NSFetchedResultsContollerDelegate for CollectionView

Question

I'd like to use the NSFetchedResultsControllerRelegate in a CollectionViewController. Therefore I just changed the method for the TableViewController for the CollectionView.

(void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
       atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {

    switch(type) {
        case NSFetchedResultsChangeInsert:
            [self.collectionView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex]];
            break;

        case NSFetchedResultsChangeDelete:
            [self.collectionView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] ];

       break;
    }
}


(void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
   atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
  newIndexPath:(NSIndexPath *)newIndexPath {

  UICollectionView *collectionView = self.collectionView;

  switch(type) {

    case NSFetchedResultsChangeInsert:
        [collectionView insertItemsAtIndexPaths:[NSArray arrayWithObject:newIndexPath]];
        break;

    case NSFetchedResultsChangeDelete:
        [collectionView deleteItemsAtIndexPaths:[NSArray arrayWithObject:indexPath]];
        break;

    case NSFetchedResultsChangeUpdate:
        [collectionView reloadItemsAtIndexPaths:[NSArray arrayWithObject:indexPath]];
        break;

    case NSFetchedResultsChangeMove:
        [collectionView deleteItemsAtIndexPaths:[NSArray arrayWithObject:indexPath]];
        [collectionView insertItemsAtIndexPaths:[NSArray arrayWithObject:newIndexPath]];
        break;
  }
}

(void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
   [self.collectionView reloadData];
}

But I do not know how to handle the WillChangeContent (beginUpdates for TableView) and DidChangeContent (endUpdates for TableView) for a CollectionView.

Everything works fine except when I move one item from one section to another section. Then I get the following error.

This is usually a bug within an observer of NSManagedObjectContextObjectsDidChangeNotification. Invalid update: invalid number of items in section 0....

Any idea how can I solve this issue?

Answer 1

Combining a fetched results controller with a collection view is a bit tricky. The problem is explained in

• http://ashfurrow.com/blog/how-to-use-nsfetchedresultscontroller-with-uicollectionview

If you're looking for how to get around the NSInternalInconsistencyException runtime exception with UICollectionView, I have an example on GitHub detailing how to queue updates from the NSFetchedResultsControllerDelegate.

The problem is that the existing UITableView class uses beginUpdates and endUpdates to submit batches to the table view. UICollectionView has a new performBatchUpdates: method, which takes a block parameter to update the collection view. That's sexy, but it doesn't work well with the existing paradigm for NSFetchedResultsController.

Fortunately, that article also provides a sample implementation:

• https://github.com/AshFurrow/UICollectionView-NSFetchedResultsController

From the README:

This is an example of how to use the new UICollectionView with NSFetchedResultsController. The trick is to queue the updates made through the NSFetchedResultsControllerDelegate until the controller finishes its updates. UICollectionView doesn't have the same beginUpdates and endUpdates that UITableView has to let it work easily with NSFetchedResultsController, so you have to queue them or you get internal consistency runtime exceptions.