WPF Behaviors

Introduction#

WPF behaviors allow a developer to alter the way WPF controls acts in response to system and user events.

Behaviors inherit from the Behavior class of the System.Windows.Interactity namespace. This namespace is a part of the overarching Expression Blend SDK, but a lighter version, suitable for behavior libraries, is available as a nuget package.

Simple Behavior to Intercept Mouse Wheel Events

Implementing the Behavior

This behavior will cause mouse wheel events from an inner ScrollViewer to bubble up to the parent ScrollViewer when the inner one is at either its upper or lower limit. Without this behavior, the events will never make it out of the inner ScrollViewer.

public class BubbleMouseWheelEvents : Behavior<UIElement>
{
    protected override void OnAttached()
    {
        base.OnAttached();
        this.AssociatedObject.PreviewMouseWheel += PreviewMouseWheel;
    }

    protected override void OnDetaching()
    {
        this.AssociatedObject.PreviewMouseWheel -= PreviewMouseWheel;
        base.OnDetaching();
    }

    private void PreviewMouseWheel(object sender, MouseWheelEventArgs e)
    {
        var scrollViewer = AssociatedObject.GetChildOf<ScrollViewer>(includeSelf: true);
        var scrollPos = scrollViewer.ContentVerticalOffset;
        if ((scrollPos == scrollViewer.ScrollableHeight && e.Delta < 0) || (scrollPos == 0 && e.Delta > 0))
        {
            UIElement rerouteTo = AssociatedObject;
            if (ReferenceEquals(scrollViewer, AssociatedObject))
            {
                rerouteTo = (UIElement) VisualTreeHelper.GetParent(AssociatedObject);
            }

            e.Handled = true;
            var e2 = new MouseWheelEventArgs(e.MouseDevice, e.Timestamp, e.Delta);
            e2.RoutedEvent = UIElement.MouseWheelEvent;
            rerouteTo.RaiseEvent(e2);
        }
    }
}

Behaviors subclass the Behavior<T> base-class, with T being the type of control that it is able to attach to, in this case UIElement. When the Behavior is instantiated from XAML, the OnAttached method is called. This method allows the behavior to hook in to events from the control it is attached to (via AssociatedControl). A similar method, OnDetached is called when the behavior need to be unhooked from the associated element. Care should be taken to remove any event handlers, or otherwise clean up objects to avoid memory leaks.

This behavior hooks in to the PreviewMouseWheel event, which gives it a change to intercept the event before the ScrollViewer has a chance to see it. It checks the position to see if it needs to forward the event up the visual tree to any ScrollViewer higher hierarchy. If so, it sets e.Handled to true to prevent the default action of the event. It then raises a new MouseWheelEvent routed to AssociatedObject. Otherwise, the event is routed as normal.

Attaching the Behavior to an Element in XAML

First, the interactivity xml-namespace must be brought in to scope before it can be used in XAML. Add the following line to the namespaces of your XAML.

xmlns:interactivity=”https://schemas.microsoft.com/expression/2010/interactivity”

The behavior can be attached like so:

<ScrollViewer>
    <!--...Content...-->
    <ScrollViewer>
        <interactivity:Interaction.Behaviors>
            <behaviors:BubbleMouseWheelEvents />
        </interactivity:Interaction.Behaviors>
        <!--...Content...-->
    </ScrollViewer>
    <!--...Content...-->.
</ScrollViewer>

This creates a Behaviors collection as an Attached Property on the inner ScrollViewer that contains a BubbleMouseWheelEvents behavior.

This particular behavior could also be attached to any existing control that contains an embedded ScrollViewer, such as a GridView, and it would still function correctly.