Why navigation controller is nil even though I push it - ios

Question

I have a LoginViewController and want to push MainTabBarController
So, I do this
let controller = MainTabBarController()
navigationController?.pushViewController(controller, animated: true)
LoginViewController has a navigation controller but MainTabBarController doesn't for some reasons.
I have two questions, if my flow is LoginViewController -> MainTabBarController -> Other Controllers embedded in MainTabBarController, and I want to log out, how do I get to the LoginViewController without memory leaks? I was thinking about something like popToRootViewController but not sure if it would work.
This is how I'm creating other controllers in MainTabBarController
private func createNewViewController(viewController: UIViewController, title: String, imageName: String) -> UIViewController {
navigationController?.navigationBar.prefersLargeTitles = true
navigationController?.tabBarItem = UITabBarItem(title: title, image: UIImage(systemName: imageName), selectedImage: nil)
navigationController?.view.backgroundColor = .white
viewController.navigationItem.title = title
return viewController
}
But it doesn't really work since navigationController is nil. I don't use storyboards at all though.
EDIT: Apparently embedding a UITabBarController inside a UINavigationController is not supported.

Answer 1

Try this design. When LoginVC appears it checks if user is logged in and re-directs to MainVC. This is just a skelton, add more details as you go forward:
class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
window = UIWindow()
window?.makeKeyAndVisible()
window?.rootViewController = LoginVC()
return true
}
}
class LoginVC: UIViewController {
var isUserLoggedIn = true
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
view.backgroundColor = .red
if isUserLoggedIn {
present(MainVC(), animated: true)
}
}
}
class MainVC: UITabBarController {
override func viewDidLoad() {
super.viewDidLoad()
modalPresentationStyle = .fullScreen
let firstVC = UIViewController()
firstVC.tabBarItem = UITabBarItem(title: "firstVC", image: nil, selectedImage: nil)
let secondVC = UIViewController()
secondVC.tabBarItem = UITabBarItem(title: "secondVC", image: nil, selectedImage: nil)
viewControllers = [firstVC, secondVC]
}
}