UIViewControllerTransitioningDelegate

ChromecastExpandedViewController+TransitioningDelegate.swift

extension ChromecastExpandedViewController: UIViewControllerTransitioningDelegate {
    public func animationController(forDismissed dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return VerticalStackDismissAnimatedTransitioning()
    }
}

VerticalStackDismissAnimatedTransitioning.swift

import UIKit

class VerticalStackDismissAnimatedTransitioning: NSObject, UIViewControllerAnimatedTransitioning {
    private let animatedTransitionDuration: TimeInterval = 0.4

    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return animatedTransitionDuration
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        guard let fromViewController = transitionContext.viewController(forKey: .from),
            let toViewController = transitionContext.viewController(forKey: .to) else {
            return
        }
        
        let originalFromViewControllerFrame = fromViewController.view.frame
        let originalToViewControllerFrame = fromViewController.view.convert(toViewController.view.frame, from: toViewController.view)
        
        let finalFromViewControllerFrame = CGRect(origin: CGPoint(x: originalToViewControllerFrame.minX, y: originalToViewControllerFrame.maxY), size: originalFromViewControllerFrame.size)
        
        let initialToViewControllerFrameOriginY = originalFromViewControllerFrame.origin.y - originalToViewControllerFrame.size.height
        let initialToViewControllerFrame = CGRect(origin: CGPoint(x: originalFromViewControllerFrame.origin.x, y: initialToViewControllerFrameOriginY), size: originalToViewControllerFrame.size)
        
        guard let snapshottedToView = toViewController.view.snapshotView(afterScreenUpdates: true) else {
            return
        }
        transitionContext.containerView.addSubview(snapshottedToView)
        snapshottedToView.frame = initialToViewControllerFrame
        snapshottedToView.alpha = 0
        
        UIView.animate(withDuration: transitionDuration(using: transitionContext), animations: {
            fromViewController.view.frame = finalFromViewControllerFrame
            snapshottedToView.frame = originalToViewControllerFrame
            snapshottedToView.alpha = 1
        }) { (_) in
            snapshottedToView.removeFromSuperview()
            transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
        }
    }
}