首页手记 JavaScript实现指定数量的并发限制

JavaScript实现指定数量的并发限制

标签：

JavaScript

作者：陈大鱼头

首发地址： https://github.com/KRISACHAN/ying-study/issues

说明：鱼头的学习记录

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Promise.allSettled去执行每一组的请求集合。

Promise.allSettled是一个新的API，跟Promise.all差不多的用法，也是接受的数组，不过不同的是Promise.allSettled会等所有任务结束之后才会返回结果，而Promise.all只要有一个reject就会返回结果。

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)
})