Touch Screen & Blocking Drop Handlers

[alanisaac]

I'm working with the drag/drop library on a Surface Pro, trying to use a Stylus. The Stylus events get correctly translated into Mouse Events, and are handled by the library appropriately.

However, in my viewmodel that handles the IDropTarget, I attempt to pop open a dialog, with a basic Window.ShowDialog() call. When I use a mouse, this works fine. But when I use a stylus (or touch), the newly opened dialog is unable to respond to stylus (or touch) events.

I believe this is the same issue noted here: http://stackoverflow.com/questions/21178504/touch-events-not-working-after-dodragdrop

And I think the solution is the same as the one here: https://social.msdn.microsoft.com/Forums/sqlserver/en-US/d9e4335b-beaa-4ecf-abd6-a900af8a9e41/show-a-dialog-inside-touchup-event?forum=wpf

The idea being that for stylus/touch events, the handler needs to return first before other UI-blocking events are invoked. The way I solved this in my own pull of the code is to make the view model code invoke asynchronously. I made the following change in DragDrop.cs:

private static void DropTarget_PreviewDrop(object sender, DragEventArgs e)
{
      var dropInfo = new DropInfo(sender, e, m_DragInfo);
      var dropHandler = TryGetDropHandler(dropInfo, sender as UIElement);
      var dragHandler = TryGetDragHandler(m_DragInfo, sender as UIElement);

      DragAdorner = null;
      EffectAdorner = null;
      DropTargetAdorner = null;

      dropHandler.DragOver(dropInfo);
      dropHandler.Drop(dropInfo);
      dragHandler.Dropped(dropInfo);

      Mouse.OverrideCursor = null;
      e.Handled = !dropInfo.NotHandled;
}

To

private static async void DropTarget_PreviewDrop(object sender, DragEventArgs e)
{
      var dropInfo = new DropInfo(sender, e, m_DragInfo);
      var dropHandler = TryGetDropHandler(dropInfo, sender as UIElement);
      var dragHandler = TryGetDragHandler(m_DragInfo, sender as UIElement);

      DragAdorner = null;
      EffectAdorner = null;
      DropTargetAdorner = null;
      Mouse.OverrideCursor = null;

      await Dispatcher.CurrentDispatcher.InvokeAsync(() => 
    {
      dropHandler.DragOver(dropInfo);
      dropHandler.Drop(dropInfo);
      dragHandler.Dropped(dropInfo);
    });

      e.Handled = !dropInfo.NotHandled;
}

This appears to allow the desired behavior, though I am not currently testing whether setting e.Handled after an await call works as expected. It does make use of async/await, so it may not be viable for the overall solution, but maybe something similar can be incorporated in the future.