[WIP] Creating a Custom Presentation Transition

AnimationControllerForPresentedController.swift

// https://developer.apple.com/reference/uikit/uiviewcontrollertransitioningdelegate

class AnimationController: NSObject, UIViewControllerAnimatedTransitioning {
    
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return 0.2 // The duration of the animation
    }
    
    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        // Since you are presenting a new view controller, you primarily need the new view controller
        guard let toViewController = transitionContext.viewController(forKey: .to) else {
            return
        }
        
        // At the end of this method, this needs to be set.
        // You place the new view controller's view at it's final frame
        // Since you will be using a transform below, you can just do it now.
        toViewController.view.frame = transitionContext.finalFrame(for: toViewController)
        
        // You also need to add the new view controller's view to the container view
        // At the latest before you call completeTransition()
        transitionContext.containerView.addSubview(toViewController.view)
        
        // Since you only have one anmination in most cases, you can use the complete duration for the animation duration
        let duration = transitionDuration(using: transitionContext)
        
        // It's almost time, hide the new view and transform it
        toViewController.view.alpha = 0.0
        toViewController.view.transform = CGAffineTransform(scaleX: 0.001, y: 0.001)
        
        // Warning If you attempt to set the frame of the new view controller, things will get messeed up
        // as the subviews will get scrambled, in this case you might need a snapshot view
        
        let animations = {
            // Set all of the new view's properties back to normal
            toViewController.view.alpha = 1.0
            toViewController.view.transform = .identity
        }
        
        let completion = { (finished: Bool) in
            // You must call this, otherwise iOS will not clear the many transitioning views
            // and your new view contrroller will be unresponsive
            transitionContext.completeTransition(finished)
        }
        
        // Animate and you're done
        UIView.animate(withDuration: duration, animations: animations, completion: completio
    }
}