Force redraw of Xamarin.Forms View with custom renderer

First, turn you BoundaryColor into a bindable property. That's not required, firing INPC event is enough, but then you can bind to it:

public static readonly BindableProperty BoundaryColorProperty =    BindableProperty.Create ("BoundaryColor", typeof(Color), typeof(MyButton), Color.Default);public Color BoundaryColor {    get { return (Color)GetValue (BoudaryColorProperty); }    set { SetValue (BoundaryColorProperty, value); }}

then, in your renderer:

protected override void OnElementPropertyChanged (object sender, PropertyChangedEventArgs e){    base.OnElementPropertyChanged (sender, e);    if (e.PropertyName == MyButton.BoundaryColorProperty.PropertyName)        SetNeedsDisplay ();}

Two modifications were required:

1. Call OnPropertyChanged within the setter of the BoundaryColor property:

   public class MyButton: Button{    Color boundaryColor = Color.Red;    public Color BoundaryColor {        get {            return boundaryColor;        }        set {            boundaryColor = value;            OnPropertyChanged();  // <-- here        }    }}

2. Subscribe to the event within the OnElementChanged method of MyButtonRenderer:

   public class MyButtonRenderer: ButtonRenderer{    protected override void OnElementChanged(ElementChangedEventArgs<Button> e)    {        base.OnElementChanged(e);        Element.PropertyChanged += (s_, e_) => SetNeedsDisplay();  // <-- here    }    public override void Draw(RectangleF rect)    {        // ...    }}

Note:It seems to be important to subscribe within OnElementChanged and not the constructor. Otherwise a System.Reflection.TargetInvocationException is raised.