Xamarin.Forms is a cross-platform, natively-backed UI toolkit abstraction that allows developers to create user interfaces that can be shared across Android, iOS, and Windows Phone. The user interfaces use the native controls of the target platform.

Hello, Xamarin.Forms!

The user-experience of a Xamarin.Forms application is usually defined in a cross-platform shared project (either a Portable Class Library or a Shared Project) and combined with platform-specific projects that, at the least, initialize the native platform rendering (note the call to Forms.Init() in the platform code samples below) and, more generally, extend the user-experience and user-interface in platform-specific manners (for instance, by accessing platform-specific sensors or capabilities).

The dependencies are one-way: the platforms depend on the shared project, but not vice-versa:

The Solution Pad for a cross-platform "Hello, World!" app might look like this:

The shared portion of the Xamarin.Forms code might look like this:

public class App
{
    public static Page GetMainPage()
    {
        return new ContentPage
        {
            Content = new Label
            {
                Text = "Hello, Forms!",
                VerticalOptions = LayoutOptions.CenterAndExpand,
                HorizontalOptions = LayoutOptions.CenterAndExpand,
            },
        };
    }
}          
          

The iOS platform code would look like this:

[Register("AppDelegate")]
public partial class AppDelegate : UIApplicationDelegate
{
    UIWindow window;

    public override bool FinishedLaunching(UIApplication app, NSDictionary options)
    {
        Forms.Init();

        window = new UIWindow(UIScreen.MainScreen.Bounds);
        window.RootViewController =  App.GetMainPage().CreateViewController();
        window.MakeKeyAndVisible();

        return true;
    }
}
          

The Android platform code would look like this:

namespace HelloXamarinFormsWorld.Android
{
    [Activity(Label = "HelloXamarinFormsWorld", MainLauncher = true)]
    public class MainActivity : AndroidActivity
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            Xamarin.Forms.Forms.Init(this, bundle);

            SetPage(App.GetMainPage());
        }
    }
}
          

And the Windows Phone code would look like this:

public partial class MainPage : PhoneApplicationPage
{
    public MainPage()
    {
        InitializeComponent();

        Forms.Init();
        Content = HelloXamarinFormsWorld.App.GetMainPage().ConvertPageToUIElement(this);
    }
}
          

The resulting app looks like this on the various device:

Elements in Xamarin.Forms

Important types in the Xamarin.Forms namespace include Xamarin.Forms.Pages, Xamarin.Forms.Views, and Xamarin.Forms.Layouts. Xamarin.Forms.Pages typically correspond with controller objects (Activities in the Android world, UIViewControllers in the iOS world), Xamarin.Forms.Views with controls or widgets, and Xamarin.Forms.Layouts arrange Xamarin.Forms.Views on other Xamarin.Forms.Views. These types are all derived from Xamarin.Forms.Element, as shown in the following diagram:

Model-View-ViewModel and Xamarin.Forms

Application developers use the Model-View-ViewModel (MVVM) pattern to create Xamarin.Forms apps that cleanly separate the concerns of data presentation and user interface from the concerns of data storage and manipulation. The Xamarin.Forms framework enables this by providing, among a few other related types, the Xamarin.Forms.BindableObject and Xamarin.Forms.BindableProperty classes. Objects that inherit from Xamarin.Forms.BindableObject can be bound to members of type Xamarin.Forms.BindableProperty on other objects.

A Xamarin.Forms.BindableObject maintains dictionary of Xamarin.Forms.BindableProperty names and an association with their corresponding binding contexts--simply the object on which the Xamarin.Forms.BindableProperty is defined and that the application developer has assigned to the Xamarin.Forms.BindingObject.BindingContext property--through a Xamarin.Forms.Binding. See Xamarin.Forms.BindableObject for more information.