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Each cell is represented by some data structure which is then visualized on the screen

headshot.Source = ...;nameEntry.Text = person.Name;emailEntry.Text = person.Email;birthday.Date = person.Dob;...

nameEntry.TextChanged +=  (sender,  e)  =>  person.Name =  nameEntry.Text;emailEntry.TextChanged +=  (sender,  e)  =>  person.Email =emailEntry.Text;

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Binding acts as an intermediary – moving the data between the source and target

Source Target

Any Object

Any public property

BindableObject

BindablePropertyBinding

Source Path Target

Can be any accessible object

Public property defined on the source object

Must be a BindableProperty

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Person person = new Person()   {  Name  =  "Homer  Simpson",   ...  };  

Entry nameEntry = new Entry();  

Binding nameBinding = new Binding();  nameBinding.Source = person;

...

Binding identifies the source of the binding data – this is where the data comes from, in this case it's a single person defined in our

application

1

Person person = new Person()   {  Name  =  "Homer  Simpson",   ...  };  

Entry nameEntry = new Entry();  

Binding nameBinding = new Binding();  nameBinding.Source = person;nameBinding.Path =  "Name";

...

Binding identifies the property path which identifies a property on the source to get the data from, in this case we want to get the value from the Person.Name

property

2

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Person person = new Person()   {  Name  =  "Homer  Simpson",   ...  };  

Entry nameEntry = new Entry();  

Binding nameBinding = new Binding();  nameBinding.Source = person;nameBinding.Path =  "Name";

...

new Binding("Property")new Binding("Property.Child")new Binding("Property[Key]")new Binding("Property[1]")new Binding("Item[Key]")new Binding(".")

More Path Examples

Person person = new Person()   {  Name  =  "Homer  Simpson",   ...  };  

Entry nameEntry = new Entry();  

Binding nameBinding = new Binding();  nameBinding.Source = person;nameBinding.Path =  "Name";

nameEntry.SetBinding(Entry.TextProperty, nameBinding);

Binding is associated to the target property using the BindableObject.SetBinding method

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Person person = new Person()   {  Name  =  "Homer  Simpson",   ...  };  

Entry nameEntry = new Entry();  

Binding nameBinding = new Binding();  nameBinding.Source = person;nameBinding.Path =  "Name";

nameEntry.SetBinding(Entry.TextProperty, nameBinding);

This is passed the specific target property the binding will work with – this must be a BindableProperty

3

Person person = new Person()   {  Name  =  "Homer  Simpson",   ...  };  

Entry nameEntry = new Entry();  

Binding nameBinding = new Binding();  nameBinding.Source = person;nameBinding.Path =  "Name";

nameEntry.SetBinding(Entry.TextProperty, nameBinding);

… and the binding which identifies the source and the property on the source to apply

3

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<StackLayout Padding="20" Spacing="20"><StackLayout.Resources>

<ResourceDictionary><Person   x:Key="homer" Name="Homer   Simpson"   .../>

</ResourceDictionary></StackLayout.Resources><Entry Text="{Binding Name,  

Source={StaticResource homer}}" />...

</StackLayout>

{Binding}  takes the Path as the first unnamed argument

Assigned to Target property

Source supplied through resource

v

v

public class Person{

public string Name { get; set; }public string Email { get; set; }public Gender Gender { get; set; }

}

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Person person = new Person()  {  Name  =  "Homer  Simpson",  ...  };  ...nameEntry.BindingContext =  person;nameEntry.SetBinding(Entry.TextProperty, new Binding("Name"));

Binding only identifies the Path – no source is set; it will then look to the BindingContext on the parent control as the source for the binding data

public partial class PersonDetailsPage : ContentPage{

public PersonDetailsPage (Person person){

BindingContext = person;InitializeComponent ();

}}

Can set the BindingContext once on the root page and it is then assigned to every control (child) on that page

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Model-View-ViewModel

ModelView ViewModel

How to display information

What to displayFlow of interaction

Business LogicData objects

Events

DataData

Model-View-ViewModel

ModelView ViewModel

Xamarin.Forms Data Binding

Events

Data

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INotifyPropertyChanged

INotifyPropertyChanged

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INotifyPropertyChanged

INotifyPropertyChanged

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Data Binding – XAML

Data Binding – XAML

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Pros Cons

§Provides higher testable surface§Centralizes the visual & business logic§Can reduce converter code used to tie

models to UI§Takes advantage of binding

infrastructure

§Requires infrastructure, more for some platforms than others

§Necessitates multiple layers which may not be worth it for smaller apps

§Bindings can be hard to debug and may not be efficient for large data sets

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BooleanToColorConverter

ArrayToStringConverter

DoubleToIntegerConverter

NotBooleanConverter

IntegerToBooleanConverter

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Menu Button ToolbarItem TextCell

other patterns abstractions

DependencyInjection

Factory and Singleton Command

Navigation Alerts + Prompts Messages

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Publisher

MessagingCenter

Subscriber Subscriber

MessagingCenter.Send<MainViewModel,  ItemViewModel>(this,  "Select",  selectedItem);

Publisher identifies sending type and parameter type through generic parameters

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MessagingCenter.Subscribe<MainViewModel,  ItemViewModel> (this,  "Select",  (mainVM,  selectedItem)  => {

//  Action  to  run  when  "Select"  is  received  //  from  MainViewModel

});

Combination of the sender type, string message, and parameter type is the key for the message recipient – these must match between publisher and subscriber

Xamarin.Forms has support for dealing with a few, very common platform-specific features

Page.DisplayAlertto show simple alert

messages

Timer management using Device.StartTimer

Device.OpenUrlto launch external apps based on a URL scheme

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UI Thread marshaling with

Device.BeginInvokeOnMainThread

Mapping and Location through

Xamarin.Forms.Maps

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platform-specific

Dialing the phone would require platform-specific code

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abstraction implemented by the target platform which defines the platform-specific functionality

PhoneDialerIOS

PhoneDialerDroid

PhoneDialerWP8

Platform projects implement the shared dialer interface using the platform-specific APIs

Shared code defines IDialer interface to represent required functionality

public interface IDialer{

bool MakeCall(string number);}

1 Define an interface or abstract class in the shared code project(PCL)

public interface IDialer{

bool MakeCall(string number);}

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2 Provide implementation of abstraction in each platform-specific project

class  PhoneDialerIOS :  IDialer{

public  bool MakeCall(string number)  {//  Implementation  goes  here

}}

3 Expose platform-specific implementation using assembly-level attribute in platform-specific project

[assembly:  Dependency(typeof(PhoneDialerIOS))]

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4Retrieve and use the dependency anywhere using DependencyService.Get<T> (both shared and platform specific projects can use this API)

IDialer dialer  =  DependencyService.Get<IDialer>();if (dialer  !=  null)  {

...}

Summary