Use the Reactive Extensions for .NET to create elegant, testable User Interfaces that run on any mobile or desktop platform.

• Xamarin.iOS
• Xamarin.Android
• Xamarin.Mac
• WPF
• Windows Phone 8
• Windows Store Apps

This library is organized into several high-level assemblies:

• ReactiveUI - Core library that doesn't rely on any particular UI framework. ReactiveObject, the base ViewModel object, as well as ReactiveCollection, a more awesome ObservableCollection, is in here.

• ReactiveUI.Xaml - Classes that require references to a Xaml'ly framework, like WPF or WinRT. ReactiveCommand, an implementation of ICommand, as well as the UserError classes are in this assembly.

• ReactiveUI.Blend - This class has several Blend Behaviors and Triggers that make attaching ViewModel changes to Visual State Manager states.

• ReactiveUI.Routing - A screens and navigation framework as well as ViewModel locator. This framework helps you to write applications using IoC containers to locate views, as well as navigating back and forwards between views.

public class ColorChooserThatDoesntLikeGreen : ReactiveObject
{
  //
  // Declaring a read/write property
  //

  byte _Red;
  public byte Red {
    get { return _Red; }
    set { this.RaiseAndSetIfChanged(value); }
  }

  byte _Green;
  public byte Green {
    get { return _Green; }
    set { this.RaiseAndSetIfChanged(value); }
  }

  byte _Blue;
  public byte Blue {
    get { return _Blue; }
    set { this.RaiseAndSetIfChanged(value); }
  }

  //
  // Declaring a Property that's based on an Observable
  // 

  ObservableAsPropertyHelper<Color> _Color;
  public Color Color {
    get { return _Color.Value; }
  }

  ReactiveCommand OkButton { get; protected set; }

  public ColorChooserThatDoesntLikeGreen()
  {
    var finalColor = this.WhenAny(x => x.Red, x => x.Green, x => x.Blue, 
        (r,g,b) => Color.FromRGB(r.Value, g.Value, b.Value));

    finalColor.ToProperty(this, x => x.Color, out _Color);

    // When the finalColor has full green, the Ok button is disabled
    OkButton = new ReactiveCommand(finalColor.Select(x => x.Green != 255));
  }
}

For more information on how to use ReactiveUI, check out ReactiveUI.