Force redraw of Xamarin.Forms View with custom renderer

Question

I have a visual element MyButton with a custom renderer implemented for iOS.

Shared:

namespace RendererTest
{
    public class MyButton: But         


        

Answer 1

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.

Answer 2

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 ();
}