UICollectionView Assertion failure

Question

I m getting this error on performing insertItemsAtIndexPaths in UICollectionView

Assertion failure in:

-[UICollectionViewD         


        

Answer 1

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]]];
 }];

Answer 2

Here's a solution for this bug I've been using in my projects I thought I'd post here in case any found it valuable.

@interface FetchedResultsViewController ()

@property (nonatomic) NSMutableIndexSet *sectionsToAdd;
@property (nonatomic) NSMutableIndexSet *sectionsToDelete;

@property (nonatomic) NSMutableArray *indexPathsToAdd;
@property (nonatomic) NSMutableArray *indexPathsToDelete;
@property (nonatomic) NSMutableArray *indexPathsToUpdate;

@end

#pragma mark - NSFetchedResultsControllerDelegate


- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
    [self resetFetchedResultControllerChanges];
}


- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
           atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type
{
    switch(type)
    {
        case NSFetchedResultsChangeInsert:
            [self.sectionsToAdd addIndex:sectionIndex];
            break;

        case NSFetchedResultsChangeDelete:
            [self.sectionsToDelete addIndex:sectionIndex];
            break;
    }
}


- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
       atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
      newIndexPath:(NSIndexPath *)newIndexPath
{
    switch(type)
    {
        case NSFetchedResultsChangeInsert:
            [self.indexPathsToAdd addObject:newIndexPath];
            break;

        case NSFetchedResultsChangeDelete:
            [self.indexPathsToDelete addObject:indexPath];
            break;

        case NSFetchedResultsChangeUpdate:
            [self.indexPathsToUpdate addObject:indexPath];
            break;

        case NSFetchedResultsChangeMove:
            [self.indexPathsToAdd addObject:newIndexPath];
            [self.indexPathsToDelete addObject:indexPath];
            break;
    }
}


- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    if (self.sectionsToAdd.count > 0 || self.sectionsToDelete.count > 0 || self.indexPathsToAdd.count > 0 || self.indexPathsToDelete > 0 || self.indexPathsToUpdate > 0)
    {
        if ([self shouldReloadCollectionViewFromChangedContent])
        {
            [self.collectionView reloadData];

            [self resetFetchedResultControllerChanges];
        }
        else
        {
            [self.collectionView performBatchUpdates:^{

                if (self.sectionsToAdd.count > 0)
                {
                    [self.collectionView insertSections:self.sectionsToAdd];
                }

                if (self.sectionsToDelete.count > 0)
                {
                    [self.collectionView deleteSections:self.sectionsToDelete];
                }

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

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

                for (NSIndexPath *indexPath in self.indexPathsToUpdate)
                {
                    [self configureCell:[self.collectionView cellForItemAtIndexPath:indexPath]
                            atIndexPath:indexPath];
                }

            } completion:^(BOOL finished) {
                [self resetFetchedResultControllerChanges];
            }];
        }
    }
}

// This is to prevent a bug in UICollectionView from occurring.
// The bug presents itself when inserting the first object or deleting the last object in a collection view.
// http://stackoverflow.com/questions/12611292/uicollectionview-assertion-failure
// This code should be removed once the bug has been fixed, it is tracked in OpenRadar
// http://openradar.appspot.com/12954582
- (BOOL)shouldReloadCollectionViewFromChangedContent
{
    NSInteger totalNumberOfIndexPaths = 0;
    for (NSInteger i = 0; i < self.collectionView.numberOfSections; i++)
    {
        totalNumberOfIndexPaths += [self.collectionView numberOfItemsInSection:i];
    }

    NSInteger numberOfItemsAfterUpdates = totalNumberOfIndexPaths;
    numberOfItemsAfterUpdates += self.indexPathsToAdd.count;
    numberOfItemsAfterUpdates -= self.indexPathsToDelete.count;

    BOOL shouldReload = NO;
    if (numberOfItemsAfterUpdates == 0 && totalNumberOfIndexPaths == 1)
    {
        shouldReload = YES;
    }

    if (numberOfItemsAfterUpdates == 1 && totalNumberOfIndexPaths == 0)
    {
        shouldReload = YES;
    }

    return shouldReload;
}

- (void)resetFetchedResultControllerChanges
{
    [self.sectionsToAdd removeAllIndexes];
    [self.sectionsToDelete removeAllIndexes];
    [self.indexPathsToAdd removeAllObjects];
    [self.indexPathsToDelete removeAllObjects];
    [self.indexPathsToUpdate removeAllObjects];
}

Answer 3

Check that you're returning the correct value in numberOfSectionsInCollectionView:

The value I was using to calculate sections was nil, thus 0 sections. This caused the exception.

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
    NSInteger sectionCount = self.objectThatShouldHaveAValueButIsActuallyNil.sectionCount;

    // section count is wrong!

    return sectionCount;
}

Answer 4

In my case, the problem was the way I was creating my NSIndexPath. For example, to delete the 3rd cell, instead of doing :

NSIndexPath* indexPath = [NSIndexPath indexPathWithIndex:2];
[_collectionView deleteItemsAtIndexPaths:@[indexPath]];

I needed to do :

NSIndexPath* indexPath = [NSIndexPath indexPathForItem:2 inSection:0];
[_collectionView deleteItemsAtIndexPaths:@[indexPath]];

Answer 5

The workaround that actually works is to return a height of 0 if the cell at your supplementary view's index path is not there (initial load, you've deleted the row, etc). See my answer here:

https://stackoverflow.com/a/18411860/917104

Answer 6

I hit this problem myself. All the answers here seemed to present problems, except for Alex L's. Referring the update seemed to be tha answer. Here is my final solution:

- (void)addItem:(id)item {
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        if (!_data) {
            _data = [NSMutableArray arrayWithObject:item];
        } else {
            [_data addObject:item];
        }
        [_collectionView insertItemsAtIndexPaths:@[[NSIndexPath indexPathForItem:_data.count-1 inSection:0]]];
    }];
}