前言
对于经常使使用Vue的前端同学来说，下面这行代码是再熟悉不过了，但不是很清楚这行代码到底做了什么。 初始化模板代码
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<title>Examples</title>
<meta name="description" content="">
<meta name="keywords" content="">
<link href="" rel="stylesheet">
</head>
<body>
<div id="app">{{ name }}</div>
</body>
</html>
new Vue({
el: '#app',
data () {
return {
name: 'rookie'
}
}
})
那么接下来，我们透过源码来一步步地分析Vue初始化的整体流程。Vue版本@2.6.x,编译版本。
Vue构造函数
我们先找到Vue这个函数所定义的文件，代码如下:
import { initMixin } from './init'
import { stateMixin } from './state'
import { renderMixin } from './render'
import { eventsMixin } from './events'
import { lifecycleMixin } from './lifecycle'
import { warn } from '../util/index'
function Vue (options) {
if (process.env.NODE_ENV !== 'production' &&
!(this instanceof Vue)
) {
warn('Vue is a constructor and should be called with the `new` keyword')
}
this._init(options)
}
initMixin(Vue)
stateMixin(Vue)
eventsMixin(Vue)
lifecycleMixin(Vue)
renderMixin(Vue)
export default Vue
我们可以看到Vue实质上就是一个构造函数，执行new Vue()操作，就是执行了实例的_init方法，options变量其实就是我们外部传入的值, 下面的函数mixin，会给Vue函数的原型上挂载方法或者一些变量。
_init
接下来我们看下具体实现,源码
/* @flow */
import config from '../config'
import { initProxy } from './proxy'
import { initState } from './state'
import { initRender } from './render'
import { initEvents } from './events'
import { mark, measure } from '../util/perf'
import { initLifecycle, callHook } from './lifecycle'
import { initProvide, initInjections } from './inject'
import { extend, mergeOptions, formatComponentName } from '../util/index'
let uid = 0
export function initMixin (Vue: Class<Component>) {
Vue.prototype._init = function (options?: Object) {
const vm: Component = this
// a uid
vm._uid = uid++
let startTag, endTag
/* istanbul ignore if */
if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
startTag = `vue-perf-start:${vm._uid}`
endTag = `vue-perf-end:${vm._uid}`
mark(startTag)
}
// a flag to avoid this being observed
vm._isVue = true
// merge options
if (options && options._isComponent) {
// optimize internal component instantiation
// since dynamic options merging is pretty slow, and none of the
// internal component options needs special treatment.
initInternalComponent(vm, options)
} else {
vm.$options = mergeOptions(
resolveConstructorOptions(vm.constructor),
options || {},
vm
)
}
/* istanbul ignore else */
if (process.env.NODE_ENV !== 'production') {
initProxy(vm)
} else {
vm._renderProxy = vm
}
// expose real self
vm._self = vm
initLifecycle(vm)
initEvents(vm)
initRender(vm)
callHook(vm, 'beforeCreate')
initInjections(vm) // resolve injections before data/props
initState(vm)
initProvide(vm) // resolve provide after data/props
callHook(vm, 'created')
/* istanbul ignore if */
if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
vm._name = formatComponentName(vm, false)
mark(endTag)
measure(`vue ${vm._name} init`, startTag, endTag)
}
if (vm.$options.el) {
vm.$mount(vm.$options.el)
}
}
}
// ... ignore code
我们可以看到_init方法其实就是在执行initMixin的时候，过载到了Vue函数的原型上，在执行了initLifecycle(初始化生命周期) ->initEvents(初始化事件监听)->initRender(初始化render相关的方法)->initState(初始化data/props/watch/methods/computed)->initInjections和initProvide(初始化依赖注入，调用时机分别在initState调用前后)->vm.$mount(挂载DOM元素)。
$mount
由于我们分析的是带编译版本的，所以$mount函数定义的文件在这里,相关代码如下:
// .. ignore code
import Vue from './runtime/index'
import { query } from './util/index'
const mount = Vue.prototype.$mount
Vue.prototype.$mount = function (
el?: string | Element,
hydrating?: boolean
): Component {
el = el && query(el)
/* istanbul ignore if */
if (el === document.body || el === document.documentElement) {
process.env.NODE_ENV !== 'production' && warn(
`Do not mount Vue to <html> or <body> - mount to normal elements instead.`
)
return this
}
const options = this.$options
// resolve template/el and convert to render function
if (!options.render) {
let template = options.template
if (template) {
if (typeof template === 'string') {
if (template.charAt(0) === '#') {
template = idToTemplate(template)
/* istanbul ignore if */
if (process.env.NODE_ENV !== 'production' && !template) {
warn(
`Template element not found or is empty: ${options.template}`,
this
)
}
}
} else if (template.nodeType) {
template = template.innerHTML
} else {
if (process.env.NODE_ENV !== 'production') {
warn('invalid template option:' + template, this)
}
return this
}
} else if (el) {
template = getOuterHTML(el)
}
if (template) {
/* istanbul ignore if */
if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
mark('compile')
}
const { render, staticRenderFns } = compileToFunctions(template, {
outputSourceRange: process.env.NODE_ENV !== 'production',
shouldDecodeNewlines,
shouldDecodeNewlinesForHref,
delimiters: options.delimiters,
comments: options.comments
}, this)
options.render = render
options.staticRenderFns = staticRenderFns
/* istanbul ignore if */
if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
mark('compile end')
measure(`vue ${this._name} compile`, 'compile', 'compile end')
}
}
}
return mount.call(this, el, hydrating)
}
/**
* Get outerHTML of elements, taking care
* of SVG elements in IE as well.
