// MARK: LoginViewController

class LoginViewController: UIViewController {

    func login(with username: String,
               password: String) {
        let authenticationSession = AuthenticationSession()
        authenticationSession.performLogin(with: username,
                                           password: password,
                                           completion: { userIdentifier in
        // Handle the login with userIdentifier

        })
    }
}

// MARK: AuthenticationSession

class AuthenticationSession {

    func performLogin(with username: String,
                      password: String,
                      completion: ((String) -> Void)) {
        let userIdentifier = "12345" //Service.login(username, password)
        completion(userIdentifier)
    }
}