethishelpermethodinPhotoCollectionViewCell.swift.funcupdateWithImage(image:UIImage?){ifletimageToDisplay=image{spinner.stopAnimating()imageView.image=imageToDisplay}else{spinner.startAnimating()imageView.image=nil}}

Itwouldbenicetoreseteachcelltothespinningstatebothwhenthecellisfirstcreatedandwhenthecellisgettingreused.ThemethodawakeFromNib()iscalledaftertheinterfacefileisloadedinandtheoutletconnectionsaremade.Thisisagoodopportunitytodoanyuserinterfacecustomization.ThemethodprepareForReuse()iscalledwhenacellisabouttobereused.

ImplementthesetwomethodsinPhotoCollectionViewCell.swifttoresetthecellbacktothespinningstate.overridefuncawakeFromNib(){super.awakeFromNib()

updateWithImage(nil)}

overridefuncprepareForReuse(){super.prepareForReuse()

updateWithImage(nil)}

Youwilluseaprototypecelltosetuptheinterfaceforthecollectionviewcellinthestoryboard,justasyoudidinChapter11forItemCell.Ifyourecall,eachprototypecell


correspondstoavisuallyuniquecellwithauniquereuseidentifier.Mostofthetime,theprototypecellswillbeassociatedwithdifferentUICollectionViewCellsubclassestoprovidebehaviorspecifictothatkindofcell.

Inthecollectionview’sattributesinspector,youcanadjustthenumberofItemsthatthecollectionviewdisplays,andeachitemcorrespondstoaprototypecellinthecanvas.ForPhotorama,youonlyneedonekindofcell:thePhotoCollectionViewCellthatdisplaysaphoto.

OpenMain.storyboardandselectthecollectionviewcell.Intheidentityinspector,changetheClasstoPhotoCollectionViewCell(Figure20.8).

Figure20.8Changingthecellclass

DraganimageviewontotheUICollectionViewCell.Addconstraintstopintheimageviewtotheedgesofthecell.OpentheattributesinspectorfortheimageviewandsettheModetoAspectFill.Thiswillcutoffpartsofthephotos,butitwillallowthephotostocompletelyfillinthecollectionviewcell.

Next,draganactivityindicatorviewontopoftheimageview.Addconstraintstocentertheactivityindicatorviewbothhorizontallyandverticallywiththeimageview.OpenitsattributesinspectorandselectHidesWhenStopped(Figure20.9).

Figure20.9Configuringtheactivityindicator

Selectthecollectionviewcellagain.Thiscanbeabittrickytodoonthecanvassincethenewlyaddedsubviewscompletelycoverthecellitself.AhelpfulInterfaceBuildertipistoholdControlandShifttogetherandthenclickontopoftheviewyouwanttoselect.Youwillbepresentedwithalistofalloftheviewsandcontrollersunderthepointyouclicked(Figure20.10).


Figure20.10Selectingthecellonthecanvas

Withthecellselected,opentheconnectionsinspectorandconnecttheimageViewandspinnerpropertiestotheimageviewandactivityindicatorviewonthecanvas(Figure20.11).


Figure20.11ConnectingPhotoCollectionViewCelloutlets

OpenPhotoDataSource.swiftandupdatethedatasourcemethodtousethePhotoCollectionViewCell.funccollectionView(collectionView:UICollectionView,cellForItemAtIndexPathindexPath:NSIndexPath)->UICollectionViewCell{

letidentifier="UICollectionViewCell"letcell=collectionView.dequeueReusableCellWithReuseIdentifier(identifier,forIndexPath:indexPath)as!PhotoCollectionViewCell

letphoto=photos[indexPath.row]cell.updateWithImage(photo.image)

returncell}

Buildandruntheapplication.Whentherecentphotosrequestcompletes,youwillseetheactivityindicatorviewsallspinning(Figure20.12).


Figure20.12Customcollectionviewsubclass


DownloadingtheImageDataNowallthatisleftisdownloadingtheimagedataforthephotosthatcomebackintherequest.Thistaskisnotverydifficult,butitrequiressomethought.Imagesarelargefiles,anddownloadingthemcouldeatupyourusers’cellulardataallowance.AsaconsiderateiOSdeveloper,youwanttomakeyourapp’sdatausageisonlywhatitneedstobe.

Consideryouroptions.YoucoulddownloadtheimagedatainviewDidLoad()whenthefetchRecentPhotos(_:)methodcallsitscompletionclosure.Atthatpoint,youalreadyassigntheincomingphotostothephotosproperty,soyoucoulditerateoverallofthosephotosanddownloadtheirimagedatarightatthatpoint.

Althoughthiswouldwork,itwouldbeverycostly.Therecouldbealargenumberofphotoscomingbackintheinitialrequest,andtheusermayneverevenscrolldownintheapplicationfarenoughtoseesomeofthem.Ontopofthat,ifyouinitializetoomanyrequestssimultaneously,someoftherequestsmaytimeoutwhilewaitingforotherrequeststofinish.Sothisisprobablynotthebestsolution.

Instead,itmakessensetodownloadtheimagedataforonlythecellsthattheuserisattemptingtoview.UICollectionViewhasamechanismtosupportthisthroughitsUICollectionViewDelegatemethodcollectionView(_:willDisplayCell:forItemAtIndexPath:).Thisdelegatemethodwillbecalledeverytimeacellisgettingdisplayedonscreenandisagreatopportunitytodownloadtheimagedata.

RecallthatthedataforthecollectionviewisdrivenbyaninstanceofPhotoDataSource,areusableclasswiththesingleresponsibilityofdisplayingphotosinacollectionview.Collectionviewsalsohaveadelegate,whichisresponsibleforhandlinguserinteractionwiththecollectionview.Thisincludestaskssuchasmanagingcellselectionandtrackingcellscomingintoandoutofview.Thisresponsibilityismoretightlycoupledwiththeviewcontrolleritself,sowhereasthedatasourceisaninstanceofPhotoDataSource,thecollectionview’sdelegatewillbethePhotosViewController.

InPhotosViewController.swift,havetheclassconformtotheUICollectionViewDelegateprotocol.classPhotosViewController:UIViewController,UICollectionViewDelegate{

(SincetheUICollectionViewDelegateprotocolonlydefinesoptionalmethods,Xcodedoesnotreportanyerrorswhenyouaddthisdeclaration.)

UpdateviewDidLoad()tosetthePhotosViewControllerasthedelegateofthecollectionview.overridefuncviewDidLoad(){super.viewDidLoad()

collectionView.dataSource=photoDataSourcecollectionView.delegate=self

...


Finally,implementthedelegatemethodinPhotosViewController.swift.funccollectionView(collectionView:UICollectionView,willDisplayCellcell:UICollectionViewCell,forItemAtIndexPathindexPath:NSIndexPath){

letphoto=photoDataSource.photos[indexPath.row]

//Downloadtheimagedata,whichcouldtakesometimestore.fetchImageForPhoto(photo){(result)->Voidin

NSOperationQueue.mainQueue().addOperationWithBlock(){

//Theindexpathforthephotomighthavechangedbetweenthe//timetherequeststartedandfinished,sofindthemost//recentindexpath

//(Note:Youwillhaveanerroronthenextline;youwillfixitsoon)letphotoIndex=self.photoDataSource.photos.indexOf(photo)!letphotoIndexPath=NSIndexPath(forRow:photoIndex,inSection:0)

//Whentherequestfinishes,onlyupdatethecellifit'sstillvisibleifletcell=self.collectionView.cellForItemAtIndexPath(photoIndexPath)as?PhotoCollectionViewCell{cell.updateWithImage(photo.image)}}}}

IftheimagealreadyexistsonthePhoto,theimagedatashouldnotbedownloadedfromthewebserviceagain.

OpenPhotoStore.swiftandupdatefetchImageForPhoto(_:completion:)toreturnthephoto’simageifithasalreadybeendownloaded.funcfetchImageForPhoto(photo:Photo,completion:(ImageResult)->Void){

ifletimage=photo.image{completion(.Success(image))return}

letphotoURL=photo.remoteURL...

Let’sfixtheerroryousawwhenfindingtheindexofphotointhephotosarray.TheindexOf(_:)methodworksbycomparingtheitemthatyouarelookingfortoeachoftheitemsinthecollection.Itdoesthisusingthe==operator.TypesthatconformtotheEquatableprotocolmustimplementthisoperator,andPhotodoesnotyetconformtoEquatable.

InPhoto.swift,declarethatPhotoconformstotheEquatableprotocolandimplementtherequiredoverloadingofthe==operator.classPhoto:Equatable{...}

func==(lhs:Photo,rhs:Photo)->Bool{//TwoPhotosarethesameiftheyhavethesamephotoIDreturnlhs.photoID==rhs.photoID}

Customoperatorshavetobedeclaredoutsideofthetypedeclaration.Thecompilerrecognizesthatthe==operatoroccursbetweentwoinstancesofPhoto(basedonthe


argumentstothefunction)andthereforevalidatesthattherequirementfortheEquatableprotocolhasbeensatisfied.

InSwift,itiscommontogrouprelatedchunksoffunctionalityintoanextension.Let’stakeashortdetourtolearnaboutextensionsandthenusethisknowledgetoseehowconformingtotheEquatableprotocolisoftendoneinpractice.

Extensions

Extensionsserveacoupleofpurposes:theyallowyoutogroupchunksoffunctionalityintoalogicalunit,andtheyalsoallowyoutoaddfunctionalitytoyourowntypesaswellastypesprovidedbythesystemorotherframeworks.Beingabletoaddfunctionalitytoatypewhosesourcecodeyoudonothaveaccesstoisaverypowerfulandflexibletool.Extensionscanbeaddedtoclasses,structs,andenums.Let’stakealookatanexample.

SayyouwantedtoaddfunctionalitytotheInttypetoprovideadoubledvalueofthatInt.Forexample:letfourteen=7.doubled//Thevalueoffourteenis'14'

YoucanaddthisfunctionalitybyextendingtheInttype:extensionInt{vardoubled:Int{returnself*2}}

Withextensions,youcanaddcomputedproperties,addmethods,andconformtoprotocols.However,youcannotaddstoredpropertiestoanextension.

Extensionsprovideagreatmechanismforgroupingrelatedpiecesoffunctionality.Theycanmakethecodemorereadableandhelpwithlong-termmaintainabilityofyourcodebase.Onecommonchunkoffunctionalitythatisoftengroupedintoanextensionisconformancetoaprotocolalongwiththemethodsofthatprotocol.

UpdatePhoto.swifttouseanextensiontoconformtotheEquatableprotocol.classPhoto:Equatable{...}

extensionPhoto:Equatable{}

func==(lhs:Photo,rhs:Photo)->Bool{//TwoPhotosarethesameiftheyhavethesamephotoIDreturnlhs.photoID==rhs.photoID}

TheextensiondeclaresthatPhotoconformstotheEquatableprotocol.Whileitmightmakesenseforthe==operatortobeincludedinthisPhotoextension,aswementionedoperatorsmustbedeclaredoutsidethescopeofaspecifictype.Therefore,topreservethegroupingasmuchaspossible,theoperatorisdefinedimmediatelyafterthe(empty)extension.

Thisisasimplifiedexample,butextensionsareverypowerfulforbothextendingexistingtypesandgroupingrelatedfunctionality.Infact,theSwiftstandardlibrarymakes


extensiveuseofextensions–andyouwill,too.

Buildandruntheapplication.Theimagedatawilldownloadforthecellsvisibleonscreen(Figure20.13).Scrolldowntomakemorecellsvisible.Atfirst,youwillseetheactivityindicatorviewsspinning,butsoontheimagedataforthosecellswillload.Ifyouscrollbackup,therewillbenodelayinloadingimagedata.Becausetheimageswerealreadydownloadedforthosecells,youwillnotseeanyactivityindicatorviewsandtheimageswillbeimmediatelyvisible.

Figure20.13Imagedownloadsinprogress


NavigatingtoaPhotoInthissection,youaregoingtoaddfunctionalitytoallowausertonavigatetoanddisplayasinglephoto.

CreateanewSwiftfilenamedPhotoInfoViewController.Inthisnewfile,declarethePhotoInfoViewControllerclassandaddanimageViewoutlet.importFoundationimportUIKit

classPhotoInfoViewController:UIViewController{

@IBOutletvarimageView:UIImageView!}

Nowsetuptheinterfaceforthisviewcontroller.OpenMain.storyboardanddraganewViewControllerontothecanvasfromtheobjectlibrary.Withthisviewcontrollerselected,openitsidentityinspectorandchangetheClasstoPhotoInfoViewController.

Whentheusertapsononeofthecollectionviewcells,theapplicationwillnavigatetothisnewviewcontroller.Control-dragfromtheUICollectionViewCelltothePhotoInfoViewControllerandselecttheShowsegue.Withthenewsegueselected,openitsattributesinspectorandgivethesegueanIdentifierofShowPhoto(Figure20.14).

Figure20.14Navigationtoaphoto

AddanimageviewtothePhotoInfoViewController’sview.SetupitsAutoLayoutconstraintstopintheimageviewtoallfoursides.OpentheattributesinspectorfortheimageviewandsetitsModetoAspectFit.


Finally,connecttheimageviewtotheimageViewoutlet.

Whentheusertapsacell,theShowPhotoseguewillbetriggered.Atthispoint,thePhotosViewControllerwillneedtopassboththePhotoandthePhotoStoretothePhotoInfoViewController.

OpenPhotoInfoViewController.swiftandaddtwoproperties.classPhotoInfoViewController:UIViewController{

@IBOutletvarimageView:UIImageView!

varphoto:Photo!{didSet{navigationItem.title=photo.title}}varstore:PhotoStore!}

Whenphotoissetonthisviewcontroller,thenavigationitemwillbeupdatedtodisplaythenameofthephoto.

NowoverrideviewDidLoad()tosettheimageontheimageViewwhentheviewisloaded.overridefuncviewDidLoad(){super.viewDidLoad()

store.fetchImageForPhoto(photo){(result)->Voidinswitchresult{caselet.Success(image):NSOperationQueue.mainQueue().addOperationWithBlock(){self.imageView.image=image}caselet.Failure(error):print("Errorfetchingimageforphoto:\(error)")}}}

InPhotosViewController.swift,implementprepareForSegue(_:sender:)topassalongthephotoandthestore.overridefuncprepareForSegue(segue:UIStoryboardSegue,sender:AnyObject?){ifsegue.identifier=="ShowPhoto"{ifletselectedIndexPath=collectionView.indexPathsForSelectedItems()?.first{

letphoto=photoDataSource.photos[selectedIndexPath.row]

letdestinationVC=segue.destinationViewControlleras!PhotoInfoViewControllerdestinationVC.photo=photodestinationVC.store=store}}}

Buildandruntheapplication.Afterthewebservicerequesthasfinished,tapononeofthephotostoseeitinthenewviewcontroller(Figure20.15).


