MainScreenViewController.swift

var handle: AuthStateDidChangeListenerHandle?


override func viewWillAppear(_ animated: Bool) {
   handle = Auth.auth().addStateDidChangeListener { (auth, user) in
       if user == nil {
           self.showFUIAuthScreen()
       }
   }
}


override func viewWillDisappear(_ animated: Bool) {
   if let handle = handle {
       Auth.auth().removeStateDidChangeListener(handle)
   }
}