Node.JS 服务器发送的事件：路由在 res.end() 之后继续到运行，导致 ERR_STREAM_WRITE_AFTER_END 错误

Question

我开始使用服务器发送事件 (SSE)，因为我的 Web 应用程序需要从服务器接收实时更新。它不需要向服务器发送任何内容，因此选择了 SSE 而不是 Websockets。

阅读一些示例后，我有以下代码：

On my server, in ./src/routers/mainRouter.js I have:

router.get('/updates', (req, res) => {
    res.writeHead(200, {
        'Content-Type': 'text/event-stream',
        'Cache-Control': 'no-cache',
        'Connection': 'keep-alive'
    })

    // Listens for 'event' and sends an 'Event triggered!' message to client when its heard.
    eventEmitter.addListener('event', () => {
        console.log('Event triggered! Sending response.')
        res.write('data: Event triggered!\n\n')
    })

    req.on('close', () => {
        console.log('Connection to client closed.')
        res.end()
    })
})

module.exports = router

On my client, in ./app/index.js I have:

const source = new EventSource('/updates')

source.onmessage = (e) => {
    console.log(e)
}

我遇到了 2 个问题：

一旦我从客户端打开一个连接然后关闭连接（通过关闭选项卡），'close' 事件触发两次导致 req.on('close') 运行内的代码块两次。我不确定为什么会这样。我的 console 在服务器端看起来如下：
```
Event triggered! Sending response.
Connection to client closed.
Connection to client closed.
```

更重要的是，虽然调用了req.end()，路由器仍然继续监听该频道上的事件并尝试发送沿着该通道的响应导致 ERR_STREAM_WRITE_AFTER_END 错误，服务器崩溃。所以最终控制台输出如下：

Event triggered! Sending response. // First event triggers.
Connection to client closed. // 'close' event fires.
Connection to client closed. // 'close' event fires a second time (not sure why).
Event triggered! Sending response. // Router continues listening for 'event' and sends another response although res.end() was called earlier
events.js:187
      throw er; // Unhandled 'error' event
      ^

Error [ERR_STREAM_WRITE_AFTER_END]: write after end

Answer 1

当流关闭时，您需要删除事件侦听器，这样您就不会再次尝试写入流。可以这样做：

router.get('/updates', (req, res) => {
    res.writeHead(200, {
        'Content-Type': 'text/event-stream',
        'Cache-Control': 'no-cache',
        'Connection': 'keep-alive'
    });

    function listener(event) {
        console.log('Event triggered! Sending response.');
        res.write('data: Event triggered!\n\n');
    }

    // Listens for 'event' and sends an 'Event triggered!' message to client when its heard.
    eventEmitter.addListener('event', listener);

    req.on('close', () => {
        // remove listener so it won't try to write to this stream any more
        eventEmitter.removeListener('event', listener);
        console.log('Connection to client closed.');
        res.end();
    });
});

module.exports = router;

仅供参考，如果您已经收到 close 事件，我认为您不需要 res.end()。如果您单方面尝试关闭与服务器的连接，您将使用 res.send()，但如果它已经关闭，我认为您不需要它和 none 我见过的代码示例使用就这样。

我想知道您的 res.end() 是否也是您收到两个 close 事件的原因。尝试删除它。

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