Figure20.15Displayingaphoto

Collectionviewsareapowerfulwaytodisplaydatausingaflexiblelayout.Youhavejustbarelytappedintothepowerofcollectionviewsinthischapter.


SilverChallenge:UpdatedItemSizesHavethecollectionviewalwaysdisplayfouritemsperrow,takingupasmuchasthescreenwidthaspossible.Thisshouldworkinbothportraitandlandscapeorientations.


GoldChallenge:CreatingaCustomLayoutCreateacustomlayoutthatdisplaysthephotosinaflipbook.Youwillneedtousethetransformpropertyonthecelllayertogetanappropriate3-Deffect.YoucansubclassUICollectionViewLayoutforthischallenge,butalsoconsidersubclassingUICollectionViewFlowLayout.CheckouttheclassreferenceforUICollectionViewLayoutformoreinformation.


21CoreData

WhendecidingbetweenapproachestosavingandloadingforiOSapplications,thefirstquestionis“Localorremote?”Ifyouwanttosavedatatoaremoteserver,youwilllikelyuseawebservice.Ifyouwanttostoredatalocally,youhavetoaskanotherquestion:“ArchivingorCoreData?”

YourHomepwnerapplicationuseskeyedarchivingtosaveitemdatatothefilesystem.Thebiggestdrawbacktoarchivingisitsall-or-nothingnature:toaccessanythinginthearchive,youmustunarchivetheentirefile,andtosaveanychanges,youmustrewritetheentirefile.CoreData,ontheotherhand,canfetchasubsetofthestoredobjects.Andifyouchangeanyofthoseobjects,youcanupdatejustthatpartofthefile.Thisincrementalfetching,updating,deleting,andinsertingcanradicallyimprovetheperformanceofyourapplicationwhenyouhavealotofmodelobjectsbeingshuttledbetweenthefilesystemandRAM.


ObjectGraphsCoreDataisaframeworkthatletsyouexpresswhatyourmodelobjectsareandhowtheyarerelatedtooneanother.Itthentakescontrolofthelifetimesoftheseobjects,makingsuretherelationshipsarekeptuptodate.Whenyousaveandloadtheobjects,CoreDatamakessureeverythingisconsistent.Thiscollectionofmodelobjectsisoftencalledanobjectgraph,astheobjectscanbethoughtofasnodesandtherelationshipsasverticesinamathematicalgraph.

OftenyouwillhaveCoreDatasaveyourobjectgraphtoaSQLitedatabase.DeveloperswhoareusedtootherSQLtechnologiesmightexpecttotreatCoreDatalikeanobject-relationalmappingsystem,butthismindsetwillleadtoconfusion.UnlikeanORM,CoreDatatakescompletecontrolofthestorage,whichjusthappenstobearelationaldatabase.Youdonothavetodescribethingslikethedatabaseschemaandforeignkeys–CoreDatadoesthat.YoujusttellCoreDatawhatneedsstoringandletitworkouthowtostoreit.

CoreDatagivesyoutheabilitytofetchandstoredatainarelationaldatabasewithouthavingtoknowthedetailsoftheunderlyingstoragemechanism.ThischapterwillgiveyouanunderstandingofCoreDataasyouaddpersistencetothePhotoramaapplication.


EntitiesArelationaldatabasehassomethingcalledatable.Atablerepresentsatype:youcanhaveatableofpeople,atableofacreditcardpurchases,oratableofrealestatelistings.Eachtablehasanumberofcolumnstoholdpiecesofinformationaboutthetype.Atablethatrepresentspeoplemighthavecolumnsforlastname,dateofbirth,andheight.Everyrowinthetablerepresentsanexampleofthetype–e.g.,asingleperson.

ThisorganizationtranslateswelltoSwift.EverytableislikeaSwifttype.Everycolumnisoneofthetype’sproperties.Everyrowisaninstanceofthattype.Thus,CoreData’sjobistomovedatatoandfromthesetworepresentations(Figure21.1).

Figure21.1RoleofCoreData

CoreDatausesdifferentterminologytodescribetheseideas:atable/typeiscalledanentity,andthecolumns/propertiesarecalledattributes.ACoreDatamodelfileisthedescriptionofeveryentityalongwithitsattributesinyourapplication.InPhotorama,youaregoingtodescribeaPhotoentityinamodelfileandgiveitattributesliketitle,remoteURL,anddateTaken.

Modelingentities

OpenPhotorama.xcodeproj.Createanewfile,butdonotmakeitaSwiftfileliketheonesyouhavecreatedbefore.Instead,selectCoreDataintheiOSsectionandcreateanewDataModel(Figure21.2).NameitPhotorama.


Figure21.2Creatingthemodelfile

ThiswillcreatethePhotorama.xcdatamodeldfileandaddittoyourproject.SelectthisfilefromtheprojectnavigatorandtheeditorareawillrevealtheuserinterfaceformanipulatingaCoreDatamodelfile.

FindtheAddEntitybuttonatthebottomleftofthewindowandclickit.Anewentitywillappearinthelistofentitiesinthelefthandtable.Double-clickthisentityandchangeitsnametoPhoto(Figure21.3).


Figure21.3CreatingthePhotoentity

NowyourPhotoentityneedsattributes.RememberthatthesewillbethepropertiesofthePhotoclass.Thenecessaryattributesarelistedbelow.Foreachattribute,clickthe+buttonintheAttributessectionandedittheAttributeandTypevalues.

photoIDisaString.

photoKeyisaString.

titleisaString.

dateTakenisaDate.

remoteURLisaTransformable.(ItisanNSURL,butthatisnotoneofthepossibilities.Wewilldiscuss“transformable”next.)

Bydefault,theCoreDatamodeleditormakesallattributesoptional.ItdoesnotmakesenseforthePhotoattributestobeoptional,solet’schangethemalltobenon-optional.

Selectalloftheattributesbyclickingonthefirstattributeinthelist,holdingtheShiftkey,andthenclickingthelastattributeinthelist.Opentheutilitiesview,openthedatamodel


inspector(the icon),andunchecktheOptionalcheckbox(Figure21.4).

Figure21.4Makingtheattributesnon-optional

Sincethesepropertiesarenolongeroptional,youwillhavetoprovidevaluesforthemwhentheobjectiscreated.Youwilldothatlaterinthischapter.

Transformableattributes

CoreDataisonlyabletostorecertaindatatypesinitsstore.NSURLisnotoneofthesetypes,soyoudeclaredtheremoteURLattributeastransformable.Withatransformableattribute,CoreDatawillconverttheobjectintoatypethatitcanstorewhensavingandthenconvertitbacktotheoriginalobjectwhenloadingfromthefilesystem.

AtransformableattributerequiresanNSValueTransformersubclasstohandletheconversionsbetweentypes.Ifyoudonotspecifyacustomsubclass,thesystemwillusethetransformernamedNSKeyedUnarchiveFromDataTransformer.ThistransformerusesarchivingtoconverttheobjecttoandfromNSData.SinceNSURLconformstoNSCoding,thedefaultNSKeyedUnarchiveFromDataTransformerwillbesufficient.IfthetypeyouwantedtotransformdidnotconformtoNSCoding,youwouldneedtowriteyourowncustomNSValueTransformersubclass.

Atthispoint,yourmodelfileissufficienttosaveandloadphotos.Inthenextsection,youwillcreateacustomsubclassforthePhotoentity.

NSManagedObjectandsubclasses

WhenanobjectisfetchedwithCoreData,itsclass,bydefault,isNSManagedObject.NSManagedObjectisasubclassofNSObjectthatknowshowtocooperatewiththerestofCoreData.AnNSManagedObjectworksabitlikeadictionary:itholdsakey-valuepairforeveryproperty(attributeorrelationship)intheentity.

AnNSManagedObjectislittlemorethanadatacontainer.Ifyouneedyourmodelobjectstodosomethinginadditiontoholdingdata,youmustsubclassNSManagedObject.Then,inyourmodelfile,youspecifythatthisentityisrepresentedbyinstancesofyoursubclass,notthestandardNSManagedObject.

WhenanewPhotoiscreated,anewUUIDneedstobegeneratedforthephotoKey.ThisfunctionalitycannotberepresentedwithNSManagedObjectitself,soyouwillcreateacustomsubclass.


XcodecangenerateNSManagedObjectsubclassesforyoubasedonwhatyouhavedefinedinyourCoreDatamodelfile.

Intheprojectnavigator,selectthePhoto.swiftfileanddeleteit.Makesureyoumoveittothetrashwhenpromptedtomakesureitdoesnotstillexistintheprojectdirectory.

Createanewfile.FromtheiOSsectionofthetemplateselectionscreen,chooseCoreData,andthenchooseNSManagedObjectsubclassandclickNext.Onthenextscreen,checktheboxforPhotoramaandclickNext.ChecktheboxforthePhotoentityandclickNext.Finally,makesurethelanguageissettoSwiftandclickCreate.

Thetemplatewillcreatetwofilesforyou:Photo.swiftandPhoto+CoreDataProperties.swift.ThetemplateplacesalloftheattributesthatyoudefinedinthemodelfileintoPhoto+CoreDataProperties.swift.Ifyoueverchangeyourentityinthemodelfile,youcansimplydeletePhoto+CoreDataProperties.swiftandrepeatthestepstogeneratetheNSManagedObjectsubclass.XcodewillrecognizethatyoualreadyhavePhoto.swiftandwillonlyrecreatePhoto+CoreDataProperties.swift.

OpenPhoto+CoreDataProperties.swiftandtakealookatwhatthetemplatecreatedforyou.

[email protected],whichisspecifictoCoreData,letsthecompilerknowthatthestorageandimplementationofthesepropertieswillbeprovidedatruntime.BecauseCoreDatawillcreatetheNSManagedObjectinstances,youcannolongeruseacustominitializer,sothepropertiesneedtobevariablesinsteadofconstants.

WhenyoumadethePhotoentity’sattributesnon-optionalinthemodelfile,youmadeacontractwithCoreDatathatthoseattributeswouldalwayshaveavaluewhenaninstanceofPhotowasbeingpersisted.Unfortunately,thatcontractwithCoreDataisnottranslatedbyXcodeintooptionalsinSwift,sowhenXcodegeneratedthePhotosubclass,itdidnotmakethepropertiesnon-optional.

ToadheretoyourcontractwithCoreData,maketheattributesinPhoto+CoreDataProperties.swiftnon-optional.(Theorderofyourattributesmightbedifferent;thatisOK.)@NSManagedvarphotoID:String?@NSManagedvarphotoKey:String?@NSManagedvartitle:String?@NSManagedvardateTaken:NSDate?@NSManagedvarremoteURL:NSObject?

InPhoto+CoreDataProperties.swift,changetheremoteURLtypetobeNSURL.RecallthatthispropertyisatransformablepropertyandwillautomaticallybeconvertedtoandfromNSDatawhenbeingstoredinandretrievedfromCoreData.@NSManagedvarphotoID:String@NSManagedvarphotoKey:String@NSManagedvartitle:String@NSManagedvardateTaken:NSDate@NSManagedvarremoteURL:NSObject@NSManagedvarremoteURL:NSURL


AnycustompropertiesorcodethatyouwanttoaddshouldbeaddedtoPhoto.swift.Let’saddbackthebehavioryoupreviouslyhad.

InPhoto.swift,addtheoptionalUIImageproperty.YouwillneedtoimportUIKitaswell.importFoundationimportUIKitimportCoreData

classPhoto:NSManagedObject{

//Insertcodeheretoaddfunctionalitytoyourmanagedobjectsubclass

varimage:UIImage?

}

Ofcourse,whenyoulaunchanapplicationthefirsttime,therearenosavedphotos.Whenthewebservicerequestreturns,thephotoswillbeaddedtothedatabase.Whenobjectsareaddedtothedatabase,theyaresentthemessageawakeFromInsert().HereiswhereyouwillsettheinitialvaluesforthepropertiesofaPhoto.

ImplementawakeFromInsert()inPhoto.swift.overridefuncawakeFromInsert(){super.awakeFromInsert()

//Givethepropertiestheirinitialvaluestitle=""photoID=""remoteURL=NSURL()photoKey=NSUUID().UUIDStringdateTaken=NSDate()}

YouhavecreatedyourmodelgraphanddefinedyourPhotoentity.ThenextstepistosetuptheCoreDatastack,whichwillmanagetheinteractionsbetweentheapplicationandCoreData.


BuildingtheCoreDataStackTheCoreDatastackconsistsoftheclassesthatinterfacewithyourentitiesaswellasthemodelfilethatdescribestheentitiestosaveandloadinstancestoastore(suchasthefilesystem).YouwilllearnabouteachcomponentinthestackandwhatroleitplaysinCoreData.

CreateanewSwiftfileandnameitCoreDataStack.OpenthisfileanddeclaretheCoreDataStackclassalongwitharequiredinitializerthatacceptsthenameofthemodelfile.DonotforgettoimporttheCoreDataframework.importFoundationimportCoreData

classCoreDataStack{

letmanagedObjectModelName:String

requiredinit(modelName:String){managedObjectModelName=modelName}

}

NSManagedObjectModel

Youworkedwiththemodelfileearlierinthechapter.Themodelfileiswhereyoudefinetheentitiesforyourapplicationalongwiththeirproperties.ThemodelfileisaninstanceofNSManagedObjectModel.

InCoreDataStack.swift,addapropertytoreadinthemodelfilefromthemainbundle.classCoreDataStack{

letmanagedObjectModelName:String

privatelazyvarmanagedObjectModel:NSManagedObjectModel={letmodelURL=NSBundle.mainBundle().URLForResource(self.managedObjectModelName,withExtension:"momd")!returnNSManagedObjectModel(contentsOfURL:modelURL)!}()

requiredinit(modelName:String){managedObjectModelName=modelName}}

First,themodelfileislocatedwithinthemainbundle.ThenaninstanceofNSManagedObjectModeliscreatedusingthisURL.

Noticethelazykeywordinthepropertydeclaration.Thelazykeywordallowsapropertytobelazilyloaded.Inotherwords,thefirsttimethatthemanagedObjectModelpropertyisaccessed,theassociatedclosurewillbeexecuted.Lazyloadingallowspropertyinitializationtobedeferreduntilthepropertyisactuallyneeded.


NSPersistentStoreCoordinator

CoreDatacanpersistdatatodiskusingafewdifferentformats:

SQLite DataissavedtodiskusingaSQLitedatabase.Thisisthemostcommonlyusedstoretype.

Atomic Dataissavedtodiskusingabinaryformat.

XML DataissavedtodiskusinganXMLformat.ThisstoretypeisnotavailableoniOS.

In-Memory Dataisnotsavedtodisk,butinsteadisstoredinmemory.

