This topic assumes that you have basic knowledge of the common language runtime (CLR) and object-oriented programming, as well as the concept of how the relationships between WPF elements can be conceptualized as a tree. In order to follow the examples in this topic, you should also understand Extensible Application Markup Language (XAML) and know how to write very basic WPF applications or pages. For more information, see Getting Started with Windows Presentation Foundation and XAML Overview.

You can think about routed events either from a functional or implementation perspective. Both definitions are presented here, because some people find one or the other definition more useful.

Functional definition: A routed event is a type of event that can invoke handlers on multiple listeners in an element tree, rather than just on the object that raised the event.

Implementation definition: A routed event is a CLR event that is backed by an instance of the RoutedEvent class and is processed by the Windows Presentation Foundation (WPF) event system.

A typical WPF application contains many elements. Whether created in code or declared in XAML, these elements exist in an element tree relationship to each other. The event route can travel in one of two directions depending on the event definition, but generally the route travels from the source element and then "bubbles" upward through the element tree until it reaches the element tree root (typically a page or a window). This bubbling concept might be familiar to you if you have worked with the DHTML object model previously.

Consider the following simple element tree:

<Border Height="50" Width="300" BorderBrush="Gray" BorderThickness="1">
  <StackPanel Background="LightGray" Orientation="Horizontal" Button.Click="CommonClickHandler">
    <Button Name="YesButton" Width="Auto" >Yes</Button>
    <Button Name="NoButton" Width="Auto" >No</Button>
    <Button Name="CancelButton" Width="Auto" >Cancel</Button>
  </StackPanel>
</Border>

This element tree produces something like the following:

In this simplified element tree, the source of a Click event is one of the Button elements, and whichever Button was clicked is the first element that has the opportunity to handle the event. But if no handler attached to the Button acts on the event, then the event will bubble upwards to the Button parent in the element tree, which is the StackPanel. Potentially, the event bubbles to Border, and then beyond to the page root of the element tree (not shown).

In other words, the event route for this Click event is:

Button-->StackPanel-->Border-->...

Top-level Scenarios for Routed Events

private void CommonClickHandler(object sender, RoutedEventArgs e)
{
  FrameworkElement feSource = e.Source as FrameworkElement;
  switch (feSource.Name)
  {
    case "YesButton":
      // do something here ...
      break;
    case "NoButton":
      // do something ...
      break;
    case "CancelButton":
      // do something ...
      break;
  }
  e.Handled=true;
}

How Routed Events Are Implemented

Public Shared ReadOnly TapEvent As RoutedEvent = EventManager.RegisterRoutedEvent("Tap", RoutingStrategy.Bubble, GetType(RoutedEventHandler), GetType(MyButtonSimple))

' Provide CLR accessors for the event
Public Custom Event Tap As RoutedEventHandler
    AddHandler(ByVal value As RoutedEventHandler)
        Me.AddHandler(TapEvent, value)
    End AddHandler

    RemoveHandler(ByVal value As RoutedEventHandler)
        Me.RemoveHandler(TapEvent, value)
    End RemoveHandler

    RaiseEvent(ByVal sender As Object, ByVal e As RoutedEventArgs)
        Me.RaiseEvent(e)
    End RaiseEvent
End Event

Routed Event Handlers and XAML

<Button Click="b1SetColor">button</Button>

Routed events use one of three routing strategies:

• Bubbling: Event handlers on the event source are invoked. The routed event then routes to successive parent elements until reaching the element tree root. Most routed events use the bubbling routing strategy. Bubbling routed events are generally used to report input or state changes from distinct controls or other UI elements.

• Direct: Only the source element itself is given the opportunity to invoke handlers in response. This is analogous to the "routing" that Windows Forms uses for events. However, unlike a standard CLR event, direct routed events support class handling (class handling is explained in an upcoming section) and can be used by EventSetter and EventTrigger.

• Tunneling: Initially, event handlers at the element tree root are invoked. The routed event then travels a route through successive child elements along the route, towards the node element that is the routed event source (the element that raised the routed event). Tunneling routed events are often used or handled as part of the compositing for a control, such that events from composite parts can be deliberately suppressed or replaced by events that are specific to the complete control. Input events provided in WPF often come implemented as a tunneling/bubbling pair. Tunneling events are also sometimes referred to as Preview events, because of a naming convention that is used for the pairs.

