并行调用一组promise，但按顺序解决它们而不等待其他promise解决

Question

我有一系列要并行调用的 promise，但要同步解决。

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 来包装本机 Promise，我可以同步检查它的已解决状态。

有没有更好的方法来完成这个不需要特殊对象的任务？

请注意。我不想使用Promise.all，因为我不想在处理承诺的效果之前等待所有承诺解决。而且我不能在我的代码库中使用async 函数。

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，如果你已经拥有一组promise，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}`);
  }
})();

由于您不能使用异步函数，您可以通过使用Array.prototype.reduce() 将promise 链接在一起来模仿for await...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

由于承诺被声明为并行实例化，我假设任何单个承诺上的错误不会传播到其他承诺，除非通过asyncForEach()（如上）构造的任何潜在脆弱链。

但我们也希望在 asyncForEach() 中将它们链接在一起时避免在 Promise 之间交叉传播错误。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承诺传播：

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，在传递给asyncForEach() 的回调中抛出的任何错误都将被链中的错误处理吞没，除了在数组最后一个索引的回调中抛出的错误。

Answer 2

我建议确实使用Promise.all - 但不是一次用于所有承诺，而是您希望在每个步骤中实现的所有承诺。您可以使用reduce 创建这个承诺的“树列表”：

function parallelPromisesSequentialReduce(promises, reducer, initial) {
  return promises.reduce((acc, promise, i) => {
    return Promise.all([acc, promise]).then(([prev, res]) => reducer(prev, res, i));
  }, Promise.resolve(initial));
}

const timedPromise = (delay, label) => new Promise(resolve =>
  setTimeout(() => {
    console.log('fulfilled ' + label + ' promise');
    resolve(label);
  }, delay)
);

parallelPromisesSequentialReduce([
  timedPromise(20, 'first'),
  timedPromise(60, 'second'),
  timedPromise(40, 'third'),
], (acc, res) => {
  console.log('combining ' + res + ' promise with previous result (' + acc + ')');
  acc.push(res);
  return acc;
}, []).then(res => {
  console.log('final result', res);
}, console.error);