ThemappingbetweenanobjectgraphandthepersistentstoreisaccomplishedusinganinstanceofNSPersistentStoreCoordinator.Thepersistentstorecoordinatorneedstoknowtwothings:“Whataremyentities?”and“WhereamIsavingtoandloadingdatafrom?”Toanswerthesequestions,youwillinstantiateanNSPersistentStoreCoordinatorwiththeNSManagedObjectModel.Thenyouwilladdapersistentstore,representingoneofthepersistenceformatsabove,tothecoordinator.

InCoreDataStack.swift,addtwopropertiestosetupthepersistentstorecoordinator.privatevarapplicationDocumentsDirectory:NSURL={leturls=NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory,inDomains:.UserDomainMask)returnurls.first!}()

privatelazyvarpersistentStoreCoordinator:NSPersistentStoreCoordinator={

varcoordinator=NSPersistentStoreCoordinator(managedObjectModel:self.managedObjectModel)

letpathComponent="\(self.managedObjectModelName).sqlite"leturl=self.applicationDocumentsDirectory.URLByAppendingPathComponent(pathComponent)

letstore=try!coordinator.addPersistentStoreWithType(NSSQLiteStoreType,configuration:nil,URL:url,options:nil)

returncoordinator}()

Afterthecoordinatoriscreated,youattempttoaddaspecificstoretothecoordinator.Ataminimum,thisstoreneedstoknowitstypeandwhereitshouldpersistthedata.

NSManagedObjectContext


TheportalthroughwhichyouinteractwithyourentitiesistheNSManagedObjectContext.Themanagedobjectcontextisassociatedwithaspecificpersistentstorecoordinator.Youcanthinkofthemanagedobjectcontextasanintelligentscratchpad.Whenyouaskthecontexttofetchsomeentities,thecontextwillworkwithitspersistentstorecoordinatortobringtemporarycopiesoftheentitiesandobjectgraphintomemory.Unlessyouaskthecontexttosaveitschanges,thepersisteddataremainsthesame.

StillinCoreDataStack.swift,addanewpropertytoholdontoaninstanceofNSManagedObjectContext.lazyvarmainQueueContext:NSManagedObjectContext={

letmoc=NSManagedObjectContext(concurrencyType:.MainQueueConcurrencyType)moc.persistentStoreCoordinator=self.persistentStoreCoordinatormoc.name="MainQueueContext(UIContext)"

returnmoc}()

YourCoreDatastackiscompletefornow.NowyoujustneedtomakethePhotoStoreresponsibleforowningtheCoreDatastack.

OpenPhotoStore.swiftandaddapropertyforaCoreDataStackinstance.classPhotoStore{

letcoreDataStack=CoreDataStack(modelName:"Photorama")


UpdatingItemsWiththeCoreDatastackcomplete,youcannowinteractwithit.Primarily,youwilldothisthroughitsmainQueueContext.Thisishowyouwillbothcreatenewentitiesandsavechanges.

Insertingintothecontext

Whenanentityiscreated,itshouldbeinsertedintoamanagedobjectcontext.

OpenFlickrAPI.swiftandimportCoreData.importFoundationimportCoreData

ThenupdatethephotoFromJSONObject(_:)methodtotakeinanadditionalargumentoftypeNSManagedObjectContext.ItwillthenusethiscontexttoinsertnewPhotoinstances.privatestaticfuncphotoFromJSONObject(json:[String:AnyObject],inContextcontext:NSManagedObjectContext)->Photo?{

guardletphotoID=json["id"]as?String,title=json["title"]as?String,dateString=json["datetaken"]as?String,photoURLString=json["url_h"]as?String,url=NSURL(string:photoURLString),dateTaken=dateFormatter.dateFromString(dateString)else{returnnil}

returnPhoto(title:title,photoID:photoID,URL:url)

varphoto:Photo!context.performBlockAndWait(){photo=NSEntityDescription.insertNewObjectForEntityForName("Photo",inManagedObjectContext:context)as!Photophoto.title=titlephoto.photoID=photoIDphoto.remoteURL=urlphoto.dateTaken=dateTaken}

returnphoto}

EachNSManagedObjectContextisassociatedwithaspecificconcurrencyqueue,andthemainQueueContextisassociatedwiththemain,oruserinterface,queue.Youhavetointeractwithacontextonthequeuethatitisassociatedwith.NSManagedObjectContexthastwomethodsthatensurethishappens:performBlock(_:)andperformBlockAndWait(_:).ThedifferencebetweenthemisthatperformBlock(_:)isasynchronousandperformBlockAndWait(_:)issynchronous.SinceyouarereturningtheresultoftheinsertoperationfromthephotoFromJSONObject(_:inContext:)method,youusethesynchronousmethod.

ThephotoFromJSONObject(_:)methodiscalledfromthemethodphotosFromJSONData(_:).Updatethismethodtotakeinacontextandpassitto


thephotoFromJSONObject(_:)method.staticfuncphotosFromJSONData(data:NSData,inContextcontext:NSManagedObjectContext)->PhotosResult{

do{letjsonObject:AnyObject=tryNSJSONSerialization.JSONObjectWithData(data,options:[])



varfinalPhotos=[Photo]()forphotoJSONinphotosArray{ifletphoto=photoFromJSONObject(photoJSON,inContext:context){finalPhotos.append(photo)}}

...

Finally,youneedtopassthemainQueueContexttotheFlickrAPIstructoncethewebservicerequestsuccessfullycompletes.

OpenPhotoStore.swiftandupdateprocessRecentPhotosRequest(data:error:).funcprocessRecentPhotosRequest(datadata:NSData?,error:NSError?)->PhotosResult{

guardletjsonData=dataelse{return.Failure(error!)}

returnFlickrAPI.photosFromJSONData(jsonData,inContext:self.coreDataStack.mainQueueContext)}

Buildandruntheapplicationnowthatallerrorshavebeenaddressed.Althoughthebehaviorremainsunchanged,theapplicationisnowbackedbyCoreData.Inthenextsection,youwillimplementsavingforboththephotosandtheirassociatedimagedata.

Savingchanges

RecallthatNSManagedObjectchangesdonotpersistuntilyoutellthecontexttosavethesechanges.

OpenCoreDataStack.swiftandaddanewmethodthatsavesthechangestothecontext.funcsaveChanges()throws{varerror:ErrorType?mainQueueContext.performBlockAndWait(){

ifself.mainQueueContext.hasChanges{do{tryself.mainQueueContext.save()}catchletsaveError{error=saveError}}


}ifleterror=error{throwerror}}

ThesaveChanges()methodwillnothandleanysaveerrorsitself.Instead,itwillthrowtheerrorsothatitispassedalongtothecaller.

OpenPhotoStore.swiftandupdatefetchRecentPhotos(_:)tosavethechangestothecontextafterPhotoentitieshavebeeninsertedintothecontext.funcfetchRecentPhotos(completioncompletion:(PhotosResult)->Void){leturl=FlickrAPI.recentPhotosURL()letrequest=NSURLRequest(URL:url)lettask=session.dataTaskWithRequest(request){(data,response,error)->Voidin

letresult=self.processRecentPhotosRequest(data:data,error:error)varresult=self.processRecentPhotosRequest(data:data,error:error)

ifcaselet.Success(photos)=result{do{tryself.coreDataStack.saveChanges()}catchleterror{result=.Failure(error)}}



UpdatingtheDataSourceOneproblemwiththeappatthemomentisthatfetchRecentPhotos(completion:)onlyreturnsthenewlyinsertedphotos.Nowthattheapplicationsupportssaving,itshouldreturnallofthephotos–thepreviouslysavedphotosaswellasthenewlyinsertedones.YouneedtoaskCoreDataforallofthePhotoentities,andyouwillaccomplishthisusingafetchrequest.

Fetchrequestsandpredicates

TogetobjectsbackfromtheNSManagedObjectContext,youmustprepareandexecuteanNSFetchRequest.Afterafetchrequestisexecuted,youwillgetanarrayofalltheobjectsthatmatchtheparametersofthatrequest.

Afetchrequestneedsanentitydescriptionthatdefineswhichentityyouwanttogetobjectsfrom.TofetchPhotoinstances,youspecifythePhotoentity.Youcanalsosettherequest’ssortdescriptorstospecifytheorderoftheobjectsinthearray.AsortdescriptorhasakeythatmapstoanattributeoftheentityandaBoolthatindicateswhethertheordershouldbeascendingordescending.

ThesortDescriptorspropertyonNSFetchRequestisanarrayofNSSortDescriptorinstances.Whyanarray?Thearrayisusefulifyouthinktheremightbecollisionswhensorting.Forexample,sayyouaresortinganarrayofpeoplebytheirlastnames.Itisentirelypossiblethatmultiplepeoplehavethesamelastname,soyoucanspecifythatpeoplewiththesamelastnameshouldbesortedbytheirfirstnames.ThiswouldbeimplementedbyanarrayoftwoNSSortDescriptorinstances.Thefirstsortdescriptorwouldhaveakeythatmapstotheperson’slastname,andthesecondsortdescriptorwouldhaveakeythatmapstotheperson’sfirstname.

ApredicateisrepresentedbytheNSPredicateclassandcontainsaconditionthatcanbetrueorfalse.Ifyouwantedtofindallphotoswithagivenidentifier,youwouldcreateapredicateandaddittothefetchrequestlikethis:letpredicate=NSPredicate.predicateWithFormat("photoID==\(someIdentifier)")request.predicate=predicate

Theformatstringforapredicatecanbeverylongandcomplex.Apple’sPredicateProgrammingGuideisacompletediscussionofwhatispossible.

YouwanttosortthereturnedinstancesofPhotobydateTakenindescendingorder.Todothis,youwillinstantiateanNSFetchRequest,givingitthe“Photo”entityname.ThenyouwillgivethefetchrequestanarrayofNSSortDescriptorinstances.ForPhotorama,thisarraywillcontainasinglesortdescriptorthatsortsphotosbytheirdateTakenproperties.Finally,youwillaskthemanagedobjectcontexttoexecutethisfetchrequest.

ImporttheCoreDataframeworkatthetopofPhotoStore.swift.importUIKitimportCoreData


StillinPhotoStore.swift,implementamethodthatwillfetchthePhotoinstancesfromthemainqueuecontext.funcfetchMainQueuePhotos(predicatepredicate:NSPredicate?=nil,sortDescriptors:[NSSortDescriptor]?=nil)throws->[Photo]{

letfetchRequest=NSFetchRequest(entityName:"Photo")fetchRequest.sortDescriptors=sortDescriptorsfetchRequest.predicate=predicate

letmainQueueContext=self.coreDataStack.mainQueueContextvarmainQueuePhotos:[Photo]?varfetchRequestError:ErrorType?mainQueueContext.performBlockAndWait(){do{mainQueuePhotos=trymainQueueContext.executeFetchRequest(fetchRequest)as?[Photo]}catchleterror{fetchRequestError=error}}

guardletphotos=mainQueuePhotoselse{throwfetchRequestError!}

returnphotos}

Afterthewebservicerequestfinishes,youneedtoreturnthephotos.InPhotoStore.swift,updatefetchRecentPhotos(completion:)tofetchthemainqueuephotoswhenthewebservicefinishes.varresult=self.processRecentPhotosRequest(data:data,error:error)

ifcaselet.Success(photos)=result{letmainQueueContext=self.coreDataStack.mainQueueContextmainQueueContext.performBlockAndWait(){try!mainQueueContext.obtainPermanentIDsForObjects(photos)}letobjectIDs=photos.map{$0.objectID}letpredicate=NSPredicate(format:"selfIN%@",objectIDs)letsortByDateTaken=NSSortDescriptor(key:"dateTaken",ascending:true)

do{tryself.coreDataStack.saveChanges()

letmainQueuePhotos=tryself.fetchMainQueuePhotos(predicate:predicate,sortDescriptors:[sortByDateTaken])result=.Success(mainQueuePhotos)}catchleterror{result=.Failure(error)}}

completion(result)

Thiswillonlyreturnthenewlyinsertedphotos.YouwanttheinterfaceforPhotosViewControllertodisplayallofthephotos.

OpenPhotosViewController.swiftandupdateviewDidLoad()tofetchanddisplayallofthephotossavedtoCoreData.overridefuncviewDidLoad()super.viewDidLoad()

collectionView.dataSource=photoDataSourcecollectionView.delegate=self



NSOperationQueue.mainQueue().addOperationWithBlock(){switchphotosResult{caselet.Success(photos):print("Successfullyfound\(photos.count)recentphotos.")self.photoDataSource.photos=photoscaselet.Failure(error):self.photoDataSource.photos.removeAll()print("Errorfetchingrecentphotos:\(error)")}self.collectionView.reloadSections(NSIndexSet(index:0))}

letsortByDateTaken=NSSortDescriptor(key:"dateTaken",ascending:true)letallPhotos=try!self.store.fetchMainQueuePhotos(predicate:nil,sortDescriptors:[sortByDateTaken])

NSOperationQueue.mainQueue().addOperationWithBlock(){self.photoDataSource.photos=allPhotosself.collectionView.reloadSections(NSIndexSet(index:0))}}}

Previouslysavedphotoswillnowbereturnedwhenthewebservicerequestfinishes.Butthereisstilloneproblem:iftheapplicationisrunmultipletimesandthesamephotoisreturnedfromthewebservicerequest,itwillbeinsertedintothecontextmultipletimes.Thisisnotgood–youdonotwantduplicatephotos.Luckilythereisauniqueidentifierforeachphoto.Whentherecentphotoswebservicerequestfinishes,theidentifierforeachphotointheincomingJSONcanbecomparedtothephotosstoredinCoreData.Ifoneisfoundwiththesameidentifier,thatphotowillbereturned.Otherwise,anewphotowillbeinsertedintothecontext.

Todothis,youneedawaytotellthefetchrequestthatitshouldnotreturnallphotosbutinsteadonlythephotosthatmatchsomespecificcriteria.Inthiscase,thespecificcriteriais“onlyphotosthathavethisspecificidentifier,”ofwhichthereshouldeitherbezerooronephoto.InCoreData,thisisdonewithapredicate.

InFlickrAPI.swift,updatephotoFromJSONObject(_:inContext:)tocheckwhetherthereisanexistingphotowithagivenIDbeforeinsertinganewone.privatestaticfuncphotoFromJSONObject(json:[String:AnyObject],inContextcontext:NSManagedObjectContext)->Photo?{

guardletphotoID=json["id"]as?String,title=json["title"]as?String,photoURLString=json["url_h"]as?String,url=NSURL(string:photoURLString),dateString=json["datetaken"]as?String,dateTaken=dateFormatter.dateFromString(dateString)else{

//Don'thaveenoughinformationtoconstructaPhotoreturnnil}

letfetchRequest=NSFetchRequest(entityName:"Photo")letpredicate=NSPredicate(format:"photoID==\(photoID)")fetchRequest.predicate=predicate

varfetchedPhotos:[Photo]!context.performBlockAndWait(){fetchedPhotos=try!context.executeFetchRequest(fetchRequest)as![Photo]}iffetchedPhotos.count>0{returnfetchedPhotos.first


}

varphoto:Photo!context.performBlockAndWait(){photo=NSEntityDescription.insertNewObjectForEntityForName("Photo",inManagedObjectContext:context)as!Photophoto.title=titlephoto.photoID=photoIDphoto.remoteURL=urlphoto.dateTaken=dateTaken}

returnphoto

}

DuplicatephotoswillnolongerbeinsertedintoCoreData.