As an application developer, you do not always need to know or care that the event you are handling is implemented as a routed event. Routed events have special behavior, but that behavior is largely invisible if you are handling an event on the element where it is raised.

Where routed events become powerful is if you use any of the suggested scenarios: defining common handlers at a common root, compositing your own control, or defining your own custom control class.

Routed event listeners and routed event sources do not need to share a common event in their hierarchy. Any UIElement or ContentElement can be an event listener for any routed event. Therefore, you can use the full set of routed events available throughout the working API set as a conceptual "interface" whereby disparate elements in the application can exchange event information. This "interface" concept for routed events is particularly applicable for input events.

Routed events can also be used to communicate through the element tree, because the event data for the event is perpetuated to each element in the route. One element could change something in the event data, and that change would be available to the next element in the route.

Other than the routing aspect, there are two other reasons that any given WPF event might be implemented as a routed event instead of a standard CLR event. If you are implementing your own events, you might also consider these principles:

• Certain WPF styling and templating features such as EventSetter and EventTrigger require the referenced event to be a routed event. This is the event identifier scenario mentioned earlier.

• Routed events support a class handling mechanism whereby the class can specify static methods that have the opportunity to handle routed events before any registered instance handlers can access them. This is very useful in control design, because your class can enforce event-driven class behaviors that cannot be accidentally suppressed by handling an event on an instance.

Each of the above considerations is discussed in a separate section of this topic.

To add an event handler in XAML, you simply add the event name to an element as an attribute and set the attribute value as the name of the event handler that implements an appropriate delegate, as in the following example.

<Button Click="b1SetColor">button</Button>

b1SetColor is the name of the implemented handler that contains the code that handles the Click event. b1SetColor must have the same signature as the RoutedEventHandler delegate, which is the event handler delegate for the Click event. The first parameter of all routed event handler delegates specifies the element to which the event handler is added, and the second parameter specifies the data for the event.

void b1SetColor(object sender, RoutedEventArgs args)
{
  //logic to handle the Click event


...


}

RoutedEventHandler is the basic routed event handler delegate. For routed events that are specialized for certain controls or scenarios, the delegates to use for the routed event handlers also might become more specialized, so that they can transmit specialized event data. For instance, in a common input scenario, you might handle a DragEnter routed event. Your handler should implement the DragEventHandler delegate. By using the most specific delegate, you can process the DragEventArgs in the handler and read the Data property, which contains the clipboard payload of the drag operation.

For a complete example of how to add an event handler to an element using XAML, see How to: Handle a Routed Event.

Adding a handler for a routed event in an application that is created in code is straightforward. Routed event handlers can always be added through a helper method AddHandler (which is the same method that the existing backing calls for add.) However, existing WPF routed events generally have backing implementations of add and remove logic that allow the handlers for routed events to be added by a language-specific event syntax, which is more intuitive syntax than the helper method. The following is an example usage of the helper method:

void MakeButton()
 {
     Button b2 = new Button();
     b2.AddHandler(Button.ClickEvent, new RoutedEventHandler(Onb2Click));
 }
 void Onb2Click(object sender, RoutedEventArgs e)
 {
     //logic to handle the Click event     
 }

The next example shows the C# operator syntax (Visual Basic has slightly different operator syntax because of its handling of dereferencing):

void MakeButton2()
{
  Button b2 = new Button();
  b2.Click += new RoutedEventHandler(Onb2Click2);
}
void Onb2Click2(object sender, RoutedEventArgs e)
{
  //logic to handle the Click event     
}

For an example of how to add an event handler in code, see How to: Add an Event Handler Using Code.

If you are using Visual Basic, you can also use the Handles keyword to add handlers as part of the handler declarations. For more information, see Visual Basic and WPF Event Handling.

The Concept of Handled

If you are defining a class that derives in some way from DependencyObject, you can also define and attach a class handler for a routed event that is a declared or inherited event member of your class. Class handlers are invoked before any instance listener handlers that are attached to an instance of that class, whenever a routed event reaches an element instance in its route.

