// MARK: LoginViewController

class LoginViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        NotificationCenter.default.addObserver(self,
                                               selector: #selector(handleLogin(notification:)),
                                               name: Notification.Name("UserIdentifier"),
                                               object: nil)
    }

    func login(with username: String,
               password: String) {
        let authenticationSession = AuthenticationSession()
        authenticationSession.performLogin(with: username,
                                           password: password)
    }

    @objc func handleLogin(notification: Notification) {
        if let userInfo = notification.userInfo {
            if let userIdentifier = userInfo["User Identifier"] as? String {
                // Handle the login with userIdentifier
            }
        }
    }
}

// MARK: AuthenticationSession

class AuthenticationSession {

    func performLogin(with username: String,
                      password: String) {
        let userIdentifier = "12345" //Service.login(username, password)
        NotificationCenter.default.post(name: Notification.Name("UserIdentifier"), object: self, userInfo: ["User Identifier": userIdentifier])
    }
}