Using [UICollectionView performBatchUpdates:] with a UICollectionViewFlowLayout that has UIDynamics

Question

Okay so basic overview. I have a UICollectionView that I needs to support adding and removing item through the performBatchUpdates: method. If I use a standard UICollectionViewFlowLayout, it works fine.

However, when I try to use a UICollectionViewFlowLayout that is powered by a UIDynamicAnimator, then I get a crash as soon as I call performBatchChanges.

On my custom UICollectionViewFlowLayout class, the prepareForCollectionViewUpdates: method never gets called. The custom UICollectionViewFlowLayout that I am using is based off of this sample.

The console output after the crash is...

*** Assertion failure in -[UICollectionViewData layoutAttributesForItemAtIndexPath:], /SourceCache/UIKit/UIKit-2903.23/UICollectionViewData.m:581
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'no UICollectionViewLayoutAttributes instance for -layoutAttributesForItemAtIndexPath: <NSIndexPath: 0xc000000000028096> {length = 2, path = 2 - 5}'
*** First throw call stack:
libc++abi.dylib: terminating with uncaught exception of type NSException

Any ideas?

Answer 1

try

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewLayoutAttributes *layoutAttributes = [self.dynamicAnimator layoutAttributesForCellAtIndexPath:indexPath];
    if(!layoutAttributes) {
        layoutAttributes = [super layoutAttributesForItemAtIndexPath:indexPath];
    }
    return layoutAttributes;
}

when you execute performBatchUpdates, [self.dynamicAnimator layoutAttributesForCellAtIndexPath: returns nil if cell which will be created by update is not visible. So just returns super(perhaps UICollectionViewFlowLayout)' s layoutAttributes for now. And when the cell about to be displayed, UIDynamicAnimator will do the job for you.