[Swift] 使用AlertView 顯示警告訊息

//如果token是空值
if (User.token.isEmpty){
                //設定UIAlertController的title,message
                let alertController = UIAlertController(title: "並未登入", message: "測試內容", preferredStyle: .alert)
                //設定action按鈕
                let okAction = UIAlertAction(title: "確定", style: .default)
                //將action加入UIAlertController
                alertController.addAction(okAction)
                //彈出UIAlertController
                self.present(alertController, animated: true, completion: nil)
                return
            }

//設定UIAlertController的title,message
        let controller = UIAlertController(title: "是否登出", message: "", preferredStyle: .alert)
                //設定ok的action按鈕，並加入按下後的動作
        let okAction = UIAlertAction(title: "確定", style: .default){(_) in
            User.token = ""
            let logoutController = UIAlertController(title: "帳號已登出", message: "", preferredStyle: .alert)
            let logoutOkAction = UIAlertAction(title: "確定", style: .default)
            logoutController.addAction(logoutOkAction)
            self.present(logoutController, animated: true, completion: nil)
        }
        //將action加入UIAlertController
        controller.addAction(okAction)
        //設定cancel的action按鈕
        let cancelAction = UIAlertAction(title: "取消", style: .cancel, handler: nil)
        //將action加入UIAlertController
        controller.addAction(cancelAction)
        //彈出UIAlertController
        self.present(controller, animated: true, completion: nil)
        return