我们知道vue中有一个api。Vue.nextTick( [callback, context] )
他的作用是在下次 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，获取更新后的 DOM。那么这个api是怎么实现的呢？你肯定也有些疑问或者好奇。下面就是我的探索，分享给大家，也欢迎大家到github上和我进行讨论哈~~
首先贴一下vue的源码,然后我们再一步步的分析
/* @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，可能猛的一看，可能有点懵，不要紧，我们一步一步抽出枝干。首先我们看一下这个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。如下所示，下面的代码估计就比较容易看懂了。把cb函数放到会掉队列里去，如果支持macroTask，则利用macroTask在下一个事件循环中执行这些异步的任务，如果不支持macroTask，那就利用microTask在下一个事件循环中执行这些异步任务。
export function nextTick (cb?: Function, ctx?: Object) {
callbacks.push(cb)
if (useMacroTask) {
macroTimerFunc()
} else {
microTimerFunc()
}
}这里再次普及一下js的event loop的相关知识，js中的两个任务队列 ：macrotasks、microtasks
macrotasks: script(一个js文件),setTimeout, setInterval, setImmediate, I/O, UI rendering
microtasks: process.nextTick, Promises, Object.observe, MutationObserver
执行过程：
1.js引擎从macrotask队列中取一个任务执行
2.然后将microtask队列中的所有任务依次执行完
3.再次从macrotask队列中取一个任务执行
4.然后再次将microtask队列中所有任务依次执行完
……
循环往复
那么我们再看我们精简掉的代码都是干什么的呢？我们往异步队列里放回调函数的时候，我们并不是直接放回调函数，而是包装了一个函数，在这个函数里调用cb，并且用try catch包裹了一下。这是因为cb在运行时出错，我们不try catch这个错误的话，会导致整个程序崩溃掉。 我们还精简掉了如下代码
if (!cb && typeof Promise !== 'undefined') {
return new Promise(resolve => {
_resolve = resolve
})
}这段代码是干嘛的呢？也就是说我们用nextTick的时候，还可以有promise的写法。如果没有向nextTick中传入cb，并且浏览器支持Promise的话，我们的nextTick返回的将是一个Promise。所以，nextTick的写法也可以是如下这样的
nextTick().then(()=>{console.log("XXXXX")})
vue源码里关于nextTick的封装的思路，也给我们一些非常有益的启示，就是我们平时在封装函数的时候，要想同时指出回调和promise的话，就可以借鉴vue中的思路。
大致的思路我们已经捋顺了。但是为什么执行macroTimerFunc或者microTimerFunc就会在下一个tick执行我们的回调队列呢？下面我们来分析一下这两个函数的定义。首先我们分析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)
}
}从上边的代码可以看出，如果浏览器支持setImmediate，我们就用setImmediate，如果浏览器支持MessageChannel，我们就用MessageChannel的异步特性，如果两者都不支持，我们就降价到setTimeout
，用setTimeout来把callbacks中的任务在下一个tick中执行
macroTimerFunc = () => {
setTimeout(flushCallbacks, 0)
}分析完macroTimerFunc，下面我们开始分析microTimerFunc，我把vue源码中关于microTimerFunc的定义稍微精简一下
let microTimerFunc;
if (typeof Promise !== 'undefined' && isNative(Promise)) {
const p = Promise.resolve()
microTimerFunc = () => {
p.then(flushCallbacks)
}
} else {
// fallback to macro
microTimerFunc = macroTimerFunc
}
从上边精简之后的代码，我们可以看到microTimerFunc的实现思路。如果支持浏览器支持promise，就用promise实现。如果不支持，就降低到用macroTimerFunc
over，整体逻辑就是这样。。看着吓人，掰开了之后好好分析一下，还是挺简单的。
