UICollectionView + NSfetchedResultsControllerDelegate

gistfile1.m

/*
The MIT License (MIT)

Copyright (c) 2013 Lucien Constantino

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath
{
    NSMutableDictionary *change = [NSMutableDictionary new];
    switch(type)
    {
        case NSFetchedResultsChangeInsert:
            change[@(type)] = newIndexPath;
            break;
        case NSFetchedResultsChangeDelete:
            change[@(type)] = indexPath;
            break;
        case NSFetchedResultsChangeUpdate:
            change[@(type)] = indexPath;
            break;
        case NSFetchedResultsChangeMove:
            change[@(type)] = @[indexPath, newIndexPath];
            break;
    }
    [_objectChanges addObject:change];
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    NSLog(@"section changes: %i", [_sectionChanges count]);
    NSLog(@"obj changes: %i", [_objectChanges count]);
    
    if ([_sectionChanges count] > 0)
    {
        [_collectionView performBatchUpdates:^{
            
            for (NSDictionary *change in _sectionChanges)
            {
                [change enumerateKeysAndObjectsUsingBlock:^(NSNumber *key, id obj, BOOL *stop) {
                    
                    NSFetchedResultsChangeType type = [key unsignedIntegerValue];
                    switch (type)
                    {
                        case NSFetchedResultsChangeInsert:
                            [self.collectionView insertSections:[NSIndexSet indexSetWithIndex:[obj unsignedIntegerValue]]];
                            break;
                        case NSFetchedResultsChangeDelete:
                            [self.collectionView deleteSections:[NSIndexSet indexSetWithIndex:[obj unsignedIntegerValue]]];
                            break;
                        case NSFetchedResultsChangeUpdate:
                            [self.collectionView reloadSections:[NSIndexSet indexSetWithIndex:[obj unsignedIntegerValue]]];
                            break;
                    }
                }];
            }
        } completion:nil];
    }
    
    if ([_objectChanges count] > 0 && [_sectionChanges count] == 0)
    {
        [self.collectionView performBatchUpdates:^{
            
            for (NSDictionary *change in _objectChanges)
            {
                [change enumerateKeysAndObjectsUsingBlock:^(NSNumber *key, id obj, BOOL *stop) {
                    
                    NSFetchedResultsChangeType type = [key unsignedIntegerValue];
                    switch (type)
                    {
                        case NSFetchedResultsChangeInsert:
                            [self.collectionView insertItemsAtIndexPaths:@[obj]];
                            break;
                        case NSFetchedResultsChangeDelete:
                            [self.collectionView deleteItemsAtIndexPaths:@[obj]];
                            break;
                        case NSFetchedResultsChangeUpdate:
                            [self.collectionView reloadItemsAtIndexPaths:@[obj]];
                            break;
                        case NSFetchedResultsChangeMove:
                            [self.collectionView moveItemAtIndexPath:obj[0] toIndexPath:obj[1]];
                            break;
                    }
                }];
            }
        } completion:^(BOOL finished) {
            
        }];
    }
    
    [_sectionChanges removeAllObjects];
    [_objectChanges removeAllObjects];
}

Hi Blake,

Indeed, it seems to be a much cleaner implementation, but why are you trying to update the whole section instead just a item? something like this:

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


    UICollectionView * __weak collectionView = self.collectionView;
    switch (type) {
        case NSFetchedResultsChangeInsert: {
            [self.collectionReloadOperation addExecutionBlock:^{
                [collectionView insertItemsAtIndexPaths:@[newIndexPath]];
            }];
            break;
        }case NSFetchedResultsChangeDelete: {
            [self.collectionReloadOperation addExecutionBlock:^{
                [collectionView deleteItemsAtIndexPaths:@[indexPath]];
            }];
            break;
        }case NSFetchedResultsChangeUpdate: {
            [self.collectionReloadOperation addExecutionBlock:^{
                [collectionView reloadItemsAtIndexPaths:@[indexPath]];
            }];
            break;
        }case NSFetchedResultsChangeMove: {
            [self.collectionReloadOperation addExecutionBlock:^{
                [collectionView moveItemAtIndexPath:indexPath toIndexPath:newIndexPath];
            }];
            break;
        }default:
            break;
    }
}

Also, the documentation for NSBlockOperation (about addExecutionBlock ) says:

The specified block should not make any assumptions about its execution environment.

Do you think it's ok to execute that operation inside performBatchUpdates?

Thanks,
StaS

Blake's approach is indeed pretty elegant, however anyone using it should be aware that NSBlockOperation doesn't give any guarantees on the block execution context (used thread). Since we're dealing with UIKit here, it's crucial to make sure that the blocks are executed on the main thread, a requirement NSBlockOperation fails to satisfy (simply verifiable by inserting assert([NSThread isMainThread]); inside the blocks).

Here's a simple solution that should be much safer, and is just as straightforward.

