nextTick:官方文档的解释是可以在DOM更新完毕之后执行一个回调;可以看作是在下次DOM更新循环结束之后执行延迟回调。在修改数据之后立即使用这个方法，获取更新之后的DOM
尽管MVVM框架并不推荐访问DOM，但有时会有操作DOM的需求，尤其是在和第三方组件配合时，免不了要进行DOM操作，而nextTick提供了一个桥梁，确保操作的是更新后的DOM
Vue如何检测到DOM更新完毕呢？
有个H5新增属性:MutationObserver 可以监听到DOM节点的改动
//MutationObserver基本用法
var observer = new MutationObserver(function(){
//这里是回调函数
console.log("DOM发生了变化")
})
var article = document.querySelector("article")
observer.observer(article)
Vue中关于nextTick的源码
/* @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
})
}
}
对是否支持Promise进行一个兼容判断，根本上是一个回调函数
for(let i =0;i<100;i++){
dom.style.left=i+"px"
}
上边的100次for循环同属于一个任务，浏览器只在该任务执行完毕之后进行一次DOM更新。
vue的数据响应过程：数据更改->通知Watcher->更新DOM.
Microtask
从名字看，可以称为微任务
每一次事件循环都包含一个微任务队列，在循环结束后会依次执行队列中的microtask并且移除，然后在开始下一次事件循环。这种方法和setTimeout类似，
在执行微任务的过程中加入微任务队列的微任务，也会在下一次事件循环之前被执行。也就是说宏任务要在微任务执行完成之后才能执行。Vue进行DOM更新内部也是调用nextTick来做异步队列控制。当我们自己调用nextTick时，它就在更新DOM的那个微任务后追加了我们自己的回调函数，从而确保了我们的代码在DOM更新后执行。
最优的微任务队列就是Promise，但是Promise是ES6新增属性，对于一些老的浏览器这种就存在bug(某些老公司会坚持使用老IE浏览器，即使IE没有了)，所以代码中就存在如果判断不支持Promise的判断，如果有这种情况，就开始从微任务降级为宏任务。
但是setTimeout不是最佳方案，因为setTimeout有着4ms的延迟，着是不可避免的。
最佳方案依次是：setimmediate、messageChannel、setTimeout。
setimmediate算是最理想的解决方案了，但是有兼容问题，IE和nodejs支持
MessageChannel的onmessage回调也是microtask，但是也有兼容性问题，所以只能是setTimeout来兜底。
总结
以上是Vue的nextTick方法的实现原理，大体是：
1.Vue用异步队列的方法来控制DOM更新和nextTick回调先后执行
2.microtask因为高优先级特性，能确保队列中的微任务在循环前执行完毕
3.因为兼容问题，不得不向下兼容，从微任务向宏任务兼容降级
