Preview events (WPF .NET)

Preview events, also known as tunneling events, are routed events that traverse downward through the element tree from the application root element to the element that raised the event. The element that raises an event is reported as the Source in the event data. Not all event scenarios support or require preview events. This article describes where preview events exist and how applications or components can interact with them. For information on how to create a preview event, see How to create a custom routed event.

Prerequisites

The article assumes a basic knowledge of routed events, and that you've read Routed events overview. To follow the examples in this article, it helps if you're familiar with Extensible Application Markup Language (XAML) and know how to write Windows Presentation Foundation (WPF) applications.

Preview events marked as handled

Be cautious when marking preview events as handled in event data. Marking a preview event as handled on an element other than the element that raised it can prevent the element that raised it from handling the event. Sometimes marking preview events as handled is intentional. For example, a composite control might suppress events raised by individual components and replace them with events raised by the complete control. Custom events for a control can provide customized event data and trigger based on component state relationships.

For input events, event data is shared by both the preview and non-preview (bubbling) equivalents of each event. If you use a preview event class-handler to mark an input event as handled, class-handlers for the bubbling input event typically won't be invoked. Or, if you use a preview event instance-handler to mark an event as handled, instance-handlers for the bubbling input event typically won't be invoked. Although you can configure class and instance handlers to be invoked even if an event is marked as handled, that handler configuration isn't common. For more information about class handling and how it relates to preview events, see Marking routed events as handled and class handling.

Working around event suppression by controls

Some composite controls suppress input events at the component-level in order to replace them with a customized high-level event. For example, the WPF ButtonBase marks the MouseLeftButtonDown bubbling input event as handled in its OnMouseLeftButtonDown method and raises the Click event. The MouseLeftButtonDown event and its event data still continue along the element tree route, but because the event is marked as Handled in event data, only handlers that are configured to respond to handled events are invoked.

If you want other elements toward the root of your application to handle a routed event that's marked as handled, you can either:

• Attach handlers by calling the UIElement.AddHandler(RoutedEvent, Delegate, Boolean) method and setting the parameter handledEventsToo to true. This approach requires attaching the event handler in code-behind, after obtaining an object reference to the element that it will attach to.

• If the event marked as handled is a bubbling event, attach handlers for the equivalent preview event if available. For instance, if a control suppresses the MouseLeftButtonDown event, you can attach a handler for the PreviewMouseLeftButtonDown event instead. This approach only works for base element input events that implement both tunneling and bubbling routing strategies and share event data.

The following example implements a rudimentary custom control named componentWrapper that contains a TextBox. The control is added to a StackPanel named outerStackPanel.

<StackPanel Name="outerStackPanel"
    VerticalAlignment="Center"
    custom:ComponentWrapper.CustomKey="Handler_PrintEventInfo"
    TextBox.KeyDown="Handler_PrintEventInfo"
    TextBox.PreviewKeyDown="Handler_PrintEventInfo" >
    <custom:ComponentWrapper
        x:Name="componentWrapper"
        TextBox.KeyDown="ComponentWrapper_KeyDown"
        custom:ComponentWrapper.CustomKey="Handler_PrintEventInfo"
        HorizontalAlignment="Center">
        <TextBox Name="componentTextBox" Width="200" KeyDown="Handler_PrintEventInfo" />
    </custom:ComponentWrapper>
</StackPanel>

The componentWrapper control listens for the KeyDown bubbling event raised by its TextBox component whenever a keystroke occurs. On that occurrence, the componentWrapper control:

1. Marks the KeyDown bubbling routed event as handled to suppress it. As a result, only the outerStackPanel handler that's configured in code-behind to respond to handled KeyDown events is triggered. The outerStackPanel handler attached in XAML for KeyDown events isn't invoked.

2. Raises a custom bubbling routed event named CustomKey, which triggers the outerStackPanel handler for the CustomKey event.

