AppRootController

AppDelegate.swift

//
//  AppDelegate.swift
//  FirebaseSample
//
//  Created by Yoonchul Koh on 2018/10/01.
//  Copyright © 2018 Yoonchul Koh. All rights reserved.
//

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?
    var rootController = AppRootController()

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        
        window = UIWindow(frame: UIScreen.main.bounds)
        window?.rootViewController = rootController
        window?.backgroundColor = UIColor.white
        window?.makeKeyAndVisible()
        
        return true
    }

}

AppRootController.swift

//
//  AppRootController.swift
//  FirebaseSample
//
//  Created by Yoonchul Koh on 2018/02/17.
//  Copyright © 2018 Yoonchul Koh. All rights reserved.
//

import UIKit
import FirebaseAuth

class AppRootController: UIViewController {

    private var currentViewController: UIViewController?
    
    override func viewDidLoad() {
        super.viewDidLoad()

        Auth.auth().addStateDidChangeListener { [weak self] (auth, user) in
            guard let strongSelf = self else { return }
            if user != nil {
                strongSelf.showMainScreen()
            } else {
                strongSelf.showLoginScreen()
            }
        }
        
        if let user = Auth.auth().currentUser {
            print(user.email)
            showMainScreen()
        } else {
            showLoginScreen()
        }
        
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
    }

    func showMainScreen() {
        let storyboard = UIStoryboard(name: "MainViewController", bundle: nil)
        let vc = storyboard.instantiateInitialViewController()!
        changeViewController(vc)
    }

    func showLoginScreen() {
        let storyboard = UIStoryboard(name: "LoginViewController", bundle: nil)
        let vc = storyboard.instantiateInitialViewController()!
        changeViewController(vc)
    }
    
    private func changeViewController(_ vc: UIViewController) {
        removeCurrentViewController()
        setCurrentViewController(vc)
    }
    
    private func setCurrentViewController(_ vc: UIViewController) {
        currentViewController = vc
        addChildViewController(vc)
        view.addSubview(vc.view)
        didMove(toParentViewController: vc)
    }
    
    private func removeCurrentViewController() {
        guard let currentViewController = currentViewController else { return }
        currentViewController.willMove(toParentViewController: nil)
        currentViewController.view.removeFromSuperview()
        currentViewController.removeFromParentViewController()
    }
}