Question

I have a wpf application with some Thumb controls which move items around a canvs. I want to detect when these thumbs are dragged over another element. However, the drag enter on the element which is dragged over is not fired. I know this code works as drag items that are not thumbs fire the event.

Is the drag event on the thumb not a drag event that other components listen to?

Any idea how to get this to work?

Answer 1

To achieve the simulation of a drag enter event when a thumb object is moved over another component I’ve had to do this:

Register event handler for the thumb drag delta

EventManager.RegisterClassHandler(typeof(Thumb), Thumb.DragDeltaEvent, new RoutedEventHandler(Thumb_DragDeltaEvent), true);

Then in the event handler see if the dragged element is over the element that is listening to the moving component

void Thumb_DragDeltaEvent(object sender, RoutedEventArgs e)
    {
        UIElement src = e.Source as UIElement ;
        if (src != null)
        {                
            Point srcPositionTopLeft = new Point(Canvas.GetLeft(src), Canvas.GetTop(src));
            Point srcPositionBottomRight = new Point(srcPositionTopLeft.X + src.ActualWidth, srcPositionTopLeft.Y + ActualHeight);
            Rect srcRect = new Rect(srcPositionTopLeft, srcPositionBottomRight);
            Rect transformedSrcRect = src.TransformToAncestor(this.Parent).TransformBounds(srcRect);

            Point trgPositionTopLeft = new Point(Canvas.GetLeft(this), Canvas.GetTop(this));
            Point trgPositionBottomRight = new Point(trgPositionTopLeft.X + this.ActualWidth, trgPositionTopLeft.Y + this.ActualHeight);
            Rect trgRect = new Rect(srcPositionTopLeft, srcPositionBottomRight);
            Rect transformedTrgRect = this.TransformToAncestor(this.Parent).TransformBounds(trgRect);

            if (transformedSrcRect.Contains(transformedTrgRect) ||
                transformedSrcRect.IntersectsWith(transformedTrgRect))
            {
                //drag is over element
            }
        }
    }

Remember to removed event handlers etc later.

Hope this helps someone in the future.