Error: Value of type 'AppDelegate?' has no member 'present'

Question

I'm getting an error of

Value of type 'AppDelegate?' has no member 'present'

when I tried to keep my users logged in even when the app was quit.

Here are my view controllers, any idea how I'd keep my users logged in? And why am I getting this error?

I'm using Firebase in my code as my Database.

LoginViewController.swift

import UIKit
import Firebase

class LoginViewController: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var emailField: UITextField!
    @IBOutlet weak var pwField: UITextField!




    func createAlert(title:String, message:String) {

        let alert=UIAlertController(title: title, message: message, preferredStyle: UIAlertControllerStyle.alert)


        alert.addAction(UIAlertAction(title: "error" , style: UIAlertActionStyle.default, handler: { (action) in alert.dismiss(animated: true, completion: nil)
        }))

        self.present(alert, animated:true, completion: nil)
    }





  /*  @IBAction func emailKeyboardField(_ sender: Any) {

       hideKeyboard()
    }

    func hideKeyboard() {
        emailField.resignFirstResponder()
        pwField.resignFirstResponder()

    }
    */

    override func viewDidLoad() {
        super.viewDidLoad()

        emailField.delegate = self


        // Do any additional setup after loading the view.
    }



    func emailKeyField(_ emailField: UITextField) -> Bool {
        self.view.endEditing(true)


        return true
    }


    @IBAction func loginPressed(_ sender: Any) {

        guard emailField.text != "", pwField.text != "" else {return}

        FIRAuth.auth()?.signIn(withEmail: emailField.text!, password: pwField.text!, completion: { (user, error) in

            if let error = error {
                print(error.localizedDescription)

                self.createAlert(title: "Error", message: "We could not locate your account. Check your email and password.")
            }


            if let user = user {
                let vc = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "usersVC")

                self.present(vc, animated: true, completion: nil)
            }
        })


    }


}

AppDelegate.swift

import UIKit
import Firebase


@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate, UIImagePickerControllerDelegate, UINavigationControllerDelegate {

    var window: UIWindow?
    var actIdc = UIActivityIndicatorView(activityIndicatorStyle: .whiteLarge)
    var container: UIView!
    private let auth = FIRAuth.auth()

    class func instance() -> AppDelegate {
        return UIApplication.shared.delegate as! AppDelegate
    }


    func showActivityIndicator() {
        if let window = window {
            container = UIView()
            container.frame = window.frame
            container.center = window.center
            container.backgroundColor = UIColor(white: 0, alpha: 0.8)

            actIdc.frame = CGRect(x: 0, y: 0, width: 40, height: 40)
            actIdc.hidesWhenStopped = true
            actIdc.center = CGPoint(x: container.frame.size.width / 2, y: container.frame.size.height / 2)

            container.addSubview(actIdc)
            window.addSubview(container)

            actIdc.startAnimating()
        }
    }

    func dismissActivityIndicatos() {
        if let _ = window {
            container.removeFromSuperview()
        }
    }

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {



                auth?.addStateDidChangeListener { [weak self] (_, user) in

                    if let user = user {
                        let vc = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "usersVC")

                        self.present(vc, animated: true, completion: nil) //error on this line
                        // user is already logged in
                    } else {
                        // user is not logged in
                    }


        }
        FIRApp.configure()
        return true
    }


    func applicationWillResignActive(_ application: UIApplication) {
        // Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
        // Use this method to pause ongoing tasks, disable timers, and invalidate graphics rendering callbacks. Games should use this method to pause the game.
    }

    func applicationDidEnterBackground(_ application: UIApplication) {
        // Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
        // If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
    }

    func applicationWillEnterForeground(_ application: UIApplication) {
        // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
    }

    func applicationDidBecomeActive(_ application: UIApplication) {
        // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
    }

    func applicationWillTerminate(_ application: UIApplication) {
        // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
    }


}

Any and all help would be appreciated.

Answer 1

App delegate is not of type ViewController, hence it doesn't have a present method to show views on it. Instead what you can do is show a view using it's window like:

let storyboard : UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
    let homeC = storyboard.instantiateViewController(withIdentifier: "YOUR_VIEWCONTROLLER_IDENTIFIER_IN_STORYBOARD") as? HomeC
    if homeC != nil {
        homeC!.view.frame = (self.window!.frame)
        self.window!.addSubview(homeC!.view)
        self.window!.bringSubview(toFront: homeC!.view)
    }