Allow dynamic droppable configuration using a function

I have a requirement where I cannot use selectors to determine if an element is droppable. For example: The "child" component must be within a "parent" component, however if the child component contains a ref attribute it can only be dragged into other parent containers that match the same ref value as the current parent. Given the above markup, the child component can be dropped into the last container but not the container with the ref of 5678. However the second child element could be dropped into any of the parent components. This would easily be accomplished if droppable would allow a fu...