ES6 Promise 替换 async.eachLimit / async.mapLimit

Question

在async 中，如果我需要对 1000 个项目应用异步函数，我可以这样做：

async.mapLimit(items, 10, (item, callback) => {
    foo(item, callback);
});

这样只能同时处理 10 个项目，从而限制开销并允许控制。

有了 ES6 承诺，虽然我可以轻松做到：

Promise.all(items.map((item) => {
    return bar(item);
}));

这将同时处理所有 1000 个项目，这可能会导致很多问题。

我知道Bluebird have ways to handle that，但我正在寻找 ES6 解决方案。

Answer 1

如果你不关心结果，那么很快就搞定一个：

Promise.eachLimit = async (funcs, limit) => {
  let rest = funcs.slice(limit);
  await Promise.all(funcs.slice(0, limit).map(async func => {
    await func();
    while (rest.length) {
      await rest.shift()();
    }
  }));
};

// Demo:

var wait = ms => new Promise(resolve => setTimeout(resolve, ms));

async function foo(s) {
  await wait(Math.random() * 2000);
  console.log(s);
}

(async () => {
  let funcs = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("").map(s => () => foo(s));
  await Promise.eachLimit(funcs, 5);
})();

一个关键性能属性是在任何函数完成后立即运行下一个可用函数。

保留结果

按顺序保留结果可能会使其不那么优雅，但还不错：

Promise.mapLimit = async (funcs, limit) => {
  let results = [];
  await Promise.all(funcs.slice(0, limit).map(async (func, i) => {
    results[i] = await func();
    while ((i = limit++) < funcs.length) {
      results[i] = await funcs[i]();
    }
  }));
  return results;
};

// Demo:

var wait = ms => new Promise(resolve => setTimeout(resolve, ms));

async function foo(s) {
  await wait(Math.random() * 2000);
  console.log(s);
  return s.toLowerCase();
}

(async () => {
  let funcs = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("").map(s => () => foo(s));
  console.log((await Promise.mapLimit(funcs, 5)).join(""));
})();

Answer 2

没有内置任何内容，但您当然可以自己将它们分组到 Promise 链中，并在生成的链数组上使用 Promise.all：

const items = /* ...1000 items... */;
const concurrencyLimit = 10;
const promise = Promise.all(items.reduce((promises, item, index) => {
    // What chain do we add it to?
    const chainNum = index % concurrencyLimit;
    let chain = promises[chainNum];
    if (!chain) {
        // New chain
        chain = promises[chainNum] = Promise.resolve();
    }
    // Add it
    promises[chainNum] = chain.then(_ => foo(item));
    return promises;
}, []));

这是一个示例，显示了在任何给定时间有多少并发承诺（并且还显示了每个“链”何时完成，并且只执行 200 而不是 1,000）：

const items = buildItems();
const concurrencyLimit = 10;
const promise = Promise.all(items.reduce((promises, item, index) => {
    const chainNum = index % concurrencyLimit;
    let chain = promises[chainNum];
    if (!chain) {
        chain = promises[chainNum] = Promise.resolve();
    }
    promises[chainNum] = chain.then(_ => foo(item));
    return promises;
}, []).map(chain => chain.then(_ => console.log("Chain done"))));
promise.then(_ => console.log("All done"));

function buildItems() {
  const items = [];
  for (let n = 0; n < 200; ++n) {
    items[n] = n;
  }
  return items;
}

var outstanding = 0;
function foo(item) {
  ++outstanding;
  console.log("Starting " + item + " (" + outstanding + ")");
  return new Promise(resolve => {
    setTimeout(_ => {
      --outstanding;
      console.log("Resolving " + item + " (" + outstanding + ")");
      resolve(item);
    }, Math.random() * 500);
  });
}

.as-console-wrapper {
  max-height: 100% !important;
}

我应该注意，如果你想跟踪每一个的结果，你必须修改上面的；它不会尝试跟踪结果（！）。 :-)

Answer 3

使用Array.prototype.splice

while (funcs.length) {
  await Promise.all( funcs.splice(0, 100).map(f => f()) )
}

Answer 4

这是最接近 async.eachLimit 的一个

Promise.eachLimit = async (coll, limit, asyncFunc) => {
let ret = [];
    const splitArr = coll.reduce((acc,item,i)=> (i%limit) ? acc :[...acc,coll.slice(i,i+limit)],[])
    for(let i =0; i< splitArr.length;i++){
        ret[i]=await Promise.all(splitArr[i].map(ele=>asyncFunc(ele)));
    }
    return ret;
}

const wait = ms => new Promise(resolve => setTimeout(resolve, ms));

async function foo(s) {
  await wait(Math.random() * 2000);
  console.log(s);
  return s.toLowerCase();
}

(async () => {
  let arr = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".split("");
  console.log((await Promise.eachLimit(arr, 5, foo)));
})();