Can two UICollectionViews respond to a single gesture?

Question

I have two fullscreen child UICollectionViews. One is a transparent overlay on the other. I'd like them both to respond when I drag around the screen - both of them when it's a horizontal drag and only one of them when it's a vertical drag, a little like some media centre home screens. Is this possible without reimplementing the private UICollectionView gesture recognisers, and if so how?

If not then any pointers to example reimplementations would be appreciated.

Some things I know, or have tried:

I have a pan gesture recogniser on the View Controller with a Delayed Begin that can detect the vertical or horizontal movement before events are sent through to the views.

I know that simply forwarding events from my parent view's touchesBegan: etc. won't work because the touches' view property is set to my parent view, and UITouches can't be copied (naively at least) since they don't implement the NSCopying protocol. Perhaps I can synthesise suitable UITouch events and forward them?

I know I can send scrollToItemAtIndexPath:atScrollPosition:animated: messages manually but I'd prefer to have the natural drag, swipe and snap paging behaviour for the Collections.

Alternatively, is it possible to modify the private gesture recognisers' delegates and implement gestureRecognizer:shouldRecognizeSimultaneouslyWithGestureRecognizer: - without explicitly accessing private APIs - to allow both collections to see the touches? Is the responder chain smart enough to call this with gesture recognisers from two sibling views?

Another approach might be to manually control the overlay, and not manage it as a Collection View, but Collection Views seem like a more natural fit, and in theory provide the interactivity I'd like out of the box. The box, at the moment, seems to need a crowbar to get in!

This question seems similar (if less explicit), and has no answers. The other questions I've looked at all seem to be about adding pinch, or having subviews of collections also respond to gestures; not quite my situation.

I'm scratching my head a little, so thanks for any pointers.

Answer 1

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.