nextTick在项目中的实践

前言

在项目中经常需要在视图层立即显示数据，而有时候由于异步数据传递的原因，在页面上并不会立即显示页面，这时候就需要使用Vue提供的nextTick这个方法，其主要原因是Vue的数据视图是异步更新的，用官方的解释就是：

Vue 实现响应式并不是数据发生变化之后 DOM 立即变化，而是按一定的策略进行 DOM 的更新。

其中说到的事件循环也是前端面试中常问到的一个点，本文不做具体展开，有兴趣的同学可参考这篇文章一次弄懂Event Loop（彻底解决此类面试问题）

踩坑目录

模板案例数据在视图上显示
兄弟组件间异步数据传递
$nextTick源码实现解析

踩坑案例

模板案例数据在视图上显示

[bug描述] 页面上点击重置后将模板视图渲染会一个固定数据下的视图

[bug分析] 点击后需要立即显示在页面上，这是典型的nextTick需要应用的场景

[解决方案]

此处还有一个坑就是对于数组类型的监听是基于一个地址的，因而如果需要Vue的Watcher能够监视到就需要符合数组监听的那几种方法，这里直接新建，相当于每次的地址都会发生变化，因而可以监听到

    async resetTemplate() {
      this.template = [];
      await this.$nextTick(function() {
          this.template = [
          {
            week: '1',
            starttime: '00:00:00',
            endtime: '00:00:00'
          },
          {
            week: '2',
            starttime: '00:00:00',
            endtime: '00:00:00'
          },
          {
            week: '3',
            starttime: '00:00:00',
            endtime: '00:00:00'
          },
          {
            week: '4',
            starttime: '00:00:00',
            endtime: '00:00:00'
          },
          {
            week: '5',
            starttime: '00:00:00',
            endtime: '00:00:00'
          },
          {
            week: '6',
            starttime: '00:00:00',
            endtime: '00:00:00'
          },
          {
            week: '7',
            starttime: '00:00:00',
            endtime: '00:00:00'
          }
        ];
      });
    }

兄弟组件间异步数据传递

[bug描述] 页面修改弹窗中的输入框字段需要复写进对应字段，利用Props传递数据进去后并不会直接修改数据

[bug分析] 此场景下数据是通过子组件emit给父组件，父组件获取数据后通过props传递给弹窗，在v-model中获取数据是异步的

[解决方案]

这是比较不常见的一种使用 $nextTick去处理v-model异步数据传递的方法(ps: 关于emit/on的发布订阅相关的介绍，有兴趣的同学可以看一下这篇文章 [vue发布订阅者模式$ emit、$on](https://blog.csdn.net/qq_42778001/article/details/96692000))，利用的是父组件的数据延迟到下一个tick去给子组件传递，子组件在对应页面上及时渲染的方法，除了这种方法还有其他方法，具体可参考这篇文章详解vue父组件传递props异步数据到子组件的问题

    edit(data) {
      this.isManu = true;
      let [content,pos] = data;
      this.manuPos = pos;
      this.form = content;
      this.$nextTick(function(){
        this.$refs.deviceEdit.form.deviceid = content.deviceId;
        this.$refs.deviceEdit.form.devicename = content.deviceName;
        this.$refs.deviceEdit.form.devicebrand = content.deviceBrand;
        this.$refs.deviceEdit.form.devicegroup = content.deviceGroup;
        this.$refs.deviceEdit.form.mediatrans = content.mediaTrans;
        this.$refs.deviceEdit.form.cloudstorage = content.cloudStorage;
        this.$refs.deviceEdit.form.longitude = content.longitude;
        this.$refs.deviceEdit.form.latitude = content.latitude;
        this.$refs.deviceEdit.form.altitude = content.altitude;
      })
    },

$nextTick源码实现解析

2.5之前的版本：

/**
 * Defer a task to execute it asynchronously.
 */
export const nextTick = (function () {
  const callbacks = []
  let pending = false
  let timerFunc

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

  // 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 if */
  if (typeof Promise !== 'undefined' && isNative(Promise)) {
    var p = Promise.resolve()
    var logError = err => { console.error(err) }
    timerFunc = () => {
      p.then(nextTickHandler).catch(logError)
      // 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)
    }
  } 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
    var counter = 1
    var observer = new MutationObserver(nextTickHandler)
    var textNode = document.createTextNode(String(counter))
    observer.observe(textNode, {
      characterData: true
    })
    timerFunc = () => {
      counter = (counter + 1) % 2
      textNode.data = String(counter)
    }
  } else {
    // fallback to setTimeout
    /* istanbul ignore next */
    timerFunc = () => {
      setTimeout(nextTickHandler, 0)
    }
  }

  return function queueNextTick (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()
    }
    if (!cb && typeof Promise !== 'undefined') {
      return new Promise((resolve, reject) => {
        _resolve = resolve
      })
    }
  }
})()

2.5之后的版本

/* @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

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 (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
    })
  }
}

不同版本主要在于timeFunc的异步函数使用优先顺序不同，2.5之后也有些许不同，但主要在于要不要暴露微任务函数和宏任务函数的问题(ps:上边的2.5之后的版本是2.6.11)

2.5之前版本： Promise => MutationObserver => setTimeout

2.5之后版本： setImmediate => MessageChannel => Promise => setTimeout

总结

js的异步执行机制是前端同学必须掌握的知识，其中nextTick就是其中一个很典型的代表，node中也有nextTick相关的方法，面试中也常常问到相关方法的实现，深刻理解js的基础方法和特性，对前端开发中避坑还是很有用处的，每每出现问题几乎在所有的面试题中都有相关知识的展现，打好基础永远是一个工程师上升的坚实的基础！

let callbacks = []
let pending = false

function nextTick (cb) {
    callbacks.push(cb)

    if (!pending) {
        pending = true
        setTimeout(flushCallback, 0)
    }
}

function flushCallback () {
    pending = false
    let copies = callbacks.slice()
    callbacks.length = 0
    copies.forEach(copy => {
        copy()
    })
}

参考