iOS Swift Firebase Checking user at app startup not working - ios

Question

I developed an app with firebase. The application checks to see if any users are logged in every boot in sceneDelegate.swift file. But its works only iOS 13 devices. iOS 12 devices users has to log in every time. How can I fix this problem? Thanks.
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
var window: UIWindow?
#available(iOS 13.0, *)
func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
// Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
// If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
// This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).
let currentUser = Auth.auth().currentUser
if currentUser != nil {
let board = UIStoryboard(name: "Main", bundle: nil)
let navigationController = board.instantiateViewController(identifier: "navigationController") as! UINavigationController
window?.rootViewController = navigationController
}
guard let _ = (scene as? UIWindowScene) else { return }
}

Answer 1

SceneDelegate was introduced in iOS 13. For iOS 12 and below you should implement this same logic in the AppDelegate's didFinishLaunchingWithOptions method like this:
#UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
window = UIWindow()
window?.makeKeyAndVisible()
let currentUser = Auth.auth().currentUser
if currentUser != nil {
let board = UIStoryboard(name: "Main", bundle: nil)
let navigationController = board.instantiateViewController(withIdentifier: "navigationController") as! UINavigationController
window?.rootViewController = navigationController
}
return true
}
}