[8]

 override func viewDidLoad() {
        super.viewDidLoad()
    
        loginButton?.isEnabled = false
        
        guard let loginTextField = loginTextField,
            let passwordTextField = passwordTextField,
            let loginButton = loginButton else {
                return
        }
        
        Observable.combineLatest(loginTextField.rx.text.orEmpty, passwordTextField.rx.text.orEmpty) { (login, password) in
                return login.isLoginValid && password.isPasswordValid
            }
            .asDriver(onErrorJustReturn: false)
            .drive(loginButton.rx.isEnabled)
            .disposed(by: disposeBag)
    }