JavaScript实现指定数量的并发限制的示例代码

Z时代
2024-01-10
分类：综合

在网上看到这么一道题：

这道题跟鱼头这篇记录『什么是时间分片（Time Slicing）？』有点相似，但不同的是这次是限制异步并发的数量。

所以话不多说，我们先来康康实现

首先我们来实现一个分割数组的函数~


const group = (list = [], max = 0) => {
  if (!list.length) {
    return list
  }
  let results = []
  for (let i = 0, len = list.length; i < len; i += max) {
    results.push(list.slice(i, i + max))
  }
  return results
}

这里就是根据指定的并发数量来分割数组。主要就是 for + slice ，这没啥好说的

接下来我们再来一个用 async + await 实现的请求集合封装。

我们通过 for...of 去遍历每一个异步函数，然后用 async + await 确保函数的执行顺序，再用 try...catch 来保证即使 reject 报错也不会导致无法继续执行任务。


const requestHandler = async (
  groupedUrl = [],
  callback = () => { }
) => {
  if (!groupedUrl.length) {
    callback()
    return groupedUrl
  }
  const newGroupedUrl = groupedUrl.map(fn => fn())
  const resultsMapper = (results) => results.map(callback)
  const data = await Promise.allSettled(newGroupedUrl).then(resultsMapper)
  return data;
}

接下来就是主函数


const sendRequest = async (
  urls = [],
  max = 0,
  callback = () => { }
) => {
  if (!urls.length) {
    return urls
  }
  const groupedUrls = group(urls, max)
  const results = []
  console.log('start !')
  for (let groupedUrl of groupedUrls) {
    try {
      const result = await requestHandler(groupedUrl, callback)
      results.push(result)
      console.log('go')
    } catch { }
  }
  console.log('done !')
  return results
}

这里就是利用了 for + async + await 来限制并发。等每次并发任务结果出来之后再执行下一次的任务。

我们执行下栗子：


const p1 = () => new Promise((resolve, reject) => setTimeout(reject, 1000, 'p1'))
const p2 = () => Promise.resolve(2)
const p3 = () => new Promise((resolve, reject) => setTimeout(resolve, 2000, 'p3'))
const p4 = () => Promise.resolve(4)
const p5 = () => new Promise((resolve, reject) => setTimeout(reject, 2000, 'p5'))
const p6 = () => Promise.resolve(6)
const p7 = () => new Promise((resolve, reject) => setTimeout(resolve, 1000, 'p7'))
const p8 = () => Promise.resolve(8)
const p9 = () => new Promise((resolve, reject) => setTimeout(reject, 1000, 'p9'))
const p10 = () => Promise.resolve(10)
const p11 = () => new Promise((resolve, reject) => setTimeout(resolve, 2000, 'p10'))
const p12 = () => Promise.resolve(12)
const p13 = () => new Promise((resolve, reject) => setTimeout(reject, 1000, 'p11'))
const p14 = () => Promise.resolve(14)
const ps = [p1, p2, p3, p4, p5, p6, p7, p8, p9, p10, p11, p12, p13, p14]
sendRequest(ps, 3, ({reason, value}) => {
  console.log(reason || value)
})