WPF 基础 - 点击事件的执行顺序及 Button 点击事件的特殊性

1. 点击事件的执行顺序

1. PreviewMouseLeftButtonDown
2. PreviewMouseDown
3. MouseLeftButtonDown
4. MouseDown
5. PreviewMouseLeftButtonUp
6. PreviewMouseUp
7. Click
8. MouseLeftButtonUp
9. MouseUp

1）PreviewMouseLeftButtonDown 是隧道事件，从根节点往下传递

2）MouseLeftButtonDown 是冒泡事件，从下往根节点传递

3）知道这两点，再来看前面从 PreviewMouseLeftButtonDown 到 MouseUp 的整个过程及 Click 在当中所处的顺序，就好理解很多

4）具体：

鼠标即将点下，从根节点往下传递

->控件接受到鼠标点下，从下往根节点冒泡

->鼠标即将弹起，从上往下传递

->控件接受到鼠标要松开，若是 ButtonBase 等有 Click 事件的控件则触发 Click 事件

->控件从下往上冒泡鼠标弹起事件。

2. 例子

<StackPanel x:Name="s0" Height="100" Width="200" Background="Red"
          PreviewMouseLeftButtonDown="PreviewMouseLeftButtonDownEventHandler"
          PreviewMouseDown="PreviewMouseDownEventHandler"
          MouseLeftButtonDown="MouseLeftButtonDownEventHandler"
          MouseDown="MouseDownEventHandler"
          PreviewMouseLeftButtonUp="PreviewMouseLeftButtonUpEventHandler"
          PreviewMouseUp="PreviewMouseUpEventHandler"
          MouseLeftButtonUp="MouseLeftButtonUpEventHandler"
          MouseUp="MouseUpEventHandler">
    <StackPanel x:Name="s1"  Height="90" Width="180" Background="Yellow"
                PreviewMouseLeftButtonDown="PreviewMouseLeftButtonDownEventHandler"
                PreviewMouseDown="PreviewMouseDownEventHandler"
                MouseLeftButtonDown="MouseLeftButtonDownEventHandler"
                MouseDown="MouseDownEventHandler"
                PreviewMouseLeftButtonUp="PreviewMouseLeftButtonUpEventHandler"
                PreviewMouseUp="PreviewMouseUpEventHandler"
                MouseLeftButtonUp="MouseLeftButtonUpEventHandler"
                MouseUp="MouseUpEventHandler">
        <Button x:Name="btn" Height="40" Width="78" Content="click"
                Click="Button_ClickEventHandler"
                MouseLeftButtonDown="MouseLeftButtonDownEventHandler"
                PreviewMouseLeftButtonDown="PreviewMouseLeftButtonDownEventHandler"
                PreviewMouseDown="PreviewMouseDownEventHandler"
                MouseDown="MouseDownEventHandler"
                PreviewMouseLeftButtonUp="PreviewMouseLeftButtonUpEventHandler"
                PreviewMouseUp="PreviewMouseUpEventHandler"
                MouseLeftButtonUp="MouseLeftButtonUpEventHandler"
                MouseUp="MouseUpEventHandler">
        </Button>
    </StackPanel>
</StackPanel>

private void PrintControlAndEvent(string controlName, string routedEventName)
{
    Console.WriteLine(string.Format("{0} : {1}", controlName, routedEventName));
}

private void PreviewMouseLeftButtonDownEventHandler(object sender, MouseButtonEventArgs e)
{
    PrintControlAndEvent((sender as FrameworkElement).Name, e.RoutedEvent.ToString());
}
...
private void Button_ClickEventHandler(object sender, RoutedEventArgs e)
{
    PrintControlAndEvent((sender as FrameworkElement).Name, e.RoutedEvent.ToString());
}

2.1 点击 Button

输出：

s0 : UIElement.PreviewMouseLeftButtonDown
s0 : Mouse.PreviewMouseDown
s1 : UIElement.PreviewMouseLeftButtonDown
s1 : Mouse.PreviewMouseDown
btn : UIElement.PreviewMouseLeftButtonDown
btn : Mouse.PreviewMouseDown
s0 : UIElement.PreviewMouseLeftButtonUp
s0 : Mouse.PreviewMouseUp
s1 : UIElement.PreviewMouseLeftButtonUp
s1 : Mouse.PreviewMouseUp
btn : UIElement.PreviewMouseLeftButtonUp
btn : Mouse.PreviewMouseUp
btn : ButtonBase.Click

2.2 点击 s1

输出：

s0 : UIElement.PreviewMouseLeftButtonDown
s0 : Mouse.PreviewMouseDown
s1 : UIElement.PreviewMouseLeftButtonDown
s1 : Mouse.PreviewMouseDown
s1 : UIElement.MouseLeftButtonDown
s1 : Mouse.MouseDown
s0 : UIElement.MouseLeftButtonDown
s0 : Mouse.MouseDown
s0 : UIElement.PreviewMouseLeftButtonUp
s0 : Mouse.PreviewMouseUp
s1 : UIElement.PreviewMouseLeftButtonUp
s1 : Mouse.PreviewMouseUp
s1 : UIElement.MouseLeftButtonUp
s1 : Mouse.MouseUp
s0 : UIElement.MouseLeftButtonUp
s0 : Mouse.MouseUp

2.3 点击 Button 后未能触发 MouseLeftButtonDown、MouseLeftButtonUp、MouseDown、MouseUp 的原因

button 的 mousedown 等已经被内部处理了，Handled 已被标记为 True。如果想强制执行，可以

btn.AddHandler(Button.MouseDownEvent, new MouseButtonEventHandler(MouseDownEventHandler), true);
btn.AddHandler(Button.MouseLeftButtonDownEvent, new MouseButtonEventHandler(MouseLeftButtonDownEventHandler), true);
btn.AddHandler(Button.MouseUpEvent, new MouseButtonEventHandler(MouseUpEventHandler), true);
btn.AddHandler(Button.MouseLeftButtonUpEvent, new MouseButtonEventHandler(MouseLeftButtonUpEventHandler), true);
第三个入参：
handledEventsToo:
    //如果为 true，则将按以下方式注册处理程序：即使路由事件在其事件数据中标记为已处理，也会调用处理程序；
    //如果为 false，则使用默认条件注册处理程序，即当路由事件被标记为已处理时，将不调用处理程序。

xaml 里写的 MouseLeftButtonDown="MouseLeftButtonDown" 的 handledEventsToo 值就是 false。

所以，在已经被内部处理的情况下，无法触发。

而即使在后台写 true，也只能触发 Button 的 MouseDownEvent，不会继续往上传递。

因为 handled 标记为 true，除非再把上层的控件也添加路由回调为 true。