UIKit present() function dismisses multiple view controllers when using a custom transitioning delegate

[juliensagot]

Description

Hi folks 👋

When using the present() method in conjunction with a custom transitioning delegate, the app dismisses multiple view controllers instead of just dismissing the intended view controller. This issue only occurs when a custom transitioning delegate is set.

let presentedVC = AnotherViewController()
presentedVC.modalPresentationStyle = .custom
presentedVC.transitioningDelegate = customViewControllerTransitioningDelegate

present(isPresented: $viewModel.isSheetDisplayed) {
  presentedVC
}

Here’s a minimal reproduction app:
SwiftNavigationDismissIssue.zip

Checklist

• I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue or discussion.

[mbrandonw]

Hi @juliensagot, thanks for the report. As far as I can tell, this issue doesn't have to do with a custom transition delegate. I observe the same behavior even without a transition delegate. Can you confirm that is the case on your end too?

[juliensagot]

@mbrandonw Interesting! While double-checking the code, I moved the declaration of the presented view controller inside the present closure instead of outside, and it works!

New:

present(isPresented: $viewModel.isSheetDisplayed) { [viewControllerTransitioningDelegate] in
  let presentedVC = ViewControllerB()
  presentedVC.modalPresentationStyle = .custom
  presentedVC.transitioningDelegate = viewControllerTransitioningDelegate
  return presentedVC
}

old:

let presentedVC = ViewControllerB()
presentedVC.modalPresentationStyle = .custom
presentedVC.transitioningDelegate = viewControllerTransitioningDelegate
    
present(isPresented: $viewModel.isSheetDisplayed) {
  presentedVC
}

I am indeed experiencing the same issue, regardless of whether a custom transitioning delegate is used in this context.
I guess I'm preventing the VC from being properly deallocated by creating it outside of the present closure?

[mbrandonw]

@mbrandonw Interesting! While double-checking the code, I moved the declaration of the presented view controller inside the present closure instead of outside, and it works!

I am not sure why declaring the presentedVC out of the present(isPresented:) trailing closure breaks things. I would have to investigate it more.

But FWIW we recommend this style anyway:

present(isPresented: $viewModel.isSheetDisplayed) { [weak self] in
  guard let self else { return UIViewController() }
  let presentedVC = ViewControllerB()
  presentedVC.modalPresentationStyle = .custom
  presentedVC.transitioningDelegate = viewControllerTransitioningDelegate
  return presentedVC
}

We don't think the presented controllers should be escaped out of the trailing closure. You should just build it up right in the closure and return it, like like how you do for SwiftUI.

I am going to convert this to a discussion for now since it everything seems to work when presenting like the above. If you investigate why the dismissal was happening and can PR a fix we would be happy to take it.