title: NSCollectionView Drag and Dropping: Most Delegate Events Not Getting Called
body: <p>I have an NSCollectionView bound to an NSArrayController. I want to get drag and drop working, so I create a delegate and implement the methods </p>
  
  <pre><code>-(BOOL)collectionView:(NSCollectionView *)collectionView canDragItemsAtIndexes:(NSIndexSet *)indexes withEvent:(NSEvent*)event
  -(BOOL)collectionView:(NSCollectionView *)collectionView acceptDrop:(id &lt; NSDraggingInfo &gt;)draggingInfo index:(NSInteger)index dropOperation:(NSCollectionViewDropOperation)dropOperation
  -(NSDragOperation)collectionView:(NSCollectionView *)collectionView validateDrop:(id &lt; NSDraggingInfo &gt;)draggingInfo proposedIndex:(NSInteger *)proposedDropIndex dropOperation:(NSCollectionViewDropOperation *)proposedDropOperation
  -(NSArray *)collectionView:(NSCollectionView *)collectionView namesOfPromisedFilesDroppedAtDestination:(NSURL *)dropURL forDraggedItemsAtIndexes:(NSIndexSet *)indexes
  </code></pre>
  
  <p>I'm returning YES for the two BOOL methods, NSDragOperationMove for the validateDrop: method, and an empty array for the namesOfPromisedFilesDroppedAtDestination: method. I also have an NSLog statement as the first line in each method so I can see when they get called. </p>
  
  <p>Right now, the only method that gets called is canDragItemsAtIndexes: (where I return YES). I see that it gets called, but any further dragging just modifies the selection. The rest never get called. </p>
  
  <p>If I make the NSCollectionView not support selections, then not even that method gets called. </p>
  
  <p>I'm sure I'm missing something super obvious, but I can't figure out what it is. Has anyone gotten drag and drop working with NSCollectionViews and can shed some light? </p>