Tell When a UIPageViewController is Scrolling (for Parallax Scrolling of an Image)

Question

I am trying to make an effect similar to that found in the new Yahoo weather app. Basically, each page in the UIPageViewController has a background image, and w

Answer 1

You are not supposed to change the delegate of the page view controller's scroll view: it can break its normal behaviour and/or not be supported later on.

Instead, you can:

1. Add a pan gesture to the page view controller's view:

   let panGesture = UIPanGestureRecognizer(target: self, action: #selector(panRecognized(gesture:)))
   view.addGestureRecognizer(panGesture)
   panGesture.delegate = self
   

2. Add the new function in order to know how the view is being scrolled.

   @objc func panRecognized(gesture: UIPanGestureRecognizer) {
       // Do whatever you need with the gesture.translation(in: view)
   }
   

3. Declare your ViewController as UIGestureRecognizerDelegate.

4. Implement this function:

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