浅谈Vue.nextTick 的实现方法(2)

日期：2020-05-21 栏目：程序人生浏览：次

只关注主流程queueNextTick函数把我们传入的() => { // todo... } 推入了callbacks内

 if (typeof Promise !== 'undefined' && isNative(Promise)) {
  var p = Promise.resolve()
  var logError = err => { console.error(err) }
  timerFunc = () => {
   p.then(nextTickHandler).catch(logError) // 重点
  }
 } else if ('!isIE MutationObserver') {
  var counter = 1
  var observer = new MutationObserver(nextTickHandler) // 重点
  var textNode = document.createTextNode(string(conter))

  observer.observe(textNode, {
   characterData: true
  })
  timerFunc = () => {
   counter = (counter + 1) % 2
   textNode.data = String(counter)
  }
 } else {
  timerFunc = () => {
   setTimeout(nextTickHandler, 0) // 重点
  }
 }

这一段我们可以看到标注的三个点表明在不同浏览器环境下使用Promise, MutationObserver或setTimeout(fn, 0) 来执行nextTickHandler

function nextTickHandler () {
  pending = false
  const copies = callbacks.slice(0) // 复制
  callbacks.length = 0 // 清空
  for (let i = 0; i < copies.length; i++) {
   copies[i]() // 逐个执行
  }
 }

nextTickHandler就是把我们之前放入callbacks的 () => { // todo... } 在当前tasks内执行。

写一个简单的nextTick

源码可能比较绕，我们自己写一段简单的nextTick

const simpleNextTick = (function () {
 let callbacks = []
 let timerFunc

 return function queueNextTick (cb) {
  callbacks.push(() => { // 给callbacks 推入cb()
   cb()
  })

  timerFunc = () => {
   return Promise.resolve().then(() => {
    const fn = callbacks.shift()
    fn()
   })
  }
  timerFunc() // 执行timerFunc，返回到是一个Promise
 }
})()

simpleNextTick(() => {
 setTimeout(console.log, 3000, 'nextTick')
})

我们可以从这里看出nextTick的原理就是返回出一个Promise，而我们todo的代码在这个Promise中执行，现在我们还可以继续简化

const simpleNextTick = (function () {
 return function queueNextTick (cb) {
  timerFunc = () => {
   return Promise.resolve().then(() => {
    cb()
   })
  }
  timerFunc()
 }
})()

simpleNextTick(() => {
 setTimeout(console.log, 3000, 'nextTick')
})

直接写成这样。

const simpleNextTick = function queueNextTick (cb) {
  timerFunc = () => {
   return Promise.resolve().then(() => {
    cb()
   })
  }
  timerFunc()
 }

simpleNextTick(() => {
 setTimeout(console.log, 3000, 'nextTick')
})

这次我们把自执行函数也简化掉

const simpleNextTick = function queueNextTick (cb) {
   return Promise.resolve().then(cb)
 }

simpleNextTick(() => {
 setTimeout(console.log, 3000, 'nextTick')
})

转载注明出处：http://www.heiqu.com/6172.html

浅谈Vue.nextTick 的实现方法(2)

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