构建此 Linq-to-Events 拖放代码的最佳方式是什么？

Question

我正在尝试处理拖放交互，其中涉及鼠标向下、鼠标移动和鼠标向上。

这是我的解决方案的简化重现：

• 鼠标按下时，创建一个椭圆并将其添加到画布中
• 鼠标移动时，重新定位椭圆以跟随鼠标

• 鼠标抬起时，更改画布的颜色，以便您拖动哪个画布一目了然。

  var mouseDown = Observable.FromEvent<MouseButtonEventArgs>(canvas, "MouseLeftButtonDown");
  var mouseUp = Observable.FromEvent<MouseButtonEventArgs>(canvas, "MouseLeftButtonUp");
  var mouseMove = Observable.FromEvent<MouseEventArgs>(canvas, "MouseMove");
  
  Ellipse ellipse = null;
  
  var q = from start in mouseDown.Do(x =>
              {
                  // handle mousedown by creating a red ellipse, 
                  // adding it to the canvas at the right position
                  ellipse = new Ellipse() { Width = 10, Height = 10, Fill = Brushes.Red };
                  Point position = x.EventArgs.GetPosition(canvas);
                  Canvas.SetLeft(ellipse, position.X);
                  Canvas.SetTop(ellipse, position.Y);
                  canvas.Children.Add(ellipse);
              })
          from delta in mouseMove.Until(mouseUp.Do(x =>
              {
                  // handle mouse up by making the ellipse green
                  ellipse.Fill = Brushes.Green;
              }))
          select delta;
  
  q.Subscribe(x =>
  {
      // handle mouse move by repositioning ellipse
      Point position = x.EventArgs.GetPosition(canvas);
      Canvas.SetLeft(ellipse, position.X);
      Canvas.SetTop(ellipse, position.Y);
  });
  

XAML 很简单

    <Canvas x:Name="canvas"/>

我不喜欢这段代码的一些地方，我需要帮助来重构它:)

首先：mousedown 和 mouseup 回调被指定为副作用。如果对q 进行了两次订阅，它们将发生两次。

其次，mouseup 回调在 mousemove 回调之前指定。这使得它有点难以阅读。

第三，对椭圆的引用似乎在一个愚蠢的地方。如果有两个订阅，该变量引用将很快被覆盖。我确信应该有某种方法可以利用 let 关键字向 linq 表达式引入一个变量，这意味着鼠标移动和鼠标向上处理程序都可以使用正确的椭圆引用

你会如何编写这段代码？

Answer 1

为避免订阅副作用，您应该发布您的 observable。我认为这样的事情就可以了：

        public MainWindow()
    {
        InitializeComponent();
        var mouseDown = Observable
            .FromEvent<MouseButtonEventArgs>(this, "MouseLeftButtonDown");
        var mouseUp = Observable
            .FromEvent<MouseButtonEventArgs>(this, "MouseLeftButtonUp");
        var mouseMove = Observable
            .FromEvent<MouseEventArgs>(this, "MouseMove");

        var ellipses = mouseDown
            .Select(args => new { 
                a = args, 
                el = new Ellipse
                {
                    Width = 10, Height = 10, Fill = Brushes.Red
                }})
            .Publish();

        ellipses
            .Subscribe(elargs =>
            {
                var position = elargs.a.EventArgs.GetPosition(canvas);
                Canvas.SetLeft(elargs.el, position.X);
                Canvas.SetTop(elargs.el, position.Y);
                canvas.Children.Add(elargs.el);
            });

        var elmove = from elargs in ellipses
                     from mm in mouseMove.TakeUntil(mouseUp)
                     select new { a = mm, el = elargs.el };

        elmove.
            Subscribe(elargs =>
            {
                var position = elargs.a.EventArgs.GetPosition(canvas);
                Canvas.SetLeft(elargs.el, position.X);
                Canvas.SetTop(elargs.el, position.Y);
            });

        var elmup = from elargs in ellipses
                    from mup in mouseUp
                    select elargs.el;

        elmup.Subscribe(el => el.Fill = Brushes.Green);

        ellipses.Connect();
    }