UICollectionView Assertion failure

Question

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.

Answer 1

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;
}

Answer 2

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

Answer 3

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 4

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 5

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];

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]]];
    }];
}