Question

I’m trying to implement a WPF application with drag&drop functionality using MouseDragElementBehavior.
But I just can’t find a way to get the dropped elements position relative to its parent Canavs.
Example code:

namespace DragTest {
    public partial class MainWindow : Window {

        private Canvas _child;

        public MainWindow() {
            InitializeComponent();

            Canvas parent = new Canvas();
            parent.Width = 400;
            parent.Height = 300;
            parent.Background = new SolidColorBrush(Colors.LightGray);    

            _child = new Canvas();
            _child.Width = 50;
            _child.Height = 50;
            _child.Background = new SolidColorBrush(Colors.Black);

            MouseDragElementBehavior dragBehavior = new MouseDragElementBehavior();
            dragBehavior.Attach(_child);
            dragBehavior.DragBegun += onDragBegun;
            dragBehavior.DragFinished += onDragFinished;

            Canvas.SetLeft(_child, 0);
            Canvas.SetTop(_child, 0);

            parent.Children.Add(_child);

            Content = parent;

        }

        private void onDragBegun(object sender, MouseEventArgs args) {
            Debug.WriteLine(Canvas.GetLeft(_child));
        }

        private void onDragFinished(object sender, MouseEventArgs args) {
            Debug.WriteLine(Canvas.GetLeft(_child));
        }
    }
}

After Dropping the child Canvas the value of Canvas.GetLeft(_child) is still 0.
Why that? Why doesn’t it change?

Sure, I can get the new position by using dragBehavior.X, but that’s the child Canvas’ position in the main window, not the position relative to the parent Canvas. There must be a way to get it…

Answer 1

I just found a workaround:

private void onDragFinished(object sender, MouseEventArgs args) {
    Point windowCoordinates = new Point(((MouseDragElementBehavior)sender).X, ((MouseDragElementBehavior)sender).Y); 
    Point screenCoordinates = this.PointToScreen(windowCoordinates);
    Point parentCoordinates = _parent.PointFromScreen(screenCoordinates);
    Debug.WriteLine(parentCoordinates);
}

So I simple convert the point to screen coordinates, then from screen coordinates to the parents coordinates.

Nevertheless there will be problems if the parent Canvas is in some ScrollView or somthing.
There doesn’t seem to be a simple solution with this Drag&Drop approach…