文章目录
js异步处理方案,js的异步串行与异步并行一、什么是串行,并行,并发二、实现异步串行三、实现异步并行js异步处理方案,js的异步串行与异步并行
一、什么是串行,并行,并发
串行:这个就像队列一样,一个执行完,下一个再执行,比如js同步执行
并发:是指这个系统拥有处理多个任务的能力,在一定时间发出,不需要同时性执行。js的异步消息队列让js拥有并发的能力
比如执行多个settimeout 它会挂载在消息队列,然后依次执行
并行:是指这个系统拥同时处理多个任务的能力,属于多线程的一种方式,而js是属于单线程 虽然可以通过worker来创建多线程,但是子线程
完全受主线程控制,没有改变JavaScript单线程的本质
串行示意图:
有人会问,异步不是都在消息队列吗,不就是一个一个执行了吗?
举个例子,如果本身就是串行执行,那么你现在执行2给个任务,一个是5秒后打印console.log(1),一个是十秒后打印console.log(2)
按照串行应该是15秒执行完毕,但实际耗时是10秒,如上这是并发,
console.time()setTimeout(() => {console.log(1)}, 5000);setTimeout(() => {console.log(2)console.timeEnd()}, 10000);
二、实现异步串行
大多时候我们可以用 promise 和async await来解决
promise
比较少的时候,可以用.then来实现串行
var a = function() {return new Promise((resolve, reject) => {setTimeout(() => {consloe.log('a')resolve('a')}, 3000)})}var b = function() {return new Promise((resolve, reject) => {setTimeout(() => {consloe.log('b')resolve('b')}, 2000)})};console.time('test')a().then((aa) => {b().then((bb) => {console.log(`${aa}-${bb}`)console.timeEnd('test')})})
async await
我们改造一下,明显使用await看起来更加清晰
var a = function() {return new Promise((resolve, reject) => {setTimeout(() => {consloe.log('a')resolve('a')}, 3000)})}var b = function() {return new Promise((resolve, reject) => {setTimeout(() => {consloe.log('b')resolve('b')}, 2000)})};(async () => {console.time('test')var aa = await a()var bb = await b()console.log(`${aa}-${bb}`)console.timeEnd('test')})()
但当我们有多个异步,比如2000个该如何
var createPromise = function(time) {// then中的回调函数 return (resolve, reject) => {return new Promise((resolve, reject) => {setTimeout(() => {//模拟请求 (真实使用把time设置为0,将resolve传入异步函数中)console.log('timein' + time)resolve(); //在异步处理结束后resolve}, time * 1000)})}}var arr = [createPromise(2), createPromise(3), createPromise(1), createPromise(4), createPromise(5)];function serpromise(arr) {arr.reduce((pre, next, index, carr) => {return pre.then(next)}, Promise.resolve())}// 相当于// Promise.resolve().then(createPromise(2)).then(createPromise(1))......serpromise(arr)
async await
var createPromise = function(time) {return () => {return new Promise((resolve, reject) => {setTimeout(() => {//模拟请求 (真实使用把time设置为0,将resolve传入异步函数中)console.log('timein' + time)resolve(); //在异步处理结束后resolve}, time * 1000)})}}var arr = [createPromise(2), createPromise(3), createPromise(1), createPromise(4), createPromise(5)];var fn = async function(promiseArr) {for (let i = 0, len = arr.length; i < len; i++) {let currentPromise = promiseArr[i]var result = await currentPromise()}}fn(arr)
通过打印可以看出如过是并发的打印会是1,2,3,4,5
三、实现异步并行
并行示意图:
js并不能并行,我们只是可以思考如何卡在abc都执行完成,这里可以用promise.all 来轻松实现
const a = new Promise((resolve, reject) => {//模拟异步任务setTimeout(function() {resolve('a');}, 1000)}).then(result => result).catch(e => {})const b = new Promise((resolve, reject) => {setTimeout(function() {// resolve('b');reject('Error in b');}, 1000)}).then(result => result).catch(e => e)Promise.all([a, b]).then(data => {console.log('data', data)}).catch(e => console.log('erro', e));
这里需要注意的是,all能接受的 resolve reject都行,所以需要每个自己去处理异常
多个异步
promise
var createPromise = function(time) {return new Promise((resolve, reject) => {setTimeout(() => {console.log('time', time);resolve(time)}, time * 1000)})}var arr = [createPromise(2), createPromise(3), createPromise(1), createPromise(4), createPromise(5)];Promise.all(arr).then(data => {console.log('data', data)})
asnyc await
var createPromise = function(time) {return new Promise((resolve, reject) => {setTimeout(() => {console.log('time', time);resolve(time)}, time * 1000)})}var arr = [createPromise(2), createPromise(3), createPromise(1), createPromise(4), createPromise(5)];var fn = async function(promiseArr) {let awaitArr = []for (let i = 0, len = arr.length; i < len; i++) {let currentPromise = promiseArr[i]let result = await currentPromiseawaitArr.push(result)}return awaitArr}// async 返回的是一个promise fn(arr).then(data => {console.log('data', data);})