UICollectionView Assertion failure

I m getting this error on performing insertItemsAtIndexPaths in UICollectionView

Assertion failure in:

-[UICollectionViewData indexPathForItemAtGlobalIndex:], 
/SourceCache/UIKit/UIKit-2372/UICollectionViewData.m:442
2012-09-26 18:12:34.432  
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', 
reason: 'request for index path for global index 805306367 
when there are only 1 items in the collection view'

I have checked and my datasource contains only one element. Any insights on why this could happen? If more information is needed I can definitely provide that.

Solution 1:

I ran into this very same problem when inserting the first cell into a collection view. I fixed the problem by changing my code so that I call the UICollectionView

- (void)reloadData

method when inserting the first cell, but

- (void)insertItemsAtIndexPaths:(NSArray *)indexPaths

when inserting all other cells.

Interestingly, I also had a problem with

- (void)deleteItemsAtIndexPaths:(NSArray *)indexPaths

when deleting the last cell. I did the same thing as before: just call reloadData when deleting the last cell.

Solution 2:

Inserting section#0 just before inserting cells seems make UICollectionView happy.

NSArray *indexPaths = /* indexPaths of the cells to be inserted */
NSUInteger countBeforeInsert = _cells.count;
dispatch_block_t updates = ^{
    if (countBeforeInsert < 1) {
        [self.collectionView insertSections:[NSIndexSet indexSetWithIndex:0]];
    }
    [self.collectionView insertItemsAtIndexPaths:indexPaths];
};
[self.collectionView performBatchUpdates:updates completion:nil];

Solution 3:

I've posted a work around for this issue here: https://gist.github.com/iwasrobbed/5528897

In the private category at the top of your .m file:

@interface MyViewController ()
{
    BOOL shouldReloadCollectionView;
    NSBlockOperation *blockOperation;
}
@end

Then your delegate callbacks would be:

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
    shouldReloadCollectionView = NO;
    blockOperation = [NSBlockOperation new];
}

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

        case NSFetchedResultsChangeDelete: {
            [blockOperation addExecutionBlock:^{
                [collectionView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex]];
            }];
            break;
        }

        case NSFetchedResultsChangeUpdate: {
            [blockOperation addExecutionBlock:^{
                [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) {
                    shouldReloadCollectionView = YES;
                } else {
                    [blockOperation addExecutionBlock:^{
                        [collectionView insertItemsAtIndexPaths:@[newIndexPath]];
                    }];
                }
            } else {
                shouldReloadCollectionView = YES;
            }
            break;
        }

        case NSFetchedResultsChangeDelete: {
            if ([self.collectionView numberOfItemsInSection:indexPath.section] == 1) {
                shouldReloadCollectionView = YES;
            } else {
                [blockOperation addExecutionBlock:^{
                    [collectionView deleteItemsAtIndexPaths:@[indexPath]];
                }];
            }
            break;
        }

        case NSFetchedResultsChangeUpdate: {
            [blockOperation addExecutionBlock:^{
                [collectionView reloadItemsAtIndexPaths:@[indexPath]];
            }];
            break;
        }

        case NSFetchedResultsChangeMove: {
            [blockOperation addExecutionBlock:^{
                [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 (shouldReloadCollectionView) {
        [self.collectionView reloadData];
    } else {
        [self.collectionView performBatchUpdates:^{
            [blockOperation start];
        } completion:nil];
    }
}

Credit for this approach goes to Blake Watters.

Solution 4:

Here is a non-hack, docs-based answer to the problem. In my case, there was a condition according to which I would return a valid or a nil supplementary view from collectionView:viewForSupplementaryElementOfKind:atIndexPath:. After encountering the crash, I checked the docs and here is what they say:

This method must always return a valid view object. If you do not want a supplementary view in a particular case, your layout object should not create the attributes for that view. Alternatively, you can hide views by setting the hidden property of the corresponding attributes to YES or set the alpha property of the attributes to 0. To hide header and footer views in a flow layout, you can also set the width and height of those views to 0.

There are other ways to do this, but the quickest seems to be:

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewFlowLayout *)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section {
    return <condition> ? collectionViewLayout.headerReferenceSize : CGSizeZero;
}

Solution 5:

My collection view was getting items from two data sources and updating them caused this issue. My workaround was to queue the data update and collection view reload together:

[[NSOperationQueue mainQueue] addOperationWithBlock:^{

                //Update Data Array
                weakSelf.dataProfile = [results lastObject]; 

                //Reload CollectionView
                [weakSelf.collectionView reloadItemsAtIndexPaths:@[[NSIndexPath indexPathForItem:0 inSection:0]]];
 }];