Called when the drag enters the handler's area.
Called when the drag leaves the handler's area or is cancelled. You should free your resources when this is called.
Called continually as the drag moves over the handler's area.
The user dropped the data and you should process it now. You can access the data through the given DropPackage.
Called when the drop is complete. You should free whatever temporary resources you were using. It is often reasonable to simply forward this call to dragLeave.
Parameters returned by DropHandler.drop.