Triple Nested UIScrollView paging issue

Backstory
I have an iPad app that needs to allow the user to navigate through groups of images. Each group is laid out in its own vertical UIScrollView (paged) so the user can swipe up and down down to see each image. Each of the group UIScrollViews is placed in a single (only one exists in the app) outer horizontal UIScrollView (also paged). This works great.... I can swipe up and down to view the images in a group and swipe left and right to go to the next or previous group.

Problem
The problem started when I needed to add zooming for each image. I accomplished this by placing each image inside its own UIScrollView. When the image is zoomed I can pan around the image and when I get to the top or the bottom of the zoomed image the group's vertical UIScrollView pages to the next or previous image as expected. Unfortunately the outer horizontal scrollview will not page to the next group when the image is zoomed and I pan to the leftmost or rightmost edge.

Is there a better(more correct) approach than triple nesting UIScrollViews or can I somehow forward touches to the outer horizontal scrollview?

Any help or suggestions would be greatly appreciated.

hope i'm not too late but I think I have a solution for your problem.

Here you can find an Xcode project demonstrating the scrollview setup you have, your problem and the proposed solution: https://bitbucket.org/reydan/threescrollviews

Basically the solution was to add 1 pixel to the contentSize.width of the vertical scrollviews. This forces the vertical scrollview to scroll a little when you pan to the edge of the zoomed image. It scrolls a little and then continues to the next vertical scrollview.

If you download the project you will see that I've created some scrollviews in the viewDidLoad method. There, I create one horizontal scrollview containing 3 vertical scrollviews, each containing 5 images. Each image is actually incapsulated in a scrollview to enable per-image zooming. In total... triple nested scrollviews.

I've also left some colored borders so that I can easily see how each scrollview scrolls.

the magenta = horizontal scrollview
the white = vertical scrollview
the blue = the image scrollview (the one that contains the image and allows for zooming)
the red = the UIImageView

You will see that I've tagged each image scrollview with value 10. This is used in the implementation of - (UIView*)viewForZoomingInScrollView:(UIScrollView *)scrollView delegate method where I return nil unless the event came from one of the image scrollviews.

If you have any questions about the project I made feel free to ask.

In the end, I would like to say that this browsing method is a little quirky for me as I sometimes scroll in the unwanted direction. Often I think I flick my finger vertically only to find the scrollview going left or right because it interpreted some tiny horizontal movement I had.

The problem I found with paging enabled for both horizontal and vertical movement is that the scrollviews are direction-locked, or so it seemed to me.

EDIT:

Today I've investigated the problem even more. These are my conclusions:

it's not a problem with zooming, it's a problem with having larger content in the innermost scrollview than the visible area(you can try this by zooming or simply initializing the content size larger than the bounds). This enables panning inside the inner-most scrollview and completely changes the behaviour of the touch events.
the bounce for a scrollview flag affects the behaviour of the panning(dragging) gesture when it reaches the edges of the content. If bounces=false then your panning gesture will stop at the edge, not forwarding the drag event up the chain (and thus not scrolling the parent scrollviews to show you other images). If bounces=true then, when you reach the edge and continue to drag the events will be forwarded to the parent scrollview and that scrollview will also be dragged. However, I've found that the dragging while bouncing reduces the distance dragged by aproximately 50%. This also happens in the Photos app.
if you start the dragging while the innermost scrollview is at the edge of the content then the scrollview is smart and will forward all events to the parent scrollview.
for some reason, triple nested scrollviews are problematic as the events are simply not forwarded between the topmost and middle scrollviews while panning inside the innermost scrollview. I have no idea why.

My solution with that +1 pixel to the content size, partially solves the problem.

EDIT 2013

Boy, these scrollviews are something out of this world :(

After more than a year of searching (just kidding... it was actually 2 days) I think I found a good elegant solution to the triple nested scrollviews. I created a test project here: https://github.com/reydanro/TripleNestedScrollViews

Inside the app, there is a switch which you can use to test with/without the fix.

The setup I am using in my app is a little different than this question. I have 1 vertical paged scrollview. Inside it, I have multiple horizontal paged scrollviews. Inside some of the horizontal scrollviews I have another vertical paged scrollview.

Without the fix, once you get to the page with the inner-most scrollview you are pretty much stuck there as the vertical scrolling gestures are not forwarded to the outer-most scroll.

The fix is a custom UIGestureRecognizer that you need to add to the inner-most scrollviews. This recognizer follows touch events and if it detects a drag beyond the contentArea, then it will temporarily disable the rest of the scrollview's recognizers. This is the only method I discovered to make the scrollview forward the events up the chain

The gesture recognizer code is very rough with limited customization but should get the job done. At the moment I am focused on the app I develop, but will continue to update the repository.

PS: I haven't tested what happens with zoom but I see no reason why this method should not work (or be adapted to work).