Multitasking in iOS - The Junction

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Sunday, July 31, 2011

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Multitasking in iOSThe Junction

Ori GoshenCo-Founder & Chief Architectmail: [email protected]: @origoshen


mailto:[email protected]




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OUTLINE

• Introduction

• Multitasking In iOS Overview

• Application Life Cycle

• Best Practices

• Multitasking & Development Tools


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Introduction


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ME

• Coder since birth

• IDF (8200) - Project manager & dev. team leader

• Fring - Product & development team leader

• Tawkon - Co-Founder & Chief Architect

• Love & hate relationship with


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ABOUT TAWKON

Drive Safely Eat Wisely Talk Responsibly


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ABOUT TAWKON


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HOW APPLE THINKS?


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MIND & SPIRIT

• Watch Apple Engineering Video Tutorials

• Watch WWDC Technical Sessions & Keynote

• Read The Guidelines! It’s NOT A Waste Of Time

• Read Coding Documentation

• Follow Apple Evangelists On Twitter

• Follow @gruber

• Go to macrumors.com

• Go to tuaw.com (how I found my iPhone)


http://www.tuaw.com/2011/03/21/another-iphone-recovery-story-with-a-twist/

http://www.tuaw.com/2011/03/21/another-iphone-recovery-story-with-a-twist/

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Multitasking In iOS Overview


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INTRODUCTION

• A major feature introduced in iOS 4

• Improves user experience

• “Every app should adopt multitasking”

• Supported on all iOS Devices

• Except old ones (iPhone, iPhone 3G, 1st gen. touch)

• Android vs. iPhone Multitasking

• Multitasking Android Campaign


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iOS MULTITASKING PHILOSOPHY

• “General purpose concurrency is not the solution for

mobile devices”

Background execution:finish task, audio, location, VoIP

Push Notification

Fast AppSwitching


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THE SERVICES

• Fast App Switching

• Resume quickly, preserve state

• Push Notifications

• Respond to a notification sent from a remote server

• Local Notifications

• Push-style notification delivered at a predetermined time

• NewsStand Push Notification (New on iOS 5)


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THE SERVICES

• Background audio

• Play audible content to the user while in the background

• Task completion

• Extra time to complete a task

• Location-Navigation

• Keep users continuously informed of their location

• Location-Significant location change, region monitoring

• Respond to location changes while in the background

• Voice over IP

• Make and receive phone calls using an Internet connection


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DO YOU REALLY NEED TO MULTITASK?


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WHAT IF... I’M DOING SOMETHING ELSE?

• Triple check if this is something that can be done remotely

• Server side push notifications

• Daemons (Jailbroken Option)

• Mmmm....What about a clipboard history app?

• Piggyback on available scenario and think of a damn good

excuse (you gonna need it!)


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FAST APP SWITCHING

• App Resumes Immediately

• App State Is Preserved

• Tight integration with multitasking UI

• Default for all apps being developed on SDK 4+

• In other words: “processes are being suspended”


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TASK COMPLETION

• Application can complete a task without remaining in

the foreground

• User does not have to wait for the task to complete

• Task duration is limited to avoid excessive battery

drain

• Examples:

• Uploading photos or videos

• Finishes applying an image filter

• Finishes downloading a magazine


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BACKGROUND AUDIO

• Plays audible content to the user

• Streams audio

• Record audio

• Continue playing/recording in the background

• Integrates with remote controls


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LOCATION NAVIGATION

• Keep users informed of their location in the

background

• Gives turn-by-turn directions


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LOCATION TRACKING

• Respond to location changes while in the

background

• Suitable for location aware apps

• Significant location changes

• Sends notification after changing cell towers

• Region monitoring

• Sends a notification upon entering and

exiting regions of interest

• Not supported on old iPhone (3G, 3GS, ..)


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LOCATION TRACKING

Significant location changes

Location changed

Location changed

Location changed

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Significant location changes


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LOCATION TRACKINGRegion monitoring

Region monitoring

Entered region

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VOIP

• Helps you maintain the connection with the VOIP server

• Keep alive with the server (min. of 10 mins.)

• Maintain a signaling connection

• Notifying the user on incoming call


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Application Life Cycle


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APPLICATION LIFE CYCLELaunch and active/inactive

Foreground

Active

Inactive

Not Running

Background

Running

Suspended


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Foreground

Active

Inactive

Not Running

Background

Running

Suspended

applicationDidFinishLaunchingWithOptions:


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Foreground

Active

Inactive

Not Running

Background

Running

Suspended


applicationDidBecomeActive:


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Foreground

Active

Inactive

Not Running

Background

Running

Suspended


applicationWillResignActive:



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APPLICATION LIFE CYCLESwitch from an app

Foreground

Active

Inactive

Not Running

Background

Running

Suspended


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Foreground

Active

Inactive

Not Running

Background

Running

Suspended



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Foreground

Active

Inactive

Not Running

Background

Running

Suspended


applicationDidEnterBackground:


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APPLICATION LIFE CYCLESwitch to an app

Foreground

Active

Inactive

Not Running

Background

Running

Suspended


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Foreground

Active

Inactive

Not Running

Background

Running

Suspended

applicationDidEnterForeground:


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Foreground

Active

Inactive

Not Running

Background

Running

Suspended


applicationDidEnterForeground:


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APPLICATION LIFE CYCLETermination

Foreground

Active

Inactive

Not Running

Background

Running

Suspended


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APPLICATION LIFE CYCLETermination

Foreground

Active

Inactive

Not Running

Background

Running

Suspended

applicationWillTerminate:


