Vue中的nextTick方法

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在官方文档中，nextTick()方法表示等待下一次DOM更新刷新的工具方法。源于Vue的异步更新机制，使得响应式数据发生改变时不会立即引起DOM视图上的更新，而是将它们存放到一个队列当中，直到下一次DOM更新之后执行。这样是为了确保每个组件无论发生多少状态改变，都仅执行一次更新。那么nextTick()的方法就是帮助我们异步的调用更新视图逻辑的回调。

运用场景

当我们开发项目时，可能会遇到一种场景，当更新了数据后，需要对新DOM做一些操作，但是按照我们之前所说的异步更新机制来讲，此时我们是获取不到更新后的DOM，因为还没有重新渲染更新后的DOM，这个时候就需要用到我们的nextTick()方法。 贴一个vue文档的例子（基于vue3）

<template>
    <button id="counter" @click="increment">{{count}}</button>
</template>
<script setup>
import {ref,nextTick} from 'vue'
let count = ref(0)
async function increment(){
    count.value++
    
    // DOM 还未更新 
    console.log(document.getElementById('counter').textContent) // 0 
    await nextTick() 
    
    // DOM 此时已经更新 
    console.log(document.getElementById('counter').textContent) // 1
}
</script>

事件循环

说到nextTick()就不得不提事件循环，因为nextTick()本质上返回的就是一个promise对象，既然涉及到promise自然需要理解js的执行过程。推荐看这篇文章👉你真的理解$nextTick么 - 掘金 (juejin.cn)

那么回到nextTick()，开头我们说的等待下一次DOM更新刷新的方法，其实就是在下次微任务执行时更新DOM，nextTick()就是将你的回调的代码添加到微任务中，等到执行栈将所有任务都执行完毕以后再去执行微任务队列。

源码

/* globals MutationObserver */

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

export let isUsingMicroTask = false

const callbacks: Array<Function> = []
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]()
  }
}

// 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 */
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
} 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)
  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)
  }
  isUsingMicroTask = true
} 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 {
  // Fallback to setTimeout.
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

export function nextTick(): Promise<void>
export function nextTick<T>(this: T, cb: (this: T, ...args: any[]) => any): void
export function nextTick<T>(cb: (this: T, ...args: any[]) => any, ctx: T): void
/**
 * @internal
 */
export function nextTick(cb?: (...args: any[]) => any, ctx?: object) {
  let _resolve
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e: any) {
        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
    })
  }
}