Vue异步更新和nextTick
Vue是异步更新的。
如果Vue中对一个Dom同时进行添加和删除的操作，这个时候到底该渲染哪一个呢？
如果对于一个属性短时间内修改很多次，但我们只需要最后修改的结果。如果是同步更新，那每修改一次，就会重新渲染一次，这样就会渲染很多次（浪费性能），所以为了上面的各种问题以及性能方面，Vue是异步更新的。
Vue的异步更新可以通过Vue.nextTick（vm.$nextTick）实现。
主要原理是JS是单线程的，且基于事件循环机制。具体可以参考我的另一篇博客。
Vue是数据驱动视图更新的，所以当数据发生改变时，会触发setter函数，通知watcher进行更新。Vue会将watcher push到一个数组中，且相同的watcher只会push一次。然后通过在微任务或宏任务中去遍历数组中的每一个watcher，并执行其update函数，这样就实现了在DOM 更新循环结束之后执行延迟回调。
Vue.nextTick和vm.$nextTick的唯一区别是后者的回调函数中的this会自动绑定到Vue实例上。
nextTick 2.5的源码分析
/* @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 micro and macro tasks.
// In < 2.4 we used micro tasks everywhere, but there are some scenarios where
// micro tasks have too high a priority and fires 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 micro task 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 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
})
}
}
在 Vue 2.4 之前的版本，nextTick 几乎都是基于 micro task 实现(MutationObserver、Promise、setTimeOut(macro task))的，但由于 micro task 的执行优先级非常高，在某些场景下它甚至要比事件冒泡还要快，就会导致一些诡异的问题，如 issue #4521、#6690、#6566；但是如果全部都改成 macro task，对一些有重绘和动画的场景也会有性能影响，如 issue #6813。所以最终 nextTick 采取的策略是默认走 micro task，对于一些 DOM 交互事件，如 v-on 绑定的事件回调函数的处理，会强制走 macro task。
这个强制是怎么做的呢，原来在 Vue.js 在绑定 DOM 事件的时候，默认会给回调的 handler 函数调用 withMacroTask 方法做一层包装，它保证整个回调函数执行过程中，遇到数据状态的改变，这些改变都会被推到 macro task 中。
对于 macro task 的执行，Vue.js 优先检测是否支持原生 setImmediate，这是一个高版本 IE 和 Edge 才支持的特性，不支持的话再去检测是否支持原生的 MessageChannel，如果也不支持的话就会降级为 setTimeout 0。
2.6.11它又变了！！！
nextTick 2.6.11的源码分析
/* @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 () {
//执行队列中的所有回调函数
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
})
}
}
nextTick返回一个回调函数或者Promise实例，如果是回调函数，则push到callbacks中，等待调用；如果是Promise实例，则将Promise的resolve赋值给_resolve.pending是一个锁，为了防止后续的 nextTick 重复执行 timerFunc。timerFunc 内部创建会一个微任务或宏任务，等待所有的 nextTick 同步执行完成后，再去执行 callbacks 内的回调。- timeFunc 优先创建微任务，其次是宏任务。优先级为：
promise.then>MutationObserver>immediate>setTimeout 无论哪种情况创建的 timerFunc，最终都会执行一个 flushCallbacks 的函数。flushCallbacks 负责执行 callbacks 里的回调。