UIViewControllerTransitioningDelegate presenting doesn't work (though dismissing does) - animation

Question

I am trying to implement a custom appearing/disappearing animation for a modal UIViewController in my app.
I have published the code showing this error here.
Here is the content related:
/// The view controller from which I'm trying to display the modal
class ViewController: UIViewController {
#IBAction func tapped() {
/// The modal showing
ModalTestViewController.show()
}
//...
}
/// The displayed modal
open class ModalTestViewController: TransitioningModalViewController {
init() {
super.init(nibName: "ModalTestViewController", bundle: .main)
transitioningDelegate = self
// 1. when I put transitioningDelegate here, case 1
}
public required init?(coder: NSCoder) {
fatalError()
}
open override func viewDidLoad() {
super.viewDidLoad()
// 2. if I put transitioningDelegate here, case 2
}
#IBAction func tapped() {
// a tap on the overlayView of my modal
dismiss(animated: true)
}
static func show() {
let modal = ModalTestViewController()
modal.modalPresentationStyle = .overCurrentContext
DispatchQueue.main.async {
UIApplication.shared.delegate?.window??.rootViewController?.present(modal, animated: true)
}
}
}
/// The default modal view controller, which all modals in my app should inherit
open class TransitioningModalViewController: UIViewController {
// MARK: View Properties
#IBOutlet weak var overlayView: UIView!
}
extension TransitioningModalViewController: UIViewControllerTransitioningDelegate {
public func animationController(
forPresented presented: UIViewController,
presenting: UIViewController,
source: UIViewController
) -> UIViewControllerAnimatedTransitioning? {
return TransitioningModalViewControllerPresenter()
}
public func animationController(
forDismissed dismissed: UIViewController
) -> UIViewControllerAnimatedTransitioning? {
return TransitioningModalViewControllerDismisser()
}
}
private final class TransitioningModalViewControllerPresenter: NSObject, UIViewControllerAnimatedTransitioning {
func transitionDuration(
using transitionContext: UIViewControllerContextTransitioning?
) -> TimeInterval {
return 0.5
}
func animateTransition(
using transitionContext: UIViewControllerContextTransitioning
) {
let toViewController: TransitioningModalViewController = transitionContext.viewController(
forKey: UITransitionContextViewControllerKey.to
) as! TransitioningModalViewController
let duration = transitionDuration(using: transitionContext)
toViewController.overlayView.alpha = 0.0
UIView.animate(
withDuration: duration
) {
toViewController.overlayView.alpha = 0.65
} completion: { result in
transitionContext.completeTransition(result)
}
}
}
private final class TransitioningModalViewControllerDismisser: NSObject, UIViewControllerAnimatedTransitioning {
func transitionDuration(
using transitionContext: UIViewControllerContextTransitioning?
) -> TimeInterval {
return 0.5
}
func animateTransition(
using transitionContext: UIViewControllerContextTransitioning
) {
let fromViewController: TransitioningModalViewController = transitionContext.viewController(
forKey: UITransitionContextViewControllerKey.from
) as! TransitioningModalViewController
let duration = transitionDuration(using: transitionContext)
UIView.animate(
withDuration: duration
) {
fromViewController.overlayView.alpha = 0.0
} completion: { result in
transitionContext.completeTransition(result)
}
}
}
The idea behind this is that the modal appearance should not be the usual bottom-to-top animation, but instead the overlay view should go from hidden to an alpha of 0,65.
Case 1: when I put transitioningDelegate = self in init(), the animation is killed and nothing happens.
Case 2: when I put it into the viewDidLoad(), the appearing animation is the default bottom-to-top one, but the disappearing one is the expecting one (with the overlay view vanishing).
It looks like something is wrong with the initial transitioningDelegate setting but I can't find what.
Thank you for your help!

Answer 1

In your original code, you are setting the delegate here:
open override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
transitioningDelegate = self
}
However, .present(...) is called before viewDidAppear(...), so the controller is presented with default slide-up animation.
Setting the delegate in init() doesn't work, because we have override the default presentation process... and the presented controller's view is never added to the view hierarchy.
This "quick fix" should do the job...
First, in ModalTestViewController, move setting the delegate to init():
init() {
super.init(nibName: "ModalTestViewController", bundle: .main)
transitioningDelegate = self
}
then, in TransitioningModalViewControllerPresenter, add these lines before the animation:
func animateTransition(
using transitionContext: UIViewControllerContextTransitioning
) {
let toViewController: TransitioningModalViewController = transitionContext.viewController(
forKey: UITransitionContextViewControllerKey.to
) as! TransitioningModalViewController
// add these lines \/
// get the "from" view controller
let fromVC = transitionContext.viewController(forKey: .from)!
// get the "to" view controller's view
let toView = transitionContext.view(forKey: .to)!
// set the frame of the "to" view to the initialFrame (the current frame) of the "from" VC
toView.frame = transitionContext.initialFrame(for: fromVC)
// get the transition container view
let container = transitionContext.containerView
// add the "to" view to the view hierarchy
container.addSubview(toView)
// add these lines /\
let duration = transitionDuration(using: transitionContext)
toViewController.overlayView.alpha = 0.0
UIView.animate(
withDuration: duration
) {
toViewController.overlayView.alpha = 0.65
} completion: { result in
transitionContext.completeTransition(result)
}
}
Personally, to make this more flexible, I would get rid of your overlayView and set the alpha on the controller's view itself.