The short answer is you can't, easily, anyway.
The approach that worked for me is a lot simpler, and cleaner: embed one collection view within another. The containing one is limited to horizontal scrolling, and the overlay one to vertical, both with paging turned on. Both share the same controller as their delegate and datasource, and - since a collection view is a subclass of scroll view - this also keeps track of which container and overlay page we're on in the scrollViewDidEndDecelerating:
method:
-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
if ([scrollView isEqual:containerCollection]) {
containerNumber = scrollView.contentOffset.x / scrollView.frame.size.width;
}
else {
overlayNumber = scrollView.contentOffset.y / scrollView.frame.size.height;
}
}
The only real bit of trickery was in my cellForItemAtIndexPath:
method where, when I instantiate the container cell, I need to register .xibs for reuse (each overlay is different) and use the remembered overlay page and issue both scrollToItemAtIndexPath:
and reloadItemsAtIndexPaths:
to the embedded overlay collection to get it to appear correctly.
I've managed to keep both cells as separate .xibs as well, with associated convenience classes for any extra data they need (and in the case of the container collection the overlay collection IBOutlet).
And not a gesture recogniser in sight.