this.$nextTick

概念：

vue的响应式并不是只有数据变化之后，dom就会立即发生变化，而是按照一定策略更新dom的。

即放到nextTick中的操作不会立即执行，而是等数据更新，dom更新完成之后执行。在准确的说就是，将nextTick的方法延迟到下次dom更新循环之后执行。

分析：

2.2.5中vue使用mutationObserver实现。

2.5.0就做降级处理，降级的方案依次是：setImmediate,MessageChannel, setTimeout ， 前两个都有兼容性问题，最后兜底的就是setTimeout了。

vue.nextTick 方法的实现原理：

1、 vue异步队列的方式来控制dom更新和nextTick回调先后执行

2、微观队列因为其高优先级特性，能确保队列中微任务在一次事件循环前被执行

3、因为兼容性问题，vue不得不做了微观事件到宏观事件的降级

总结:

因为vue的异步更新策略，导致我们数据的修改不会立即体现到dom变化上。

mutationObserver 是html5新增的属性，用于监听dom修改事件，能够监听节点属性，文本内容，子节点等改动。

 

nextTick的源码 兼容判断，先判断是否支持promise (es6特性，兼容性)  ---mutationObserver(ios有bug 选择使用settimeout) ------ setImmediate  (兼容ie和node) ------  setTimeout

nextTick总是 由于 setTimeout 执行的。

源码文件路径：vue/src/core/util/next-tick.js

/* 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)) { //---存在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) //ios环境
  }
  isUsingMicroTask = true
} else if (!isIE && typeof MutationObserver !== 'undefined' && ( //不是ie 且存在mutationObserver
  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
    })
  }
}