Buildandruntheapplication.ThephotoswillappearjustastheydidbeforeintroducingCoreData.AsyoudidinChapter15,closetheapplicationusingtheHomebutton(orShift-Command-Hinthesimulator).LaunchtheapplicationagainandyouwillseethephotosthatCoreDatasavedinthecollectionview.


SavingImagestoDiskAlthoughthePhotoentitiesarebeingpersisted,theimagedataassociatedwitheachphotoisnot.Largeblobsofdata,suchasanimage,shouldnotbestoredinCoreDataduetoperformanceconcerns.Instead,youshouldstoredatalikethisonthefilesystem.

Fortheimagedata,youwillusethesameapproachthatyouusedinyourHomepwnerapplication.Infact,youwillusethesameImageStoreclassthatyouwroteforthatproject.

OpenHomepwner.xcodeprojanddragtheImageStore.swiftfilefromtheHomepwnerapplicationtothePhotoramaapplication.MakesuretochooseCopyitemsifneeded.OncetheImageStore.swiftfilehasbeenaddedtoPhotorama,youcanclosetheHomepwnerproject.

BackinPhotorama,openPhotoStore.swiftandgiveitapropertyforanImageStore.classPhotoStore{

letcoreDataStack=CoreDataStack(modelName:"Photorama")letimageStore=ImageStore()

ThenupdatefetchImageForPhoto(_:completion:)tosavetheimagesusingtheimageStore.funcfetchImageForPhoto(photo:Photo,completion:(ImageResult)->Void){

letphotoKey=photo.photoKeyifletimage=photo.image{ifletimage=imageStore.imageForKey(photoKey){photo.image=imagecompletion(.Success(image))return}


lettask=session.dataTaskWithRequest(request){(data,response,error)->Voidinletresult=self.processImageRequest(data:data,error:error)

ifcaselet.Success(image)=result{photo.image=imageself.imageStore.setImage(image,forKey:photoKey)}


Buildandruntheapplication.Nowwhentheimagedataisdownloaded,itwillbesavedtothefilesystem.Thenexttimethatphotoisrequested,itwillbeloadedfromthefilesystemifitisnotcurrentlyinmemory.

ThePhotoramaapplicationisnowpersistingitsdatabetweenruns.ThephotometadataisbeingpersistedusingCoreData,andtheimagedataisbeingpersisteddirectlytothefilesystem.Asyouhaveseen,thereisnoone-size-fits-allapproachtodatapersistence.Instead,eachpersistencemechanismhasitsownsetofbenefitsanddrawbacks.Inthis


chapter,youhaveexploredoneofthose,CoreData,butyouhaveonlyseenthetipoftheiceberg.InChapter22,youwillexploretheCoreDataframeworkfurthertolearnaboutrelationshipsandperformance.


BronzeChallenge:PhotoViewCountAddanattributetothePhotoentitythattrackshowmanytimesaphotoisviewed.DisplaythisnumbersomewhereonthePhotoInfoViewControllerinterface.


22CoreDataRelationships

CoreDataisnotthatexcitingwithjustoneentity.MuchofthepowerbehindCoreDatacomestolightwhentherearemultipleentitiesthatarerelatedtooneanother,becauseCoreDatamanagesrelationshipsbetweenentities.

Inthischapter,youaregoingtoaddtagstothephotosinPhotoramawithlabelssuchas“Nature,”“Electronics,”or“Selfies.”Userswillbeabletoaddoneormoretagstophotosandalsocreatetheirowncustomtags(Figure22.1).

Figure22.1FinalPhotoramaapplication


RelationshipsOneofthebenefitsofusingCoreDataisthatentitiescanberelatedtooneanotherinawaythatallowsyoutodescribecomplexmodels.Relationshipsbetweenentitiesarerepresentedbyreferencesbetweenobjects.Therearetwokindsofrelationships:to-oneandto-many.

Whenanentityhasato-onerelationship,eachinstanceofthatentitywillhaveareferencetoaninstanceintheentityithasarelationshipto.

Whenanentityhasato-manyrelationship,eachinstanceofthatentityhasareferencetoaSet.Thissetcontainstheinstancesoftheentitythatithasarelationshipwith.Toseethisinaction,youaregoingtoaddanewentitytothemodelfile.

ReopenthePhotoramaapplication.InPhotorama.xcdatamodeld,addanotherentitycalledTag.GiveitanattributecallednameoftypeString.Tagwillallowuserstotagphotos.

Aphotomighthavemultipletagsthatdescribeit,andatagmightbeassociatedwithmultiplephotos.Forexample,apictureofaniPhonemightbetagged“Electronics”and“Apple,”andapictureofaBetamaxplayermightbetagged“Electronics”and“Rare.”TheTagentitywillhaveato-manyrelationshiptothePhotoentitybecausemanyinstancesofPhotocanhavethesameTag.AndthePhotoentitywillhaveato-manyrelationshiptotheTagentitybecauseaphotocanbeassociatedwithmanyTags.

AsFigure22.2shows,aPhotowillhaveareferencetoasetofTags,andaTagwillhaveareferencetoasetofPhotos.


Figure22.2EntitiesinPhotorama

Whentheserelationshipsaresetup,youwillbeabletoaskaPhotoobjectforthesetofTagobjectsthatitisassociatedwithandaskaTagobjectforthesetofPhotoobjectsthatitisassociatedwith.

Toaddthesetworelationshipstothemodelfile,firstselecttheTagentityandclickthe+buttonintheRelationshipssection.IntheRelationshipcolumn,nametherelationshipphotos.IntheDestinationcolumn,selectPhoto.Inthedatamodelinspector,changetheTypedropdownfromToOnetoToManyandunchecktheOptionalcheckbox(Figure22.3).

Figure22.3Creatingthephotosrelationship


Next,selectthePhotoentity.AddarelationshipnamedtagsandpickTagasitsdestination.Inthedatamodelinspector,changetheTypedropdowntoToManyanduncheckitsOptionalcheckbox.

Nowthatyouhavetwounidirectionalrelationships,youcanmakethemintoaninverserelationship.Aninverserelationshipisabidirectionalrelationshipbetweentwoentities.WithaninverserelationshipsetupbetweenPhotoandTag,CoreDatacanensurethatyourobjectgraphremainsinaconsistentstatewhenanychangesaremade.

Tocreatetheinverserelationship,clickNoInverseintheInversecolumnandselectphotos(Figure22.4).IfyoureturntotheTagentity,youwillseethatthephotosrelationshipnowshowstagsasitsinverse.

Figure22.4Creatingthetagsrelationship

NowopenPhoto+CoreDataProperties.swift.Addthetagsproperty,whichwillbeaSetofNSManagedObjectinstances.extensionPhoto{

@NSManagedvarphotoID:String@NSManagedvarphotoKey:String@NSManagedvartitle:String@NSManagedvardateTaken:NSDate@NSManagedvarremoteURL:NSURL@NSManagedvartags:Set<NSManagedObject>

}


AddingTagstotheInterfaceWhenusersnavigatetoaspecificphoto,theycurrentlyseeonlythetitleofthephotoandtheimageitself.Let’supdatetheinterfacetoincludeaphoto’sassociatedtags.

OpenMain.storyboardandnavigatetotheinterfaceforPhotoInfoViewController.Addatoolbartothebottomoftheview.UpdatetheAutoLayoutconstraintssothatthetoolbarisanchoredtothebottom,justasitwasinHomepwner.ThebottomconstraintfortheimageViewshouldbeanchoredtothetopofthetoolbarinsteadofthebottomofthesuperview.AddaUIBarButtonItemtothetoolbar,ifoneisnotalreadypresent,andgiveitatitleofTags.YourinterfacewilllooklikeFigure22.5.

Figure22.5PhotoInfoViewControllerinterface

CreateanewSwiftfilenamedTagsViewController.OpenthisfileanddeclaretheTagsViewControllerclassasasubclassofUITableViewController.ImportUIKitandCoreDatainthisfile.importFoundationimportUIKitimportCoreData


classTagsViewController:UITableViewController{

}

TheTagsViewControllerwilldisplayalistofallthetags.Theuserwillseeandbeabletoselectthetagsthatareassociatedwithaspecificphoto.Theuserwillalsobeabletoaddnewtagsfromthisscreen.ThecompletedinterfacewilllooklikeFigure22.6.

Figure22.6TagsViewController

GivetheTagsViewControllerclassapropertytoreferencethePhotoStoreaswellasaspecificPhoto.Youwillalsoneedapropertytokeeptrackofthecurrentlyselectedtags,whichyouwilltrackusinganarrayofNSIndexPathinstances.classTagsViewController:UITableViewController{

varstore:PhotoStore!varphoto:Photo!

varselectedIndexPaths=[NSIndexPath]()}

Thedatasourceforthetableviewwillbeaseparateclass.AswediscussedwhenyoucreatedPhotoDataSourceinChapter20,anapplicationwhosetypeshaveasingleresponsibilityiseasiertoadapttofuturechanges.Thisclasswillberesponsiblefordisplayingthelistoftagsinthetableview.

CreateanewSwiftfilenamedTagDataSource.swift.DeclaretheTagDataSourceclassandimplementthetableviewdatasourcemethods.YouwillneedtoimportUIKitandCoreData.importFoundationimportUIKitimportCoreData

classTagDataSource:NSObject,UITableViewDataSource{

vartags:[NSManagedObject]=[]

functableView(tableView:UITableView,


numberOfRowsInSectionsection:Int)->Int{returntags.count}

functableView(tableView:UITableView,cellForRowAtIndexPathindexPath:NSIndexPath)->UITableViewCell{

letcell=tableView.dequeueReusableCellWithIdentifier("UITableViewCell",forIndexPath:indexPath)

lettag=tags[indexPath.row]letname=tag.valueForKey("name")as!Stringcell.textLabel?.text=name

returncell}

}

OpenPhotoStore.swiftanddefineanewmethodthatfetchesallthetagsfromthemainqueue.funcfetchMainQueueTags(predicatepredicate:NSPredicate?=nil,sortDescriptors:[NSSortDescriptor]?=nil)throws->[NSManagedObject]{letfetchRequest=NSFetchRequest(entityName:"Tag")fetchRequest.predicate=predicatefetchRequest.sortDescriptors=sortDescriptors

letmainQueueContext=self.coreDataStack.mainQueueContextvarmainQueueTags:[NSManagedObject]?varfetchRequestError:ErrorType?mainQueueContext.performBlockAndWait({do{mainQueueTags=trymainQueueContext.executeFetchRequest(fetchRequest)as?[NSManagedObject]}catchleterror{fetchRequestError=error}})

guardlettags=mainQueueTagselse{throwfetchRequestError!}

returntags}

Withthedatasourceandfetchrequestcomplete,youhavefinishedtheimplementationofTagsViewController.

OpenTagsViewController.swiftandsetthedataSourceforthetableviewtobeaninstanceofTagDataSource.classTagsViewController:UITableViewController{

varstore:PhotoStore!varphoto:Photo!

varselectedIndexPaths=[NSIndexPath]()

lettagDataSource=TagDataSource()


tableView.dataSource=tagDataSource}}

Nowfetchthemainqueuetagsandassociatethemwiththetagspropertyonthedata


source.overridefuncviewDidLoad(){super.viewDidLoad()

tableView.dataSource=tagDataSource

updateTags()}

funcupdateTags(){lettags=try!store.fetchMainQueueTags(predicate:nil,sortDescriptors:[NSSortDescriptor(key:"name",ascending:true)])tagDataSource.tags=tags}

TheTagsViewControllerneedstomanagetheselectionoftagsandupdatethePhotoinstancewhentheuserselectsordeselectsatag.

AddtheappropriateUITableViewDelegatemethodsinTagsViewController.swift.AlsoaddtheappropriateindexpathstotheselectedIndexPathsarray.overridefuncviewDidLoad(){super.viewDidLoad()

tableView.dataSource=tagDataSourcetableView.delegate=selfupdateTags()}

funcupdateTags(){lettags=try!store.fetchMainQueueTags(predicate:nil,sortDescriptors:[NSSortDescriptor(key:"name",ascending:true)])tagDataSource.tags=tags

fortaginphoto.tags{ifletindex=tagDataSource.tags.indexOf(tag){letindexPath=NSIndexPath(forRow:index,inSection:0)selectedIndexPaths.append(indexPath)}}}

overridefunctableView(tableView:UITableView,didSelectRowAtIndexPathindexPath:NSIndexPath){

lettag=tagDataSource.tags[indexPath.row]

ifletindex=selectedIndexPaths.indexOf(indexPath){selectedIndexPaths.removeAtIndex(index)}else{selectedIndexPaths.append(indexPath)}

tableView.reloadRowsAtIndexPaths([indexPath],withRowAnimation:.Automatic)}

overridefunctableView(tableView:UITableView,willDisplayCellcell:UITableViewCell,forRowAtIndexPathindexPath:NSIndexPath){

ifselectedIndexPaths.indexOf(indexPath)!=nil{cell.accessoryType=.Checkmark}else{cell.accessoryType=.None}}

Currently,thetagsarenotbeingaddedorremovedfromthePhoto.Addingand


removingobjectsfromaCoreDatato-manyrelationshipisalittlebizarreduetothedynamicnatureofNSManagedObjectcombinedwiththeimmutabilityoftheSetthatCoreDatareturns.Toallowforchanges,ApplehasprovidedthemethodmutableSetValueForKey(_:).Thismethodreturnsanobjectthatisamutableset,andyouthenmakeanychangestothisobject.

OpenPhoto.swiftanddefinetwomethodsthathandleaddingandremovingTaginstancesfromthetagsset.funcaddTagObject(tag:NSManagedObject){letcurrentTags=mutableSetValueForKey("tags")currentTags.addObject(tag)}

funcremoveTagObject(tag:NSManagedObject){letcurrentTags=mutableSetValueForKey("tags")currentTags.removeObject(tag)}

ByusingthemutableSetValueForKey(_:)method,CoreDatawillbeinformedofanychangestotheto-manyrelationshipandwillallowthechangestopersist.

YoucannowusethesetwomethodsonPhototoupdatethetagsassociatedwithaphoto.

