快速地以编程方式创建NavigationController时，ViewController出现了多次

Question

I have been creating my app completely from scratch programmatically, without using Storyboards .

My app is integrated with Firebase, and uses Facebook login.

My setup is fairly simple:

1. Launch the app -> takes you to the first VC called WelcomeViewController .
2. There is a check that happens in the viewDidLoad method to see if a user is already signed in and exists. If there is, it sends you straight to the second VC called FilmsViewController
3. The FilmsViewController is a collectionViewController that displays films. The user can press a film, and it takes them to more information about that film.

(For reference, I am already signed in with Facebook in my app)

I have a current issue, where when step 2 above happens, it transitions to the FilmsViewController , but it does it like 2 or 3 times. So you see the new VC appear like 2 or 3 times, then the content loads. If you press the Back button in the nav bar, it takes you back through the 2 or 3 viewControllers that it loaded before taking you back to the WelcomeViewController .

I have set my views up as follows.

In AppDelegate.swift :

var window: UIWindow?
var navController: UINavigationController?

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    // Override point for customization after application launch.

    navController = UINavigationController()
    let firstViewController: WelcomeViewController = WelcomeViewController()
    self.navController!.pushViewController(firstViewController, animated: true)

    window = UIWindow(frame: UIScreen.main.bounds)
    window?.rootViewController = navController
    window?.makeKeyAndVisible()

    return true

}

In the WelcomeViewController in the viewDidLoad :

FIRAuth.auth()?.addStateDidChangeListener { auth, user in

        if let user = user {

            // User is signed in.
            // Direct the user to the home screen

            let toFilmListVC = FilmsViewController(collectionViewLayout: UICollectionViewFlowLayout())
            self.navigationController?.pushViewController(toFilmListVC, animated: true)

        } else { ...
        }
}

I have looked loads for a solution - and nothing. I've only found one post on this issue, where someone said the solution was to change the class name of that controller, which I have already done and it didn't change anything.

Can anyone help me resolve this, please? Thank you.

Answer 1

The addStateDidChangeListener is probably being called multiple times.

You should modify it to check whether a FilmsViewController has already been pushed, to prevent pushing another one:

FIRAuth.auth()?.addStateDidChangeListener { auth, user in

    if let user = user {

        // User is signed in.
        // Direct the user to the home screen

        // Only push one FilmsViewController onto the navigation stack!
        var shouldPush = true
        if let navigationController = self.navigationController {
            for viewController in navigationController.viewControllers {
                if viewController is FilmsViewController {
                    shouldPush = false
                }
            }
        }

        if shouldPush {
            let toFilmListVC = FilmsViewController(collectionViewLayout: UICollectionViewFlowLayout())
            self.navigationController?.pushViewController(toFilmListVC, animated: true)
        }
    } else { ...
    }
}

Answer 2

I know the above solutions works, but to make it look extremely simple. here is the code i wrote in my project.

let someController = SomeController.someController()
if !(self.navigationController!.viewControllers.contains(someController)){
   self.navigationController?.pushViewController(someController, animated:true)
 }

only push the controller to navigation bar if the controller is not exist.