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        // Attach a handler on outerStackPanel that will be invoked by handled KeyDown events.
        outerStackPanel.AddHandler(KeyDownEvent, new RoutedEventHandler(Handler_PrintEventInfo), 
            handledEventsToo: true);
    }

    private void ComponentWrapper_KeyDown(object sender, System.Windows.Input.KeyEventArgs e)
    {
        Handler_PrintEventInfo(sender, e);

        Debug.WriteLine("KeyDown event marked as handled on componentWrapper.\r\n" +
            "CustomKey event raised on componentWrapper.");

        // Mark the event as handled.
        e.Handled = true;

        // Raise the custom click event.
        componentWrapper.RaiseCustomRoutedEvent();
    }

    private void Handler_PrintEventInfo(object sender, System.Windows.Input.KeyEventArgs e)
    {
        string senderName = ((FrameworkElement)sender).Name;
        string sourceName = ((FrameworkElement)e.Source).Name;
        string eventName = e.RoutedEvent.Name;
        string handledEventsToo = e.Handled ? " Parameter handledEventsToo set to true." : "";

        Debug.WriteLine($"Handler attached to {senderName} " +
            $"triggered by {eventName} event raised on {sourceName}.{handledEventsToo}");
    }

    private void Handler_PrintEventInfo(object sender, RoutedEventArgs e)
    {
        string senderName = ((FrameworkElement)sender).Name;
        string sourceName = ((FrameworkElement)e.Source).Name;
        string eventName = e.RoutedEvent.Name;
        string handledEventsToo = e.Handled ? " Parameter handledEventsToo set to true." : "";

        Debug.WriteLine($"Handler attached to {senderName} " +
            $"triggered by {eventName} event raised on {sourceName}.{handledEventsToo}");
    }

    // Debug output:
    //
    // Handler attached to outerStackPanel triggered by PreviewKeyDown event raised on componentTextBox.
    // Handler attached to componentTextBox triggered by KeyDown event raised on componentTextBox.
    // Handler attached to componentWrapper triggered by KeyDown event raised on componentTextBox.
    // KeyDown event marked as handled on componentWrapper.
    // CustomKey event raised on componentWrapper.
    // Handler attached to componentWrapper triggered by CustomKey event raised on componentWrapper.
    // Handler attached to outerStackPanel triggered by CustomKey event raised on componentWrapper.
    // Handler attached to outerStackPanel triggered by KeyDown event raised on componentTextBox. Parameter handledEventsToo set to true.
}

public class ComponentWrapper : StackPanel
{
    // Register a custom routed event using the Bubble routing strategy.
    public static readonly RoutedEvent CustomKeyEvent = 
        EventManager.RegisterRoutedEvent(
            name: "CustomKey",
            routingStrategy: RoutingStrategy.Bubble,
            handlerType: typeof(RoutedEventHandler),
            ownerType: typeof(ComponentWrapper));

    // Provide CLR accessors for assigning an event handler.
    public event RoutedEventHandler CustomKey
    {
        add { AddHandler(CustomKeyEvent, value); }
        remove { RemoveHandler(CustomKeyEvent, value); }
    }

    public void RaiseCustomRoutedEvent()
    {
        // Create a RoutedEventArgs instance.
        RoutedEventArgs routedEventArgs = new(routedEvent: CustomKeyEvent);

        // Raise the event, which will bubble up through the element tree.
        RaiseEvent(routedEventArgs);
    }
}

Partial Public Class MainWindow
        Inherits Window

        Public Sub New()
        InitializeComponent()

        ' Attach a handler on outerStackPanel that will be invoked by handled KeyDown events.
        outerStackPanel.[AddHandler](KeyDownEvent, New RoutedEventHandler(AddressOf Handler_PrintEventInfo),
                                     handledEventsToo:=True)
    End Sub

    Private Sub ComponentWrapper_KeyDown(sender As Object, e As KeyEventArgs)
        Handler_PrintEventInfo(sender, e)
        Debug.WriteLine("KeyDown event marked as handled on componentWrapper." &
                        vbCrLf & "CustomKey event raised on componentWrapper.")

