Is it possible to swap 2 cells in a UICollectionVi

Question

Say I have a UICollectionView using a UICollectionViewFlowLayout and the following format:

1 2 3
4 5 6
7 8 9

I touch cell 9 and drag it over cell 6, moving cell 9 to where cell 6 is and cell 6 to where cell 9 was:

1 2 3
4 5 9
7 8 6

Is there any way to easily move and animate cell 6 to where cell 9 is as well as move and animate cell 9 to where cell 6 is? Or do I have to subclass UICollectionViewLayout? If I have to subclass UICollectionViewLayout, how would this be done? I've tried explicitly setting the centers of the cells in

-(UICollectionViewLayoutAttributes*)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath

but to no success.

Answer 1

After much head banging, the solution I found is to use moveItemAtIndexPath:toIndexPath inside of performBatchUpdates:completion. According to the documentation for performBatchUpdates:completion:

You can use this method in cases where you want to insert, delete, reload or move cells around the collection view in one single animated operation, as opposed to in several separate animations. Use the blocked passed in the updates parameter to specify all of the operations you want to perform.

So, what I essentially did was

[self performBatchUpdates:^{
    [self moveItemAtIndexPath:fromIp toIndexPath:toIp];
    [self moveItemAtIndexPath:toIp toIndexPath:fromIp];
} completion:^(BOOL finished) {
    // update data source here
    // e.g [dataSource exchangeObjectAtIndex:fromIp.row withObjectAtIndex:toIp.row];
}];

Answer 2

Swift code:

        self.collectionView.performBatchUpdates({
            self.collectionView.moveItem(at: selectedIndexPath, to: targetIndexPath)
            self.collectionView.moveItem(at: targetIndexPath, to: selectedIndexPath)
        }, completion: nil)

Answer 3

Have you tried re-ordering the items directly using moveItemAtIndexPath:toIndexPath: ?

-make sure you actually re-order the underlying model collection so that the changes stick when you do a full reload next time.