let alertController = UIAlertController(title: "系统提示",
                                                    message: "请打开手机蓝牙", preferredStyle: .alert)
            let cancelAction = UIAlertAction(title: "取消", style: .cancel, handler: nil)
            let okAction = UIAlertAction(title: "好的", style: .default, handler: {
                action in
                 print("点击了确定")
             })
             alertController.addAction(cancelAction)
             alertController.addAction(okAction)
             self.present(alertController, animated: true, completion: nil)

第二种底部弹出

 let alertController = UIAlertController(title: "保存或删除数据", message: "删除数据将不可恢复",
                                         preferredStyle: .actionSheet)
 let cancelAction = UIAlertAction(title: "取消", style: .cancel, handler: nil)
 let deleteAction = UIAlertAction(title: "删除", style: .destructive, handler: nil)
 let archiveAction = UIAlertAction(title: "保存", style: .default, handler: nil)
 
alertController.addAction(cancelAction)
alertController.addAction(deleteAction)
alertController.addAction(archiveAction)
self.present(alertController, animated: true, completion: nil)
警告   红色文字按钮
let okAction = UIAlertAction(title: "好的", style: .destructive, handler: nil)
弹出输入用户名
let alertController = UIAlertController(title: "系统登录",message: "请输入用户名和密码", preferredStyle: .alert
             alertController.addTextField { (textField: UITextField!) -> Void in
                          textField.placeholder = "用户名"
                   }
              alertController.addTextField { (textField: UITextField!) -> Void in
                          textField.placeholder = "密码"
                          textField.isSecureTextEntry = true
                     }
            let cancelAction = UIAlertAction(title: "取消", style: .cancel, handler: nil)
             let okAction = UIAlertAction(title: "好的", style: .default, handler: {
                       action in
                        //也可以用下标的形式获取textField let login = alertController.textFields![0]
                       let login = alertController.textFields!.first!
                       let password = alertController.textFields!.last!
                      print("用户名：\(login.text) 密码：\(password.text)")
                    })
                 alertController.addAction(cancelAction)
                 alertController.addAction(okAction)
                 self.present(alertController, animated: true, completion: nil)