Some WPF controls have inherent class handling for certain routed events. This might give the outward appearance that the routed event is not ever raised, but in reality it is being class handled, and the routed event can potentially still be handled by your instance handlers if you use certain techniques. Also, many base classes and controls expose virtual methods that can be used to override class handling behavior. For more information both on how to work around undesired class handling and on defining your own class handling in a custom class, see Marking Routed Events as Handled, and Class Handling.

The XAML language also defines a special type of event called an attached event. An attached event enables you to add a handler for a particular event to an arbitrary element. The element handling the event need not define or inherit the attached event, and neither the object potentially raising the event nor the destination handling instance must define or otherwise "own" that event as a class member.

The WPF input system uses attached events extensively. However, nearly all of these attached events are forwarded through base elements. The input events then appear as equivalent non-attached routed events that are members of the base element class. For instance, the underlying attached event Mouse..::.MouseDown can more easily be handled on any given UIElement by using MouseDown on that UIElement rather than dealing with attached event syntax either in XAML or code.

For more information about attached events in WPF, see Attached Events Overview.

Another syntax usage that resembles typename.eventname attached event syntax but is not strictly speaking an attached event usage is when you attach handlers for routed events that are raised by child elements. You attach the handlers to a common parent, to take advantage of event routing, even though the common parent might not have the relevant routed event as a member. Consider this example again:

<Border Height="50" Width="300" BorderBrush="Gray" BorderThickness="1">
  <StackPanel Background="LightGray" Orientation="Horizontal" Button.Click="CommonClickHandler">
    <Button Name="YesButton" Width="Auto" >Yes</Button>
    <Button Name="NoButton" Width="Auto" >No</Button>
    <Button Name="CancelButton" Width="Auto" >Cancel</Button>
  </StackPanel>
</Border>

Here, the parent element listener where the handler is added is a StackPanel. However, it is adding a handler for a routed event that was declared and will be raised by the Button class (ButtonBase actually, but available to Button through inheritance). Button "owns" the event, but the routed event system permits handlers for any routed event to be attached to any UIElement or ContentElement instance listener that could otherwise attach listeners for a common language runtime (CLR) event. The default xmlns namespace for these qualified event attribute names is typically the default WPF xmlns namespace, but you can also specify prefixed namespaces for custom routed events. For more information about xmlns, see XAML Namespaces and Namespace Mapping.

One frequent application of routed events within the WPF platform is for input events. In WPF, tunneling routed events names are prefixed with the word "Preview" by convention. Input events often come in pairs, with one being the bubbling event and the other being the tunneling event. For example, the KeyDown event and the PreviewKeyDown event have the same signature, with the former being the bubbling input event and the latter being the tunneling input event. Occasionally, input events only have a bubbling version, or perhaps only a direct routed version. In the documentation, routed event topics cross-reference similar routed events with alternative routing strategies if such routed events exist, and sections in the managed reference pages clarify the routing strategy of each routed event.

WPF input events that come in pairs are implemented so that a single user action from input, such as a mouse button press, will raise both routed events of the pair in sequence. First, the tunneling event is raised and travels its route. Then the bubbling event is raised and travels its route. The two events literally share the same event data instance, because the RaiseEvent method call in the implementing class that raises the bubbling event listens for the event data from the tunneling event and reuses it in the new raised event. Listeners with handlers for the tunneling event have the first opportunity to mark the routed event handled (class handlers first, then instance handlers). If an element along the tunneling route marked the routed event as handled, the already-handled event data is sent on for the bubbling event, and typical handlers attached for the equivalent bubbling input events will not be invoked. To outward appearances it will be as if the handled bubbling event has not even been raised. This handling behavior is useful for control compositing, where you might want all hit-test based input events or focus-based input events to be reported by your final control, rather than its composite parts. The final control element is closer to the root in the compositing, and therefore has the opportunity to class handle the tunneling event first and perhaps to "replace" that routed event with a more control-specific event, as part of the code that backs the control class.

