nextTick的原理及运行机制

for (macroTask of macroTaskQueue) {
    // 1. Handle current MACRO-TASK
    handleMacroTask();
      
    // 2. Handle all MICRO-TASK
    for (microTask of microTaskQueue) {
        handleMicroTask(microTask);
    }
}

<div id="app">
    <p ref="message">{{ message }}</p>
    <button @click="handleClick">updateMessage</button>
</div>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<script>
    var vm = new Vue({
        el: '#app',
        data: {
            message: '未更新'
        },
        methods: {
            handleClick () {
                vm.message = '已更新';
                console.log(111, vm.$refs.message.innerText); // => 111 "未更新"
                vm.$nextTick(() => {
                    console.log(222, vm.$refs.message.innerText); // => 222 "已更新"
                })
            }
        }
    })
</script>

/* @flow */
/* globals MutationObserver */

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

export let isUsingMicroTask = false

const callbacks = []
let pending = false

/**
 * 对所有callback进行遍历，然后指向响应的回调函数
 * 使用 callbacks 保证了可以在同一个tick内执行多次 nextTick，不会开启多个异步任务，而把这些异步任务都压成一个同步任务，在下一个 tick 执行完毕。
*/

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

// Here we have async deferring wrappers using microtasks.
// In 2.5 we used (macro) tasks (in combination with microtasks).
// However, it has subtle problems when state is changed right before repaint
// (e.g. #6813, out-in transitions).
// Also, using (macro) tasks in event handler would cause some weird behaviors
// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
// So we now use microtasks everywhere, again.
// A major drawback of this tradeoff is that there are some scenarios
// where microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690, which have workarounds)
// or even between bubbling of the same event (#6566).

let timerFunc

// The nextTick behavior leverages the microtask queue, which can be accessed
// via either native Promise.then or MutationObserver.
// MutationObserver has wider support, however it is seriously bugged in
// UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
// completely stops working after triggering a few times... so, if native
// Promise is available, we will use it:
/* istanbul ignore next, $flow-disable-line */
/**
* timerFunc 实现的就是根据当前环境判断使用哪种方式实现
* 就是按照 Promise.then和 MutationObserver以及setImmediate的优先级来判断，支持哪个就用哪个，如果执行环境不支持，会采用setTimeout(fn, 0)代替；
*/

// 判断是否支持原生 Promise
if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  timerFunc = () => {
    p.then(flushCallbacks)
    // In problematic UIWebViews, Promise.then doesn't completely break, but
    // it can get stuck in a weird state where callbacks are pushed into the
    // microtask queue but the queue isn't being flushed, until the browser
    // needs to do some other work, e.g. handle a timer. Therefore we can
    // "force" the microtask queue to be flushed by adding an empty timer.
    if (isIOS) setTimeout(noop)
  }
  isUsingMicroTask = true
  // 不支持 Promise的话，再判断是否原生支持 MutationObserver
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
  isNative(MutationObserver) ||
  // PhantomJS and iOS 7.x
  MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
  // Use MutationObserver where native Promise is not available,
  // e.g. PhantomJS, iOS7, Android 4.4
  // (#6466 MutationObserver is unreliable in IE11)
  // 新建一个 textNode的DOM对象，使用 MutationObserver 绑定该DOM并传入回调函数，在DOM发生变化的时候会触发回调，该回调会进入主线程（比任务队列优先执行）
  let counter = 1
  const observer = new MutationObserver(flushCallbacks)
  const textNode = document.createTextNode(String(counter))
  observer.observe(textNode, {
    characterData: true
  })
  timerFunc = () => {
    counter = (counter + 1) % 2
    // 此时便会触发回调
    textNode.data = String(counter)
  }
  isUsingMicroTask = true
  // 不支持的 MutationObserver 的话，再去判断是否原生支持 setImmediate
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  // Fallback to setImmediate.
  // Technically it leverages the (macro) task queue,
  // but it is still a better choice than setTimeout.
  timerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else {
  // Promise，MutationObserver, setImmediate 都不支持的话，最后使用 setTimeout(fun, 0)
  // Fallback to setTimeout.
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

// 该函数的作用就是延迟 cb 到当前调用栈执行完成之后执行
export function nextTick (cb?: Function, ctx?: Object) {
  // 传入的回调函数会在callbacks中存起来
  let _resolve
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  // pending是一个状态标记，保证timerFunc在下一个tick之前只执行一次
  if (!pending) {
    pending = true
    /**
    * timerFunc 实现的就是根据当前环境判断使用哪种方式实现
    * 就是按照 Promise.then和 MutationObserver以及setImmediate的优先级来判断，支持哪个就用哪个，如果执行环境不支持，会采用setTimeout(fn, 0)代替；
    */
    timerFunc()
  }
  // 当nextTick不传参数的时候，提供一个Promise化的调用
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}