Vue原始碼簡析之檢視生成（上）

摘要： new Vue({ el: '#app', template: '<div>{{msg}}</div>', data: { msg: 'hello vue' } }) 複製程式碼 最終會生成 ...

new Vue({
el: '#app',
template: '<div>{{msg}}</div>',
data: {
msg: 'hello vue'
}
})
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最終會生成 <div>hello vue</div> 的檢視，下面來分析一下流程。

new Vue()

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)
}
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_init

// core/instance/inti.js
Vue.prototype._init = function (options?: Object) {
const vm: Component = this
// a uid
vm._uid = uid++
....
...
// 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.$options.el)
}
}
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new Vue()的時候會執行_init方法，做一些初始化工作後執行vm.$mount,在這之前會判斷有沒有el屬性，沒有的話需要手動呼叫。就像這樣:

new Vue({
template: '<div>{{msg}}</div>',
data: {
msg: 'hello vue'
}
}).$mount('#app')
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$mount

// platforms/web/entry-runtime-with-compilers.js
const mount = Vue.prototype.$mount// 快取$mount方法
Vue.prototype.$mount = function (
el?: string | Element,
hydrating?: boolean
): Component {
el = el && query(el)
...
...
const options = this.$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, {
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 ${this._name} compile`, 'compile', 'compile end')
}
}
}
return mount.call(this, el, hydrating)
}
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這裡先快取了$mount方法，然後又重新定義了$mount方法，新方法中主要加了template轉render函式的部分，這就對應了vue的 only-runtime 版本和 runtime-with-compiler 版本，compileToFunction的實現暫不深究，就是用來生成render函式，例子中的template轉成的render函式如下：

快取的mount方法：

//platforms/web/runtime/index.js
Vue.prototype.$mount = function (
el?: string | Element,
hydrating?: boolean
): Component {
el = el && inBrowser ? query(el) : undefined
return mountComponent(this, el, hydrating)
}
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就是執行了mountComponent方法

mountComponent

export function mountComponent (
vm: Component,
el: ?Element,
hydrating?: boolean
): Component {
vm.$el = el
...

callHook(vm, 'beforeMount')

let updateComponent
/* istanbul ignore if */
if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
...
} else {
updateComponent = () => {
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 mounted hook), which relies on vm._watcher being already defined
new Watcher(vm, updateComponent, noop, {
before () {
if (vm._isMounted) {
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
}
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這裡主要是定義了 updateComponent 函式，例項化一個watcher並將updateComponent作為引數傳入，在watcher的建構函式中會執行。也就執行了下面這句：

vm._update(vm._render(), hydrating)
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整個DOm的生成和掛載都是由這一句程式碼完成的，可以分成兩部分：

• render函式執行返回Vnode
• _update函式執行生成對應的DOM並掛載到頁面上

render

// core/instance/render.js
Vue.prototype._render = function (): VNode {
const vm: Component = this
const { render, _parentVnode } = vm.$options
...
let vnode
try {
vnode = render.call(vm._renderProxy, vm.$createElement)
} catch (e) {
...
}
// return empty vnode in case the render function errored out
if (!(vnode instanceof VNode)) {
if (process.env.NODE_ENV !== 'production' && Array.isArray(vnode)) {
warn(
'Multiple root nodes returned from render function. Render function ' +
'should return a single root node.',
vm
)
}
vnode = createEmptyVNode()
}
// set parent
vnode.parent = _parentVnode
return vnode
}
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核心程式碼是：

vnode = render.call(vm._renderProxy, vm.$createElement)
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Vue是支援手寫render函式的，接受一個引數，其實就是這裡的vm.$createElement, 該函式也定義在同一個檔案中。

vm.$createElement = (a, b, c, d) => createElement(vm, a, b, c, d, true)
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最終還是呼叫了createElement函式

createElement

// core/vdom/render.js
export function createElement (
context: Component,
tag: any,
data: any,
children: any,
normalizationType: any,
alwaysNormalize: boolean
): VNode | Array<VNode> {
if (Array.isArray(data) || isPrimitive(data)) {
normalizationType = children
children = data
data = undefined
}
if (isTrue(alwaysNormalize)) {
normalizationType = ALWAYS_NORMALIZE
}
return _createElement(context, tag, data, children, normalizationType)
}

export function _createElement (
context: Component,
tag?: string | Class<Component> | Function | Object,
data?: VNodeData,
children?: any,
normalizationType?: number
): VNode | Array<VNode> {
 ...
// support single function children as default scoped slot
if (Array.isArray(children) &&
typeof children[0] === 'function'
) {
data = data || {}
data.scopedSlots = { default: children[0] }
children.length = 0
}
if (normalizationType === ALWAYS_NORMALIZE) {
children = normalizeChildren(children)
} else if (normalizationType === SIMPLE_NORMALIZE) {
children = simpleNormalizeChildren(children)
}
let vnode, ns
if (typeof tag === 'string') {
let Ctor
ns = (context.$vnode && context.$vnode.ns) || config.getTagNamespace(tag)
if (config.isReservedTag(tag)) {
// platform built-in elements
vnode = new VNode(
config.parsePlatformTagName(tag), data, children,
undefined, undefined, context
)
} else if (isDef(Ctor = resolveAsset(context.$options, 'components', tag))) {
// component
vnode = createComponent(Ctor, data, context, children, tag)
} else {
// unknown or unlisted namespaced elements
// check at runtime because it may get assigned a namespace when its
// parent normalizes children
vnode = new VNode(
tag, data, children,
undefined, undefined, context
)
}
} else {
// direct component options / constructor
vnode = createComponent(tag, data, context, children)
}
if (Array.isArray(vnode)) {
return vnode
} else if (isDef(vnode)) {
if (isDef(ns)) applyNS(vnode, ns)
if (isDef(data)) registerDeepBindings(data)
return vnode
} else {
return createEmptyVNode()
}
}
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createElement內部又呼叫了_createElemnet，這裡就是生成Vnode的部分了，Vnode可以理解為用來描述dom節點的js物件，整個VirtualDOM就是由Vnode構成的Tree型結構。_createElement方法中有很多判斷，是web還是weex，是元件還是簡單的節點，對children格式化等，例子比較簡單就不討論這些了。最終會生成Vnode並返回，看下例子中的Vnode長啥樣：

注意幾個屬性，tag表示標籤名，context表示當前的Vue例項，children表示子節點，這裡是我們的hello vue文字節點，見下圖：

到這裡render函式執行完畢，生成了Vnode並傳入_update方法，最終由_update生成DOM並完成掛載，這部分下篇再說。