Intercepting pan gestures over a UIScrollView breaks scrolling

Question

I have a vertically-scrolling UIScrollView. I want to also handle horizontal pans on it, while allowing the default vertical scroll behavior. I've put a transparent UIView over the scroll view, and added a pan gesture recognizer to it. This way I can get the pans just fine, but then the scroll view doesn't receive any gestures.

I've implemented the following UIPanGestureRecognizerDelegate methods, hoping to limit my gesture recognizer to horizontal pans only, but that didn't help:

- (BOOL)gestureRecognizerShouldBegin:(UIPanGestureRecognizer *)gestureRecognizer {
    // Only accept horizontal pans here.
    // Leave the vertical pans for scrolling the content.
    CGPoint translation = [gestureRecognizer translationInView:self.view];
    BOOL isHorizontalPan = (fabsf(translation.x) > fabsf(translation.y));
    return  isHorizontalPan;
}

- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer {
    return (otherGestureRecognizer == _scrollView.panGestureRecognizer);
}

Answer 1

OK, I figured it out. I needed to do 2 things to make this work:

1) Attach my own pan recognizer to the scroll view itself, not to another view on top of it.

2) This UIGestureRecognizerDelegate method prevents the goofy behavior that happens when both the default scrollview and my own one are invoked simultaneously.

-(BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer {
    return YES;
}

Answer 2

I had the same problem to solve and I did this:

1) Attach my own pan recognizer to the scroll view.

2) Return YES on: – gestureRecognizer:shouldRecognizeSimultaneouslyWithGestureRecognizer:

This will allow both gestures to work. So what that means is that on vertical scroll, both your panGesture delegate and scrollView Delegate will be fired. If it is a horizontal scroll, it will only call your panGesture delegate.

3) in my panGesture delegate, detect if it is a horizontal scroll, if it is not, ignore.

Answer 3

Swift answer:

let scrollViewPanGesture = UIPanGestureRecognizer(target: self, action: #selector(onPan(_:)))
scrollViewPanGesture.delegate = self
scrollView.addGestureRecognizer(scrollViewPanGesture)

extension ViewController: UIGestureRecognizerDelegate {
    func gestureRecognizer(_ gestureRecognizer: UIGestureRecognizer, shouldRecognizeSimultaneouslyWith otherGestureRecognizer: UIGestureRecognizer) -> Bool {
        return true
    }
}