iOS UICollectionView, how to animate datasource change? - ios

Question

I'm working with a UICollection view and would like to animate changes to it's datasource. New datasource shares a lot of items with the old one, and I would like to animate insertion of new items and deletion of items that are no longer in the datasource, while keeping all other cells intact:
Current: [1,2,8,9]
Updated: [1,2,3,9]
Changes: insert 3, delete 8
Are there examples of swapping datasources for UICollectionView with animation?
I tried this, which causes the collection view to flash, but no animations are being played:
[self.collectionView performBatchUpdates:^{
[self.collectionView reloadSections:[NSIndexSet indexSetWithIndex:0]];
} completion:^(BOOL finished) {
}];
This method complains about numbers of items not matching:
-(void)setDatasource:(NSMutableArray *)datasource
{
NSMutableArray* _datasourceBackup = _datasource;
//find out what is being added and deleted
NSMutableArray* itemsToDelete = [NSMutableArray arrayWithArray:_datasource];
DLog(#"delete raw: %lu", (unsigned long)itemsToDelete.count);
[itemsToDelete removeObjectsInArray:datasource];
DLog(#"delete filtered: %lu", (unsigned long)itemsToDelete.count);
NSMutableArray* itemsToAdd = [NSMutableArray arrayWithArray:datasource];
DLog(#"add raw: %lu", (unsigned long)itemsToAdd.count);
[itemsToAdd removeObjectsInArray:_datasource];
DLog(#"add filtered: %lu", (unsigned long)itemsToAdd.count);
//1 update datasource
_datasource = datasource;
if(_datasourceBackup.count == 0)
{
//original load
[self.collectionView reloadData];
}else
{
[self.collectionView performBatchUpdates:^{
//convert items to add/delete into index paths and give them to collection view to animate
NSMutableArray* indexPathsToDelete = [NSMutableArray array];
for(NSString* identifier in itemsToDelete)
{
NSInteger index = [_datasourceBackup indexOfObject:identifier];
[indexPathsToDelete addObject: [NSIndexPath indexPathForRow:index inSection:0]];
}
if(indexPathsToDelete.count > 0 )
{
[self.collectionView deleteItemsAtIndexPaths:indexPathsToDelete];
}
NSMutableArray* indexPathsToInsert = [NSMutableArray arrayWithCapacity:itemsToAdd.count];
for(NSString* identifier in itemsToAdd)
{
NSInteger index = [_datasource indexOfObject:identifier];
[indexPathsToInsert addObject: [NSIndexPath indexPathForRow:index inSection:0]];
}
if(indexPathsToInsert.count > 0 )
{
[self.collectionView insertItemsAtIndexPaths:indexPathsToInsert];
}
} completion:nil];
}
}