UIViewPropertyAnimator different behaviour on iOS10 and iOS11 reversing an animation. Bug on `isReversed` and `fractionComplete` properties?

Question

THE PROBLEM
Running the same code on iOS10 and iOS11 my UIViewPropertyAnimator has a different behaviour just after changing of his .isReversed

Answer 1

I have tried many solutions, but no one didn't work for me. I wrote my solution and everything is fine now. My solution:

1. take an image of the screen and show it

2. finish animation

3. start new animation for old state

4. pause the animation and set progress (1 - original progress)

5. remove screen image and continue animation

   switch pan.state {
   ...
   case .ended, .cancelled, .failed:
       let velocity = pan.velocity(in: view)
       let reversed: Bool
   
       if abs(velocity.y) < 200 {
           reversed = progress < 0.5
       } else {
           switch state {
           case .shown:
               reversed = velocity.y < 0
           case .minimized:
               reversed = velocity.y > 0
           }
       }
   
       if reversed {
           let overlayView = UIScreen.main.snapshotView(afterScreenUpdates: false)
           view.addSubview(overlayView)
           animator?.stopAnimation(false)
           animator?.finishAnimation(at: .end)
           startAnimation(state: state.opposite)
           animator?.pauseAnimation()
           animator?.fractionComplete = 1 - progress
           overlayView.removeFromSuperview()
           animator?.continueAnimation(withTimingParameters: nil, durationFactor: 0.5)
       } else {
           animator?.continueAnimation(withTimingParameters: nil, durationFactor: 0)
       }
   

And the animation curve option must be linear.

    animator = UIViewPropertyAnimator(duration: 0.3, curve: .linear) {
        startAnimation()
    }