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

Question

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 way 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.

have you found anything yet? I am trying to find the answer on how to do the drag and drop behaviour using pure xaml on the view as well :( — user802356
@Anon, I'm pretty sure that functionality isn't built into XAML. You'll need to at least implement a handler for the Drop event. It's available on UIElement. — Phil Gan
In my case, I'm not looking for the drop event. I'm trying to apply a style. I've clarified above. — Mark A. Donohoe

atomicode atomicode · Accepted Answer · 2015-01-30T13:15:11

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

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);
}

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);
    }
}

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);
    }
}

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>

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

Update

3 Answers