Xamarin.Forms力重绘查看自定义渲染器 [英] Force redraw of Xamarin.Forms View with custom renderer

问题描述

我有一个视觉元素用作MyButton 与iOS版实现了自定义渲染

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

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的：

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);
            }
        }
    }
}

当按下按钮，红色的边界应该变成蓝色。显然，渲染器不注意到更改的属性。我怎样才能触发重绘？

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?

（这个例子是适用于iOS。但我的问题适用于Android作为好。）

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

推荐答案

被要求两个修改：

1. 呼叫 OnPropertyChanged 在 BoundaryColor setter方法内：

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
        }
    }
}

订阅中事件的 OnElementChanged MyButtonRenderer 方法：

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：
这似乎是很重要的 OnElementChanged ，而不是在构造函数中订阅。否则， System.Reflection.TargetInvocationException 升高。

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

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