javascript - Node 7 : EventEmitter + await/async

Question

我们如何通过传递给事件发射器的回调结束 async 函数而不 promise 事件发射器？

也不使用外部模块，只是简单的NodeJS 7.x/8.x(支持Es6语法和async/等待。

我们主要是想将 async function ... 与事件发射器混合，以便在事件发射器发出 end 信号时解析。

另请记住，在使用 await 完成一些其他异步函数之前，我们不会从事件发射器开始。

如果我们有一个“new Promise(...)”，我们会调用 resolve()；头痛也就过去了，但是在“async”中没有“resolve”，而且我们不能使用“return”，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处完成，我想道歉。所做的研究显示了与混合异步和同步逻辑相关的问题，但我对此一无所知。

标题中的类似问题是 this 'write async function with EventEmitter'但与本题无关。

Answer 1

Can we end an async function from a callback passed to an event emitter without promisifing the event emitter?

否。 async/await 语法只是 then 调用的糖分，并且依赖于 promises。

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 >:)');
}