NodeJS 7：EventEmitter + 等待/异步

Question

我们如何从传递给事件发射器的回调中结束 async 函数而不承诺事件发射器？

同样不使用外部模块，只需简单的NodeJS 7.x/8.x（支持Es6语法和async/await。

我们希望基本上将async function ... 与事件发射器混合，以便在事件发射器发出end 信号时解决。

另外请记住，在使用 await 完成其他一些异步函数之前，我们不会从事件发射器开始。

如果我们有一个“新的 Promise(...)”，我们会调用 resolve();并且头痛会结束，但是在“异步”中没有“解决”，而且我们不能使用“返回”，因为我们在回调中。

/*
 * Example of mixing Events + async/await.
 */

// Supose a random pomise'd function like:
function canIHazACheezBurger () {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(Math.random() > 0.5);
    }, 500 + Math.random() * 500)
  });
}

/**
 * Then, we want to mix an event emitter with this logic,
 * what we want is that this function resolves the promise
 * when the event emitter signals 'end' (for example).
 * Also bear in mind that we won't start with the event emitter
 * until done with the above function.
 * If I had a "new Promise(...)" I would call resolve(); and the
 * headache would be over, but in 'async' there's no 'resolve',
 * plus I cannot use 'return' because I'm inside a callback.
 */
async function bakeMeSomeBurgers () {
  let canIHave = await canIHazACheezBurger();
  // Do something with the result, as an example.
  if (canIHave) {
    console.log('Hehe, you can have...');
  } else {
    console.log('NOPE');
  }
  // Here invoke our event emitter:
  let cook = new BurgerCooking('cheez');
  // Assume that is a normal event emitter, like for handling a download.
  cook.on('update', (percent) => {
    console.log(`The burger is ${percent}% done`);
  });
  // Here lies the problem:
  cook.on('end', () => {
    console.log('I\'ve finished the burger!');
    if (canIHave) {
      console.log('Here, take it :)');
    } else {
      console.log('Too bad you can\'t have it >:)');
    }
    // So, now... What?
    // resolve(); ? nope
    // return; ?
  });
}

免责声明

如果这个问题已经在某个地方完成，我想道歉。所做的研究显示了与混合异步与同步逻辑相关的问题，但我对此一无所知。

标题中类似的问题是this 'write async function with EventEmitter'，但与本问题无关。

Answer 1

我们能否在不承诺事件发射器的情况下从传递给事件发射器的回调中结束异步函数？

否。 async/await 语法只是 then 调用的糖并依赖于承诺。

async function bakeMeSomeBurgers () {
  let canIHave = await canIHazACheezBurger();
  if (canIHave)
    console.log('Hehe, you can have...');
  else
    console.log('NOPE');

  // Here we create and await our promise:
  await new Promise((resolve, reject) => {
    // Here invoke our event emitter:
    let cook = new BurgerCooking('cheez');
    // a normal event callback:
    cook.on('update', percent => {
      console.log(`The burger is ${percent}% done`);
    });
    cook.on('end', resolve); // call resolve when its done
    cook.on('error', reject); // don't forget this
  });

  console.log('I\'ve finished the burger!');
  if (canIHave)
    console.log('Here, take it :)');
  else
    console.log('Too bad, you can\'t have it >:)');
}