Nightmare with performBatchUpdates crash

Question

I am facing a nightmare of a crash during performBatchUpdates on a collection view.

The problem is basically this: I have a lot of images on a directory on a server. I want to show the thumbnails of those files on a collection view. But the thumbnail have to be downloaded from the server asynchronously. As they arrive they will be inserted on the collection view using something like this:

dispatch_async(dispatch_get_main_queue(),
             ^{
               [self.collectionView performBatchUpdates:^{

                 if (removedIndexes && [removedIndexes count] > 0) {
                   [self.collectionView deleteItemsAtIndexPaths:removedIndexes];
                 }

                 if (changedIndexes && [changedIndexes count] > 0) {
                   [self.collectionView reloadItemsAtIndexPaths:changedIndexes];
                 }

                 if (insertedIndexes && [insertedIndexes count] > 0) {
                   [self.collectionView insertItemsAtIndexPaths:insertedIndexes];
                 }

               } completion:nil];
             });

the problem is this (I think). Suppose that at time = 0, the collection view has 10 items. I then add 100 more files to the server. The application sees the new files and start downloading the thumbnails. As the thumbnails download they will be inserted on the collection view. But because the downloads can take different times and this download operation is asynchronous, at one point iOS will lost track of how many elements the collection has and the whole thing will crash with this catastrophic infamous message.

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid update: invalid number of items in section 0. The number of items contained in an existing section after the update (213) must be equal to the number of items contained in that section before the update (154), plus or minus the number of items inserted or deleted from that section (40 inserted, 0 deleted) and plus or minus the number of items moved into or out of that section (0 moved in, 0 moved out).'

The proof I have something fishy is going on is that if I print the count of items on the data set I see exactly 213. So, the dataset matches the correct number and the message is nonsense.

I have had this problem before, here but that was an iOS 7 project. Somehow the problem returned now on iOS 8 and the solutions there are not working and now the dataset IS IN SYNC.

Answer 1

It sounds like you need to do a bit extra work with batching which images have appeared for each animation group. From dealing with crashes like this before, the way performBatchUpdates works is

1. Before invoking your block, it double checks all the item counts and saves them by calling numberOfItemsInSection (this is the 154 in your error message).
2. It runs the block, tracking the inserts/deletes, and calculates what the final number of items should be based on the insertions and deletions.
3. After the block is run, it double checks the counts it calculated to the actual counts when it asks your dataSource numberOfItemsInSection (this is the 213 number). If it doesn't match, it will crash.

Based on your variables insertedIndexes and changedIndexes, you're pre-calculating which things need to show up based on the download response from server, and then running the batch. However I'm guessing your numberOfItemsInSection method is always just returning the 'true' count of items.

So if a download completes during step 2, when it performs the sanity check in '3', your numbers won't line up anymore.

Easiest solution: Wait until all files have downloaded, then do a single batchUpdates. Probably not the best user experience but it avoids this issue.

Harder solution: Perform batches as needed, and track which items have already shown up / are currently animating separately from the total number of items. Something like:

BOOL _performingAnimation;
NSInteger _finalItemCount;

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
    return _finalItemCount;
}

- (void)somethingDidFinishDownloading {
    if (_performingAnimation) {
        return;
    }
    // Calculate changes.
    dispatch_async(dispatch_get_main_queue(),
             ^{
                _performingAnimation = YES;
               [self.collectionView performBatchUpdates:^{

                 if (removedIndexes && [removedIndexes count] > 0) {
                   [self.collectionView deleteItemsAtIndexPaths:removedIndexes];
                 }

                 if (changedIndexes && [changedIndexes count] > 0) {
                   [self.collectionView reloadItemsAtIndexPaths:changedIndexes];
                 }

                 if (insertedIndexes && [insertedIndexes count] > 0) {
                   [self.collectionView insertItemsAtIndexPaths:insertedIndexes];
                 }

                 _finalItemCount += (insertedIndexes.count - removedIndexes.count);
               } completion:^{
                 _performingAnimation = NO;
               }];
             });
}

The only thing to solve after that would be to make sure you run one final check for leftover items if the last item to download finished during an animation (maybe have a method performFinalAnimationIfNeeded that you run in the completion block)