Vue虚拟dom被创建的方法

 ...
import { parse } from './parser/index'
import { optimize } from './optimizer'
import { generate } from './codegen/index'
...
const ast = parse(template.trim(), options)
if (options.optimize !== false) {
optimize(ast, options)
}
const code = generate(ast, options)
...

export function generate (
ast: ASTElement | void,
options: CompilerOptions
): CodegenResult {
const state = new CodegenState(options)
const code = ast ? genElement(ast, state) : '_c("div")'
return {
render: `with(this){return $[code]}`,
staticRenderFns: state.staticRenderFns
}
}

export function genElement (el: ASTElement, state: CodegenState): string {
if (el.staticRoot && !el.staticProcessed) {
return genStatic(el, state)
} else if (el.once && !el.onceProcessed) {
return genOnce(el, state)
} else if (el.for && !el.forProcessed) {
return genFor(el, state)
} else if (el.if && !el.ifProcessed) {
return genIf(el, state)
} else if (el.tag === 'template' && !el.slotTarget) {
return genChildren(el, state) || 'void 0'
} else if (el.tag === 'slot') {
return genSlot(el, state)
} else {
// component or element
let code
if (el.component) {
code = genComponent(el.component, el, state)
} else {
const data = el.plain ? undefined : genData(el, state)
const children = el.inlineTemplate ? null : genChildren(el, state, true)
code = `_c('${el.tag}'${        data ? `,${data}` : '' // data      }${        children ? `,${children}` : '' // children      })`
}
// module transforms
for (let i = 0; i < state.transforms.length; i++) {
code = state.transforms[i](el, code)
}
return code
}
}

<div id="app">
<ul>
<li v-for="item in items">
itemid:{{item.id}}
</li>
</ul>
</div>

with(this){
return
_c('div',{
attrs:{"id":"app"}
},
[_c('ul',_l((items),function(item){
return _c('li',
[_v("\n      itemid:"+_s(item.id)+"\n    ")]
)}
)
)]
)}

return {
ast, //AST模型
render: code.render, //render函数字符串
staticRenderFns: code.staticRenderFns //静态节点render函数字符串
}

  vm._c = (a, b, c, d) => createElement(vm, a, b, c, d, false)
// normalization is always applied for the public version, used in
// user-written render functions.
vm.$createElement = (a, b, c, d) => createElement(vm, a, b, c, d, true)

export function renderMixin (Vue: Class<Component>) {
// install runtime convenience helpers
installRenderHelpers(Vue.prototype)
Vue.prototype.$nextTick = function (fn: Function) {
return nextTick(fn, this)
}
Vue.prototype._render = function (): VNode {
const vm: Component = this
const { render, _parentVnode } = vm.$options
// reset _rendered flag on slots for duplicate slot check
if (process.env.NODE_ENV !== 'production') {
for (const key in vm.$slots) {
// $flow-disable-line
vm.$slots[key]._rendered = false
}
}
if (_parentVnode) {
vm.$scopedSlots = _parentVnode.data.scopedSlots || emptyObject
}
// set parent vnode. this allows render functions to have access
// to the data on the placeholder node.
vm.$vnode = _parentVnode
// render self
let vnode
try {
vnode = render.call(vm._renderProxy, vm.$createElement)
} catch (e) {
handleError(e, vm, `render`)
// return error render result,
// or previous vnode to prevent render error causing blank component
/* istanbul ignore else */
if (process.env.NODE_ENV !== 'production') {
if (vm.$options.renderError) {
try {
vnode = vm.$options.renderError.call(vm._renderProxy, vm.$createElement, e)
} catch (e) {
handleError(e, vm, `renderError`)
vnode = vm._vnode
}
} else {
vnode = vm._vnode
}
} else {
vnode = vm._vnode
}
}
// 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
}
}

new Vue({
el:"#app",
render(createElement){
return createElement("div",{
attr:{
id:"app"
}
},this.message)
},
data(){
return {
message:"hello"
}
}
})

/** * 创建vnode节点, 本质上是调_createElement方法, 这里做一层封装, 来兼容更多情况 */
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
}
// 根据normalizationType的不同, 后续处理children的方式也不同, 详见core/vdom/helpers/normalzie-children.js
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> {
// 如果节点的data已经是响应式, 即已有__ob__属性, 直接返回空节点并发出警告
if (isDef(data) && isDef((data: any).__ob__)) {
process.env.NODE_ENV !== 'production' && warn(
'Avoid using observed data object as vnode data:' + ` ${JSON.stringify(data)}\n` +
'Always create fresh vnode data objects in each render!',
context
)
return createEmptyVNode()
}
// 处理动态组件
// object syntax in v-bind
if (isDef(data) && isDef(data.is)) {
tag = data.is
}
// 没有tag字段的组件直接返回一个空节点
if (!tag) {
// in case of component :is set to falsy value
return createEmptyVNode()
}
// warn against non-primitive key
// 检查key是否是string or number
if (process.env.NODE_ENV !== 'production' &&
isDef(data) && isDef(data.key) && !isPrimitive(data.key)
) {
if (!__WEEX__ || !('@binding' in data.key)) {
warn(
'Avoid using non-primitive value as key, ' +
'use string/number value instead.',
context
)
}
}
// support single function children as default scoped slot
// 对于children数组的两种处理方式, 详见helper/normalize-children.js
if (Array.isArray(children) &&
typeof children[0] === 'function'
) {
data = data || {}
data.scopedSlots = { default: children[0] }
children.length = 0
}
if (normalizationType === ALWAYS_NORMALIZE) {
children = normalizeChildren(children)
} else if (normalizationType === SIMPLE_NORMALIZE) {
children = simpleNormalizeChildren(children)
}
let vnode, ns
/**   * 先对tag进行判断 如果是 string ，接着判断是否是dom内置的节点，如果是则直接创建一个普通 VNode   * 如果是为已注册的组件名，则通过 createComponent 创建一个组件类型的 VNode   * 否则创建一个未知的标签的 VNode   *    * 如果tag是Component类型, 通过createComponent创建一个节点   */
if (typeof tag === 'string') {
let Ctor
ns = (context.$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)
}
// 对vnode进行标准化处理, 保证每次都返回一个标准格式的vnode
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()
}
}
// 对每个children节点应用命名空间
function applyNS (vnode, ns, force) {
vnode.ns = ns
if (vnode.tag === 'foreignObject') {
// use default namespace inside foreignObject
ns = undefined
force = true
}
if (isDef(vnode.children)) {
for (let i = 0, l = vnode.children.length; i < l; i++) {
const child = vnode.children[i]
if (isDef(child.tag) && (
isUndef(child.ns) || (isTrue(force) && child.tag !== 'svg'))) {
applyNS(child, ns, force)
}
}
}
}
// ref #5318
// necessary to ensure parent re-render when deep bindings like :style and
// :class are used on slot nodes
// 对于style和class进行深度递归绑定
function registerDeepBindings (data) {
if (isObject(data.style)) {
traverse(data.style)
}
if (isObject(data.class)) {
traverse(data.class)
}
}

export function simpleNormalizeChildren (children: any) {
for (let i = 0; i < children.length; i++) {
if (Array.isArray(children[i])) {
return Array.prototype.concat.apply([], children)
}
}
return children
}
export function normalizeChildren (children: any): ?Array<VNode> {
return isPrimitive(children)
? [createTextVNode(children)]
: Array.isArray(children)
? normalizeArrayChildren(children)
: undefined
}