使用你的静态方法快速编写有逻辑：

static func createLoginDialog(buttonPressed: @escaping (_ username:String, _ pw:String) -> Void) -> UIAlertController {
    let alert = UIAlertController(title: "Login", message: "", preferredStyle: .alert)
    
    let loginAction = UIAlertAction(title: "Log in", style: .default, handler: { _ in
        let u = alert.textFields![0].text!
        let p = alert.textFields![1].text!
        buttonPressed(u,p)
    })
    
    alert.addAction(loginAction)
    loginAction.isEnabled = false
    
    var textFields: [UITextField] = []
    
    alert.addTextField { textField in
        textField.placeholder = "Enter Username"
        textFields.append(textField)
        NotificationCenter.default.addObserver(forName: UITextField.textDidChangeNotification,
                                               object: textField,
                                               queue: .main) { _ in
            loginAction.isEnabled = textFieldsAreValid(textFields)
        }
    }
    
    alert.addTextField { textField in
        textField.placeholder = "Enter Password"
        textFields.append(textField)
        NotificationCenter.default.addObserver(forName: UITextField.textDidChangeNotification,
                                               object: textField,
                                               queue: .main) { _ in
            loginAction.isEnabled = textFieldsAreValid(textFields)
        }
    }
    
    func textFieldsAreValid(_ textFields: [UITextField]) -> Bool {
        textFields.allSatisfy { !($0.text ?? "").isEmpty } //Do the needed tests, I just check if not empty
    }
    
    return alert
}

但正如Vadian所说，在UIViewController上扩展可能更好。
我们可以想象你有

protocol LoginProtocol: Protocol {
}

extension LoginProtocol where Self: UIViewController {
  
    func loginAlert(buttonPressed: @escaping (_ username:String, _ pw:String) -> Void) -> UIAlertController {
        //The previous code
    }

    func showLoginAlert(buttonPressed: @escaping (_ username:String, _ pw:String) -> Void) {
        let alert = loginAlert(buttonPressed: buttonPressed)
        present(alert, animated: true)
    }
}

我知道有时候，你想从不是UIViewController的类中调用它，但理论上，该对象（无论是子视图等）应该告诉其父ViewController显示登录警报，而不是它自己。这就是为什么我更喜欢使用这种方式。
在每个可以显示警报的ViewController上：

extension MyCustomVC: LoginProtocol {}

它可以调用showLoginAlert(...)。