如果鼠标失去对对象的关注，为什么 MouseMove 事件不起作用？

Question

我的程序中基本上有一个简单的问题，我只想确保一切正常。它应该在单击鼠标按钮时添加MouseEventHandler，然后与鼠标一起移动圆圈，直到事件处理程序被删除。我将代码简化为非常基础的代码：

XAML：

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
    <Grid Name="grid1" Background="White" MouseLeftButtonUp="grid_MouseUp">
        <Ellipse Height="50" HorizontalAlignment="Left" Margin="12,12,0,0" Name="ellipse1" Stroke="{x:Null}" VerticalAlignment="Top" Width="50" Fill="Black" MouseLeftButtonDown="ellipse1_MouseDown" />
    </Grid>
</Window>

C＃：

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private static Point _oldPoint = new Point(), _newPoint = new Point();

    private void ellipse1_MouseDown(object sender, MouseButtonEventArgs e)
    {
        _oldPoint = e.GetPosition(grid1);
        grid1.MouseMove += new MouseEventHandler(grid_MouseMove);
    }

    private void grid_MouseUp(object sender, MouseButtonEventArgs e)
    {
        grid1.MouseMove -= new MouseEventHandler(grid_MouseMove);
    }

    private void grid_MouseMove(object sender, MouseEventArgs e)
    {
        _newPoint = e.GetPosition(grid1);
        ellipse1.Margin = new Thickness(ellipse1.Margin.Left - _oldPoint.X + _newPoint.X, ellipse1.Margin.Top - _oldPoint.Y + _newPoint.Y, 0, 0);
        _oldPoint = _newPoint;
    }
}

现在总的来说，这段代码工作正常，我认为非常简洁，因为它在实际按下按钮之前不会检查鼠标的移动。但是，我的问题如下：

我不得不将MouseMove事件添加到网格而不是圆，因为一旦鼠标指针失去圆的焦点（通过移动鼠标太快），它就不再触发 MouseMove 事件。但为什么会这样呢？在事件开始时，鼠标肯定在圆圈上方，然后它移动了。是的，它离开了圆圈，但不应该仍然触发事件吗？

Answer 1

您可以捕获鼠标并处理椭圆中的所有事件。

<Grid Name="grid1" Background="White">
    <Ellipse Height="50" HorizontalAlignment="Left" Margin="12,12,0,0" Name="ellipse1" Stroke="{x:Null}" VerticalAlignment="Top" Width="50" Fill="Black"
             MouseLeftButtonDown="ellipse1_MouseDown" MouseLeftButtonUp="ellipse1_MouseUp" />
</Grid>

后面有这个代码

private void ellipse1_MouseDown(object sender, MouseButtonEventArgs e)
{
    Mouse.Capture(ellipse1);
    _oldPoint = e.GetPosition(grid1);
    ellipse1.MouseMove += new MouseEventHandler(ellipse1_MouseMove);
}

private void ellipse1_MouseUp(object sender, MouseButtonEventArgs e)
{
    Mouse.Capture(null);
    ellipse1.MouseMove -= new MouseEventHandler(ellipse1_MouseMove);
}

我已将 grid_MouseMove 移动并重命名为 ellipse1_MouseMove。