InTagsViewController.swift,updatetableView(_:didSelectRowAtIndexPath:)toaddorremovetheassociatedTagfromthePhoto.Alsosavethechangestothecontextwhentheusermakesaselection.overridefunctableView(tableView:UITableView,didSelectRowAtIndexPathindexPath:NSIndexPath){

lettag=tagDataSource.tags[indexPath.row]

ifletindex=find(selectedIndexPaths,indexPath){selectedIndexPaths.removeAtIndex(index)photo.removeTagObject(tag)}else{selectedIndexPaths.append(indexPath)photo.addTagObject(tag)}

tableView.reloadRowsAtIndexPaths([indexPath],withRowAnimation:.Automatic)

do{trystore.coreDataStack.saveChanges()}catchleterror{print("CoreDatasavefailed:\(error)")}}

Let’ssetupTagsViewControllertobepresentedmodallywhentheusertapstheTagsbarbuttonitemonthePhotoInfoViewController.

OpenMain.storyboardanddragaNavigationControllerontothecanvas.ThisshouldgiveyouaUINavigationControllerwitharootviewcontrollerthatisaUITableViewController.IftherootviewcontrollerisnotaUITableViewController,deletetherootviewcontroller,dragaTableViewControllerontothecanvas,andmakeittherootviewcontrolleroftheNavigationController.

Control-dragfromtheTagsitemonPhotoInfoViewControllertothenewNavigationControllerandWOW! eBook

www.wowebook.org

selectthePresentModallyseguetype(Figure22.7).OpentheattributesinspectorforthesegueandgiveitanIdentifiernamedShowTags.

Figure22.7Addingthetagsviewcontroller

SelecttheRootViewControllerthatyoujustaddedtothecanvasandopenitsidentityinspector.ChangeitsClasstoTagsViewController.Thisnewviewcontrollerdoesnothaveanavigationitemassociatedwithit,sofindNavigationItemintheobjectlibraryanddragitontotheviewcontroller.Double-clickthenewnavigationitem’sTitlelabelandchangeittoTags.

Next,theUITableViewCellontheTagsViewControllerinterfaceneedstomatchwhattheTagDataSourceexpects.Itneedstousethecorrectstyleandhavethecorrectreuseidentifier.


SelecttheUITableViewCell.(Itmightbeeasiertoselectinthedocumentoutline.)Openitsattributesinspector.ChangetheStyletoBasicandsettheIdentifiertoUITableViewCell(Figure22.8).

Figure22.8ConfiguringtheUITableViewCell

Now,theTagsViewControllerneedstwobarbuttonitemsonitsnavigationbar:aDonebuttonthatdismissestheviewcontrolleranda+buttonthatallowstheusertoaddanewtag.

DragabarbuttonitemtotheleftandrightbarbuttonitemslotsfortheTagsViewController.SettheleftitemtousetheDonestyleandsystemitem.SettherightitemtousetheBorderedstyleandanAddsystemitem(Figure22.9).

Figure22.9Barbuttonitemattributes

CreateandconnectanactionforeachoftheseitemstotheTagsViewController.TheDoneitemshouldbeconnectedtoamethodnameddone(_:),andthe+itemshouldbeconnectedtoamethodnamedaddNewTag(_:).ThetwomethodsinTagsViewController.swiftwillbe:@IBActionfuncdone(sender:AnyObject){

}

@IBActionfuncaddNewTag(sender:AnyObject){

}

Theimplementationofdone(_:)issimple:theviewcontrollerjustneedstobe


dismissed.Implementthisfunctionalityindone(_:).@IBActionfuncdone(sender:AnyObject){presentingViewController?.dismissViewControllerAnimated(true,completion:nil)}

Whentheusertapsthe+item,analertwillbepresentedthatwillallowtheusertotypeinthenameforanewtag.

Figure22.10Addinganewtag

SetupandpresentaninstanceofUIAlertControllerinaddNewTag(_:).@IBActionfuncaddNewTag(sender:AnyObject){letalertController=UIAlertController(title:"AddTag",message:nil,preferredStyle:.Alert)

alertController.addTextFieldWithConfigurationHandler({(textField)->VoidintextField.placeholder="tagname"textField.autocapitalizationType=.Words})

letokAction=UIAlertAction(title:"OK",style:.Default,handler:{(action)->Voidin

})alertController.addAction(okAction)

letcancelAction=UIAlertAction(title:"Cancel",style:.Cancel,handler:nil)alertController.addAction(cancelAction)

presentViewController(alertController,animated:true,completion:nil)}

UpdatethecompletionhandlerfortheokActiontoinsertanewTagintothecontext.Thensavethecontext,updatethelistoftags,andreloadthetableviewsection.letokAction=UIAlertAction(title:"OK",style:.Default,handler:{(action)->Voidin

iflettagName=alertController.textFields?.first!.text{letcontext=self.store.coreDataStack.mainQueueContextletnewTag=NSEntityDescription.insertNewObjectForEntityForName("Tag",inManagedObjectContext:context)newTag.setValue(tagName,forKey:"name")

do{tryself.store.coreDataStack.saveChanges()}catchleterror{


print("CoreDatasavefailed:\(error)")}self.updateTags()self.tableView.reloadSections(NSIndexSet(index:0),withRowAnimation:.Automatic)}})alertController.addAction(okAction)

Finally,whentheTagsbarbuttonitemonPhotoInfoViewControlleristapped,thePhotoInfoViewControllerneedstopassalongitsstoreandphotototheTagsViewController.

OpenPhotoInfoViewControllerandimplementprepareForSegue(_:).overridefuncprepareForSegue(segue:UIStoryboardSegue,sender:AnyObject?){ifsegue.identifier=="ShowTags"{letnavController=segue.destinationViewControlleras!UINavigationControllerlettagController=navController.topViewControlleras!TagsViewController

tagController.store=storetagController.photo=photo}}

Buildandruntheapplication.NavigatetoaphotoandtaptheTagsitemonthetoolbaratthebottom.TheTagsViewControllerwillbepresentedmodally.Tapthe+item,enteranewtag,andselectthenewtagtoassociateitwiththephoto.


Parent-ChildContextsYoulearnedinChapter21thataninstanceofNSManagedObjectContextisassociatedwithaspecificqueue,whichisthemainqueueinthecaseofthemainQueueContext.AsinglecontextassociatedwiththemainqueueistheminimumthatyouneedforaworkingCoreDataapplication,butanyCoreDataoperationsthattakealongtimewillnoticeablyblockthemainqueue,resultinginanunresponsiveapplication.

Toaddressthisproblem,applicationsusingCoreDataoftenhavemultiplecontexts.Photoramawillhaveonecontextassociatedwiththemainqueueandanothercontextassociatedwithabackgroundqueue.

OpenCoreDataStack.swift.Addanothercontextthatisassociatedwithaprivate,orbackground,queue.lazyvarprivateQueueContext:NSManagedObjectContext={

letmoc=NSManagedObjectContext(concurrencyType:.PrivateQueueConcurrencyType)moc.parentContext=self.mainQueueContextmoc.name="PrimaryPrivateQueueContext"

returnmoc}()

AnNSManagedObjectContextcaneitherbeassociatedwithanNSPersistentStoreCoordinatororwithaparentNSManagedObjectContext.Thecontextatthe“top”ofthestackmustbeassociatedwithapersistentstorecoordinator.

Sohowdoesthiswork?Whenyousaveacontextassociatedwithapersistentstorecoordinator,thosechangesarepersistedtothestore.Whenyousaveacontextthathasaparentcontext,anychangesfromthechildcontextpropagateuponlyoneleveltotheparentcontext.Atthispoint,theparentcontextwillhavenewchanges.Thesenewchangeswillnotautomaticallybesaved,soyouwillalsoneedtosavetheparentcontext.

InCoreDataStack.swift,updatesaveChanges(_:)tosupportthisfunctionality.funcsaveChanges()throws{varerror:ErrorType?

privateQueueContext.performBlockAndWait{()->Voidinifself.privateQueueContext.hasChanges{do{tryself.privateQueueContext.save()}catchletsaveError{error=saveError}}}

ifleterror=error{throwerror}

mainQueueContext.performBlockAndWait{()->Voidinifself.mainQueueContext.hasChanges{do{tryself.mainQueueContext.save()}


catchletsaveError{error=saveError}}}

ifleterror=error{throwerror}}

First,anychangesfromtheprivateQueueContextarepropagateduptothemainQueueContext.Then,anychangestothemainQueueContextwillbepersistedusingitspersistentstorecoordinator.

Nowlet’susethisnewcontexttoimprovetheperformanceofPhotorama.Currently,thePhotoentitiesareaddedonthemainqueue,whichcanblockotheroperations.Youaregoingtoupdatetheapplicationtoaddentitiesonthebackgroundqueueinstead.

OpenPhotoStore.swiftandupdateprocessRecentPhotosRequest(data:error:)tousetheprivateQueueContext.funcprocessRecentPhotosRequest(datadata:NSData?,error:NSError?)->PhotosResult{

guardletjsonData=dataelse{return.Failure(error!)}

returnFlickrAPI.photosFromJSONData(jsonData,inContext:self.coreDataStack.mainQueueContext)

returnFlickrAPI.photosFromJSONData(jsonData,inContext:self.coreDataStack.privateQueueContext)}

NowupdatefetchRecentPhotos(completion:)togetthepermanentIDsforthephotosfromtheprivatequeuecontext,sincethatisthecontextthatthephotosarebeingaddedtonow.funcfetchRecentPhotos(completioncompletion:(PhotosResult)->Void){leturl=FlickrAPI.recentPhotosURL()letrequest=NSURLRequest(URL:url)lettask=session.dataTaskWithRequest(request,completionHandler:{(data,response,error)->Voidin

varresult=self.processRecentPhotosRequest(data:data,error:error)

ifcaselet.Success(photos)=result{letmainQueueContext=self.coreDataStack.mainQueueContextmainQueueContext.performBlockAndWait({try!mainQueueContext.obtainPermanentIDsForObjects(photos)})

letprivateQueueContext=self.coreDataStack.privateQueueContextprivateQueueContext.performBlockAndWait({try!privateQueueContext.obtainPermanentIDsForObjects(photos)})

...

Buildandruntheapplication.Althoughthebehaviorhasnotchanged,theapplicationisnolongerindangerofbecomingunresponsivewhilenewphotosarebeingadded.Asthescaleofyourapplicationsincreases,handlingCoreDataentitiessomewhereotherthanthemainqueue(likeyouhavedonehere)canresultinhugeperformancewins.


Congratulations!Overthepastfourchapters,youhaveworkedonarathercomplexapp.Photoramaisabletomakemultiplewebservicecalls,displayphotosinagrid,cacheimagedatatothefilesystem,andpersistphotodatausingCoreData.Toaccomplishthis,youusedknowledgethatyouhavegainedthroughoutthisbook,andyouappliedthatknowledgetocreateanawesomeappthatisalsorobustandmaintainable.Itwashardwork,andyoushouldbeproudofyourself.


SilverChallenge:FavoritesAllowtheusertofavoritephotos.Becreativeinhowyoupresentthefavoritephotostotheuser.TwopossibilitiesincludeviewingthemusingaUITabBarControlleroraddingaUISegmentedControltothePhotosViewControllerthatswitchesbetweenallphotosandfavoritephotos.(Hint:youwillneedtoaddanewattributetothePhotoentity.)


23Afterword

Welcometotheendofthebook!Youshouldbeveryproudofallyourworkandallthatyouhavelearned.Nowthereisgoodnewsandbadnews:

Thegoodnews:ThestuffthatleavesprogrammersbefuddledwhentheycometotheiOSplatformisbehindyounow.YouareaniOSdeveloper.

Thebadnews:YouareprobablynotaverygoodiOSdeveloper.


WhattoDoNextItisnowtimetomakesomemistakes,readsomereallytediousdocumentation,andbehumbledbytheheartlessexpertswhowillridiculeyourquestions.Hereiswhatwerecommend:

Writeappsnow.Ifyoudonotimmediatelyusewhatyouhavelearned,itwillfade.Exerciseandextendyourknowledge.Now.

Godeep.Thisbookhasconsistentlyfavoredbreadthoverdepth;anychaptercouldhavebeenexpandedintoanentirebook.Findatopicthatyoufindinterestingandreallywallowinit–dosomeexperiments,readApple’sdocsonthetopic,readapostingonablogoronStackOverflow.

Connect.ThereisaniOSDeveloperMeetupinmostcities,andthetalksaresurprisinglygood.ThereareCocoaHeadschaptersaroundtheworld.Therearediscussiongroupsonline.Ifyouaredoingaproject,findpeopletohelpyou:designers,testers(AKAguineapigs),andotherdevelopers.

Makemistakesandfixthem.Youwilllearnalotonthedayswhenyousay,“Thisapplicationhasbecomeaballofcrap!I’mgoingtothrowitawayandwriteitagainwithanarchitecturethatmakessense.”Politeprogrammerscallthisrefactoring.

Giveback.Sharetheknowledge.Answeradumbquestionwithgrace.Giveawaysomecode.


ShamelessPlugsYoucanfindusonTwitter,wherewekeepyouinformedaboutprogrammingandentertainedaboutlife:@cbkeurand@aaronhillegass.

KeepaneyeoutforotherguidesfromBigNerdRanch.Wealsoofferweek-longcoursesfordevelopers.Andifyoujustneedsomecodewritten,wedocontractprogramming.Formoreinformation,visitourwebsiteathttp://www.bignerdranch.com/.

