Changed implementation to use array of raw blocks instead of NSBlockOperation so it's easy to run them on the main thread.

NSFetchedResultsController-UICollectionView-Example.m

// Properties:

@property (nonatomic) BOOL shouldReloadCollectionView;
@property (strong, nonatomic) NSMutableArray *collectionViewUpdateBlocks;

// NSFetchedResultsController Delegate Methods:

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
    self.shouldReloadCollectionView = NO;
    self.collectionViewUpdateBlocks = [NSMutableArray array];
    
}

- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id<NSFetchedResultsSectionInfo>)sectionInfo
           atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type
{
    __weak UICollectionView *collectionView = self.collectionView;
    switch (type) {
        case NSFetchedResultsChangeInsert: {
            [self.collectionViewUpdateBlocks addObject:^{
                [collectionView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex]];
            }];
            break;
        }
            
        case NSFetchedResultsChangeDelete: {
            [self.collectionViewUpdateBlocks addObject:^{
                [collectionView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex]];
            }];
            break;
        }
            
        case NSFetchedResultsChangeUpdate: {
            [self.collectionViewUpdateBlocks addObject:^{
                [collectionView reloadSections:[NSIndexSet indexSetWithIndex:sectionIndex]];
            }];
            break;
        }
            
        default:
            break;
    }
}

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath
{
    __weak UICollectionView *collectionView = self.collectionView;
    switch (type) {
        case NSFetchedResultsChangeInsert: {
            if ([self.collectionView numberOfSections] > 0) {
                if ([self.collectionView numberOfItemsInSection:indexPath.section] == 0) {
                    self.shouldReloadCollectionView = YES;
                } else {
                    [self.collectionViewUpdateBlocks addObject:^{
                        [collectionView insertItemsAtIndexPaths:@[newIndexPath]];
                    }];
                }
            } else {
                self.shouldReloadCollectionView = YES;
            }
            break;
        }
            
        case NSFetchedResultsChangeDelete: {
            if ([self.collectionView numberOfItemsInSection:indexPath.section] == 1) {
                self.shouldReloadCollectionView = YES;
            } else {
                [self.collectionViewUpdateBlocks addObject:^{
                    [collectionView deleteItemsAtIndexPaths:@[indexPath]];
                }];
            }
            break;
        }
            
        case NSFetchedResultsChangeUpdate: {
            [self.collectionViewUpdateBlocks addObject:^{
                [collectionView reloadItemsAtIndexPaths:@[indexPath]];
            }];
            break;
        }
            
        case NSFetchedResultsChangeMove: {
            [self.collectionViewUpdateBlocks addObject:^{
                [collectionView moveItemAtIndexPath:indexPath toIndexPath:newIndexPath];
            }];
            break;
        }
            
        default:
            break;
    }
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    // Checks if we should reload the collection view to fix a bug @ http://openradar.appspot.com/12954582
    if (self.shouldReloadCollectionView) {
        [self.collectionView reloadData];
    } else {
        [self.collectionViewUpdateBlocks enumerateObjectsUsingBlock:^(void (^updateBlock)(void), NSUInteger idx, BOOL * _Nonnull stop) {
            updateBlock();
        }];
    }
}