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How to: Handle a Routed Event

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This example shows how bubbling events work and how to write a handler that can process the routed event data.

Example

In Windows Presentation Foundation (WPF), elements are arranged in an element tree structure. The parent element can participate in the handling of events that are initially raised by child elements in the element tree. This is possible because of event routing.

Routed events typically follow one of two routing strategies, bubbling or tunneling. This example focuses on the bubbling event and uses the ButtonBase.Click event to show how routing works.

The following example creates two Button controls and uses XAML attribute syntax to attach an event handler to a common parent element, which in this example is StackPanel. Instead of attaching individual event handlers for each Button child element, the example uses attribute syntax to attach the event handler to the StackPanel parent element. This event-handling pattern shows how to use event routing as a technique for reducing the number of elements where a handler is attached. All the bubbling events for each Button route through the parent element.

Note that on the parent StackPanel element, the Click event name specified as the attribute is partially qualified by naming the Button class. The Button class is a ButtonBase derived class that has the Click event in its members listing. This partial qualification technique for attaching an event handler is necessary if the event that is being handled does not exist in the members listing of the element where the routed event handler is attached.

<StackPanel
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  x:Class="SDKSample.RoutedEventHandle"
  Name="dpanel"
  Button.Click="HandleClick"
>
  <StackPanel.Resources>
      <Style TargetType="{x:Type Button}">
        <Setter Property="Height" Value="20"/>
        <Setter Property="Width" Value="250"/>
        <Setter Property="HorizontalAlignment" Value="Left"/>
      </Style>
  </StackPanel.Resources>
  <Button Name="Button1">Item 1</Button>
  <Button Name="Button2">Item 2</Button>
  <TextBlock Name="results"/>
</StackPanel>

The following example handles the Click event. The example reports which element handles the event and which element raises the event. The event handler is executed when the user clicks either button.

public partial class RoutedEventHandle : StackPanel
{
    StringBuilder eventstr = new StringBuilder();
    void HandleClick(object sender, RoutedEventArgs args)
    {
        // Get the element that handled the event.
        FrameworkElement fe = (FrameworkElement)sender;
        eventstr.Append("Event handled by element named ");
        eventstr.Append(fe.Name);
        eventstr.Append("\n");

        // Get the element that raised the event.
        FrameworkElement fe2 = (FrameworkElement)args.Source;
        eventstr.Append("Event originated from source element of type ");
        eventstr.Append(args.Source.GetType().ToString());
        eventstr.Append(" with Name ");
        eventstr.Append(fe2.Name);
        eventstr.Append("\n");

        // Get the routing strategy.
        eventstr.Append("Event used routing strategy ");
        eventstr.Append(args.RoutedEvent.RoutingStrategy);
        eventstr.Append("\n");

        results.Text = eventstr.ToString();
    }
}

Private eventstr As New Text.StringBuilder()

Private Sub HandleClick(ByVal sender As Object, ByVal args As RoutedEventArgs)
    ' Get the element that handled the event.
    Dim fe As FrameworkElement = DirectCast(sender, FrameworkElement)
    eventstr.Append("Event handled by element named ")
    eventstr.Append(fe.Name)
    eventstr.Append(vbLf)

    ' Get the element that raised the event. 
    Dim fe2 As FrameworkElement = DirectCast(args.Source, FrameworkElement)
    eventstr.Append("Event originated from source element of type ")
    eventstr.Append(args.Source.[GetType]().ToString())
    eventstr.Append(" with Name ")
    eventstr.Append(fe2.Name)
    eventstr.Append(vbLf)

    ' Get the routing strategy.
    eventstr.Append("Event used routing strategy ")
    eventstr.Append(args.RoutedEvent.RoutingStrategy)
    eventstr.Append(vbLf)

    results.Text = eventstr.ToString()
End Sub

See also