Assertion Failure in UICollectionViewData indexPat

问题:

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 ?

回答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!

回答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!

回答3:

reloadData doesn't work for me, because the whole purpose of using performBatchUpdates is to get the changes animated. If you use reloadData you only refresh the data, but without animations.

So suggestions of "replace performBatchUpdates with reloadData" is pretty much saying "give up on what you're trying to do."

I'm sorry, I'm just frustrated because this error keeps coming up for me while I'm trying to do some great animated updates and my model is 100 % correct, it's some iOS magic inside getting broken and forcing me to change my solutions completely.

My opinion is that Collection Views are still buggy and can't do complicated animated refreshes, even though they should be able to. Because this used to be the same thing for Table Views but those are now pretty stable (it took time, though).

//Edit (Sep 1, 2013) The reported bug is closed now so this issues seems to have been resolved by Apple already.

回答4:

I have been having the same problem.

I have tried a number of variations, but the final one that seems to work is [self.collectionView reloadData], where "self.collectionView"is the name of your collection view.

I have tried the following methods, straight from the "UICollectionView Class Reference": inserting, moving, and deleting items.

These were used at first, to "move" the item from one section to another.

• deleteItemsAtIndexPaths:

• insertItemsAtIndexPaths:

Next, I tried moveItemAtIndexPath:toIndexPath:.

They all produced the following error:

Assertion failure in -[UICollectionViewData indexPathForItemAtGlobalIndex:], /SourceCache/UIKit_Sim/UIKit-2372/UICollectionViewData.m:442

So, try the "reloadData" method.

回答5:

If you remove the last cell from a section containing header/footer the bug appears.

I tried to return nil for header/footer size/element at that time and this sometimes fixes the issue.

Options:

1. Reload the whole table view instead of animating the removal of the last item.
2. Add an additional invisible, basic cell with a size less than 1.

回答6:

A cheeseball mistake that can lead to this error is reusing the same UICollectionViewFlowLayout on multiple collectionViews on the same viewcontroller! Just init different flowLayouts for each collectionview and you'll be good to go!

回答7:

I ran into this problem when I delete one of the cells from my collection view.

The problem was that I use a custom layout, and the call layoutAttributesForElementsInRect was returning more than the number of cells in the collection view after the delete.

Apparently UICollectionView just iterates through the array returned by the method without checking the number of cells.

Modifying the method to return the same number of layout attributes solved the crash.

回答8:

I still couldn't figure out how the global index was incremented so much, but I solved my problem by inserting a temporary item in the underlying datasource array i.e. cellItems and calling [self.collectionview reloadData] in viewDidLoad().

This inserts a placeholder cell temporarily in the collection view until I trigger the actual process using performBatchUpdates().