我们晓得vue中有一个api。Vue.nextTick( [callback, context] )
他的作用是鄙人次 DOM 更新轮回完毕以后实行耽误回调。在修正数据以后马上运用这个要领，猎取更新后的 DOM。那末这个api是怎样完成的呢？你一定也有些疑问或许猎奇。下面就是我的探究，分享给人人，也迎接人人到github上和我举行议论哈~~
起首贴一下vue的源码,然后我们再一步步的剖析
/* @flow */
/* globals MessageChannel */
import { noop } from 'shared/util'
import { handleError } from './error'
import { isIOS, isNative } from './env'
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 both microtasks and (macro) tasks.
// In < 2.4 we used microtasks everywhere, but there are some scenarios where
// microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690) or even between bubbling of the same
// event (#6566). However, using (macro) tasks everywhere also has subtle problems
// when state is changed right before repaint (e.g. #6813, out-in transitions).
// Here we use microtask by default, but expose a way to force (macro) task when
// needed (e.g. in event handlers attached by v-on).
let microTimerFunc
let macroTimerFunc
let useMacroTask = false
// Determine (macro) task defer implementation.
// Technically setImmediate should be the ideal choice, but it's only available
// in IE. The only polyfill that consistently queues the callback after all DOM
// events triggered in the same loop is by using MessageChannel.
/* istanbul ignore if */
if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
macroTimerFunc = () => {
setImmediate(flushCallbacks)
}
} else if (typeof MessageChannel !== 'undefined' && (
isNative(MessageChannel) ||
// PhantomJS
MessageChannel.toString() === '[object MessageChannelConstructor]'
)) {
const channel = new MessageChannel()
const port = channel.port2
channel.port1.onmessage = flushCallbacks
macroTimerFunc = () => {
port.postMessage(1)
}
} else {
/* istanbul ignore next */
macroTimerFunc = () => {
setTimeout(flushCallbacks, 0)
}
}
// Determine microtask defer implementation.
/* istanbul ignore next, $flow-disable-line */
if (typeof Promise !== 'undefined' && isNative(Promise)) {
const p = Promise.resolve()
microTimerFunc = () => {
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)
}
} else {
// fallback to macro
microTimerFunc = macroTimerFunc
}
/**
* Wrap a function so that if any code inside triggers state change,
* the changes are queued using a (macro) task instead of a microtask.
*/
export function withMacroTask (fn: Function): Function {
return fn._withTask || (fn._withTask = function () {
useMacroTask = true
const res = fn.apply(null, arguments)
useMacroTask = false
return res
})
}
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
if (useMacroTask) {
macroTimerFunc()
} else {
microTimerFunc()
}
}
// $flow-disable-line
if (!cb && typeof Promise !== 'undefined') {
return new Promise(resolve => {
_resolve = resolve
})
}
}
这么多代码，能够猛的一看，能够有点懵，没关系，我们一步一步抽出枝干。起首我们看一下这个js文件里的nextTick的定义
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
if (useMacroTask) {
macroTimerFunc()
} else {
microTimerFunc()
}
}
// $flow-disable-line
if (!cb && typeof Promise !== 'undefined') {
return new Promise(resolve => {
_resolve = resolve
})
}
}我将代码精简一下。以下所示，下面的代码预计就比较轻易看懂了。把cb函数放到会掉行列里去，假如支撑macroTask，则应用macroTask鄙人一个事宜轮回中实行这些异步的使命，假如不支撑macroTask，那就应用microTask鄙人一个事宜轮回中实行这些异步使命。
export function nextTick (cb?: Function, ctx?: Object) {
callbacks.push(cb)
if (useMacroTask) {
macroTimerFunc()
} else {
microTimerFunc()
}
}这里再次提高一下js的event loop的相干学问，js中的两个使命行列 ：macrotasks、microtasks
macrotasks: script(一个js文件),setTimeout, setInterval, setImmediate, I/O, UI rendering
microtasks: process.nextTick, Promises, Object.observe, MutationObserver
实行历程：
1.js引擎从macrotask行列中取一个使命实行
2.然后将microtask行列中的一切使命顺次实行完
3.再次从macrotask行列中取一个使命实行
4.然后再次将microtask行列中一切使命顺次实行完
……
轮回往复
那末我们再看我们精简掉的代码都是干什么的呢？我们往异步行列里放回调函数的时刻，我们并非直接放回调函数，而是包装了一个函数，在这个函数里挪用cb，而且用try catch包裹了一下。这是由于cb在运行时失足，我们不try catch这个毛病的话，会致使全部顺序崩溃掉。 我们还精简掉了以下代码
if (!cb && typeof Promise !== 'undefined') {
return new Promise(resolve => {
_resolve = resolve
})
}这段代码是干吗的呢？也就是说我们用nextTick的时刻，还能够有promise的写法。假如没有向nextTick中传入cb，而且浏览器支撑Promise的话，我们的nextTick返回的将是一个Promise。所以，nextTick的写法也能够是以下如许的
nextTick().then(()=>{console.log("XXXXX")})
vue源码里关于nextTick的封装的思绪，也给我们一些异常有益的启发，就是我们日常平凡在封装函数的时刻，要想同时指出回折衷promise的话，就能够自创vue中的思绪。
大抵的思绪我们已捋顺了。然则为何实行macroTimerFunc或许microTimerFunc就会鄙人一个tick实行我们的回调行列呢？下面我们来剖析一下这两个函数的定义。起首我们剖析macroTimerFunc
let macroTimerFunc
if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
macroTimerFunc = () => {
setImmediate(flushCallbacks)
}
} else if (typeof MessageChannel !== 'undefined' && (
isNative(MessageChannel) ||
// PhantomJS
MessageChannel.toString() === '[object MessageChannelConstructor]'
)) {
const channel = new MessageChannel()
const port = channel.port2
channel.port1.onmessage = flushCallbacks
macroTimerFunc = () => {
port.postMessage(1)
}
} else {
/* istanbul ignore next */
macroTimerFunc = () => {
setTimeout(flushCallbacks, 0)
}
}从上边的代码能够看出，假如浏览器支撑setImmediate，我们就用setImmediate，假如浏览器支撑MessageChannel，我们就用MessageChannel的异步特征，假如两者都不支撑，我们就贬价到setTimeout
，用setTimeout来把callbacks中的使命鄙人一个tick中实行
macroTimerFunc = () => {
setTimeout(flushCallbacks, 0)
}剖析完macroTimerFunc，下面我们最先剖析microTimerFunc，我把vue源码中关于microTimerFunc的定义轻微精简一下
let microTimerFunc;
if (typeof Promise !== 'undefined' && isNative(Promise)) {
const p = Promise.resolve()
microTimerFunc = () => {
p.then(flushCallbacks)
}
} else {
// fallback to macro
microTimerFunc = macroTimerFunc
}
从上边精简以后的代码，我们能够看到microTimerFunc的完成思绪。假如支撑浏览器支撑promise，就用promise完成。假如不支撑，就降低到用macroTimerFunc
over，团体逻辑就是如许。。看着吓人，掰开了以后好好剖析一下，照样挺简朴的。
