Bug with multiple screen transitions in UIKit?

[dazy1030]

Hi, I'm just checking the specifications for screen transitions in UINavigationController using the observe method, and I found a bug-like behaviour.
If I create multiple screen transition origins with a single observe method, the first transition I describe will immediately return to the screen from which I made the transition, as shown in the attached video.

Simulator.Screen.Recording.-.iPhone.15.Pro.-.2024-04-14.at.00.04.01.mp4

I have prepared a minimal sample that confirms this problem.
https://github.com/dazy1030/UIKitPlayground/tree/tca/multi-navigation-observe

I have confirmed that this problem can be avoided by writing only one transition in the observe method and using two observe methods.

Thanks.

Environments
macOS 14.2.1, Xcode 15.2, iPhone 15 Pro(17.2 simulator)

[mbrandonw]

Hi @dazy1030, this is happening because of these lines here:

if let store = store.scope(state: \.second, action: \.second.presented) {
    let vc = SecondViewController(store: store)
    navigationController?.pushViewController(vc, animated: true)
} else {
    navigationController?.popToViewController(self, animated: true)
}
if let store = store.scope(state: \.third, action: \.third.presented) {
    let vc = ThirdViewController(store: store)
    navigationController?.pushViewController(vc, animated: true)
} else {
    navigationController?.popToViewController(self, animated: true)
}

When the second state becomes non-nil the SecondViewController is pushed onto the nav controller, but then immediately after it is popped off because third == nil.

You need to follow the pattern we show in our Tic-Tac-Toe demo:

swift-composable-architecture/Examples/TicTacToe/tic-tac-toe/Sources/LoginUIKit/LoginViewController.swift

Lines 101 to 122 in fd4b169

	if let store = store.scope(state: \.alert, action: \.alert),
	alertController == nil
	{
	alertController = UIAlertController(store: store)
	present(alertController!, animated: true, completion: nil)
	} else if store.alert == nil, alertController != nil {
	alertController?.dismiss(animated: true)
	alertController = nil
	}
	if let store = store.scope(state: \.twoFactor, action: \.twoFactor.presented),
	twoFactorController == nil
	{
	twoFactorController = TwoFactorViewController(store: store)
	navigationController?.pushViewController(
	twoFactorController!,
	animated: true
	)
	} else if store.twoFactor == nil, twoFactorController != nil {
	navigationController?.popToViewController(self, animated: true)
	twoFactorController = nil
	}

You must keep track of the second and third view controllers so that you know whether or not it is currently presented. Here is how it should look in your situation:

var secondViewController: SecondViewController?
var thirdViewController: ThirdViewController?

observe { [weak self] in
  guard let self else { return }

  if 
    secondViewController == nil,
    let store = store.scope(state: \.second, action: \.second.presented)
  {
    let vc = SecondViewController(store: store)
    secondViewController = vc
    navigationController?.pushViewController(vc, animated: true)
  } else if secondViewController != nil, store.second == nil {
    navigationController?.popToViewController(self, animated: true)
    secondViewController = nil
  }

  if
    thirdViewController == nil,
    let store = store.scope(state: \.third, action: \.third.presented)
  {
    let vc = ThirdViewController(store: store)
    thirdViewController = vc
    navigationController?.pushViewController(vc, animated: true)
  } else if thirdViewController != nil, store.third == nil {
    navigationController?.popToViewController(self, animated: true)
    thirdViewController = nil
  }
}

I will also say that this is not correct:

if !isMovingToParent {
  store.send(.someViewDismissed)
}

This is going to dismiss both controllers whenever this controller appears again. Again I encourage you to look at how we do this in the Tic-Tac-Toe demo:

swift-composable-architecture/Examples/TicTacToe/tic-tac-toe/Sources/LoginUIKit/LoginViewController.swift

Lines 129 to 131 in fd4b169

	if !isMovingToParent && store.twoFactor != nil {
	store.twoFactorDismissed()
	}

[dazy1030]

Hello, mbrandonw.

Oh, it was a mistake in my code. I mistakenly thought that the processing related to a specific screen could be written closed in an if-else statement. I should have remembered that both expressions are evaluated.

I will also say that this is not correct:

I had just discovered this PR and corrected it. Thank you very much!
https://github.com/pointfreeco/swift-composable-architecture/pull/2986/files

In the latest commit of the sample, I combined it with Destination Reducer based on the advice I received.
https://github.com/dazy1030/UIKitPlayground/tree/tca/multi-navigation-observe