Conditional dropping with DnD.

Question

I need some help here, please.
I've seen the examples for drag and drop implementations. However most of the implementations that need to do determination of whether the item is droppable assume that you have a reference to the object being transferred.
I my case I am dragging a set of files from the file system to a tree, but they can only be dropped on certain nodes. It seems that you only have the DataFlavors available from the DropTargetDragEvent, but no direct reference to the data. Assuming that .exe files could only be dragged to one type of node, and .bat files to another, how can I make the dragOver methods do the determination against that actual data being transferred as opposed to simple just the type of data?
Any suggestions on this?
Thanks,
Bill

Answer 1

I guess I'm not making my problem clear, or I'm missing something.
The decision is not made simply on the basis of the DataFlavors. Instead it is based on a combination of the DataFlavor, the drag object, and the drop target selection.
Maybe this is a better example.
I want to be able to drag nodes inside of a tree. However the rule is that you can't drop a node onto a child of itself.
In order to evaluate the rule you need to know if the DataFlavor is accepted, AND whether the node we are dragging is a parent of the potential drop node. I can't make this determination without having a reference to the Transferable object being dragged.
Now, I've seen this done in trees where in the drag gesture they cache internally a reference to the node that was selected when the drag gesture was recognized. I could do this, but it seems like a hack to me, as there can be other drag sources where you might still want the same type of drop potential information, such as the native file system, where you don't have access to caching the drag object.
The best solution I have been able to come up with is to create my own "clipboard" type object where I cache the selected node for a drag if the drag originated from the tree. When I recieve dragEvents in the tree of the appropriate data flavor I would go out to the "clipboard" and get the data and use it to see if the drop should be allowed at that point.
What really frustrates me though is that the Transferable object simply isn't available to the DropTargetDragEvent objects. It seems overly simplistic to me to believe that all potiental determinations of the ability to drop something can be made on the DataFlavor alone. The examples even show for checking the data "during" a drop process, but it seems stupid to indicate a drop is acceptable to the user and then fail when they actually try to do the operation.
Bill