加深vue渲染页面原理记录
1. 栗子
先来了解一个例子关于vue中数据和DOM更新机制以及vm.nextTick作用。
template
<template>
<div>
<div ref="msg">{{msg}}</div>
<button @click="changeMsg">点击</button>
</div>
</template>
script
export default {
data() {
return {
msg:"default",
}
},
methods: {
changeMsg() {
//把 nextTick放在前面结果也是一样。
// this.$nextTick(() => {
// console.log("我是nextTick:" + this.$refs.msg.textContent)//改变最新的值
// })
this.msg = new Date().getTime();
console.log(this.msg)
this.msg = new Date().getTime();
console.log(this.msg)
this.msg = new Date().getTime();
console.log(this.msg)
this.msg = new Date().getTime();
console.log(this.msg)// 改变最新的值
console.log("我是外边：" + this.$refs.msg.textContent)//初始值
setTimeout(() => {
console.log("我是setTimeout:" + this.$refs.msg.textContent)//改变最新的值
}, 0)
this.$nextTick(() => {
console.log("我是nextTick:" + this.$refs.msg.textContent)//改变最新的值
})
}
},
}
点击按钮 控制台结果图
nextTick.jpg
通过图上可知：DOM更新是异步的，我们只能通过异步的手法让获取DOM值是最新改变的值。并且nextTick和传统的setTimeout有异曲同工之处。
2. 什么是异步更新队列
官方文档解释
Vue 在更新 DOM 时是异步执行的。只要侦听到数据变化，Vue 将开启一个队列，并缓冲在同一事件循环中发生的所有数据变更。如果同一个 watcher 被多次触发，只会被推入到队列中一次。这种在缓冲时去除重复数据对于避免不必要的计算和 DOM 操作是非常重要的。然后，在下一个的事件循环“tick”中，Vue 刷新队列并执行实际 (已去重的) 工作。Vue 在内部对异步队列尝试使用原生的 Promise.then、MutationObserver 和 setImmediate，如果执行环境不支持，则会采用 setTimeout(fn, 0) 代替。
例如，当你设置 vm.someData = 'new value'，该组件不会立即重新渲染。当刷新队列时，组件会在下一个事件循环“tick”中更新。多数情况我们不需要关心这个过程，但是如果你想基于更新后的 DOM 状态来做点什么，这就可能会有些棘手。虽然 Vue.js 通常鼓励开发人员使用“数据驱动”的方式思考，避免直接接触 DOM，但是有时我们必须要这么做。为了在数据变化之后等待 Vue 完成更新 DOM，可以在数据变化之后立即使用 Vue.nextTick(callback)。这样回调函数将在 DOM 更新完成后被调用
注意：
看了大堆文字说明还是懵逼的感觉，简单来说：这也和vue数据响应原理有关联的，异步更新队列意思，就当vue检测到data变化以后，由于watcher触发了，所以会被推入到队列中，这也是操作DOM的时候，是不会立即更新DOM的，所以用nextTick(),可以立即更新DOM,获取最新的DOM状态。
要想理解官方文档的说明，我们必须先了解js中的事件循环机制和事件队列的概念可以借鉴
Promise 解析和事件循环机制
应用场景：
在Vue生命周期的created()钩子函数进行的DOM操作一定要放在Vue.nextTick()的回调函数中。
3. Vue 2.6.10 源码浅析
/* @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 () {
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.
// 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
})
}
}
