按下按钮后UIAlert验证中的Swift3 UITextField

// 创建UIAlertController
let alertController = UIAlertController(title: "验证", message: "请输入用户名和密码", preferredStyle: .alert)

// 添加用户名输入框
alertController.addTextField { (textField) in
    textField.placeholder = "用户名"
}

// 添加密码输入框
alertController.addTextField { (textField) in
    textField.placeholder = "密码"
    textField.isSecureTextEntry = true
}

// 添加登录按钮
let loginAction = UIAlertAction(title: "登录", style: .default) { (action) in
    // 处理登录逻辑
    if let username = alertController.textFields?[0].text,
       let password = alertController.textFields?[1].text {
        // 进行用户名和密码的验证
        if username.isEmpty || password.isEmpty {
            // 用户名或密码为空，显示错误提示
            let errorAlert = UIAlertController(title: "错误", message: "用户名和密码不能为空", preferredStyle: .alert)
            let okAction = UIAlertAction(title: "确定", style: .default, handler: nil)
            errorAlert.addAction(okAction)
            self.present(errorAlert, animated: true, completion: nil)
        } else {
            // 用户名和密码验证通过，进行登录操作
            // TODO: 处理登录逻辑
        }
    }
}

// 添加取消按钮
let cancelAction = UIAlertAction(title: "取消", style: .cancel, handler: nil)

// 将按钮添加到UIAlertController
alertController.addAction(loginAction)
alertController.addAction(cancelAction)

// 显示UIAlertController
self.present(alertController, animated: true, completion: nil)