Assertion Failure in UICollectionViewData indexPathForItemAtGlobalIndex

Question

I am using performBatchUpdates() to update my collection view, where I am doing a complete refresh, i.e. delete whatever was in it and re-insert everything. The batch updates are done as part of an Observer which is attached to a NSMutableArray (bingDataItems).

cellItems is the array containing items that are or will be inserted into the collection view.

Here is the code:

- (void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {     cultARunner *_cultARunner = [cultARunner getInstance];     if ( [[_cultARunner bingDataItems] count] ) {         [self.collectionView reloadData];         [[self collectionView] performBatchUpdates: ^{              int itemSize = [cellItems count];             NSMutableArray *arrayWithIndexPaths = [NSMutableArray array];              // first delete the old stuff             if (itemSize == 0) {                 [arrayWithIndexPaths addObject: [NSIndexPath indexPathForRow: 0 inSection: 0]];             }             else {                 for( int i = 0; i < cellItems.count; i++ ) {                     [arrayWithIndexPaths addObject:[NSIndexPath indexPathForRow:i inSection:0]];                 }             }             [cellItems removeAllObjects];             if(itemSize) {                 [self.collectionView deleteItemsAtIndexPaths:arrayWithIndexPaths];             }              // insert the new stuff             arrayWithIndexPaths = [NSMutableArray array];             cellItems = [_cultARunner bingDataItems];             if ([cellItems count] == 0) {                 [arrayWithIndexPaths addObject: [NSIndexPath indexPathForRow: 0 inSection: 0]];             }             else {                               for( int i = 0; i < [cellItems count]; i++ ) {                     [arrayWithIndexPaths addObject:[NSIndexPath indexPathForRow:i inSection:0]];                 }             }             [self.collectionView insertItemsAtIndexPaths:arrayWithIndexPaths];         }         completion:nil];     } } 

I get this error, but not all of the times (why ?)

2012-12-16 13:17:59.789 [16807:19703] *** Assertion failure in -[UICollectionViewData indexPathForItemAtGlobalIndex:], /SourceCache/UIKit_Sim/UIKit-2372/UICollectionViewData.m:442 2012-12-16 13:17:59.790 [16807:19703] DEBUG:    request for index path for global index 1342177227 when there are only 53 items in the collection view 

I checked the only thread that mentioned the same problem here: UICollectionView Assertion failure, but it is not very clear i.e. doing [collectionview reloadData] is not advisable in the performBatchUpdates() block.

Any suggestions on what might be going wrong here ?

Answer 1

Finally! Ok, here's what was causing this crash for me.

As previously noted, I was creating supplementary views in order to provide custom-styled section headers for my collection view.

The problem is this: it appears that the indexPath of a supplementary view MUST correspond to the indexPath of an extant cell in the collection. If the supplementary view's index path has no corresponding ordinary cell, the application will crash. I believe that the collection view attempts to retrieve information for a supplementary view's cell for some reason during the update procedure. It crashes when it cannot find one.

Hopefully this will solve your problem too!

Answer 2

This is the proper workaround to this crash:

Each of your supplementary views are associated with a certain index path. If you don't have a cell at that index path (initial load, you've deleted the row, etc), return a height of 0 for your supplementary view via your layout's delegate.

So, for a flow layout, implement UICollectionViewDelegateFlowLayout's

(CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section 

method (and the corresponding footer method, if you're using footers) with the following logic

if ( you-have-a-cell-at-the-row-for-this-section )     return myNormalHeaderSize; else return CGSizeMake( 0,0 ); 

Hope this helps!