You,dearreader,makeourlivesofwriting,coding,andteachingpossible.Sothankyouforbuyingourbook.


http://twitter.com/cbkeur

http://twitter.com/aaronhillegass

http://www.bignerdranch.com/

IndexABCDEFGHIJKLMNOPQRSTUVWX

Symbols

.xcassets(assetcatalog),ApplicationIcons

.xcdatamodeld(datamodelfile),Modelingentities//MARK:,//MARK:@IBInspectable,@IBInspectable

A

accesscontrol,NSURLComponentsaccessoryindicator(UITableViewCell),UITableViewCellsactionmethods

connectingininterfacefile,Addingacamerabutton

defining,Definingactionmethods

implementing,Implementingactionmethods

andUIControl,FortheMoreCurious:UIControl

activestate,ApplicationStatesandTransitionsaddSubview(_:),ViewsandFramesalerts,displaying,DisplayingUserAlertsalignmentrectangles,Alignmentrectangleandlayoutattributesanchors,AnchorsanimateWithDuration:animations:,BasicAnimationsanimations

animatingconstraints,AnimatingConstraints

basic,BasicAnimations

markingcompletionof,AnimationCompletion

spring-like,BronzeChallenge:SpringAnimations

timingfunctions,TimingFunctions

anti-aliasing,FortheMoreCurious:RetinaDisplayAPIReference,Archivingappend(_:),Instancemethodsapplicationbundle

explained,FortheMoreCurious:TheApplicationBundle


andinternationalization,Baseinternationalization,FortheMoreCurious:NSBundle’sRoleinInternationalization

applicationsandbox,ApplicationSandbox,FortheMoreCurious:TheApplicationBundleapplicationstates,ApplicationStatesandTransitions,FortheMoreCurious:ApplicationStateTransitionsapplicationDidBecomeActive:,ApplicationStatesandTransitions,FortheMoreCurious:ApplicationStateTransitionsapplicationDidEnterBackground(_:),NSKeyedArchiverandNSKeyedUnarchiver,FortheMoreCurious:ApplicationStateTransitionsapplicationDidEnterBackground:,ApplicationStatesandTransitionsapplications

(seealsoapplicationbundle,debugging,projects)

building,Runningonthesimulator,Localization

cleaning,Localization

datastorage,ApplicationSandbox

directoriesin,ApplicationSandbox

iconsfor,ApplicationIcons

launchimagesfor,LaunchScreen

multiplethreadsin,TheMainThread

runningoniPad,CreatinganXcodeProject

runningonsimulator,Runningonthesimulator

applicationWillEnterForeground:,ApplicationStatesandTransitions,FortheMoreCurious:ApplicationStateTransitionsapplicationWillResignActive:,ApplicationStatesandTransitions,FortheMoreCurious:ApplicationStateTransitionsarchiving

vs.CoreData,CoreData

described,Archiving

implementing,Archiving

withNSKeyedArchiver,NSKeyedArchiverandNSKeyedUnarchiver

arrays

about,Collectiontypes

append(_:),Instancemethods

count,Properties

reverse(),Instancemethods

subscripting,Literalsandsubscripting


andtraps,Literalsandsubscripting

writingtofilesystem,FortheMoreCurious:ReadingandWritingtotheFilesystem

assetcatalogs(Xcode),ApplicationIconsassistanteditor(Xcode),Addingacamerabuttonattributes(CoreData),EntitiesAutoLayout

(seealsoconstraints(AutoLayout),InterfaceBuilder)

alignmentrectangles,Alignmentrectangleandlayoutattributes

autoresizingmasksand,FortheMoreCurious:NSAutoresizingMaskLayoutConstraint

dynamiccellheights,DynamicCellHeights

introductionto,AbriefintroductiontoAutoLayout

layoutattributes,Alignmentrectangleandlayoutattributes

purposeof,TheAutoLayoutSystem

autoresizingmasks,ProgrammaticConstraints,FortheMoreCurious:NSAutoresizingMaskLayoutConstraintawakeFromInsert,NSManagedObjectandsubclasses

B

backgroundstate,ApplicationStatesandTransitions,FortheMoreCurious:ApplicationStateTransitionsBaseinternationalization,Baseinternationalizationbaselines,Alignmentrectangleandlayoutattributesbasicanimations,BasicAnimationsbecomeFirstResponder,DismissingthekeyboardBool,NumberandBooleantypesbooleantypes,NumberandBooleantypesbundles

application(seeapplicationbundle)

NSBundle,FortheMoreCurious:NSBundle’sRoleinInternationalization,FortheMoreCurious:TheApplicationBundle

buttons

addingtonavigationbars,Addingbuttonstothenavigationbar

camera,Addingacamerabutton

C

CALayer,TheViewHierarchycallbacks,Delegation


(seealsodelegation,target-actionpairs)

camera

(seealsoimages)

takingpictures,Addingacamerabutton

cancelsTouchesInView,UIPanGestureRecognizerandsimultaneousrecognizerscanPerformAction(_:withSender:),FortheMoreCurious:UIMenuControllerandUIResponderStandardEditActionscells

(seealsoUITableViewCell)

addingpaddingto,ContentInsets

changingcellclass,CreatingItemCell

customizinglayoutof,CustomizingtheLayout

dynamiccellheights,DynamicCellHeights

prototype,ReusingUITableViewCells,CreatingaCustomUICollectionViewCell

reusing,ReusingUITableViewCells

CGPoint,ViewsandFramesCGRect,ViewsandFramesCGSize,ViewsandFramesclosures,Closures,ConstructingafileURLcollectionview

customizinglayoutof,CustomizingtheLayout

displaying,DisplayingtheGrid

downloadingimagedata,DownloadingtheImageData

layoutobject,CollectionViews

navigatingto/displayingphotos,NavigatingtoaPhoto

settingdatasource,CollectionViewDataSource

colors

background,ViewsandFrames,DisplayingtheGrid

customizing,Customizingthelabels

commonancestor,Activatingconstraintsconcurrency,TheMainThreadconditionals

if-let,Optionals

switch,EnumerationsandtheSwitchStatement


connections(inInterfaceBuilder),Makingconnectionsconnectionsinspector,Summaryofconnectionsconsole

printingto,Usingadelegate

viewinginplayground,Optionals

constants,UsingStandardTypesconstraints(AutoLayout)

activatingprogrammatically,Activatingconstraints

addingtolabels,Preparingforlocalization

animating,AnimatingConstraints

clearing,AbriefintroductiontoAutoLayout,Addingmoreconstraints

collectionview,DisplayingtheGrid

creatingexplicitconstraints,Explicitconstraints

creatinginInterfaceBuilder,Constraints

creatingprogrammatically,ProgrammaticConstraints

implicit,Implicitconstraints

nearestneighborand,Constraints

overview,Constraints

pin,AddingconstraintsinInterfaceBuilder

resolvingunsatisfiable,FortheMoreCurious:NSAutoresizingMaskLayoutConstraint

specifying,AbriefintroductiontoAutoLayout

constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:,Explicitconstraintscontentcompression,ContentcompressionresistanceprioritiescontentMode(UIImageView),DisplayingImagesandUIImageViewcontentView(UITableViewCell),UITableViewCellscontrolevents,FortheMoreCurious:UIControlcontrollerobjects,Model-View-Controllercontrols,programmatic,ProgrammaticControlsCoreData

vs.archiving,CoreData

attributes,Entities

fetchrequests,Fetchrequestsandpredicates

persistentstoreformats,NSPersistentStoreCoordinator

relationshipmanagementwith,CoreDataRelationships


roleof,Entities

subclassingNSManagedObject,NSManagedObjectandsubclasses

transformingvalues,Transformableattributes

CoreGraphics,FortheMoreCurious:RetinaDisplaycount(arrays),PropertiescurrentLocale,Formatters

D

datasourcemethods,Implementingdatasourcemethods,DesignPatterns,CollectionViewDataSource,UpdatingtheDataSourcedatastorage

(seealsoarchiving,CoreData)

forapplicationdata,ApplicationSandbox

binary,WritingtotheFilesystemwithNSData,FortheMoreCurious:ReadingandWritingtotheFilesystem

withNSData,WritingtotheFilesystemwithNSData

dataSource(UITableView),UITableViewController,UITableView’sDataSource,Implementingdatasourcemethodsdebugging

(seealsodebuggingtools,exceptions)

debuggingtools

issuenavigator,BuildingtheFinishedApplication

Xcode,Optionals

declarations

protocol,Conformingtoaprotocol

default:(switchstatement),EnumerationsandtheSwitchStatementdelegation

designpattern,DesignPatterns

forUIImagePickerController,Settingtheimagepicker’sdelegate

forUITableView,UITableViewController

overview,Delegation

UICollectionViewDelegate,DownloadingtheImageData

deleteRowsAtIndexPaths(_:withRowAnimation:),DeletingRowsdependencyinjection,Givingthecontrolleraccesstothestoredependencyinversionprinciple,Givingthecontrolleraccesstothestore


designpatterns,DesignPatternsDetailViewController,WrappingUpImageStore,SizeClassesdeveloperdocumentation,Archivingdevices

checkingforcamera,Settingtheimagepicker’ssourceType

displayresolution,ViewsandFrames

Retinadisplay,ApplicationIcons,FortheMoreCurious:RetinaDisplay

dictionaries

(seealsoJSONdata)

about,Collectiontypes

accessing,Subscriptingdictionaries

subscripting,Subscriptingdictionaries

using,CreatingandUsingKeys

writingtofilesystem,FortheMoreCurious:ReadingandWritingtotheFilesystem

directories

application,ApplicationSandbox

Documents,ApplicationSandbox

Library/Caches,ApplicationSandbox

Library/Preferences,ApplicationSandbox

lproj,Baseinternationalization,FortheMoreCurious:NSBundle’sRoleinInternationalization

temporary,ApplicationSandbox

displayresolution,ViewsandFramesdocumentoutline(InterfaceBuilder),InterfaceBuilderdocumentation

developer,Archiving

opening,ControllingAnimations

forSwift,ExploringApple’sSwiftDocumentation

Documentsdirectory,ApplicationSandboxDouble,NumberandBooleantypesdrawing(seeviews)drill-downinterface,UINavigationControllerDynamicType,DynamicType

E


editButtonItem,Addingbuttonstothenavigationbarediting(UITableView,UITableViewController),EditingModeeditorarea(Xcode),InterfaceBuilderencodeInteger(_:forKey:),ArchivingencodeObject(_:forKey:),ArchivingencodeWithCoder(_:),NSKeyedArchiverandNSKeyedUnarchiverencodeWithCoder:,ArchivingendEditing(_:),Dismissingbytappingelsewhereentities

creating,UpdatingItems

defined,Entities

modeling,Modelingentities

relationshipsbetween,Relationships

savingchangesto,Savingchanges

enumerate(),LoopsandStringInterpolationenums(enumerations)

defined,EnumerationsandtheSwitchStatement

overviewof,Enumerationsandassociatedvalues

andrawvalues,Enumerationsandrawvalues

andswitchstatements,EnumerationsandtheSwitchStatement

errorhandling,ErrorHandling,DownloadingtheImageDataerrors

dealingwith,Optionals

inplaygrounds,UsingStandardTypes

traps,Literalsandsubscripting

eventhandling,Eventhandlingbasicsevents

control,ProgrammaticControls,FortheMoreCurious:UIControl

touch,Eventhandlingbasics,TouchEvents

(seealsotouchevents)

exceptions

vs.errorhandling,FortheMoreCurious:ReadingandWritingtotheFilesystem

internalinconsistency,AddingRows

Swiftvs.otherlanguages,FortheMoreCurious:ReadingandWritingtotheFilesystem

expressions,stringinterpolationand,LoopsandStringInterpolationWOW! eBook

www.wowebook.org

extensions,Extensions

F

fallthrough(switchstatement),EnumerationsandtheSwitchStatementfetchrequests,Fetchrequestsandpredicatesfileinspector,LocalizationfileURLs,retrieving,ConstructingafileURLfilesystem,writingto,WritingtotheFilesystemwithNSData,FortheMoreCurious:ReadingandWritingtotheFilesystemfirstresponder

becoming,Dismissingthekeyboard

andnil-targetedactions,FortheMoreCurious:UIControl

overview,DismissingtheKeyboard

resigning,Dismissingthekeyboard,DismissingbypressingtheReturnkey,Dismissingbytappingelsewhere

andresponderchain,FortheMoreCurious:TheResponderChain

andUIMenuController,UIMenuController

Flickr,WebServicesFloat,NumberandBooleantypesfloating-pointtypes,NumberandBooleantypes,Initializersfonts

changingpreferredsize,DynamicType

customizingsize,Customizingthelabels

for-in,LoopsandStringInterpolationforcedunwrapping(ofoptionals),Optionalsframe(UIView),ViewsandFramesframeworks

CoreData(seeCoreData)

definitionof,ViewsandFrames

linkingmanually,SettingtheInitialViewController

functions

(seealsoindividualfunctionnames)

callback,Delegation

G

genstrings,NSLocalizedStringandstringstables


gesturerecognizer(seeUIGestureRecognizer)gestures

(seealsoUIGestureRecognizer,UIScrollView)

discretevs.continuous,UILongPressGestureRecognizer

longpress,UILongPressGestureRecognizer

panning,UILongPressGestureRecognizer,UIPanGestureRecognizerandsimultaneousrecognizers

taps,DetectingTapswithUITapGestureRecognizer

GUIDs,CreatingandUsingKeys

H

Hashable,Collectiontypesheaderview(UITableView),EditingModehierarchies,view,TheViewHierarchyHomepwnerapplication

addinganimagestore,CreatingImageStore

addingdrill-downinterface,UINavigationController,UINavigationController

addingitemimages,Camera

adjustingviewpropertiespersizeclass,AnotherSizeClass

applicationsandbox,NSKeyedArchiverandNSKeyedUnarchiver

creatingnestedstackviews,StackViews

enablingediting,EditingUITableView

objectdiagrams,UITableView’sDataSource,UINavigationController

storingimages,WritingtotheFilesystemwithNSData

horizontalambiguity,AddingconstraintsinInterfaceBuilderHTTPprotocol,FortheMoreCurious:HTTP

I

IBAction,Definingactionmethods,Addingacamerabutton@IBInspectable,@IBInspectableIBOutlet,Declaringoutlets,HookingUptheContenticons

(seealsoimages)

application,ApplicationIcons


assetcatalogsfor,ApplicationIcons

camera,Addingacamerabutton

if-let,Optionalsimagepicker(seeUIImagePickerController)imageNamed:,FortheMoreCurious:RetinaDisplayimagePickerController:didFinishPickingMediaWithInfo:,Settingtheimagepicker’sdelegateimagePickerControllerDidCancel:,Settingtheimagepicker’sdelegateimages

(seealsocamera,icons,UIImageView)

accessingfromthecache,CreatingandUsingKeys

caching,WritingtotheFilesystemwithNSData

displayinginUIImageView,DisplayingImagesandUIImageView

downloadingimagedata,DownloadingandDisplayingtheImageData,DownloadingtheImageData

fetching,GivingViewControllersAccesstotheImageStore

modelingPhotoclass,ModelingthePhoto

navigatingtophotos,NavigatingtoaPhoto

forRetinadisplay,FortheMoreCurious:RetinaDisplay

saving,Savingtheimage

savingtodisk,SavingImagestoDisk

storing,CreatingImageStore

wrappinginNSData,WritingtotheFilesystemwithNSData

imageWithContentsOfFile(_:),WritingtotheFilesystemwithNSDataimplementationfiles,navigating,FortheMoreCurious:NavigatingImplementationFilesimplicitconstraints,Implicitconstraintsinactivestate,ApplicationStatesandTransitionsinequalityconstraints,Preparingforlocalizationinit(coder:),Archivinginit(frame:),ViewsandFramesinit?(contentsOfFile:encoding:error:),FortheMoreCurious:ReadingandWritingtotheFilesysteminitialviewcontroller,SettingtheInitialViewControllerinitializers

about,Initializers

convenience,Custominitializers

custom,Custominitializers


designated,Custominitializers

free,Custominitializers

member-wise,Structs

forstandardtypes,Initializers

initWithCoder:,Archivinginspectors(Xcode)

connections,Summaryofconnections

file,Localization

instancevariables,NSKeyedArchiverandNSKeyedUnarchiver

(seealsopointers,properties)

instances,InitializersInt,NumberandBooleantypesintegertypes,NumberandBooleantypesInterfaceBuilder