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    self.updateBlocks = [NSMutableArray new];
}

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
    __weak UICollectionView *collectionView = self.galleryView;
    switch (type) {
        case NSFetchedResultsChangeInsert: {
            [self.updateBlocks addObject:^{
                [collectionView insertItemsAtIndexPaths:@[newIndexPath]];
            }];
            break;
        }
        case NSFetchedResultsChangeDelete: {
            [self.updateBlocks addObject:^{
                [collectionView deleteItemsAtIndexPaths:@[indexPath]];
            }];
            break;
        }
        case NSFetchedResultsChangeUpdate: {
            [self.updateBlocks addObject:^{
                [collectionView reloadItemsAtIndexPaths:@[indexPath]];
            }];
            break;
        }
        case NSFetchedResultsChangeMove: {
            [self.updateBlocks addObject:^{
                [collectionView moveItemAtIndexPath:indexPath toIndexPath:newIndexPath];
            }];
            break;
        }
        default:
            break;
    }
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [self.galleryView performBatchUpdates:^{
        for (void (^updateBlock)(void) in self.updateBlocks) {
            updateBlock();
        }
    } completion:nil];
}

@matej very nice work!

[self.collectionView performBatchUpdates:^{
    [self.blockOperation start];
} completion:^(BOOL finished) {
    // Do whatever
}];

this way always results in crash.

but the following way did works

[self.galleryView performBatchUpdates:^{
        for (void (^updateBlock)(void) in self.updateBlocks) {
            updateBlock();
        }
 } completion:nil];

I just implemented that with Swift. So I would like to share my implementation.
First initialise an array of NSBlockOperations:

    var blockOperations: [NSBlockOperation] = []

In controller will change, re-init the array:

func controllerWillChangeContent(controller: NSFetchedResultsController) {
    blockOperations.removeAll(keepCapacity: false)
}

In the did change object method:

    func controller(controller: NSFetchedResultsController, didChangeObject anObject: AnyObject, atIndexPath indexPath: NSIndexPath?, forChangeType type: NSFetchedResultsChangeType, newIndexPath: NSIndexPath?) {

    if type == NSFetchedResultsChangeType.Insert {
        println("Insert Object: \(newIndexPath)")

        blockOperations.append(
            NSBlockOperation(block: { [weak self] in
                if let this = self {
                    this.collectionView!.insertItemsAtIndexPaths([newIndexPath!])
                }
            })
        )
    }
    else if type == NSFetchedResultsChangeType.Update {
        println("Update Object: \(indexPath)")
        blockOperations.append(
            NSBlockOperation(block: { [weak self] in
                if let this = self {
                    this.collectionView!.reloadItemsAtIndexPaths([indexPath!])
                }
            })
        )
    }
    else if type == NSFetchedResultsChangeType.Move {
        println("Move Object: \(indexPath)")

        blockOperations.append(
            NSBlockOperation(block: { [weak self] in
                if let this = self {
                    this.collectionView!.moveItemAtIndexPath(indexPath!, toIndexPath: newIndexPath!)
                }
            })
        )
    }
    else if type == NSFetchedResultsChangeType.Delete {
        println("Delete Object: \(indexPath)")

        blockOperations.append(
            NSBlockOperation(block: { [weak self] in
                if let this = self {
                    this.collectionView!.deleteItemsAtIndexPaths([indexPath!])
                }
            })
        )
    }
}

In the did change section method:

func controller(controller: NSFetchedResultsController, didChangeSection sectionInfo: NSFetchedResultsSectionInfo, atIndex sectionIndex: Int, forChangeType type: NSFetchedResultsChangeType) {

    if type == NSFetchedResultsChangeType.Insert {
        println("Insert Section: \(sectionIndex)")

        blockOperations.append(
            NSBlockOperation(block: { [weak self] in
                if let this = self {
                    this.collectionView!.insertSections(NSIndexSet(index: sectionIndex))
                }
            })
        )
    }
    else if type == NSFetchedResultsChangeType.Update {
        println("Update Section: \(sectionIndex)")
        blockOperations.append(
            NSBlockOperation(block: { [weak self] in
                if let this = self {
                    this.collectionView!.reloadSections(NSIndexSet(index: sectionIndex))
                }
            })
        )
    }
    else if type == NSFetchedResultsChangeType.Delete {
        println("Delete Section: \(sectionIndex)")

        blockOperations.append(
            NSBlockOperation(block: { [weak self] in
                if let this = self {
                    this.collectionView!.deleteSections(NSIndexSet(index: sectionIndex))
                }
            })
        )
    }
}

And finally, in the did controller did change content method:

func controllerDidChangeContent(controller: NSFetchedResultsController) {        
    collectionView!.performBatchUpdates({ () -> Void in
        for operation: NSBlockOperation in self.blockOperations {
            operation.start()
        }
    }, completion: { (finished) -> Void in
        self.blockOperations.removeAll(keepCapacity: false)
    })
}

I personally added some code in the deinit method as well, in order to cancel the operations when the ViewController is about to get deallocated:

deinit {
    // Cancel all block operations when VC deallocates
    for operation: NSBlockOperation in blockOperations {
        operation.cancel()
    }

    blockOperations.removeAll(keepCapacity: false)
}

@AppsTitude The comments above your Swift implementation made a convincing case against using NSBlockOperation.

Just implemented this pod: https://github.com/k06a/ABCollectionViewFRC