UIView custom transition snaps back on completion - swift

Question

I have implemented a class BubbleAnimator, that should create a bubble-like transition between views and added it through the UIViewControllerTransitioningDelegate-protocol. The presenting animation works fine so far (that's why I haven't added all the code for this part).
But on dismissing the view, the 'fromViewController' flashes up at the very end of the animation. After this very short flash, the correct toViewController is displayed again, but this glitch is very annoying.
The following is the relevant animateTransition-method:
//Get all the necessary views from the context
let containerView = transitionContext.containerView()
let fromViewController = transitionContext.viewControllerForKey(UITransitionContextFromViewControllerKey)
let toViewController = transitionContext.viewControllerForKey(UITransitionContextToViewControllerKey)
//Presenting
if self.reverse == false {
//Add the destinationvc as subview
containerView!.addSubview(fromViewController!.view)
containerView!.addSubview(toViewController!.view)
/*...Animating the layer goes here... */
//Dismissing
} else {
containerView!.addSubview(toViewController!.view)
containerView!.addSubview(fromViewController!.view)
//Init the paths
let circleMaskPathInitial = UIBezierPath(ovalInRect: self.originFrame)
let extremePoint = CGPoint(x: originFrame.origin.x , y: originFrame.origin.y - CGRectGetHeight(toViewController!.view.bounds) )
let radius = sqrt((extremePoint.x*extremePoint.x) + (extremePoint.y*extremePoint.y))
let circleMaskPathFinal = UIBezierPath(ovalInRect: CGRectInset(originFrame, -radius, -radius))
//Create a layer
let maskLayer = CAShapeLayer()
maskLayer.path = circleMaskPathFinal.CGPath
fromViewController!.view.layer.mask = maskLayer
//Create and add the animation
let animation = CABasicAnimation(keyPath: "path")
animation.toValue = circleMaskPathInitial.CGPath
animation.fromValue = circleMaskPathFinal.CGPath
animation.duration = self.transitionDuration(transitionContext)
animation.delegate = self
maskLayer.addAnimation(animation, forKey: "path")
}
The cleanup takes place in the delegate method:
override public func animationDidStop(anim: CAAnimation, finished flag: Bool) {
self.transitionContext?.completeTransition(!(self.transitionContext?.transitionWasCancelled())!)
}
I guess, that I am doing something wrong with adding the views to the containerView, but I couldn't figure it out. Another possibility is, that the view's layer mask gets reset, when the function completeTransition is called.

Answer 1

Thanks to this blogpost I have finally been able to solve this problem. Short explanation:
The CAAnimation only manipulates the presentation-layer of the view, but does not change the model-layer. When the animation now finishes, it's value snaps back to the original and unchanged value of the model-layer.
Short and simply workaround:
animation.fillMode = kCAFillModeForwards
animation.removedOnCompletion = false
The better solution, as it doesn't prevent the animation from being removed is to manually set the final value of the layer's position before the animation starts. This way, the model-layer is assigned the correct value:
maskLayer.path = circleMaskPathInitial.CGPath
//Create and add the animation below..