/// <summary>
/// ItemsSourceChanged Behavior uses an Attached Dependency Property
/// to add and raise a rotued event whenever an ItemsControl's ItemsSource property
/// changes. Also looks for INotifyCollectionChanged on the ItemsSource and raises the
/// event on every collection changed event
/// </summary>

public static  class ItemsSourceChangedBehavior
{
  /// <summary>
  /// ItemsSourceChanged Attached Dependency Property with Callback method
  /// </summary>
  public static readonly DependencyProperty ItemsSourceChangedProperty =
                                            DependencyProperty.RegisterAttached("ItemsSourceChanged",
                                             typeof(bool), typeof(ItemsSourceChangedBehavior),
                                             new FrameworkPropertyMetadata(false, OnItemsSourceChanged));

  /// <summary>
  /// Static Get method allowing easy Xaml usage and to simplify the
  /// GetValue process
  /// </summary>
  /// <param name="obj">The dependency obj.</param>
  /// <returns>True or False</returns>
  public static bool GetItemsSourceChanged(DependencyObject obj)
  {
    return (bool)obj.GetValue(ItemsSourceChangedProperty);
  }

  /// <summary>
  /// Static Set method allowing easy Xaml usage and to simplify the
  /// Setvalue process
  /// </summary>
  /// <param name="obj">The obj.</param>
  /// <param name="value">if set to <c>true</c> [value].</param>
  public static void SetItemsSourceChanged(DependencyObject obj, bool value)
  {
    obj.SetValue(ItemsSourceChangedProperty, value);
  }

  /// <summary>
  /// Dependency Property Changed Call Back method. This will be called anytime
  /// the ItemsSourceChangedProperty value changes on a Dependency Object
  /// </summary>
  /// <param name="obj">The obj.</param>
  /// <param name="e">The <see cref="System.Windows.DependencyPropertyChangedEventArgs"/> instance containing the event data.</param>
  private static void OnItemsSourceChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
  {
    ItemsControl itemsControl = obj as ItemsControl;

    if (itemsControl == null)
      return;

    bool oldValue = (bool)e.OldValue;
    bool newValue = (bool)e.NewValue;

    if (!oldValue && newValue) // If changed from false to true
    {
      // Create a binding to the ItemsSourceProperty on the ItemsControl
      Binding b = new Binding
                      {
                         Source = itemsControl,
                         Path = new PropertyPath(ItemsControl.ItemsSourceProperty)
                      };

      // Since the ItemsSourceListenerProperty is now bound to the
      // ItemsSourceProperty on the ItemsControl, whenever the 
      // ItemsSourceProperty changes the ItemsSourceListenerProperty
      // callback method will execute
      itemsControl.SetBinding(ItemsSourceListenerProperty, b);
    }
    else if (oldValue && !newValue) // If changed from true to false
    {
      // Clear Binding on the ItemsSourceListenerProperty
      BindingOperations.ClearBinding(itemsControl, ItemsSourceListenerProperty);
    }
  }

  #endregion

  #region Items Source Listener Property

  /// <summary>
  /// The ItemsSourceListener Attached Dependency Property is a private property
  /// the ItemsSourceChangedBehavior will use silently to bind to the ItemsControl
  /// ItemsSourceProperty.
  /// Once bound, the callback method will execute anytime the ItemsSource property changes
  /// </summary>
  private static readonly DependencyProperty ItemsSourceListenerProperty =
       DependencyProperty.RegisterAttached("ItemsSourceListener",
                                           typeof(object), typeof(ItemsSourceChangedBehavior),
                                           new FrameworkPropertyMetadata(null, OnItemsSourceListenerChanged));


  /// <summary>
  /// Dependency Property Changed Call Back method. This will be called anytime
  /// the ItemsSourceListenerProperty value changes on a Dependency Object
  /// </summary>
  /// <param name="obj">The obj.</param>
  /// <param name="e">The <see cref="System.Windows.DependencyPropertyChangedEventArgs"/> instance containing the event data.</param>
  private static void OnItemsSourceListenerChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
  {
    ItemsControl itemsControl = obj as ItemsControl;

    if (itemsControl == null)
      return;

   INotifyCollectionChanged collection = e.NewValue as INotifyCollectionChanged;

   if (collection != null)
   {
     collection.CollectionChanged += delegate
                                           {
                                             itemsControl.RaiseEvent(new RoutedEventArgs(ItemsSourceChangedEvent));
                                           };
   }

   if (GetItemsSourceChanged(itemsControl))
     itemsControl.RaiseEvent(new RoutedEventArgs(ItemsSourceChangedEvent));
 }

 #endregion

 #region Items Source Changed Event

 /// <summary>
 /// Routed Event to raise whenever the ItemsSource changes on an ItemsControl
 /// </summary>
 public static readonly RoutedEvent ItemsSourceChangedEvent =
      EventManager.RegisterRoutedEvent("ItemsSourceChanged",
                                       RoutingStrategy.Bubble,
                                       typeof(RoutedEventHandler),
                                       typeof(ItemsControl));

 #endregion
}