As an illustration of how input event processing works, consider the following input event example. In the following tree illustration, leaf element #2 is the source of both a PreviewMouseDown and then a MouseDown event.

Input Event Bubbling and Tunneling

The order of event processing is as follows:

1. PreviewMouseDown (tunnel) on root element.

2. PreviewMouseDown (tunnel) on intermediate element #1.

3. PreviewMouseDown (tunnel) on source element #2.

4. MouseDown (bubble) on source element #2.

5. MouseDown (bubble) on intermediate element #1.

6. MouseDown (bubble) on root element.

A routed event handler delegate provides references to two objects: the object that raised the event and the object where the handler was invoked. The object where the handler was invoked is the object reported by the sender parameter. The object where the event was first raised is reported by the Source property in the event data. A routed event can still be raised and handled by the same object, in which case sender and Source are identical (this is the case with Steps 3 and 4 in the event processing example list).

Because of tunneling and bubbling, parent elements receive input events where the Source is one of their child elements. When it is important to know what the source element is, you can identify the source element by accessing the Source property.

Usually, once the input event is marked Handled, further handlers are not invoked. Typically, you should mark input events as handled as soon as a handler is invoked that addresses your application-specific logical handling of the meaning of the input event.

The exception to this general statement about Handled state is that input event handlers that are registered to deliberately ignore Handled state of the event data would still be invoked along either route. For more information, see Preview Events or Marking Routed Events as Handled, and Class Handling.

The shared event data model between tunneling and bubbling events, and the sequential raising of first tunneling then bubbling events, is not a concept that is generally true for all routed events. That behavior is specifically implemented by how WPF input devices choose to raise and connect the input event pairs. Implementing your own input events is an advanced scenario, but you might choose to follow that model for your own input events also.

Certain classes choose to class-handle certain input events, usually with the intent of redefining what a particular user-driven input event means within that control and raising a new event. For more information, see Marking Routed Events as Handled, and Class Handling.

For more information on input and how input and events interact in typical application scenarios, see Input Overview.

In styles, you can include some pre-declared XAML event handling syntax in the markup by using an EventSetter. When the style is applied, the referenced handler is added to the styled instance. You can declare an EventSetter only for a routed event. The following is an example. Note that the b1SetColor method referenced here is in a code-behind file.

<StackPanel
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  x:Class="SDKSample.EventOvw2"
  Name="dpanel2"
  Initialized="PrimeHandledToo"
>
  <StackPanel.Resources>
    <Style TargetType="{x:Type Button}">
      <EventSetter Event="Click" Handler="b1SetColor"/>
    </Style>
  </StackPanel.Resources>
  <Button>Click me</Button>
  <Button Name="ThisButton" Click="HandleThis">
    Raise event, handle it, use handled=true handler to get it anyway.
  </Button>
</StackPanel>

The advantage gained here is that the style is likely to contain a great deal of other information that could apply to any button in your application, and having the EventSetter be part of that style promotes code reuse even at the markup level. Also, an EventSetter abstracts method names for handlers one step further away from the general application and page markup.

Another specialized syntax that combines the routed event and animation features of WPF is an EventTrigger. As with EventSetter, only routed events may be used for an EventTrigger. Typically, an EventTrigger is declared as part of a style, but an EventTrigger can also be declared on page-level elements as part of the Triggers collection, or in a ControlTemplate. An EventTrigger enables you to specify a Storyboard that runs whenever a routed event reaches an element in its route that declares an EventTrigger for that event. The advantage of an EventTrigger over just handling the event and causing it to start an existing storyboard is that an EventTrigger provides better control over the storyboard and its run-time behavior. For more information, see How to: Use Event Triggers to Control a Storyboard After It Starts.

This topic mainly discusses routed events from the perspective of describing the basic concepts and offering guidance on how and when to respond to the routed events that are already present in the various base elements and controls. However, you can create your own routed event on your custom class along with all the necessary support, such as specialized event data classes and delegates. The routed event owner can be any class, but routed events must be raised by and handled by UIElement or ContentElement derived classes in order to be useful. For more information about custom events, see How to: Create a Custom Routed Event.

Concepts

Reference