I'm struggling with how to keep user logged in even after he closes the app completely. Heard something about userDefaults but didn't work out so well. The code below is used to check logging in

@IBOutlet weak var logInButton: UIButton!
@IBOutlet weak var signUpButton: UIButton!

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    navigationController?.isNavigationBarHidden = true
}

override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    navigationController?.isNavigationBarHidden = false
}

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view.
    logInButton.backgroundColor = .clear
    logInButton.layer.cornerRadius = 27
    logInButton.layer.borderWidth = 1
    logInButton.layer.borderColor = UIColor.systemGreen.cgColor
    if Auth.auth().currentUser != nil {
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        let secondVC = storyboard.instantiateViewController(identifier: "EcoMapViewController")

        show(secondVC, sender: self)
    } else {
        do {
            try Auth.auth().signOut()
            navigationController?.popToRootViewController(animated: true)
        } catch let signOutError as NSError {
            print("Error signing out: %@", signOutError)
        }
    }
}