如果鼠标移到画布边界之外，如何处理矩形绘制

Question

我有一些代码可以在Canvas上绘制矩形。 这一切都很好，但是，当鼠标离开Canvas的边界时，矩形的图形将冻结并且什么也没有发生。 一些代码

<Canvas x:Name="canvas" MouseDown="Canvas_MouseDown" MouseMove="Canvas_MouseMove" MouseUp="Canvas_MouseUp" Background="Transparent" />

private Point startPoint;
private Rectangle rect;

private void Canvas_MouseDown(object sender, MouseButtonEventArgs e)
{
    startPoint = e.GetPosition(canvas);

    rect = new Rectangle
    {
        Stroke = Brushes.LightBlue,
        StrokeThickness = 2
    };
    Canvas.SetLeft(rect,startPoint.X);
    Canvas.SetTop(rect,startPoint.X);
    canvas.Children.Add(rect);
}

private void Canvas_MouseMove(object sender, MouseEventArgs e)
{
    if(e.LeftButton == MouseButtonState.Released || rect == null)
        return;

    var pos = e.GetPosition(canvas);

    var x = Math.Min(pos.X, startPoint.X);
    var y = Math.Min(pos.Y, startPoint.Y);

    var w = Math.Max(pos.X, startPoint.X) - x;
    var h = Math.Max(pos.Y, startPoint.Y) - y;

    rect.Width = w;
    rect.Height = h;

    Canvas.SetLeft(rect, x);
    Canvas.SetTop(rect, y);
}

private void Canvas_MouseUp(object sender, MouseButtonEventArgs e)
{
    rect = null;
}

我想要的是继续绘制矩形，即使我超出了Canvas的边界，但很明显，矩形的尺寸已裁剪到Canvas的边界。

实现此目标的最佳方法是什么？

Answer 1

您应该在MouseDown处理程序中捕获鼠标

private void Canvas_MouseDown(object sender, MouseButtonEventArgs e)
{
    canvas.CaptureMouse();
    ...
}

并在MouseUp处理程序中释放捕获

private void Canvas_MouseUp(object sender, MouseButtonEventArgs e)
{
    ...
    canvas.ReleaseMouseCapture();
}