Can't transition from UIScrollView to UIPanGesture in one movement

Question

What I want to achieve: http://youtu.be/SUWxVtpc0JA

What I can achieve: http://youtu.be/b1xH6Xsvxcg

I’m trying to do the same vertical parallax scrolling effect that the Year Walk Companion app does when you change from text page to menu.

I’ve been able to do this effect on UIScrollViews that only move horizontally, by using a UIPanGestureRecognizer to move up and down, (imagine swiping between nested arrays). However when I have a UIScrollView with the vertical scrolling it breaks.

When I scroll down the UIScrollView it moves as you would expect, same if you move up. However the issues start when I want the UIPanGestureRecognizer to come in and move the UIScrollView itself vertically, not it’s content. To do this so far I have subclassed UIScrollView to make a test on the function gestureRecognizerShouldBegin :

-(BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer{
    UIPanGestureRecognizer* pan = (UIPanGestureRecognizer*)gestureRecognizer;
    CGPoint panGestureTranslation = [pan translationInView:self];
    if (self.contentOffset.y == 0 && panGestureTranslation.y > 0 ) {
        return NO;
    }
    return YES;
}

It returns NO when my UIScrollView is at the top, and allows my other UIPanGesture for the parallax scrolling between pages to start again.

My problem is, when my UIScrollView is scrolling and reached the top, the function gestureRecognizerShouldBegin will not be called and force me to release the touch before allowing the UIPanGestureRecognizer to parallax scroll.

The question: How can I stop the handle of the touch by the UIScrollView and let the UIPanGestureRecognizer get the touch going up without release my finger from the screen? (like in the Year Walk Companion app).

Answer 1

gestureRecognizerShouldBegin isn't called because a new touch 'session' hasn't started. The existing one is still in progress and the gesture said it didn't want to start.

You could create your own gesture subclass which stays possible and only starts when the content offset < 0. Or you could use the scroll view delegate methods as your input and not use gestures at all.