TransitionAnimator.swift

final class TransitionAnimator: NSObject, UIViewControllerAnimatedTransitioning {
    // 1
    let presenting: Bool

    // 2
    init(presenting: Bool) {
        self.presenting = presenting
    }

    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        // 3
        return TimeInterval(UINavigationControllerHideShowBarDuration)
    }

    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        // 4
        guard let fromView = transitionContext.view(forKey: .from) else { return }
        guard let toView = transitionContext.view(forKey: .to) else { return }

        // 5
        let duration = transitionDuration(using: transitionContext)

        // 6
        let container = transitionContext.containerView
        if presenting {
            container.addSubview(toView)
        } else {
            container.insertSubview(toView, belowSubview: fromView)
        }

        // 7
        let toViewFrame = toView.frame
        toView.frame = CGRect(x: presenting ? toView.frame.width : -toView.frame.width, y: toView.frame.origin.y, width: toView.frame.width, height: toView.frame.height)

        let animations = {
            UIView.addKeyframe(withRelativeStartTime: 0.0, relativeDuration: 0.5) {
                toView.alpha = 1
                if self.presenting {
                    fromView.alpha = 0
                }
            }

            UIView.addKeyframe(withRelativeStartTime: 0.0, relativeDuration: 1) {
                toView.frame = toViewFrame
                fromView.frame = CGRect(x: self.presenting ? -fromView.frame.width : fromView.frame.width, y: fromView.frame.origin.y, width: fromView.frame.width, height: fromView.frame.height)
                if !self.presenting {
                    fromView.alpha = 0
                }
            }

        }

        UIView.animateKeyframes(withDuration: duration,
                                delay: 0,
                                options: .calculationModeCubic,
                                animations: animations,
                                completion: { finished in
                                    // 8
                                    container.addSubview(toView)
                                    transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
        })
    }
}