Assertion failure in -[UICollectionView _endItemAnimations] - objective-c

Question

I'm quite a newbie in objective-c programming. I'm trying to make my first app and I can't solve one issue.
I use UICollectionView with NSFetchedResultController in my app. The app downloads data from the web and uses NSOperationQueue to do it asynchronously. Everything works fine when I am testing my app in a simulator but when I'm testing it on iPad 3 quite often I'm getting this error.
* Assertion failure in -[UICollectionView _endItemAnimations], /SourceCache/UIKit/UIKit-2380.17/UICollectionView.m:2801
* 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 (28) must be equal to the number of items contained in that section before the update (26), plus or minus the number of items inserted or deleted from that section (1 inserted, 0 deleted) and plus or minus the number of items moved into or out of that section (0 moved in, 0 moved out).'
I do not know why because I set maxConcurrentOperationCount to 1 and I thought that in this case all operations resulting in inserting items to UICollectionView in order… What could be wrong and what should I change?
mateusz
If anyone is still interested that's my code:
I'm calling this method when my NSOperation is finished:
- (void)collectionViewUpdaterDidFinish:(CollectionViewUpdater *)updater {
[Artwork artworkWithBehanceInfo:self.projectsData
andImageData:updater.imageData
atIndex:updater.projectIndex
forIndexInDataBase:updater.indexPathInCollectionView.item withDataHelper:self.dataHelper];
NSIndexPath *indexPath = updater.indexPathInCollectionView;
[self.dataHelper fetchData];
[self.myCollectionView insertItemsAtIndexPaths:#[indexPath]];
[self.pendingOperations.downloadsInProgress removeObjectForKey:indexPath];
if ([self.dataHelper.fetchedResultsController.fetchedObjects count]==self.numberOfLastItemOnPage){
[self.refresh endRefreshing];
[self stopAnimation];
self.progressView.hidden = YES;
[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
}
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"ResultCell";
ResultCellView *cell = [collectionView dequeueReusableCellWithReuseIdentifier:CellIdentifier forIndexPath:indexPath];
Artwork *artwork = [self.dataHelper.fetchedResultsController objectAtIndexPath:indexPath];
cell.title.text = [artwork.title stringByAppendingString:#"\n "];
cell.owner.text = [NSString stringWithFormat:#"%#",artwork.owner];
cell.fields.text = artwork.fields;
cell.numberOfViews.text = [NSString stringWithFormat:#"%#",artwork.views];
cell.numberOfLikes.text = [NSString stringWithFormat:#"%#",artwork.likes];
cell.numberOfComments.text = [NSString stringWithFormat:#"%#",artwork.comments];
cell.publishedDate.text = artwork.publishedDate;
if ([artwork.hasThumbnail boolValue]) {
dispatch_queue_t downloadQueue = dispatch_queue_create("imagecache", NULL);
dispatch_async(downloadQueue, ^{
UIImage *productImage = [UIImage imageWithData:artwork.thumbnail];
CGSize imageSize = productImage.size;
UIGraphicsBeginImageContext(imageSize);
[productImage drawInRect:CGRectMake(0, 0, imageSize.width, imageSize.height)];
productImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
dispatch_async(dispatch_get_main_queue(), ^ {
cell.thumbnailImage.image = productImage;
cell.thumbnailImage.hidden = NO;
});
});
}
else {
NSLog(#"Has to download thumbnail");
cell.thumbnailImage.hidden = YES;
[self startOperationsForPhotoRecord:artwork atIndexPath:indexPath];
}
if ([self.dataHelper.fetchedResultsController.fetchedObjects count]==self.numberOfLastItemOnPage) {
self.cellRefreshed = YES;
}
// it must be redifined for collections in behance!
if (indexPath.item >=(self.numberOfLastItemOnPage-10) &&self.cellRefreshed &&([self.projectsData count]%12==0)) {
self.numberOfFirstItemOnPage = self.numberOfLastItemOnPage+1;
self.numberOfLastItemOnPage +=12;
[self downloadMoreProjects];
self.cellRefreshed = NO;
}
cell.selectedBackgroundView = [[UIView alloc] initWithFrame:CGRectZero];
cell.selectedBackgroundView.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.5f];
return cell;
}