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LIFE CYCLE NOTIFICATIONS

UIApplicationDelegate Callback Notification

application:didFinishLaunchingWithOptions: UIApplicationDidFinishLaunchingNotification

applicationWillTerminate: UIApplicationWillTerminateNotification

applicationDidBecomeActive: UIApplicationDidBecomeActiveNotification

applicationWillResignActive: UIApplicationWillResignActiveNotification

applicationDidEnterBackground: UIApplicationDidEnterBackgroundNotification

applicationWillEnterForeground: UIApplicationWillEnterForegroundNotification


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Best Practices


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BEST PRACTICES OUTLINE

• System Resources

• Memory

• OpenGL

• Gracefully resuming from the background

• Preserving state

• Networking

• System notifications


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SYSTEM RESOURCES

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RESOURCE MANAGEMENT

• Apps share system resources:

• CPU

• I/O

• Memory

• GPU

• Network


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RESOURCE MANAGEMENT

• Goals

• Preserve foreground app usability

• Preserve battery life

• System resources are shared by all apps

• Multitasking apps should minimize CPU & memory

footprint in the background


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MEMORY

• Apps share a limited amount of memory

• iOS ensures it has the memory it needs

• Sending apps memory warnings

• Terminates apps

• Priority

• Suspended (high memory usage)

• Background (high memory usage)

• Suspended (low memory usage)

• Background (low memory usage)

• ...

• “Termination due to low memory is a normal condition”


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MEMORY

• iOS sends notifications to running apps to free memory

• Only sends warnings when freeing memory is crucial

• Suspended apps do not receive memory warnings!!!

• Reduce memory use in applicationDidEnterBackground:

Memory Warnings


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MEMORY

• OS and frameworks free some memory going to the background

• Apps using less than 16 MB of dirty memory are written to disk

• Balance memory footprint and speed to resume


Backing Store

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MEMORYView Backing Stores

CALayer

UIView


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MEMORYView Backing Stores

• Every UIView has a CALayer

• UIViews that draw themselves have bitmap backing stores

• iOS frees backing stores

• iOS may reclaim backing stores while app is running in the

background

• If reclaimed, iOS will call your -drawRect method

Backing Store

CALayer

UIView


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MEMORYUIImage Cache

• UIKit caches images loaded with -[UIImage imageName:]

• Cache is purged on entering the background


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MEMORYDisk Caches

• Many frameworks cache data in memory

• SQLite

• Core Data

• NSCache

• ...

• Caches are emptied automatically when entering background


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MEMORYUIImageViews

CALayer

UIView

• UIImageViews also have CALayer

• CALayer uses the image directly

• Images are not automatically reclaimed

• Detach large images from the view hierarchy

• Unload offscreen UIImageViews

• Beware of decompression on resume

• “The more you release the more time you need to resume”


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MEMORYCaches

• Flush application caches

• But not if resuming takes as long as resuming

• Consider using NSCache and NSPurgeableData:

• NSCache

• Objects are in memory

• Evicts objects as necessary

• Evicts objects when entering the background

• NSPurgeableData objects in an NSCache are not evicted

• Instead they become reclaimable when not in use


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MEMORYSummary

• Free memory when you receive a memory warning

• Remove UIImageViews from the view hierarchy unless

decompression is a problem

• Use NSCache and memory-mapped files when appropriate

• Balance memory footprint and speed to resume


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OPEN GL

• iOS terminates apps that use OpenGL in the background

• Stop animation timer when entering the background


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OPEN GL

Exception Type: EXC_CRASH (SIGABRT)

...

Thread 0 Crashed:0 libsystem_kernel.dylib 0x33b35a1c __pthread_kill

1 libsystem_c.dylib 0x30bac3b4 pthread_kill

2 libsystem_c.dylib 0x30ba4bf8 abort

3 IMGSGX535GLDriver 0x355229ae glrReturnNotPermittedKillClient

...

7 OpenGLES 0x354ade4e glFinish

...

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Foreground

Active

Inactive

Not Running

Background

Running

Suspended

Try it yourself


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PRESERVING STATECommon State

• Return to exactly where you left off

• Save common UI states

• Selected tab bar

• Scroll position


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PRESERVING STATEApp-specific content

• User input, like the last number entered in Calculator

• For networking apps, save last search query

• ...


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PRESERVING STATEGames

• Turn-by-turn games should save after each turn

• Games with substantial, frequent state updates save periodically

• When the application enters the background

• Between levels, rooms


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NETWORKINGSockets and suspension

• Sockets may disconnect while suspended

• Be prepared for errors on resume

• Suspended apps cannot accept incoming connections

• Close listening sockets before suspend

• Reopen listening sockets on resume


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SYSTEM NOTIFICATIONS

• System change notifications not delivered to suspended app

• System queues notifications

• Delivered when app resumes

• Preferences and Locale may be changed in settings app

Event Notification

Preference changed in Settings NSUserDefaultsDidChangeNotification

Language or locale change NSCurrentLocaleDidChangeNotification


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Multitasking And Development


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INSTRUMENTS

• App life cycles events are flagged• App life cycle events are flagged

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SIMULATOR

• Simulator is not a replacement for device

• Supported

• Fast app switching

• Task completion

• Local notifications

• Multitasking UI (tray bar)

• Not supported

• Background audio, location and VoIP

• Significant location change, region monitoring

• Push notifications


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DEBUGGER

• Debugging app changes some behavior!!!

• Must-have to test outside the debugger

• No time limit in UIApplicationDelegate callbacks

• No time limit in task completion handler


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It takes leadership to innovate


Multitasking in iOS - The Junction

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