结合Event Loop谈谈对Vue中nextTick的理解

// src\core\util\next-tick.js

import { noop } from 'shared/util'
import { handleError } from './error'
import { isIE, isIOS, isNative } from './env'

const callbacks = []
let pending = false

function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]( "i")
  }
}

let timerFunc

// nextTick行为利用了微任务队列，微任务队列可以通过原生Promise.then或MutationObserver访问到。 
// MutationObserver具有更广泛的支持，但是在iOS> = 9.3.3中的UIWebView中，在触摸事件处理程序中触发时会发生错误。触发几次后，它将完全停止工作
// 因此，如果原生Promise可用，优先使用Promise：

if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  timerFunc = () => {
    p.then(flushCallbacks)
    // 在有问题的UIWebViews中，会出现奇怪的状态：微任务队列中有回调但是不被清空，直到浏览器有其他任务，例如处理计时器
    // 因此此处使用一个空计时器，来强制触发微任务队列执行
    if (isIOS) setTimeout(noop)
  }
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
  isNative(MutationObserver) ||
  // PhantomJS and iOS 7.x
  MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
  let counter = 1
  const observer = new MutationObserver(flushCallbacks)
  const textNode = document.createTextNode(String(counter))
  observer.observe(textNode, {
    characterData: true
  })
  timerFunc = () => {
    counter = (counter + 1) % 2
    textNode.data = String(counter)
  }
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  // setImmediate,宏任务,但是相比 setTimeout 是个更好的选择
  timerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else {
  // setTimeout 0 宏任务
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

export function nextTick (cb?: Function, ctx?: Object) {
  let _resolve
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  if (!pending) {
    pending = true
    timerFunc()
  }
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}

..
<template>
	<div id="chart" :style='{width:chartWidth,height:"200px"}'
</template>

...
this.chartWidth = getWidthByData(data)
this.nextTick(()=>{
	let chart= echarts.init(document.getElementById('chart'))
	chart.setOption({...})  //echarts渲染
})