(seealsoXcode)

addingconstraintsin,AddingconstraintsinInterfaceBuilder

attributesinspector,ViewsandFrames

canvas,InterfaceBuilder

connectingobjects,Makingconnections

connectingwithsourcefiles,ExposingthePropertiesofItemCell

documentoutline,InterfaceBuilder

modifyingviewattributes,@IBInspectable

andproperties,ExposingthePropertiesofItemCell

scene,InterfaceBuilder

settingoutletsin,Settingoutlets,HookingUptheContent

settingtarget-actionin,Settingtargetsandactions

sizeinspector,ViewsandFrames

interfacefiles

badconnectionsin,HookingUptheContent

Baseinternationalizationand,Baseinternationalization

connectingwithsourcefiles,Addingacamerabutton

makingconnectionsin,Addingacamerabutton


internalinconsistencyexception,AddingRowsinternationalization,Localization,FortheMoreCurious:NSBundle’sRoleinInternationalization

(seealsolocalization)

intrinsiccontentsize,Intrinsiccontentsizeinverserelationships,RelationshipsiOSsimulator

runningapplicationson,Runningonthesimulator

sandboxlocation,NSKeyedArchiverandNSKeyedUnarchiver

savingimagesto,Presentingtheimagepickermodally

viewingapplicationbundlein,FortheMoreCurious:TheApplicationBundle

iPad

(seealsodevices)

applicationiconsfor,ApplicationIcons

runningiPhoneapplicationson,CreatinganXcodeProject

isEmpty(strings),PropertiesisSourceTypeAvailable:,Settingtheimagepicker’ssourceTypeissuenavigator,BuildingtheFinishedApplication

J

JSONdata,JSONData

K

key-valuepairs

indictionaries,Collectiontypes

inJSONdata,JSONData

inwebservices,FormattingURLsandrequests

keyboard

attributes,Keyboardattributes

dismissing,Dismissingthekeyboard,DismissingtheKeyboard

numberpad,BronzeChallenge:DisplayingaNumberPad

keys

creating/using,CreatingandUsingKeys

indictionaries,Collectiontypes


L

labels

adding,ViewsandFrames

addingadditional,AnotherLabel

addingconstraintsto,Preparingforlocalization

addingtotabbar,Tabbaritems

customizing,Customizingthelabels

updatingpreferredtextsize,Respondingtouserchanges

languagesettings,Localization

(seealsolocalization)

launchimages,LaunchScreenlayers(ofviews),TheViewHierarchylayoutattributes,Alignmentrectangleandlayoutattributeslayoutguides,Layoutguides,SilverChallenge:LayoutGuideslazyloading,TheViewofaViewController,LoadedandAppearingViewslet,UsingStandardTypeslibraries

(seealsoframeworks)

object,Creatingviewobjects

Library/Cachesdirectory,ApplicationSandboxLibrary/Preferencesdirectory,ApplicationSandboxliteralvalues,LiteralsandsubscriptingloadView,TheViewofaViewController,CreatingaViewProgrammaticallylocalvariables,LoopsandStringInterpolationlocalization

Baseinternationalizationand,Baseinternationalization

internationalization,Localization,FortheMoreCurious:NSBundle’sRoleinInternationalization

lprojdirectories,Baseinternationalization,FortheMoreCurious:NSBundle’sRoleinInternationalization

NSBundle,FortheMoreCurious:NSBundle’sRoleinInternationalization

stringstables,NSLocalizedStringandstringstables

usersettingsfor,Localization

XLIFFdatatype,FortheMoreCurious:ImportingandExportingasXLIFF

locationInView:,MultipleGestureRecognizers


loops

examininginValueHistory,LoopsandStringInterpolation

for,LoopsandStringInterpolation

for-in,LoopsandStringInterpolation

inSwift,LoopsandStringInterpolation

low-memorywarnings,Savingtheimagelprojdirectories,Baseinternationalization,FortheMoreCurious:NSBundle’sRoleinInternationalization

M

mainbundle,Baseinternationalization,FortheMoreCurious:NSBundle’sRoleinInternationalization

(seealsoapplicationbundle)

maininterface,SettingtheInitialViewControllermainthread,TheMainThreadmainBundle,FortheMoreCurious:TheApplicationBundlemargins,Margins,Explicitconstraints//MARK:,//MARK:member-wiseinitializers,Structsmemorymanagement

memorywarnings,Savingtheimage

UITableViewCell,ReusingUITableViewCells

menus(UIMenuController),UIMenuController,FortheMoreCurious:UIMenuControllerandUIResponderStandardEditActionsmessages

(seealsomethods)

action,FortheMoreCurious:UIControl,UIGestureRecognizerSubclasses,UIMenuController

log,Handlingmultipletouches

UIResponder,TouchEvents

methods

(seealsoindividualmethodnames)

action,Definingactionmethods,FortheMoreCurious:UIControl

datasource,Implementingdatasourcemethods

defined,TypesinSwift,Instancemethods

protocol,Moreonprotocols

static,TypesinSwift


minimumPressDuration,UILongPressGestureRecognizermodalviewcontroller,DisplayingUserAlerts,Presentingtheimagepickermodallymodellayer,Model-View-ControllerModel-View-Controller(MVC),Model-View-Controller,UITableViewController,DesignPatternsmulti-threading,TheMainThreadmultipleTouchEnabled(UIView),Handlingmultipletouchesmultitouch,enabling,HandlingmultipletouchesmutableSetValueForKey(_:),AddingTagstotheInterfaceMVC(Model-View-Controller),Model-View-Controller,UITableViewController,DesignPatterns

N

namingconventions

cellreuseidentifiers,ReusingUITableViewCells

delegateprotocols,Conformingtoaprotocol

navigationcontrollers(seeUINavigationController)navigationItem(UIViewController),UINavigationBarnavigators(Xcode)

defined,CreatinganXcodeProject

issue,BuildingtheFinishedApplication

project,CreatinganXcodeProject

nearestneighbor,ConstraintsnextResponder,FortheMoreCurious:TheResponderChainnil-targetedactions,FortheMoreCurious:UIControlNSBundle,FortheMoreCurious:NSBundle’sRoleinInternationalizationNSCoder,ArchivingNSCodingprotocol,ArchivingNSData,WritingtotheFilesystemwithNSDataNSDate,FortheMoreCurious:ReadingandWritingtotheFilesystemNSDateFormatter,FormattersNSFetchRequest,FetchrequestsandpredicatesNSIndexPath,CreatingandretrievingUITableViewCells,DeletingRowsNSJSONSerialization,NSJSONSerializationNSKeyedArchiver,NSKeyedArchiverandNSKeyedUnarchiverNSKeyedUnarchiver,LoadingfilesNSLocale,FormattersNSLocalizedString(),NSLocalizedStringandstringstablesNSManagedObject,NSManagedObjectandsubclasses,RelationshipsNSNumber,FortheMoreCurious:ReadingandWritingtotheFilesystemNSString

conversionto,Archiving


propertylistserializable,FortheMoreCurious:ReadingandWritingtotheFilesystem

NSTemporaryDirectory,ApplicationSandboxNSURL,NSURLComponentsNSURLRequest,SendingtheRequest,FortheMoreCurious:HTTPNSURLSession,SendingtheRequestNSURLSessionDataTask,NSURLSession,ParsingJSONdata,TheMainThreadNSURLSessionTask,FortheMoreCurious:HTTPNSUserDefaults,ApplicationSandboxNSUUID,CreatingandUsingKeysNSValueTransformer,Transformableattributesnumberformatters,Numberformatters,Formattersnumberpad,BronzeChallenge:DisplayingaNumberPad

O

objectgraphs,ObjectGraphsobjectlibrary,Creatingviewobjectsobjects

(seealsomemorymanagement)

propertylistserializable,FortheMoreCurious:ReadingandWritingtotheFilesystem

optional,Moreonprotocolsoptionalbinding,Optionalsoptionalmethods(protocols),Moreonprotocolsoptionals

about,Optionals

anddictionarysubscripting,Subscriptingdictionaries

forcedunwrappingof,Optionals

if-let,Optionals

andoptionalbinding,Optionals

syntaxfor,Optionals

unwrapping,Optionals

outlets

autogenerating/connecting,HookingUptheContent

connectingconstraintsto,AnimatingConstraints

connectingwithsourcefiles,ExposingthePropertiesofItemCell

defined,Makingconnections

setting,Makingconnections


settinginInterfaceBuilder,HookingUptheContent

P

padding,ContentInsetsparallelcomputing,TheMainThreadparent-childcontexts,Parent-ChildContextsPhotoramaapplication

addingpersistenceto,ObjectGraphs

addingtagstophotos,CoreDataRelationships

collectionview,CollectionViews

downloadingimagedata,DownloadingandDisplayingtheImageData

webservicerequests,StartingthePhotoramaApplication

photos(seecamera,images)pixels,ViewsandFramesplaygrounds(Xcode),UsingStandardTypes

errorsin,UsingStandardTypes

ValueHistory,LoopsandStringInterpolation

viewingconsolein,Optionals

pointers

inInterfaceBuilder(seeoutlets)

points(vs.pixels),ViewsandFramespredicates,Fetchrequestsandpredicatespreferences,ApplicationSandbox

(seealsoDynamicType,localization)

prepareForSegue:sender:,PassingDataAroundpresentViewController:animated:completion:,Presentingtheimagepickermodallypreviewassistant,Preparingforlocalizationprojectnavigator,CreatinganXcodeProjectprojects

cleaningandbuilding,Localization

creating,CreatinganXcodeProject

targetsettingsin,FortheMoreCurious:TheApplicationBundle

properties

creatinginInterfaceBuilder,ExposingthePropertiesofItemCell

defined,Properties


propertylistserializableobjects,FortheMoreCurious:ReadingandWritingtotheFilesystempropertyobserver,ImplementingtheTemperatureConversionprotocol,Conformingtoaprotocolprotocols

conformingto,Conformingtoaprotocol

declaring,Conformingtoaprotocol

delegate,Conformingtoaprotocol

NSCoding,Archiving

optionalmethodsin,Moreonprotocols

requiredmethodsin,Moreonprotocols

structureof,Conformingtoaprotocol

UIApplicationDelegate,ApplicationStatesandTransitions

UICollectionViewDataSourceprotocol,CollectionViewDataSource

UICollectionViewDelegate,DownloadingtheImageData

UIGestureRecognizerDelegate,UIPanGestureRecognizerandsimultaneousrecognizers

UIImagePickerControllerDelegate,Settingtheimagepicker’sdelegate,Savingtheimage

UINavigationControllerDelegate,Savingtheimage

UIResponderStandardEditActions,FortheMoreCurious:UIMenuControllerandUIResponderStandardEditActions

UITableViewDataSource,UITableViewController,Implementingdatasourcemethods,CreatingandretrievingUITableViewCells,DeletingRows,MovingRows

UITableViewDelegate,UITableViewController

UITextFieldDelegate,Conformingtoaprotocol,DismissingtheKeyboard

pseudolanguage,Preparingforlocalization

Q

Quartz,FortheMoreCurious:RetinaDisplay(seeCoreGraphics)queryitems,FormattingURLsandrequestsQuickHelp,ArchivingQuickHelp(Xcode),InferringtypesQuizapplication,CreatinganXcodeProject

R

Range,LoopsandStringInterpolationrawValue(enums),Enumerationsandrawvalues


referencepages,Archivingreferencetypes,Valuetypesvs.referencetypesregionsettings,Localizationreorderingcontrols,MovingRowsrequiredmethods(protocols),MoreonprotocolsrequireGestureRecognizerToFail(_:),MoreonUIGestureRecognizerresignFirstResponder,Dismissingthekeyboard,DismissingbypressingtheReturnkeyresources

assetcatalogsfor,ApplicationIcons

defined,ApplicationIcons,FortheMoreCurious:TheApplicationBundle

responderchain,FortheMoreCurious:TheResponderChainresponders(seefirstresponder,UIResponder)Retinadisplay,ApplicationIcons,FortheMoreCurious:RetinaDisplayreuseidentifiers,DisplayingtheGridreuseIdentifier(UITableViewCell),ReusingUITableViewCellsreverse(),Instancemethodsrootviewcontroller(UINavigationController),UINavigationControllerrows(UITableView)

adding,AddingRows

deleting,DeletingRows

moving,MovingRows

S

sandbox,application,ApplicationSandbox,FortheMoreCurious:TheApplicationBundleschemes,Runningonthesimulatorsections(UITableView),Implementingdatasourcemethods,EditingModesegues,SeguessendAction(_:to:from:forEvent:),FortheMoreCurious:UIControlsendActionsForControlEvents(_:),FortheMoreCurious:UIControlsetEditing:animated:,EditingMode,Addingbuttonstothenavigationbarsets,Collectiontypes,Initializerssettings(seepreferences)Settingsapplication,ApplicationSandboxsimulator

runningapplicationson,Runningonthesimulator

sandboxlocation,NSKeyedArchiverandNSKeyedUnarchiver

savingimagesto,Presentingtheimagepickermodally

viewingapplicationbundlein,FortheMoreCurious:TheApplicationBundle

sizeclasses,SizeClassesWOW! eBook

www.wowebook.org

sortdescriptors(NSFetchRequest),FetchrequestsandpredicatessourceType(UIImagePickerController),TakingPicturesandUIImagePickerControllerstackviews,StackViewsstates,application,ApplicationStatesandTransitionsstaticmethods,TypesinSwiftString

internationalizing,NSLocalizedStringandstringstables

writingtofilesystem,WritingtotheFilesystemwithNSData

stringinterpolation,LoopsandStringInterpolationstrings

(seealsoNSString)

initializersfor,Initializers

interpolation,LoopsandStringInterpolation

isEmpty,Properties

literal,Literalsandsubscripting

stringstables,NSLocalizedStringandstringstablesstructs,Structssubscripting

arrays,Literalsandsubscripting

dictionaries,Subscriptingdictionaries

subviews,TheViewHierarchy,Accessingsubviewssuperview,ViewsandFramessuspendedstate,ApplicationStatesandTransitionsSwift

about,TheSwiftLanguage

documentationfor,ExploringApple’sSwiftDocumentation

enumerationsandswitchstatement,EnumerationsandtheSwitchStatement

extensionsin,Extensions

loopsandstringinterpolation,LoopsandStringInterpolation

optionaltypesin,Optionals,ErrorHandling

typesin,TypesinSwift

usingstandardtypes,UsingStandardTypes

valuetypes,Valuetypesvs.referencetypes

switch,EnumerationsandtheSwitchStatementswitchstatements,EnumerationsandtheSwitchStatement


