如何从 Xamarin 窗体自定义呈现器内的视图访问方法

Question

我有以下代码：

public partial class PhrasesFrameRendererClass : Frame
{
    .....
    void getRandomWords() {
       // more code here that involves getting random numbers 
       // and updating a grid's bindingcontext
    }
}

在我的自定义渲染器中，我希望能够在向左滑动手势时调用 getRandomWords，如下所示：

public class PhraseFrameCustomRenderer : FrameRenderer
{
   UISwipeGestureRecognizer leftSwipeGestureRecognizer;
   protected override void OnElementChanged(ElementChangedEventArgs<Frame> e)
   {
       base.OnElementChanged(e);
       leftSwipeGestureRecognizer = new UISwipeGestureRecognizer();
       leftSwipeGestureRecognizer.Direction = UISwipeGestureRecognizerDirection.Left;
       leftSwipeGestureRecognizer.NumberOfTouchesRequired = 1;
       leftSwipeGestureRecognizer.AddTarget((obj) =>
       {
          // Call getRandomWords() here
       });
   }
}

这可能吗？关于如何做到这一点的任何想法？

Answer 1

 base.OnElementChanged(e);
   leftSwipeGestureRecognizer = new UISwipeGestureRecognizer();
   leftSwipeGestureRecognizer.Direction = UISwipeGestureRecognizerDirection.Left;
   leftSwipeGestureRecognizer.NumberOfTouchesRequired = 1;
   leftSwipeGestureRecognizer.AddTarget((obj) =>
   {
      // Call getRandomWords() here
      var frame = Element as PhrasesFrameRendererClass ;
      if(frame!=null){
           frame.getRandomWords();
      }
   });

Answer 2

您可以在自定义帧类中创建 Command 类型的 BindableProperty，从渲染器调用该 Command，并将 ViewModel 的 getRandomWords 方法绑定为命令

//Your custom control in your PCL project
public partial class PhrasesFrameRendererClass : Frame
{
    public static readonly BindableProperty SwipeLeftCommandProperty = 
    BindableProperty.Create(nameof(SwipeLeftCommand), typeof(ICommand), typeof(PhrasesFrameRendererClass ), null);
    public ICommand SwipeLeftCommand
    {
        get { return (ICommand)GetValue(CommandProperty); }
        set { SetValue(CommandProperty, value); }
    }
}
//Your custom control renderer
public class PhraseFrameCustomRenderer : FrameRenderer
{
   UISwipeGestureRecognizer leftSwipeGestureRecognizer;
   protected override void OnElementChanged(ElementChangedEventArgs<Frame> e)
   {
       base.OnElementChanged(e);
       leftSwipeGestureRecognizer = new UISwipeGestureRecognizer();
       leftSwipeGestureRecognizer.Direction = UISwipeGestureRecognizerDirection.Left;
       leftSwipeGestureRecognizer.NumberOfTouchesRequired = 1;
       leftSwipeGestureRecognizer.AddTarget((obj) =>
       {
           var myFrame = Element as PhrasesFrameRendererClassl
           if(myFrame != null){
               if(myFrame.SwipeLeftCommand != null && myFrame.SwipeLeftCommand.CanExecute()){
                   myFrame.SwipeLeftCommand.Execute();
               }
           }
       });
   }
}
//Your ViewModel
public class PhrasesViewModel{
    public Command GetRandomWordsCommand {get;set;}
    public PhrasesViewModel(){
        GetRandomWordsCommand = new Command(ExecuteGetRandomWords);
    }
    private void ExecuteGetRandomWords(){
    //Your method goes here
    }
}
//Your XAML
<yourControls:PhrasesFrameRendererClass SwipeLeftCommand="{Binding GetRandomWordsCommand }"/>

这样看起来可能更复杂，但使用命令可以将应用程序代码(例如获取随机短语(与呈现代码分开