问如何在关闭app后保持用户登录？

EN

import UIKit
import Firebase

class WelcomeViewController: UIViewController {

    @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)
            }
        }
    }
}