Custom UIViewController transition bug in iOS 9 - ios

Question

I encountered a strange bug. I am just using iOS's custom transitioning method for UIViewControllers using UIViewControllerTransitioningDelegate together with an implementation of UIViewControllerAnimatedTransitioning. It all seems to work fine, until I do exactly the following:
open the app
present another view controller with my custom transition
rotate to landscape
dismiss the just presented view controller
That's all! What happens now is the following: I see a large black bar on the right side of the initial view controller (as if that controller's view wasn't rotated to landscape).
The funny thing is this only goes wrong in iOS 9, in iOS 8 everything seems to work just fine. Did anything change with custom transition API I don't know of? Or is this simply a really nasty iOS 9 bug? If anyone can tell me what I did wrong or if anyone can provide me with a workaround I would really appreciate that!
These classes reproduce the problem:
import UIKit
class ViewController: UIViewController, UIViewControllerTransitioningDelegate {
override func viewDidLoad() {
super.viewDidLoad()
let tapGestureRecognizer = UITapGestureRecognizer(target: self, action: "tap")
view.addGestureRecognizer(tapGestureRecognizer)
}
func tap() {
let controller = ModalViewController()
controller.transitioningDelegate = self
presentViewController(controller, animated: true, completion: nil)
}
func animationControllerForPresentedController(presented: UIViewController,
presentingController presenting: UIViewController,
sourceController source: UIViewController) -> UIViewControllerAnimatedTransitioning? {
return Transitioning()
}
func animationControllerForDismissedController(dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
return Transitioning()
}
}
The presented view controller:
import UIKit
class ModalViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
view.backgroundColor = UIColor.redColor()
let tapGestureRecognizer = UITapGestureRecognizer(target: self, action: "tap")
view.addGestureRecognizer(tapGestureRecognizer)
}
func tap() {
dismissViewControllerAnimated(true, completion: nil)
}
}
And finally the UIViewControllerAnimatedTransitioning implementation:
import UIKit
class Transitioning: NSObject, UIViewControllerAnimatedTransitioning {
func transitionDuration(transitionContext: UIViewControllerContextTransitioning?) -> NSTimeInterval {
return 0.5
}
func animateTransition(transitionContext: UIViewControllerContextTransitioning) {
let fromView = transitionContext.viewForKey(UITransitionContextFromViewKey)
let toView = transitionContext.viewForKey(UITransitionContextToViewKey)
let containerView = transitionContext.containerView()
if let fromView = fromView, toView = toView {
containerView?.addSubview(fromView)
containerView?.addSubview(toView)
toView.alpha = 0
UIView.animateWithDuration(0.5, animations: {
toView.alpha = 1
}, completion: {
finished in
transitionContext.completeTransition(true)
})
}
}
}

Answer 1

I generally use the following in animateTransition:
toView.frame = fromView.frame
FYI, you don't have to add fromView to the hierarchy, as it's already there.