How does animateAlongsideTransition work?

This is my part of my subclass UIPresentationController

:

override func presentationTransitionWillBegin() {

    if let containerView = containerView {
        dimmingView.frame = containerView.bounds
        dimmingView.backgroundColor = UIColor.yellowColor()
        containerView.insertSubview(dimmingView, atIndex: 0)

        presentedViewController.transitionCoordinator()?.animateAlongsideTransition({ context in

            self.dimmingView.backgroundColor = UIColor.greenColor() //line A

            }, completion: nil)
    }
}

      

        
        
        
      

    

The problem is I can't see yellow even for a millisecond. What for? What does it mean with the transition? What is the transition? My transition time is set UIViewControllerAnimatedTransitioning

up to 5 seconds .

If I remove my line A then I only see yellow and everything is fine. Why doesn't this animation (yellow to green) take 5 seconds?