Open Radar

 
Summary:

Manual animation of the UICollectionView's bounds does not properly create cells during the animation (as it does when setting the animated: argument to YES).

Steps to Reproduce:

I have tried a good number of approaches here. The closest solution is to do what we would normally do for UIScrollView animation (by setting the animated: argument to NO and wrapping it in a UIView animation block). This works perfectly fine for the scrollview. However, this screws with the UICollectionView creation process for some reason.

I have included an example below using two approaches. Each approach assumes that you have 4 sections with 4 items in each section. In addition, the animation assumes you are moving from 0,0 to 3,3.

Using Default Animation

Part of the issue here certainly seems tied to UICollectionView. If you take the following approach (using the default animation option) - all works fine:

[self.collectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:3 inSection:3]
                                atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally
                                        animated:YES];
When this is being executed, each cell in-between the current visible cells and the destination cell is created. I have included logging on the collectionView:cellForItemAtIndexPath: method:

2013-05-18 09:33:24.366 DEF-CV-Testing[75463:c07] Transition
Cell Created for Index Path: <NSIndexPath 0x8913f40> 2 indexes [0, 1]
Cell Created for Index Path: <NSIndexPath 0x75112e0> 2 indexes [0, 2]
Cell Created for Index Path: <NSIndexPath 0xfe1a6c0> 2 indexes [0, 3]
Cell Created for Index Path: <NSIndexPath 0x89159e0> 2 indexes [1, 0]
Cell Created for Index Path: <NSIndexPath 0x8a10e70> 2 indexes [1, 1]
Cell Created for Index Path: <NSIndexPath 0x7510d90> 2 indexes [1, 2]
Cell Created for Index Path: <NSIndexPath 0x75112a0> 2 indexes [1, 3]
Cell Created for Index Path: <NSIndexPath 0x8915a00> 2 indexes [2, 0]
Cell Created for Index Path: <NSIndexPath 0x75111c0> 2 indexes [2, 1]
Cell Created for Index Path: <NSIndexPath 0xfe17f30> 2 indexes [2, 2]
Cell Created for Index Path: <NSIndexPath 0xfe190c0> 2 indexes [2, 3]
Cell Created for Index Path: <NSIndexPath 0xfe16920> 2 indexes [3, 0]
Cell Created for Index Path: <NSIndexPath 0x75112a0> 2 indexes [3, 1]
Cell Created for Index Path: <NSIndexPath 0xfe1a4f0> 2 indexes [3, 2]
Cell Created for Index Path: <NSIndexPath 0x75142d0> 2 indexes [3, 3]
Using Custom Animation

When wrapping the scrollToItemAtIndexPath: method in a UIView animation block, items are not created correctly. See code sample here:

[UIView animateWithDuration:5.0 delay:0.0 options:0 animations:^{
    [self.collectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:3 inSection:3]
                                atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally
                                        animated:NO];
} completion:^(BOOL finished) {
    NSLog(@"Completed");
}];

The currently visible cells disappear and only the destination one is created. I have included the same logging on the collectionView:cellForItemAtIndexPath: method:

Transition
Cell Created for Index Path: <NSIndexPath 0x71702f0> 2 indexes [3, 3]
Completed

Expected Results:
In both of the methods described above, the UICollectionView should act identically (as far as how cells are created).

Actual Results:
When trying to perform the animation using a UIView animation block, the currently visible cell disappears and no new cells are created except for the cell which is the 'end' of the animation.