Restrict dragging range

Nice work!

Not sure if this is possible, but I'd like be able to restrict the drag of an object to a range defined by the object. My use case is a series of operations which have some dependencies. I'd like the user to be able to reorder operations, but not go past dependency boundaries. Is there any way to do this with the current APIs? It doesn't seem like the callbacks can return any information, so all I think I can do right now is put up cursors.