title: Cannot get drag and drop to work onto NSCollectionView
body: <p>There is probably a simple mistake that I'm making, but I simply cannot get dropping of files onto an NSCollectionView to work even in the most basic way.</p>
  
  <p>In a test project, I have an NSCollectionView on a window, and the view controller is both its delegate and data source. I want to be able to drag files from the Finder onto this collection view.</p>
  
  <p>From reading the docs, all I should have to do is:</p>
  
  <p>Register for dragged type(s):</p>
  
  <pre><code>- (void)viewDidLoad {
  
      [super viewDidLoad];
      // Do any additional setup after loading the view.
  
      NSLog(@"Registering dragged types for collection view: %@", self.collectionView);
      [self.collectionView registerForDraggedTypes:@[NSFilenamesPboardType]];
      [self.collectionView setDraggingSourceOperationMask:NSDragOperationEvery forLocal:YES];
      [self.collectionView setDraggingSourceOperationMask:NSDragOperationEvery forLocal:NO];
  }
  </code></pre>
  
  <p>And then implement these two methods:</p>
  
  <pre><code>-(NSDragOperation)collectionView:(NSCollectionView *)collectionView validateDrop:(id&lt;NSDraggingInfo&gt;)draggingInfo proposedIndex:(NSInteger *)proposedDropIndex dropOperation:(NSCollectionViewDropOperation *)proposedDropOperation {
  
      NSLog(@"Validate drop: %@", draggingInfo);
  
      return NSDragOperationMove;
  }
  
  -(BOOL)collectionView:(NSCollectionView *)collectionView acceptDrop:(id&lt;NSDraggingInfo&gt;)draggingInfo index:(NSInteger)index dropOperation:(NSCollectionViewDropOperation)dropOperation {
  
      NSLog(@"Accept drop: %@", draggingInfo);
  
      return YES;
  }
  </code></pre>
  
  <p>But none of the two methods is ever called, when I try to drag an item onto the collection view, which makes me think that the <code>registerForDraggedTypes:</code> call is not working as expected.</p>
  
  <p>What can be the issue here? What else do I have to look into?</p>