javascript - 为事件处理程序编写中间件

Question

用例:

我必须处理几个需要“可用客户端”的事件。所以在每个事件处理程序中，我首先必须尝试获取可用的客户端。如果没有可用的客户端，我将回复一条“服务不可用”消息。现在我已经实现了这样的要求:

public constructor(consumer: RpcConsumer) {
  consumer.on('requestA', this.onRequestA);
}

private onRequestA = async (msg: RpcConsumerMessage) {
    const client: RequestClient = this.getClient(msg);
    if (client == null) {
      return;
    }

   msg.reply(await client.getResponseA());
}

private getClient(msg: RpcConsumerMessage): RequestClient {
    const client: RequestClient= this.clientManager.getClient();
    if (client == null) {
      const err: Error = new Error('Currently there is no client available to process this request');
      msg.reply(undefined, MessageStatus.ServiceUnavailable, err);

      return;
    }

    return client;
}

问题:

我不想一次又一次地检查所有事件处理程序中的可用客户端。相反，我认为中间件非常适合这个用例。它会检查可用的客户端并传递客户端实例(如果有的话)。如果没有可用的客户端，它将以错误消息作为响应。

问题:

我将如何为这种情况编写这样的中间件？

Answer 1

为此构建一个柯里化(Currying)方法:

 private withClient(cb: (client: RequestClient) => string | Promise<string>) {
  return function(msg: RpcConsumerMessage) {
    const client: RequestClient= this.clientManager.getClient();
    if (client == null) {
      const err: Error = new Error('Currently there is no client available to process this request');
      msg.reply(undefined, MessageStatus.ServiceUnavailable, err);

      return;
    }

    msq.reply(await cb(client));
   };
 }

因此您可以将其用作:

 private onRequestA = withClient(client => client.getResponseA());