WPF 已知问题 Popup 吃掉 PreviewMouseDown 事件

在 WPF 中，使用 Popup 也许会看到 PreviewMouseDown 事件被吃掉

因为 PreviewMouseDown 是 RoutingStrategy.Direct 路由事件，不能在多个视觉树使用，在设置 Popup 点击界面 StaysOpen=”False” 的逻辑就在下面代码

private void OnPreviewMouseButton(MouseButtonEventArgs e)
{
    // We should only react to mouse buttons if we are in an auto close mode (where we have capture)
    if (_cacheValid[(int)CacheBits.CaptureEngaged] && !StaysOpen)
    {
        Debug.Assert( Mouse.Captured == _popupRoot.Value, "_cacheValid[(int)CacheBits.CaptureEngaged] == true but Mouse.Captured != _popupRoot");

        // If we got a mouse press/release and the mouse isn't on the popup (popup root), dismiss.
        // When captured to subtree, source will be the captured element for events outside the popup.
        if (_popupRoot.Value != null && e.OriginalSource == _popupRoot.Value)
        {
            // When we have capture we will get all mouse button up/down messages.
            // We should close if the press was outside.  The MouseButtonEventArgs don't tell whether we get this
            // message because we have capture or if it was legit, so we have to do a hit test.
            if (_popupRoot.Value.InputHitTest(e.GetPosition(_popupRoot.Value)) == null)
            {
                // The hit test didn't find any element; that means the click happened outside the popup.
                SetCurrentValueInternal(IsOpenProperty, BooleanBoxes.FalseBox);
            }
        }
    }
}

如果写一个 CheckBox 放在界面上，运行代码可以看到可以被打勾但是没有事件

<Grid>
    <StackPanel>
        <Button Margin="10,10,10,10" Content="Open Popup" Click="OpenPopup_OnClick"></Button>
        <CheckBox PreviewMouseDown="UIElement_OnPreviewMouseDown"></CheckBox>
    </StackPanel>
    <Popup x:Name="Popup" StaysOpen="False">
        <Grid Width="100" Height="100" Background="White">
            <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center" Text="Popup"></TextBlock>
        </Grid>
    </Popup>
</Grid>

在 UIElement_OnPreviewMouseDown 添加输出内容，代码如下，可以看到，没有符合预期输出

private void UIElement_OnPreviewMouseDown(object sender, MouseButtonEventArgs e)
{
    Debug.WriteLine("PreviewMouseDown");
}
 
private void OpenPopup_OnClick(object sender, RoutedEventArgs e)
{
    Popup.PlacementTarget = (UIElement) sender;
    Popup.Placement = PlacementMode.Mouse;
    Popup.IsOpen = true;
}