alert的用法(Swift)

@IBAction func change(_ sender: Any) {

            let deepAlert2 = UIAlertController(title: "不保存", message: "不保存刚刚的修改吗", preferredStyle: .alert)
            let yesAction = UIAlertAction(title: "不保存", style: .default, handler: { (action:UIAlertAction) in
                self.presentedViewController?.dismiss(animated: true, completion: nil)
                self.navigationController?.popViewController(animated: true)
                // 我这里是直接通过popViewController来进行返回上一层的操作的。因为用了这个changeBtn来覆盖掉原来NavigationController自带的那个返回按钮。
            })
            let noAction = UIAlertAction(title: "返回继续编辑", style: .default, handler: { (action:UIAlertAction) in
                self.presentedViewController?.dismiss(animated: true, completion: nil)
                // do nothing
            })
            deepAlert2.addAction(yesAction)
            deepAlert2.addAction(noAction)
            // self.presentedViewController?.dismiss(animated: true, completion: nil)
            self.present(deepAlert2, animated: true, completion: nil)
 
    }