private bool CanExecuteDrag(DragCommandParameter parameter)
{
// Determine the data that is dragged and store it in parameter.Tag.
// If a ContentControl is dragged, use ContentControl.Content.
var contentControl = AssociatedObject as ContentControl;
if (contentControl != null)
{
// The dragged element is not within an ItemsControl.
// --> Select the data-context.
parameter.Tag = contentControl.Content;
return(true);
}
// If an item of an ItemsControl is dragged, use ItemContainer.DataContext.
var itemsControl = AssociatedObject as ItemsControl;
if (itemsControl != null)
{
// The dragged element is within an ItemsControl.
// --> Select the clicked item.
var visual = parameter.HitObject as Visual;
Debug.Assert(visual != null, "HitObject should always be set.");
var itemContainer = itemsControl.ContainerFromElement(visual) as FrameworkElement;
if (itemContainer != null)
{
parameter.Tag = itemContainer.DataContext;
return(true);
}
}
return(false);
}
private void DisableDrag()
{
// Abort current drag operation.
_dragCommandParameter = null;
// Unregister event handlers.
AssociatedObject.PreviewMouseLeftButtonDown -= OnPreviewMouseLeftButtonDown;
AssociatedObject.MouseLeftButtonDown -= OnMouseLeftButtonDown;
AssociatedObject.PreviewMouseLeftButtonUp -= OnPreviewMouseLeftButtonUp;
AssociatedObject.PreviewMouseMove -= OnPreviewMouseMove;
}
private void OnPreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs eventArgs)
{
if (eventArgs.Handled || !UsePreviewEvent)
{
return;
}
_dragCommandParameter = new DragCommandParameter
{
AssociatedObject = AssociatedObject,
HitObject = eventArgs.OriginalSource as DependencyObject,
MousePosition = eventArgs.GetPosition(AssociatedObject),
};
OnMouseLeftButtonDown(eventArgs);
}
private void OnPreviewMouseMove(object sender, MouseEventArgs eventArgs)
{
if (eventArgs.Handled)
{
return;
}
if (_dragCommandParameter == null)
{
return;
}
// Avoid reentrance
if (_inDragDrop)
{
// Not sure why this happens. It can happen when dragging from a Button...
return;
}
// Only start drag-and-drop when user moved the mouse by a reasonable amount.
if (!ExceedsMinimumDragDistance(_dragCommandParameter.MousePosition, eventArgs.GetPosition(AssociatedObject)))
{
return;
}
_inDragDrop = true;
// The user wants to drag something.
try
{
var dragCommand = DragCommand ?? DefaultDragCommand;
if (dragCommand.CanExecute(_dragCommandParameter))
{
// Call DragCommand to start the drag-and-drop operation.
dragCommand.Execute(_dragCommandParameter);
eventArgs.Handled = true;
}
}
finally
{
_inDragDrop = false;
_dragCommandParameter = null;
}
}
private void OnMouseLeftButtonDown(MouseButtonEventArgs eventArgs)
{
_dragCommandParameter = new DragCommandParameter
{
AssociatedObject = AssociatedObject,
HitObject = eventArgs.OriginalSource as DependencyObject,
MousePosition = eventArgs.GetPosition(AssociatedObject),
};
// Call DragCommand.CanExecute() and let the view model decide whether
// dragging is possible. The drag-and-drop starts in the mouse move
// event when the user moves the mouse by a significant distance.
var dragCommand = DragCommand ?? DefaultDragCommand;
if (!dragCommand.CanExecute(_dragCommandParameter))
{
_dragCommandParameter = null;
}
}
private void ExecuteDrag(DragCommandParameter parameter)
{
// The data was determined in CanExecuteDrag and stored in the Tag.
var data = parameter.Tag;
if (data == null)
{
return;
}
// Wrap data in IDataObject.
var dataObject = SetData(data);
_sourceUpdated = false;
// Execute drag-and-drop.
var effect = DragDrop.DoDragDrop(parameter.AssociatedObject, dataObject, DefaultEffects);
// Update source if not already handled by DropCommand.
if ((effect & DragDropEffects.Move) != 0 && !_sourceUpdated)
{
// Remove data from source.
var contentControl = parameter.AssociatedObject as ContentControl;
var itemsControl = parameter.AssociatedObject as ItemsControl;
if (contentControl != null)
{
// Source is a ContentControl.
// --> Remove content.
contentControl.Content = null;
}
else if (itemsControl != null)
{
// Source is an ItemsControl.
// --> Remove item.
RemoveItemFromItemsControl(itemsControl, data);
}
}
}
private void OnPreviewMouseLeftButtonUp(object sender, MouseButtonEventArgs eventArgs)
{
// Cancel drag operation.
_dragCommandParameter = null;
}