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.

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.

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];

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.

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;
}

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]]];
 }];

Check that you are returning the correct number of elements in the UICollectionViewDataSource methods:

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section

and

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView

I ran into this problem as well. Here's what happened to me:

1. I subclassed UICollectionViewController and, on initWithCollectionViewLayout:, was initializing my NSFetchedResultsController.
2. Have a shared class fetch results from an NSURLConnection and parse the JSON string (different thread)
3. Loop through the feed and create my NSManagedObjects, add them to my NSManagedObjectContext, which has the main thread's NSManagedObjectContext as a parentContext.
4. Save my context.
5. Have my NSFetchedResultsController pick up the changes and queue them up.
6. On - (void)controllerDidChangeContent:, I would process the changes and apply them to my UICollectionView.

Intermittently, I would get the error the OP is getting and couldn't figure out why.

To fix this issue, I moved the NSFetchedResultsController initialization and performFetch to my - viewDidLoad method and this problem is now gone. No need to call [collectionView reloadData] or anything and all the animations are working properly.

Hope this helps!

It seems that the problem occurs when you either insert or move a cell to a section that contains a supplementary header or footer view (with UICollectionViewFlowLayout or a layout derived from that) and the section has a count of 0 cells before the insertion / move.

I could only circumvent the crash and still maintain the animations by having an empty and invisible cell in the section containing the supplementary header view like this:

1. Make - (NSInteger)collectionView:(UICollectionView *)view numberOfItemsInSection:(NSInteger)section return the actual cell `count + 1 for that section where the header view is.

2. In - (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath return

    if ((indexPath.section == YOUR_SECTION_WITH_THE_HEADER_VIEW) && (indexPath.item == [self collectionView:collectionView numberOfItemsInSection:indexPath.section] - 1)) {
            cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"EmptyCell" forIndexPath:indexPath];
    }
   

   ... an empty cell for that position. Remember to register the cell reuse in viewDidLoad or wherever you initialize your UICollectionView:

    [self.collectionView registerClass:[UICollectionReusableView class] forCellWithReuseIdentifier:@"EmptyCell"];
   

3. Use moveItemAtIndexPath: or insertItemsAtIndexPaths: without crashing.

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];
}

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]];

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;
}

Just for the record, I ran into the same problem and for me the solution was to remove the header (disable them in the .xib) and as them were not needed anymore removed this method. After that everything seems fine.

- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementary
ElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath

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]]];
    }];
}

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