簡単なAlert機能

extension UIViewController {  
    func alert(title: String, message: String) {  
        let alertController = UIAlertController(title: title, message: message, preferredStyle: .alert)
        let alertAction = UIAlertAction(title: "OK", style: .default, handler: nil)
        alertController.addAction(alertAction)
        self.present(alertController, animated: true, completion: nil)
    }

    func alert(title: String) {
        alert(title: title, message: "")
    }
}