使用自定义渲染器强制重绘Xamarin.Forms视图

Question

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

Shared:

namespace RendererTest
{
    public class MyButton: Button
    {
        public Color BoundaryColor { get; set; }
    }

    public static class App
    {
        public static Page GetMainPage()
        {    
            var button = new MyButton { Text = "Click me!", BoundaryColor = Color.Red };
            button.Clicked += (sender, e) => (sender as MyButton).BoundaryColor = Color.Blue;
            return new ContentPage { Content = button };
        }
    }
}

iOS:

[assembly:ExportRenderer(typeof(MyButton), typeof(MyButtonRenderer))]

namespace RendererTest.iOS
{
    public class MyButtonRenderer: ButtonRenderer
    {
        public override void Draw(RectangleF rect)
        {
            using (var context = UIGraphics.GetCurrentContext()) {
                context.SetFillColor(Element.BackgroundColor.ToCGColor());
                context.SetStrokeColor((Element as MyButton).BoundaryColor.ToCGColor());
                context.SetLineWidth(10);
                context.AddPath(CGPath.FromRect(Bounds));
                context.DrawPath(CGPathDrawingMode.FillStroke);
            }
        }
    }
}

When pressing the button, the red boundary should become blue. Apparently the renderer does not notice the changed property. How can I trigger a redraw?

(This example is for iOS. But my question applies to Android as well.)

Answer 1

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 ();
}

Answer 2

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.