使用自定义渲染器强制重绘Xamarin.Forms视图

Question

我有一个MyButton带有为iOS实现的自定义渲染器的可视元素.

共享:

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版:

[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);
            }
        }
    }
}

按下按钮时,红色边界应变为蓝色.显然渲染器没有注意到已更改的属性.如何触发重绘？

(此示例适用于iOS.但我的问题也适用于Android.)

Answer 1

首先,将您BoundaryColor转变为可绑定的属性.这不是必需的,触发INPC事件就足够了,但是你可以绑定它:

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); }
}

然后,在你的渲染器中:

protected override void OnElementPropertyChanged (object sender, PropertyChangedEventArgs e)
{
    base.OnElementPropertyChanged (sender, e);

    if (e.PropertyName == MyButton.BoundaryColorProperty.PropertyName)
        SetNeedsDisplay ();
}

Answer 2

需要进行两项修改:

1. OnPropertyChanged在酒店的设置者内打电话BoundaryColor:

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

2. 在以下OnElementChanged方法中订阅活动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)
       {
           // ...
       }
   }
   

注意: 在内部订阅OnElementChanged而不是构造函数似乎很重要.否则a System.Reflection.TargetInvocationException被提出.