xcode CollectionViewController scrollToItemAtIndexPath not working

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.