JavaScript实现指定数量的并发限制的示例代码
时间：2020-12-20 12:30:28 | 栏目：JavaScript代码 | 点击：次
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)
})
OK，我们看到结果是如我们所愿的