override public func presentationTransitionWillBegin() {
        super.presentationTransitionWillBegin()
        dimmingView.frame = presentingViewController.view.bounds
        containerView?.addSubview(dimmingView)
        guard let coordinator = presentedViewController.transitionCoordinator else {
            return
        }
        coordinator.animate(alongsideTransition: { (context) in
            self.dimmingView.alpha = 0.5
        }, completion: nil)
    }
    override public func dismissalTransitionWillBegin() {
        super.dismissalTransitionWillBegin()
        guard let coordinator = presentedViewController.transitionCoordinator else {
            return
        }
        coordinator.animate(alongsideTransition: { (context) in
            self.dimmingView.alpha = 0.0
        }, completion: nil)
    }