简单了解 $nextTick
我们知道对于 Vue 来说,从数据变化到执行 DOM 更新,这个过程是异步的,发生在下一个 tick 里。
它会创建一个更新队列 queue
,队列中维护着各个属性的 watcher
,在需要时执行、更新它们。
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。
Vue.nextTick()
.then(function () {
// DOM 更新了
})
那么针对这样一个核心功能,Vue 2.5 与 Vue 2.6 的实现有什么不同呢?
可能需要你简单了解下 js 的 event loop。模拟实现 JS 引擎:深入了解 JS机制 以及 Microtask and Macrotask
Vue 2.5 nextTick 实现
在 Vue 2.5 中,nextTick 的实现是 microTimerFunc
、macroTimerFunc
组合实现的,具体见源码。
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
})
}
}
比较关键的几行
let useMacroTask = false
if (!pending) {
pending = true
if (useMacroTask) {
macroTimerFunc()
} else {
microTimerFunc()
}
}
这里说明,Vue 2.5 会优先使用 microTimerFunc
,如果存在兼容性问题,则降级为 macroTimerFunc
。
microTimerFunc 的实现:原生的 Promise
;
macroTimerFunc 的实现:setImmediate
|| MessageChannel
|| setTimeout
。
同时 Vue 2.5 的 next-tick 还对外暴露了两个函数:nextTick
以及 withMacroTask
(用于处理一些 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)。
补充,更详细的可以参考 @marsprince 完成的vue2.6更新研究之nextTick总是使用microTask。
通读源码,发现逻辑很清晰,也完成优雅渐进,那么发什么了导致 Vue 在 2.6 对其进行了 fix 呢?来看一段 Vue 2.6 的注释
// 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).
主要阐明了两个问题:
- 在重绘之前状态发生改变会有轻微的问题;
- 利用 macro task 处理事件时,会产生一系列无法规避的诡异问题。
简单地描述下这两个问题:
第一个问题,具体见图
试一试,具体描述见github.com/vuejs/vue/i…,本质上就是在 css 中定义了 @media 媒体查询,js 中 window 监听了 resize 事件,那么当触发固定阈值时,state 发生了变化、样式也需要重绘,这就产生了问题。
第二个问题,一般可以概括为由于使用 macroTask 处理 DOM 操作,会使得有些时候触发和执行之间间隔太大,例如在移动端,单击的 handler 和音频播放功能不在同一 tick 里。
Vue 2.6 nextTick 实现
由于以上问题,所以在 Vue 2.6 实现过程中,利用 microtasks
代替之前的解决方案,具体见源码。
/* @flow */
/* globals MutationObserver */
import { noop } from 'shared/util'
import { handleError } from './error'
import { isIE, 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 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)
}
} 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)
}
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
// Fallback to setImmediate.
// Techinically 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
})
}
}
Vue 2.6 利用最典型的两个 microTask,promise.then
以及 Mutation observers
,并添加 setImmediate
、setTimeout
,作为降级方案。
只对外暴露了一个接口 next-tick,同时用 microTask 来处理 event handler。这种实现方法解决了上述问题,但是也有一个很明显的弊端。由于 microTask 的优先级太高,导致当连续触发 event 事件时产生问题,具体见注释。