*/
function getOuterHTML (el: Element): string {
if (el.outerHTML) {
return el.outerHTML
} else {
const container = document.createElement('div')
container.appendChild(el.cloneNode(true))
return container.innerHTML
}
}
Vue.compile = compileToFunctions
export default Vue
Vue.prototype.mount方法。 执行函数后，会对el进行取值，接着进行判断，如果el是body或者是html元素，则抛出异常警告，Vue不允许对body或者html元素进行替换。接着会对template属性进行判断，后面经过一系列的判断处理之后，会把template属性变成render函数，最后会调用mount方法(即原先Vue.prototype.$mount)，我们再看看相应的实现 传送门:
Vue.prototype.$mount = function (
el?: string | Element,
hydrating?: boolean
): Component {
el = el && inBrowser ? query(el) : undefined
return mountComponent(this, el, hydrating)
}
首先会对el进行取值，取值操作也很简单，判断el是否存在并且当前环境是否是浏览器，在进行下一步操作,query函数实现如下:
export function query (el: string | Element): Element {
if (typeof el === 'string') {
const selected = document.querySelector(el) // 就是通过document.querySelector的Api选中元素
if (!selected) {
process.env.NODE_ENV !== 'production' && warn(
'Cannot find element: ' + el
)
return document.createElement('div')
}
return selected
} else {
return el
}
}
对el取完值之后，就执行mountComponent方法。
mountComponent
export function mountComponent (
vm: Component,
el: ?Element,
hydrating?: boolean
): Component {
vm.$el = el
// ignore code
callHook(vm, 'beforeMount')
let updateComponent
/* istanbul ignore if */
if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
// ignore code
} else {
updateComponent = () => { // 一般来说会走到这一分支，重点分析
vm._update(vm._render(), hydrating)
}
}
// we set this to vm._watcher inside the watcher's constructor
// since the watcher's initial patch may call $forceUpdate (e.g. inside child
// component's mounted hook), which relies on vm._watcher being already defined
new Watcher(vm, updateComponent, noop, {
before () {
if (vm._isMounted && !vm._isDestroyed) {
callHook(vm, 'beforeUpdate')
}
}
}, true /* isRenderWatcher */)
hydrating = false
// manually mounted instance, call mounted on self
// mounted is called for render-created child components in its inserted hook
if (vm.$vnode == null) {
vm._isMounted = true
callHook(vm, 'mounted')
}
return vm
}
首先el会赋值给vm.$el，接着会执行beforeMount钩子函数，updateComponent也被相应的赋值，接着执行了new Watcher()。
Watcher
/* @flow */
import {
warn,
remove,
isObject,
parsePath,
_Set as Set,
handleError,
noop
} from '../util/index'
import { traverse } from './traverse'
import { queueWatcher } from './scheduler'
import Dep, { pushTarget, popTarget } from './dep'
import type { SimpleSet } from '../util/index'
let uid = 0
/**
* A watcher parses an expression, collects dependencies,
* and fires callback when the expression value changes.
* This is used for both the $watch() api and directives.
*/
export default class Watcher {
vm: Component;
expression: string;
cb: Function;
id: number;
lazy: boolean;
// ignore code
constructor (
vm: Component,
expOrFn: string | Function,
cb: Function,
options?: ?Object,
isRenderWatcher?: boolean
) {
this.vm = vm
if (isRenderWatcher) {
vm._watcher = this
}
vm._watchers.push(this)
// options
if (options) {
this.deep = !!options.deep
this.user = !!options.user
this.lazy = !!options.lazy
this.sync = !!options.sync
this.before = options.before
} else {
this.deep = this.user = this.lazy = this.sync = false
}
// parse expression for getter
if (typeof expOrFn === 'function') {
this.getter = expOrFn
}
this.value = this.lazy
? undefined
: this.get()
}
/**
* Evaluate the getter, and re-collect dependencies.
*/
get () {
pushTarget(this)
let value
const vm = this.vm
try {
value = this.getter.call(vm, vm)
} catch (e) {
if (this.user) {
handleError(e, vm, `getter for watcher "${this.expression}"`)
} else {
throw e
}
} finally {
// "touch" every property so they are all tracked as
// dependencies for deep watching
if (this.deep) {
traverse(value)
}
popTarget()
this.cleanupDeps()
}
return value
}
}
为了方便理解，我只截取了本次执行时，所需要的核心代码。更完整的源码，可以点击这里查看。
new Watcher(vm, updateComponent, noop, {
before () {
if (vm._isMounted && !vm._isDestroyed) {
callHook(vm, 'beforeUpdate')
}
}
}, true /* isRenderWatcher */)
第一个参数是vm实例，第二个参数是upadteComponent函数，第三个noop就是一个空函数，第四个参数是一个对象，第五个参数为true，即为渲染watcher。 把参数带入之后，new Watcher的时候就会执行this.get()，而执行get方法就是去执行this.getter，对于渲染watcher来说就是updateComponent方法，而updateComponent方法本质上就是生成虚拟DOM，并将虚拟DOM映射生成成真实DOM。
Vue初始化的过程，我们先分析到这里，updateComponent方法的分析，将放到下一篇进行分析，如有不足或有误之处，望指出！
