Swift - UIViewControllerAnimatedTransitioning not transform as expected with Swift4 - ios

Question

I just upgraded to XCode9 with Swift4 today.
And I found that my UIViewControllerAnimatedTransitioning doesn't work as expected anymore.
The effect of this animation is that the fromView will scale down to 0.95 and the toView will slide in from the right side. The pop operation will do it reversely.
But now when I hit the back button of the NavigationBar, the start position of the toView isn't right. It displayed a original size toView and then scale up to 1.05.
Here's how I implement the transition animator.
// animate a change from one viewcontroller to another
func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
// get reference to our fromView, toView and the container view that we should perform the transition in
let container = transitionContext.containerView
let fromView = transitionContext.view(forKey: UITransitionContextViewKey.from)!
let toView = transitionContext.view(forKey: UITransitionContextViewKey.to)!
// set up from 2D transforms that we'll use in the animation
let offScreenRight = CGAffineTransform(translationX: container.frame.width, y: 0)
let offScreenDepth = CGAffineTransform(scaleX: 0.95, y: 0.95)
// start the toView to the right of the screen
if( presenting ){
toView.transform = offScreenRight
container.addSubview(fromView)
container.addSubview(toView)
}
else{
toView.transform = offScreenDepth
container.addSubview(toView)
container.addSubview(fromView)
}
// get the duration of the animation
// DON'T just type '0.5s' -- the reason why won't make sense until the next post
// but for now it's important to just follow this approach
let duration = self.transitionDuration(using: transitionContext)
// perform the animation!
// for this example, just slid both fromView and toView to the left at the same time
// meaning fromView is pushed off the screen and toView slides into view
// we also use the block animation usingSpringWithDamping for a little bounce
UIView.animate(withDuration: duration, delay: 0.0, options: .curveEaseOut, animations: {
if( self.presenting ){
fromView.transform = offScreenDepth
}
else{
fromView.transform = offScreenRight
}
toView.transform = .identity
}, completion: { finished in
transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
})
}
I didn't found anything special in this migration guide page.
https://swift.org/migration-guide-swift4/
What should I do to make the transition works again?

Answer 1

Before setting anything else, try to reset transformations of the fromView to identity:
// animate a change from one viewcontroller to another
func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
...
UIView.animate(withDuration: duration, delay: 0.0, options: .curveEaseOut, animations: {
if( self.presenting ){
fromView.transform = offScreenDepth
}
else{
fromView.transform = offScreenRight
}
toView.transform = .identity
}, completion: { finished in
fromView.transform = .identity
transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
})
....
}