NSTreeController and Drag and Drop

I've posted enough about this to cocoa-dev that I think it's time to actually write it down. In Tiger, using NSTreeController with NSOutlineView means drag-and-drop doesn't work as expected. This definitely does not mean you should abandon bindings, especially if you're using Core Data. There are two key things you need to know.

One: Unlike NSTableView, NSOutlineView expects you to implement all of the "required" datasource methods if you plan to use any of them. The simple solution is to provide dummy implementations:

#pragma mark -
#pragma mark NSOutlineView Hacks for Drag and Drop

- (BOOL) outlineView: (NSOutlineView *)ov
         isItemExpandable: (id)item { return NO; }

- (int)  outlineView: (NSOutlineView *)ov
         numberOfChildrenOfItem:(id)item { return 0; }

- (id)   outlineView: (NSOutlineView *)ov
         child:(int)index
         ofItem:(id)item { return nil; }

- (id)   outlineView: (NSOutlineView *)ov
         objectValueForTableColumn:(NSTableColumn*)col
         byItem:(id)item { return nil; }

Two: When using NSTreeController, the "item" objects handed to you in the drag-and-drop datasource methods are actually instances of the private _NSArrayControllerTreeNode class. There's no real way to communicate with them directly. This may seem like a dead end, but through the magic of runtime binding, we can go ahead and use some private API to get at the "real" object:

- (BOOL) outlineView: (NSOutlineView *)ov
          acceptDrop: (id )info
                item: (id)item
          childIndex: (int)index
{
    item = [item observedObject];
    
    // do whatever you would normally do with the item
}

This will generate a compiler warning but you can make it go away by making up some random NSObject category that declares -observedObject. As an extra bonus, when you're passed an array of items, you can take advantage of NSArray's -valueForKey: implementation:

NSArray *myItems = [items valueForKey:@"observedObject"];

That's all! I haven't tried this with the browser view, but I expect it's basically the same there.

Don't let this turn you off to NSTreeController in general. The things it does it does really well. In particular, recursive child relationships "just work," which is really useful when you have an entity with a relationship that references itself.

Some folks run for the hills at the first sign of trouble in bindings. In most cases, you'll end up with far less code by implementing a few workarounds. Make things easy on yourself and only write code where you have to.

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