        ' Mark the event as handled.
        e.Handled = True

        ' Raise the custom click event.
        componentWrapper.RaiseCustomRoutedEvent()
    End Sub

    Private Sub Handler_PrintEventInfo(sender As Object, e As KeyEventArgs)
        Dim senderName As String = CType(sender, FrameworkElement).Name
        Dim sourceName As String = CType(e.Source, FrameworkElement).Name
        Dim eventName As String = e.RoutedEvent.Name
        Dim handledEventsToo As String = If(e.Handled, " Parameter handledEventsToo set to true.", "")
        Debug.WriteLine($"Handler attached to {senderName} " &
                        $"triggered by {eventName} event raised on {sourceName}.{handledEventsToo}")
    End Sub

    Private Sub Handler_PrintEventInfo(sender As Object, e As RoutedEventArgs)
        Dim senderName As String = CType(sender, FrameworkElement).Name
        Dim sourceName As String = CType(e.Source, FrameworkElement).Name
        Dim eventName As String = e.RoutedEvent.Name
        Dim handledEventsToo As String = If(e.Handled, " Parameter handledEventsToo set to true.", "")
        Debug.WriteLine($"Handler attached to {senderName} " &
                        $"triggered by {eventName} event raised on {sourceName}.{handledEventsToo}")
    End Sub

    ' Debug output
    '
    ' Handler attached to outerStackPanel triggered by PreviewKeyDown event raised on componentTextBox.
    ' Handler attached to componentTextBox triggered by KeyDown event raised on componentTextBox.
    ' Handler attached to componentWrapper triggered by KeyDown event raised on componentTextBox.
    ' KeyDown event marked as handled on componentWrapper.
    ' CustomKey event raised on componentWrapper.
    ' Handler attached to componentWrapper triggered by CustomKey event raised on componentWrapper.
    ' Handler attached to outerStackPanel triggered by CustomKey event raised on componentWrapper.
    ' Handler attached to outerStackPanel triggered by KeyDown event raised on componentTextBox. Parameter handledEventsToo set to true.
End Class

    Public Class ComponentWrapper
        Inherits StackPanel

        ' Register a custom routed event with the Bubble routing strategy.
        Public Shared ReadOnly CustomKeyEvent As RoutedEvent =
            EventManager.RegisterRoutedEvent(
                name:="CustomKey",
                routingStrategy:=RoutingStrategy.Bubble,
                handlerType:=GetType(RoutedEventHandler),
                ownerType:=GetType(ComponentWrapper))

        ' Provide CLR accessors to support event handler assignment.
        Public Custom Event CustomKey As RoutedEventHandler

            AddHandler(value As RoutedEventHandler)
                [AddHandler](CustomKeyEvent, value)
            End AddHandler

            RemoveHandler(value As RoutedEventHandler)
                [RemoveHandler](CustomKeyEvent, value)
            End RemoveHandler

            RaiseEvent(sender As Object, e As RoutedEventArgs)
                [RaiseEvent](e)
            End RaiseEvent

        End Event

    Public Sub RaiseCustomRoutedEvent()
        ' Create a RoutedEventArgs instance & raise the event,
        ' which will bubble up through the element tree.
        Dim routedEventArgs As New RoutedEventArgs(routedEvent:=CustomKeyEvent)
            [RaiseEvent](routedEventArgs)
        End Sub
    End Class

The example demonstrates two workarounds for getting the suppressed KeyDown routed event to invoke an event handler attached to the outerStackPanel:

• Attach a PreviewKeyDown event handler to the outerStackPanel. Since a preview input routed event precedes the equivalent bubbling routed event, the PreviewKeyDown handler in the example runs ahead of the KeyDown handler that suppresses both preview and bubbling events through their shared event data.

• Attach a KeyDown event handler to the outerStackPanel by using the UIElement.AddHandler(RoutedEvent, Delegate, Boolean) method in code-behind, with the handledEventsToo parameter set to true.

See also

• Marking routed events as handled, and class handling
• Routed events overview
• How to create a custom routed event