nextTick引出的问题
任务类型
JS中有两种任务类型：微任务（microtask）和宏任务（macrotask），在ES6中，microtask称为 jobs，macrotask称为 task。
宏任务： script （主代码块）、setTimeout 、setInterval 、setImmediate 、I/O 、UI rendering
微任务：process.nextTick（Nodejs） 、promise 、Object.observe 、MutationObserver
它们之间区别如下：
宏任务是每次执行栈执行的代码（包括每次从事件队列中获取一个事件回调并放到执行栈中执行）
浏览器为了能够使得JS引擎线程与GUI渲染线程有序切换，会在当前宏任务结束之后，下一个宏任务执行开始之前，对页面进行重新渲染（宏任务 > 渲染 > 宏任务 > …）
微任务是在当前宏任务执行结束之后立即执行的任务（在当前 宏任务执行之后，UI渲染之前执行的任务）。微任务的响应速度相比setTimeout（下一个宏任务）会更快，因为无需等待UI渲染。
当前宏任务执行后，会将在它执行期间产生的所有微任务都执行一遍。
根据事件循环机制，重新梳理一下流程：
执行一个宏任务（首次执行的主代码块或者任务队列中的回调函数）
执行过程中如果遇到微任务，就将它添加到微任务的任务队列中
宏任务执行完毕后，立即执行当前微任务队列中的所有任务（依次执行）
JS引擎线程挂起，GUI线程执行渲染
GUI线程渲染完毕后挂起，JS引擎线程执行任务队列中的下一个宏任务
VUE各版本的nextTick（截止最新的2.6.11）
因为宏任务对渲染性能的影响以及微任务的优先级过高在事件冒泡等情况下会有问题等因素，源码关于nextTick的实现有多次改动。
2.4版本之前 微任务
(Promise/MutationObserver)
2.4之后出现过如下顺序，宏任务（V.2.5.0）
- setImmediate（only ie10+）
- MessageChannel
- Promise
- setTimeout
2.5之后（可选使用宏任务或者微任务，默认使用微任务）（V2.5.22）
宏任务：
- setImmediate（only ie10+）
- MessageChannel
- setTimeout
微任务
- promise （若环境不支持promise，则使用宏任务）
2.6之后的顺序 （微任务）
- promise
- MutationObserver
- setImmediate
- setTimeout
查看 【V2.6.11】版本的源码
/* @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
})
}
}
参考资料：
https://juejin.im/post/5ae3f0956fb9a07ac90cf43e
https://juejin.im/post/5cd9854b5188252035420a13
https://www.jianshu.com/p/4f07ef18b5d7
