等待 process.nextTick() 上触发的多个事件


当使用 events.once() 函数等待在同一批 process.nextTick() 操作中触发的多个事件时,或者同步触发多个事件时,有一个边缘情况值得注意。 具体来说,因为 process.nextTick() 队列在 Promise 微任务队列之前被排空,并且因为 EventEmitter 同步触发所有事件,所以 events.once() 有可能错过事件。

const { EventEmitter, once } = require('node:events');

const myEE = new EventEmitter();

async function foo() {
  await once(myEE, 'bar');
  console.log('bar');

  // 此 Promise 永远不会被解决,
  // 因为 'foo' 事件在 Promise 被创建之前就已经触发了。
  await once(myEE, 'foo');
  console.log('foo');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));

要捕获这两个事件,则在等待其中一个之前创建每个 Promise,然后可以使用 Promise.all()Promise.race()Promise.allSettled()

const { EventEmitter, once } = require('node:events');

const myEE = new EventEmitter();

async function foo() {
  await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
  console.log('foo', 'bar');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));

There is an edge case worth noting when using the events.once() function to await multiple events emitted on in the same batch of process.nextTick() operations, or whenever multiple events are emitted synchronously. Specifically, because the process.nextTick() queue is drained before the Promise microtask queue, and because EventEmitter emits all events synchronously, it is possible for events.once() to miss an event.

const { EventEmitter, once } = require('node:events');

const myEE = new EventEmitter();

async function foo() {
  await once(myEE, 'bar');
  console.log('bar');

  // This Promise will never resolve because the 'foo' event will
  // have already been emitted before the Promise is created.
  await once(myEE, 'foo');
  console.log('foo');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));

To catch both events, create each of the Promises before awaiting either of them, then it becomes possible to use Promise.all(), Promise.race(), or Promise.allSettled():

const { EventEmitter, once } = require('node:events');

const myEE = new EventEmitter();

async function foo() {
  await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
  console.log('foo', 'bar');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));