How can we activate a WPF trigger in XAML when a drag operation is occurring?

We're looking for a way to base a WPF trigger in XAML on whether or not we're in a drag-drop operation. Depending on if we are or not, we want different hovering behaviors which is why this is needed.

The only way I can think of is to handle the drag start and end events and manually track the state, but that requires a code-behind, not pure XAML. Plus it seems like complete overkill, especially since we'd have to do it on every potential drop target which is a real pain.

So is there an easy w开发者_Go百科ay to say 'Hey... I'm in a drag-drop operation so make this trigger active' or am I out of luck here?

Update

To clarify what we are trying to do, currently in pure XAML, you can create a style, then set a style trigger to examine the IsMouseOver property to say, draw a background highlight. Well, we want to do this, but we want to say if 'IsMouseOver' is true and if IsDragging = true then apply this trigger.

I've just had this problem, my solution consists of using an attached property that supplies the missing IsDragging:

1. Define the attached Property

   public static readonly DependencyProperty IsDraggingProperty =
       DependencyProperty.RegisterAttached
       (
           "IsDragging",
           typeof(bool),
           typeof(ClassContainingThisProperty),
           new UIPropertyMetadata(false)
       );
   
   public static bool GetIsDragging(DependencyObject source)
   {
       return (bool)source.GetValue(IsDraggingProperty);
   }
   
   public static void SetIsDragging(DependencyObject target, bool value)
   {
       target.SetValue(IsDraggingProperty, value);
   }
   

2. Create this extension methods to help you set the Property

   public static TParent FindParent<TParent>(this DependencyObject child) where TParent : DependencyObject
   {
       DependencyObject current = child;
       while(current != null && !(current is TParent))
       {
           current = VisualTreeHelper.GetParent(current);
       }
       return current as TParent;
   }
   
   public static void SetParentValue<TParent>(this DependencyObject child, DependencyProperty property, object value) where TParent : DependencyObject
   {
       TParent parent = child.FindParent<TParent>();
       if(parent != null)
       {
           parent.SetValue(property, value);
       }
   }
   

3. Handle DragDrop events according to the Control used (e.g. ListView), to set the attached property on the elements.

   private void OnDragEnter(object sender, DragEventArgs e)
   {
       DependencyObject source = e.OriginalSource as DependencyObject;
       if (source != null)
       {
           source.SetParentValue<ListViewItem>(ClassContainingTheProperty.IsDraggingProperty, true);
       }
   }
   
   private void OnDragLeave(object sender, DragEventArgs e)
   {
       DependencyObject source = e.OriginalSource as DependencyObject;
       if(source != null)
       {
           source.SetParentValue<ListViewItem>(ClassContainingTheProperty.IsDraggingProperty, false);
       }
   }
   
   private void OnDrop(object sender, DragEventArgs e)
   {
       DependencyObject source = e.OriginalSource as DependencyObject;
       if(source != null)
       {
           source.SetParentValue<ListViewItem>(ClassContainingTheProperty.IsDraggingProperty, false);
       }
   }
   

4. Use the Property in your Trigger

   <ControlTemplate TargetType="{x:Type ListViewItem}">
     <ControlTemplate.Triggers>
       <Trigger Property="namespace:ClassContainingTheProperty.IsDragging" Value="True">
         <Setter Property="Background" Value="White" />
         <Setter Property="BorderBrush" Value="Black" />
       </Trigger>
     </ControlTemplate.Triggers>
   </ControlTemplate>
   

I've only ever seen drag-drop implemented with event handlers, so I think you're out of luck. I suggest you create your own dependency property to indicate a drag drop in progress.

if it is a real DragDrop event just watch for the DragOver event...

            <EventTrigger RoutedEvent="DragOver">
                <BeginStoryboard>
                    <Storyboard Storyboard.TargetProperty="Fill.Color">
                        <ColorAnimation From="White" To="Black" Duration="0:0:1" />
                    </Storyboard>
                </BeginStoryboard>
            </EventTrigger>