Doubleclicking in Silverlight

Mike Snow recently posted an article on how to do double clicks in Silverlight. However, his approach isn't very reusable, doesn't allow for multiple listeners and doesn't check to see if the mouse moved between the two clicks. Below is my solution, which uses extension methods and attached properties to track clicks and event handlers.

To use it, first include the namespace in your code:

using MyApplication.Extensions;

This will add AddDoubleClick and RemoveDoubleClick methods to any UIElement. You can then add an event handler to your element like so:

MyDblClickElement.AddDoubleClick(MyDblClickElement_MouseDoubleClick);

The double click handler signature is the same as for mouse down. Example:

private void MyDblClickElement_MouseDoubleClick(object sender, MouseButtonEventArgs e)
{
Point position = e.GetPosition(this);
MessageBox.Show(string.Format("dblclick at {0},{1}", position.X, position.Y));
} 

Here's the code:

using System;
using System.Collections.Generic;
using System.Windows;
using System.Windows.Input;
using System.Windows.Threading;
namespace MyApplication.Extensions
{
    public static class Mouse
    {
        private const int doubleClickInterval = 200;
        private static readonly DependencyProperty DoubleClickTimerProperty = DependencyProperty.RegisterAttached("DoubleClickTimer", typeof(DispatcherTimer), typeof(UIElement), null);
        private static readonly DependencyProperty DoubleClickHandlersProperty = DependencyProperty.RegisterAttached("DoubleClickHandlers", typeof(List<MouseButtonEventHandler>), typeof(UIElement), null);
        private static readonly DependencyProperty DoubleClickPositionProperty = DependencyProperty.RegisterAttached("DoubleClickPosition", typeof(Point), typeof(UIElement), null);
        /// <summary>
        /// Adds a double click event handler.
        /// </summary>
        /// <param name="element">The Element to listen for double clicks on.</param>
        /// <param name="handler">The handler.</param>
        public static void AddDoubleClick(this UIElement element, MouseButtonEventHandler handler)
        {
            element.MouseLeftButtonDown += element_MouseLeftButtonDown;
            List<MouseButtonEventHandler> handlers;
            handlers = element.GetValue(DoubleClickHandlersProperty) as List<MouseButtonEventHandler>;
            if (handlers == null)
            {
                handlers = new List<MouseButtonEventHandler>();
                element.SetValue(DoubleClickHandlersProperty, handlers);
            }
            handlers.Add(handler);
        }
        /// <summary>
        /// Removes a double click event handler.
        /// </summary>
        /// <param name="element">The element.</param>
        /// <param name="handler">The handler.</param>
        public static void RemoveDoubleClick(this UIElement element, MouseButtonEventHandler handler)
        {
            element.MouseLeftButtonDown -= element_MouseLeftButtonDown;
            List<MouseButtonEventHandler> handlers = element.GetValue(DoubleClickHandlersProperty) as List<MouseButtonEventHandler>;
            if (handlers != null)
            {
                handlers.Remove(handler);
                if (handlers.Count == 0)
                    element.ClearValue(DoubleClickHandlersProperty);
            }
        }
        private static void element_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            UIElement element = sender as UIElement;
            Point position = e.GetPosition(element);
            DispatcherTimer timer = element.GetValue(DoubleClickTimerProperty) as DispatcherTimer;
            if (timer != null) //DblClick
            {
                timer.Stop();
                Point oldPosition = (Point)element.GetValue(DoubleClickPositionProperty);
                element.ClearValue(DoubleClickTimerProperty);
                element.ClearValue(DoubleClickPositionProperty);
                if (Math.Abs(oldPosition.X - position.X) < 1 && Math.Abs(oldPosition.Y - position.Y) < 1) //mouse didn't move => Valid double click
                {
                    List<MouseButtonEventHandler> handlers = element.GetValue(DoubleClickHandlersProperty) as List<MouseButtonEventHandler>;
                    if (handlers != null)
                    {
                        foreach (MouseButtonEventHandler handler in handlers)
                        {
                            handler(sender, e);
                        }
                    }
                    return;
                }
            }
            //First click or mouse moved. Start a new timer
            timer = new DispatcherTimer() { Interval = TimeSpan.FromMilliseconds(doubleClickInterval) };
            timer.Tick += new EventHandler((s, args) =>
            {  //DblClick timed out
                (s as DispatcherTimer).Stop();
                element.ClearValue(DoubleClickTimerProperty); //clear timer
                element.ClearValue(DoubleClickPositionProperty); //clear first click position
            });
            element.SetValue(DoubleClickTimerProperty, timer);
            element.SetValue(DoubleClickPositionProperty, position);
            timer.Start();
        }
    }
}