Specifies the number of valid items for a drop operation.
- macOS 10.7+
During draggingEntered: or draggingUpdated:, you are responsible for returning the drag operation. In some cases, you may accept some, but not all items on the dragging pasteboard. (For example, your application may only accept image files.)
If you only accept some of the items, set this property to the number of items accepted so the drag manager can update the drag count badge.
update is called, you should set the image of non-valid dragging items to
nil. If none of the drag items are valid then you should not
update, simply return
NSDrag from your implementation of draggingEntered: and, or draggingUpdated: and do not modify any drag item properties.