T

tabbarcontrollers(seeUITabBarController)tabbaritems,Tabbaritemstableviewcells(seeUITableViewCell)tableviewcontrollers(seeUITableViewController)tableviews(seeUITableView)tables(database),EntitiestableView,ContentInsetstableView(_:commitEditingStyle:forRowAtIndexPath:),DeletingRowstableView(_:moveRowAtIndexPath:toIndexPath:),MovingRowstableView:cellForRowAtIndexPath:,Implementingdatasourcemethods,CreatingandretrievingUITableViewCellstableView:numberOfRowsInSection:,Implementingdatasourcemethodstags

addingtophotos,AddingTagstotheInterface

addingtotheinterface,AddingTagstotheInterface

creatingrelationshipsbetween,Relationships

target-actionpairs

defined,Definingactionmethods,DesignPatterns

settingprogrammatically,Addingbuttonstothenavigationbar


andUIGestureRecognizer,UIGestureRecognizerSubclasses

targets,buildsettingsfor,FortheMoreCurious:TheApplicationBundletemplates(Xcode),StyleChoicestext

(seealsoAutoLayout)

aligning,TextEditing

compressionof,Contentcompressionresistancepriorities

customizingappearance,Customizingthelabels,TextEditing

dynamicstylingof,DynamicType

input,TextInputandDelegation

textFieldShouldReturn:,DismissingtheKeyboardthreads,TheMainThreadtimingfunctions,TimingFunctionstmpdirectory,ApplicationSandboxto-manyrelationships,Relationshipsto-onerelationships,Relationships


toggleEditingMode:,EditingModetoolbars

adding,Addingacamerabutton

addingbuttonsto,Addingacamerabutton

addingconstraintsto,Addingacamerabutton

anchoring,AddingTagstotheInterface

topViewController(UINavigationController),UINavigationControllertouchevents

basicsof,TouchEvents

defined,Eventhandlingbasics

enablingmultitouch,Handlingmultipletouches


andtarget-actionpairs,FortheMoreCurious:UIControl


touchesBegan(_:withEvent:),TouchEventstouchesCancelled(_:withEvent:),TouchEventstouchesEnded(_:withEvent:),TouchEventstouchesMoved(_:withEvent:),TouchEventsTouchTrackerapplication

creating,CreatingtheTouchTrackerApplication

drawinglines,CreatingtheLineStruct

recognizinggestures,UIGestureRecognizerandUIMenuController

transformableattributes(CoreData),TransformableattributestranslationInView(_:),UIPanGestureRecognizerandsimultaneousrecognizerstraps,Literalsandsubscriptingtuples,LoopsandStringInterpolationtypeinference,Inferringtypestypes

boolean,NumberandBooleantypes

floating-point,NumberandBooleantypes,Initializers

hashable,Collectiontypes

inferenceof,Inferringtypes

instancesof,Initializers

integer,NumberandBooleantypes

sets,Collectiontypes,InitializersWOW! eBook

www.wowebook.org

specifying,Specifyingtypes

tuples,LoopsandStringInterpolation

U

UIthread,TheMainThreadUIAlertController,DisplayingUserAlertsUIApplication

andevents,TouchEvents

andresponderchain,FortheMoreCurious:TheResponderChain,FortheMoreCurious:UIControl

UIApplicationDelegate,ApplicationStatesandTransitionsUIBarButtonItem,UINavigationBar,AddingacamerabuttonUICollectionViewCell,CreatingaCustomUICollectionViewCellUICollectionViewDataSourceprotocol,CollectionViewDataSourceUICollectionViewDelegateprotocol,DownloadingtheImageDataUICollectionViewFlowLayout,CollectionViewsUIColor,ViewsandFramesUIControl,FortheMoreCurious:UIControlUIControlEvent.TouchUpInside,FortheMoreCurious:UIControlUIControlEvents,ProgrammaticControlsUIGestureRecognizer

actionmessagesof,UIGestureRecognizerSubclasses,UILongPressGestureRecognizer

cancelsTouchesInView,UIPanGestureRecognizerandsimultaneousrecognizers

chainingrecognizers,MoreonUIGestureRecognizer

delayingtouches,MoreonUIGestureRecognizer

described,UIGestureRecognizerandUIMenuController

detectingtaps,DetectingTapswithUITapGestureRecognizer

enablingsimultaneousrecognizers,UIPanGestureRecognizerandsimultaneousrecognizers

implementingmultiple,MultipleGestureRecognizers,UIPanGestureRecognizerandsimultaneousrecognizers

interceptingtouchesfromview,UIGestureRecognizerSubclasses,UIPanGestureRecognizerandsimultaneousrecognizers

locationInView:,MultipleGestureRecognizers

longpress,UILongPressGestureRecognizer

panning,UILongPressGestureRecognizer,UIPanGestureRecognizerandsimultaneousrecognizers


state(property),UILongPressGestureRecognizer,UIPanGestureRecognizerandsimultaneousrecognizers,MoreonUIGestureRecognizer

subclasses,Dismissingthekeyboard,UIGestureRecognizerSubclasses,MoreonUIGestureRecognizer

subclassing,MoreonUIGestureRecognizer

translationInView(_:),UIPanGestureRecognizerandsimultaneousrecognizers

andUIRespondermethods,UIPanGestureRecognizerandsimultaneousrecognizers

UIGestureRecognizerDelegate,UIPanGestureRecognizerandsimultaneousrecognizersUIImage,WritingtotheFilesystemwithNSData

(seealsoimages,UIImageView)

UIImageJPEGRepresentation,WritingtotheFilesystemwithNSDataUIImagePickerController

instantiating,TakingPicturesandUIImagePickerController

presenting,Presentingtheimagepickermodally

UIImagePickerControllerDelegate,Settingtheimagepicker’sdelegate,SavingtheimageUIImageView,DisplayingImagesandUIImageViewUIInterpolatingMotionEffect,BasicAnimationsUIKit,ViewsandFramesUILongPressGestureRecognizer,UILongPressGestureRecognizerUIMenuController,UIMenuController,FortheMoreCurious:UIMenuControllerandUIResponderStandardEditActionsUINavigationBar,UINavigationControllerUINavigationController

(seealsoviewcontrollers)

addingviewcontrollersto,AppearingandDisappearingViews

described,UINavigationController

instantiating,UINavigationController

managingviewcontrollerstack,UINavigationController

rootviewcontroller,UINavigationController

instoryboards,Segues

topViewController,UINavigationController

andUINavigationBar,UINavigationBar

view,UINavigationController

viewControllers,UINavigationController

viewWillAppear:,AppearingandDisappearingViews


viewWillDisappear:,AppearingandDisappearingViews

UINavigationControllerDelegate,SavingtheimageUINavigationItem,UINavigationBarUIPanGestureRecognizer,UILongPressGestureRecognizer,UIPanGestureRecognizerandsimultaneousrecognizersUIResponder

menuactions,FortheMoreCurious:UIMenuControllerandUIResponderStandardEditActions


andtouchevents,TouchEvents

UIResponderStandardEditActions(protocol),FortheMoreCurious:UIMenuControllerandUIResponderStandardEditActionsUIScrollView,ContentInsetsUIStackView,UsingUIStackViewUIStoryboardSegue,SeguesUITabBarController

implementing,UITabBarController

vs.UINavigationController,UINavigationController

view,UITabBarController

UITabBarItem,TabbaritemsUITableView,UITableViewandUITableViewController

(seealsoUITableViewCell,UITableViewController)

addingrowsto,AddingRows

deletingrowsfrom,DeletingRows

editingmodeof,EditingMode,SubclassingUITableViewCell,Addingbuttonstothenavigationbar

editingproperty,EditingMode

footerview,EditingMode

headerview,EditingMode

movingrowsin,MovingRows

populating,UITableView’sDataSource

sections,Implementingdatasourcemethods,EditingMode

view,SubclassingUITableViewController

UITableViewCell

cellstyles,UITableViewCells

contentView,UITableViewCells


editingstyles,DeletingRows

retrievinginstancesof,CreatingandretrievingUITableViewCells

reusinginstancesof,ReusingUITableViewCells

subclassing,SubclassingUITableViewCell

UITableViewCellStyle,UITableViewCells

UITableViewCellEditingStyleDelete,DeletingRowsUITableViewController

(seealsoUITableView)

addingrows,AddingRows

datasourcemethods,Implementingdatasourcemethods

dataSource,UITableView’sDataSource

deletingrows,DeletingRows

described,UITableViewController

editingproperty,EditingMode

movingrows,MovingRows

returningcells,CreatingandretrievingUITableViewCells

subclassing,SubclassingUITableViewController

tableView,ContentInsets

UITableViewDataSource(protocol),UITableViewController,Implementingdatasourcemethods,CreatingandretrievingUITableViewCells,DeletingRows,MovingRowsUITableViewDelegate,UITableViewControllerUITapGestureRecognizer,Dismissingthekeyboard,DetectingTapswithUITapGestureRecognizerUITextField

asfirstresponder,DismissingtheKeyboard,FortheMoreCurious:UIControl

andkeyboard,DismissingtheKeyboard

settingattributesof,BronzeChallenge:DisplayingaNumberPad

textediting,TextEditing

UITextFieldDelegate,Conformingtoaprotocol,DismissingtheKeyboardUIToolbar,UINavigationBar,AddingacamerabuttonUITouch,TouchEvents,TurningTouchesintoLines,HandlingmultipletouchesUIView

(seealsoUIViewController,views)

animationdocumentation,ControllingAnimations

defined,ViewsandtheViewHierarchy


frame,ViewsandFrames

instantiating,ViewsandFrames

superview,ViewsandFrames

UIViewController

(seealsoUIView,viewcontrollers)

loadView,TheViewofaViewController,CreatingaViewProgrammatically

navigationItem,UINavigationBar

tabBarItem,Tabbaritems

view,TheViewofaViewController,FortheMoreCurious:TheResponderChain

viewDidLoad,Accessingsubviews

viewWillAppear:,Accessingsubviews,Savingtheimage

UIWindow

purposeof,TheViewHierarchy


unarchiveObjectWithFile(_:),LoadingfilesURLForResource(_:withExtension:),FortheMoreCurious:NSBundle’sRoleinInternationalizationURLs,FormattingURLsandrequests

(seealsoNSURL)

useralerts,displaying,DisplayingUserAlertsuserinterface

(seealsoAutoLayout,views)

drill-down,UINavigationController

keyboard,DismissingtheKeyboard

usersettings(seepreferences)UUIDs,CreatingandUsingKeys,NSManagedObjectandsubclasses

V

valuetypes,Valuetypesvs.referencetypesvar,UsingStandardTypesvariables,UsingStandardTypes

(seealsoinstancevariables,localvariables,pointers,properties)

view(UIViewController),TheViewofaViewControllerviewcontrollers


(seealsoUIViewController,views)

allowingaccesstoimagestore,GivingViewControllersAccesstotheImageStore

interactingwith,InteractingwithViewControllersandTheirViews

lazyloadingofviews,TheViewofaViewController,LoadedandAppearingViews

modal,DisplayingUserAlerts,Presentingtheimagepickermodally

navigatingbetween,Segues

presenting,UITabBarController

reloadingsubviews,Savingtheimage

root,UINavigationController

settinginitial,SettingtheInitialViewController

andviewhierarchy,TheViewofaViewController,CreatingaViewProgrammatically

viewhierarchy,TheViewHierarchy,CreatingaViewProgrammaticallyviewControllers(UINavigationController),UINavigationControllerviewDidLoad,ViewsandFrames,Accessingsubviewsviews

(seealsoAutoLayout,touchevents,UIView,viewcontrollers)

addingtowindow,TheViewHierarchy,CreatingaViewProgrammatically

animating,ControllingAnimations

appearing/disappearing,AppearingandDisappearingViews

contentcompressionresistancepriorities,Contentcompressionresistancepriorities

contenthuggingpriorities,Contenthuggingpriorities

creatingprogrammatically,CreatingaViewProgrammatically

defined,ViewsandtheViewHierarchy

drawingtoscreen,TheViewHierarchy

inhierarchy,TheViewHierarchy

layersand,TheViewHierarchy

lazyloadingof,TheViewofaViewController,LoadedandAppearingViews

misplaced,Misplacedviews

modalpresentationof,Presentingtheimagepickermodally

inModel-View-Controller,Model-View-Controller

removingfromstoryboard,ProgrammaticViews

rendering,TheViewHierarchy


resizing,DisplayingImagesandUIImageView

scroll,DisplayingtheGrid

sizeandpositionof,ViewsandFrames

stackview,StackViews,AnotherSizeClass

andsubviews,TheViewHierarchy

viewWillAppear:,Accessingsubviews,AppearingandDisappearingViews,SavingtheimageviewWillDisappear:,AppearingandDisappearingViews

W

webservices

andHTTPprotocol,FortheMoreCurious:HTTP

withJSONdata,JSONData

andNSURLSession,SendingtheRequest

overview,WebServices

requestingdatafrom,BuildingtheURL

wildcardAnyWidth/Heightlayout,SizeClassesworkspaces(Xcode),CreatinganXcodeProjectWorldTrotterapplication

addingtabbarcontroller,UITabBarController

configuring,CreatingaNewProject

implementingtemperatureconversion,ImplementingtheTemperatureConversion

interfacelayout,ViewsandFrames

localizing,Internationalization

multipleviewcontrollersfor,ViewControllers

programmaticviewsin,ProgrammaticViews

textinput,TextInputandDelegation

writeToURL(_:atomically:),WritingtotheFilesystemwithNSDatawriteToURL(_:atomically:encoding:error:),FortheMoreCurious:ReadingandWritingtotheFilesystem

X

.xcassets(assetcatalog),ApplicationIcons

.xcdatamodeld(datamodelfile),ModelingentitiesXcode


(seealsodebuggingtools,InterfaceBuilder,projects,iOSsimulator)

APIReference,Archiving

assetcatalogs,ApplicationIcons

assistanteditor,Addingacamerabutton

creatingprojectsin,CreatinganXcodeProject

documentation,ControllingAnimations

editorarea,InterfaceBuilder

fileinspector,Localization

issuenavigator,BuildingtheFinishedApplication

navigatorarea,CreatinganXcodeProject

navigators,CreatinganXcodeProject

objectlibrary,Creatingviewobjects

organizingmethodswith//MARK:,//MARK:

playgrounds,UsingStandardTypes

QuickHelp,Inferringtypes,Archiving

schemes,Runningonthesimulator

sourceeditorjumpbar,FortheMoreCurious:NavigatingImplementationFiles

versions,CreatinganXcodeProject

workspaces,CreatinganXcodeProject

XLIFFdatatype,FortheMoreCurious:ImportingandExportingasXLIFFXMLpropertylists,FortheMoreCurious:ReadingandWritingtotheFilesystem


iOS Programming The Big Nerd Ranch Guide, 5th

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