xcode CollectionViewController scrollToItemAtIndexPath not working

Question

I have created a CollectionView Control and filled it with images. Now I want to scroll to item at a particular index on start. I have tried out scrollToItemAtIndexPath as follows:

[self.myFullScreenCollectionView scrollToItemAtIndexPath:indexPath 
atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:YES];

However, I am getting following exception. Could anyone guide me on where am I going wrong.

2013-02-20 02:32:45.219 ControlViewCollection1[1727:c07] *** Assertion failure in 
-[UICollectionViewData layoutAttributesForItemAtIndexPath:], /SourceCache/UIKit_Sim/UIKit-2380.17
/UICollectionViewData.m:485 2013-02-20 02:32:45.221 ControlViewCollection1[1727:c07] must return a 
UICollectionViewLayoutAttributes instance from -layoutAttributesForItemAtIndexPath: for path 
<NSIndexPath 0x800abe0> 2 indexes [0, 4]

Answer 1

Whether it's a bug or a feature, UIKit throws this error whenever scrollToItemAtIndexPath:atScrollPosition:Animated is called before UICollectionView has laid out its subviews.

As a workaround, move your scrolling invocation to a place in the view controller lifecycle where you're sure it has already computed its layout, like so:

@implementation CollectionViewControllerSubclass

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    // scrolling here doesn't work (results in your assertion failure)
}

- (void)viewDidLayoutSubviews
{
    [super viewDidLayoutSubviews];

    NSIndexPath *indexPath = // compute some index path

    // scrolling here does work
    [self.collectionView scrollToItemAtIndexPath:indexPath
                                atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally
                                        animated:YES];
}

@end

At the very least, the error message should probably be more helpful. I've opened a rdar://13416281; please dupe.

Answer 2

If you are trying to scroll when the view controller is loading, make sure to call layoutIfNeeded on the UICollectionView before you call scrollToItemAtIndexPath. This is better than putting the scroll logic in viewDidLayoutSubviews because you won't perform the scroll operation every time the parent view's subviews are laid out.