SettingsViewController.swift

var handle: AuthStateDidChangeListenerHandle?

override func viewWillAppear(_ animated: Bool) {
    handle = Auth.auth().addStateDidChangeListener { (auth, user) in
        if user != nil {
            self.nameTextField.text = user?.displayName
        } else {
            self.navigationController?.popToRootViewController(animated: true)
        }
    }
}

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