Tuesday, February 19, 2013

Routed Events in WPF / Silverlight

Routed events are events which navigate up or down the visual tree acording to their RoutingStrategy. The routing strategy can be bubble, tunnel or direct. You can hook up event handlers on the element that raises the event or also on other elements above or below it by using the attached event syntax: Button.Click="Button_Click".
Routed events normally appear as pair. The first is a tunneling event called PreviewMouseDown and the second is the bubbling called MouseDown. They don't stop routing if the reach an event handler. To stop routing then you have to set e.Handled = true;
  • Tunneling The event is raised on the root element and navigates down to the visual tree until it reaches the source element or until the tunneling is stopped by marking the event as handeld. By naming convention it is called Preview... and appears before corresponding bubbling event.
  • Bubbling The event is raised on the source element and navigates up to the visual tree until it reaches the root element or until the bubbling is stopped by marking the event as handled. The bubbling event is raised after the tunneling event.
  • Direct The event is raised on the source element and must be handled on the source element itself. This behavior is the same as normal .NET events.
Create a Custom Routed Event
// Register routed event
public static readonly RoutedEvent SelectedEvent = 
    EventManager.RegisterRoutedEvent( "Selected", RoutingStrategy.Bubble, 
    typeof(RoutedEventHandler), typeof(MyCustomControl));
 
// .NET wrapper
public event RoutedEventHandler Selected
{
    add { AddHandler(SelectedEvent, value); } 
    remove { RemoveHandler(SelectedEvent, value); }
}
 
// Raise the routed event "selected"
RaiseEvent(new RoutedEventArgs(MyCustomControl.SelectedEvent));

 


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