javascript - 并行调用一系列 Promise，但按顺序解决它们，而不等待其他 Promise 解决

Question

我有一系列的 promise ，我想并行调用，但同步解析。

我编写了这段代码来完成所需的任务，但是，我需要创建自己的对象QueryablePromise来包装我可以同步检查的 native Promise查看其已解决状态。

有没有更好的方法来完成这个不需要特殊对象的任务？

Please note. I do not want to use Promise.all as I don't want to have to wait for all promises to resolve before processing the effects of the promises. And I cannot use async functions in my code base.

const PROMISE = Symbol('PROMISE')
const tap = fn => x => (fn(x), x)

class QueryablePromise {
  resolved = false
  rejected = false
  fulfilled = false
  constructor(fn) {
    this[PROMISE] = new Promise(fn)
      .then(tap(() => {
        this.fulfilled = true
        this.resolved = true
      }))
      .catch(x => {
        this.fulfilled = true
        this.rejected = true
        throw x
      })
  }
  then(fn) {
    this[PROMISE].then(fn)
    return this
  }
  catch(fn) {
    this[PROMISE].catch(fn)
    return this
  }
  static resolve(x) {
    return new QueryablePromise((res) => res(x))
  }
  static reject(x) {
    return new QueryablePromise((_, rej) => rej(x))
  }
}

/**
 * parallelPromiseSynchronousResolve
 *
 * Call array of promises in parallel but resolve them in order
 *
 * @param  {Array<QueryablePromise>}  promises
 * @praram {Array<fn>|fn}  array of resolver function or single resolve function
 */
function parallelPromiseSynchronousResolve(promises, resolver) {
  let lastResolvedIndex = 0
  const resolvePromises = (promise, i) => {
    promise.then(tap(x => {
      // loop through all the promises starting at the lastResolvedIndex
      for (; lastResolvedIndex < promises.length; lastResolvedIndex++) {
        // if promise at the current index isn't resolved break the loop
        if (!promises[lastResolvedIndex].resolved) {
          break
        }
        // resolve the promise with the correct resolve function
        promises[lastResolvedIndex].then(
          Array.isArray(resolver)
            ? resolver[lastResolvedIndex]
            : resolver
        )
      }
    }))
  }
  
  promises.forEach(resolvePromises)
}

const timedPromise = (delay, label) => 
  new QueryablePromise(res => 
    setTimeout(() => {
      console.log(label)
      res(label)
    }, delay)
  )

parallelPromiseSynchronousResolve([
  timedPromise(20, 'called first promise'),
  timedPromise(60, 'called second promise'),
  timedPromise(40, 'called third promise'),
], [
  x => console.log('resolved first promise'),
  x => console.log('resolved second promise'),
  x => console.log('resolved third promise'),
])

<script src="https://codepen.io/synthet1c/pen/KyQQmL.js"></script>

为任何帮助干杯。

Answer 1

使用 for await...of循环，如果你已经有了 Promise 数组，你可以很好地做到这一点:

const delay = ms => new Promise(resolve => { setTimeout(resolve, ms); });
const range = (length, mapFn) => Array.from({ length }, (_, index) => mapFn(index));

(async () => {
  const promises = range(5, index => {
    const ms = Math.round(Math.random() * 5000);
    return delay(ms).then(() => ({ ms, index }));
  });

  const start = Date.now();

  for await (const { ms, index } of promises) {
    console.log(`index ${index} resolved at ${ms}, consumed at ${Date.now() - start}`);
  }
})();

由于您无法使用异步函数，因此您可以通过使用 Array.prototype.reduce() 将 promise 链接在一起来模仿 for wait...of 的效果，并为每个链同步调度回调:

const delay = ms => new Promise(resolve => { setTimeout(resolve, ms); });
const range = (length, mapFn) => Array.from({ length }, (_, index) => mapFn(index));

const asyncForEach = (array, cb) => array.reduce(
  (chain, promise, index) => chain.then(
    () => promise
  ).then(
    value => cb(value, index)
  ),
  Promise.resolve()
);

const promises = range(5, index => {
  const ms = Math.round(Math.random() * 5000);
  return delay(ms).then(() => ms);
});

const start = Date.now();

asyncForEach(promises, (ms, index) => {
  console.log(`index ${index} resolved at ${ms}, consumed at ${Date.now() - start}`);
});

错误处理

由于 promise 被声明为并行实例化，因此我假设任何单个 promise 的错误都不会传播到其他 promise ，除非通过 asyncForEach() 构造的任何潜在的脆弱链(例如上)。

但是，当在 asyncForEach() 中将 Promise 链接在一起时，我们还希望避免 Promise 之间的交叉传播错误。这是一种稳健地安排错误回调的方法，其中错误只能从原始 promise 传播:

const delay = ms => new Promise(resolve => { setTimeout(resolve, ms); });
const maybe = p => p.then(v => Math.random() < 0.5 ? Promise.reject(v) : v);
const range = (length, mapFn) => Array.from({ length }, (_, index) => mapFn(index));

const asyncForEach = (array, fulfilled, rejected = () => {}) => array.reduce(
  (chain, promise, index) => {
    promise.catch(() => {}); // catch early rejection until handled below by chain
    return chain.then(
      () => promise,
      () => promise // catch rejected chain and settle with promise at index
    ).then(
      value => fulfilled(value, index),
      error => rejected(error, index)
    );
  },
  Promise.resolve()
);

const promises = range(5, index => {
  const ms = Math.round(Math.random() * 5000);
  return maybe(delay(ms).then(() => ms)); // promises can fulfill or reject
});

const start = Date.now();

const settled = state => (ms, index) => {
  console.log(`index ${index} ${state}ed at ${ms}, consumed at ${Date.now() - start}`);
};

asyncForEach(
  promises,
  settled('fulfill'),
  settled('reject') // indexed callback for rejected state
);

这里唯一需要注意的是，传递给 asyncForEach() 的回调中抛出的任何错误都将被链中的错误处理所吞没，除了最后一个索引上的回调中抛出的错误之外数组的。