override func transition(
    container: UIView,
    fromViewController: UIViewController,
    toViewController: UIViewController,
    isDismissing: Bool,
    duration: TimeInterval,
    completion: @escaping () -> Void) {
    if isDismissing {
        closeAnimation(
            container: container,
            fromViewController: fromViewController,
            toViewController: toViewController,
            duration: duration,
            completion: completion)
    } else {
        openAnimation(
            container: container,
            fromViewController: fromViewController,
            toViewController: toViewController,
            duration: duration,
            completion: completion)
    }
}

func openAnimation (
    container: UIView,
    fromViewController: UIViewController,
    toViewController: UIViewController,
    duration: TimeInterval,
    completion: @escaping () -> Void) {
    
    container.addSubview(toViewController.view)
    toViewController.view.top = fromViewController.view.bottom
    toViewController.view.addGestureRecognizer(UIPanGestureRecognizer(target: self, action: #selector(handlePan(pan:))))
    panningViewController = toViewController
    
    UIView.animate(
        withDuration: duration,
        animations: {
            toViewController.view.top = 0
    },
        completion: { finished in
            completion()
    })
}

func closeAnimation (
    container: UIView,
    fromViewController: UIViewController,
    toViewController: UIViewController,
    duration: TimeInterval,
    completion: @escaping () -> Void) {
    
    container.addSubview(toViewController.view)
    container.bringSubviewToFront(fromViewController.view)
    
    UIView.animate(
        withDuration: duration,
        animations: {
            fromViewController.view.frame = CGRect(x: 60, y: 400, width: 120, height: 120)
    },
        completion: { finished in
            completion()
    })
}

@objc func handlePan(pan: UIPanGestureRecognizer) {
    let percent = pan.translation(in: pan.view!).y / pan.view!.bounds.size.height
    switch pan.state {
    case .began:
        interactionTransitionController = UIPercentDrivenInteractiveTransition()
        panningViewController?.dismiss(animated: true, completion: {
            self.interactionTransitionController?.finish()
        })
    case .changed:
        interactionTransitionController!.update(percent)
    case .ended:
        if percent > 0.5 {
            interactionTransitionController!.finish()
            panningViewController = nil
        } else {
            interactionTransitionController!.cancel()
        }
        interactionTransitionController = nil
    default:
        return
    }
}