先来看生成虚构dom的入口文件:
... 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) ...咱们看到了AST通过三轮加工，最终本性难移成为render function code。那么咱们这一章就来讲AST的第三次试炼。入口文件./codegen/index
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}}咱们来看generate函数外部结构，返回render是一个with(this){return ${code}}包起来的一串货色，staticRenderFns是在编译过程中会把那些不会变的动态节点打上标记，设置为true，而后在渲染阶段独自渲染。
那么genElement函数的作用是什么呢？
export function genElement (el: ASTElement, state: CodegenState): string { if (el.staticRoot && !el.staticProcessed) { return genStatic(el, state) } else if (el.once && !el.onceProcessed) { return genOnce(el, state) } else if (el.for && !el.forProcessed) { return genFor(el, state) } else if (el.if && !el.ifProcessed) { return genIf(el, state) } else if (el.tag === 'template' && !el.slotTarget) { return genChildren(el, state) || 'void 0' } else if (el.tag === 'slot') { return genSlot(el, state) } else { // component or element let code if (el.component) { code = genComponent(el.component, el, state) } else { const data = el.plain ? undefined : genData(el, state) const children = el.inlineTemplate ? null : genChildren(el, state, true) code = `_c('${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 }}咱们能够看到有有限个if else在嵌套，而且once、for、if、tag就是Vue自身的屠非凡标签，并且最初有return code 那么就是拼接字符串。
那么咱们来看一看生成的with(this){return ${code}}长啥样：
<div id="app"> <ul> <li v-for="item in items"> itemid:{{item.id}} </li> </ul></div>通过compile之后转化成：
with(this){ return _c('div',{ attrs:{"id":"app"} }, [_c('ul',_l((items),function(item){ return _c('li', [_v("\n itemid:"+_s(item.id)+"\n ")] )} ) )] )}到这里咱们就把tamplate三次锻炼讲完了，那么render function code 有了，那真正的渲染函数在哪呢？进一步来揭晓:
return { ast, //AST模型 render: code.render, //render函数字符串 staticRenderFns: code.staticRenderFns //动态节点render函数字符串 }咱们裸露进来一个ast、render、staticRenderFns别离为AST模型，render函数字符串，动态的render函数字符串,那么咱们的render函数在那里呢？
咱们晓得在Vue初始化的时候initRender:
vm._c = (a, b, c, d) => createElement(vm, a, b, c, d, false) // normalization is always applied for the public version, used in // user-written render functions. vm.$createElement = (a, b, c, d) => createElement(vm, a, b, c, d, true)该办法向vue上挂载了两个办法，一个服务于用户手写的render函数，一个则用于template模板。
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 }}向vue原型上挂载了_render办法，该办法在mount的过程中会被调用生成一个vnode实例用于update比照生成一个新的dom对象并对原dom节点进行替换,该办法将会拿到option上定义render办法：
- 用户自定义的
render tamplate用户自定义的是这样的- 参考vue实战视频解说：进入学习
new Vue({ el:"#app", render(createElement){ return createElement("div",{ attr:{ id:"app" } },this.message) }, data(){ return { message:"hello" } }})一种则是应用的template形式，然而该办法最终在mount的过程中通过调用compileToFunctions会被转化render函数，也就是说，最终供_render办法应用的实际上就是咱们自定义的render函数,在初始化render中会代理Vue实例vm._renderProxy，其实就是vue，那么vm.$createElement就是增加在Vue原型上的一个办法（initRender阶段），所以就是createElement办法
/** * 创立vnode节点, 实质上是调_createElement办法, 这里做一层封装, 来兼容更多状况 */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 } // 依据normalizationType的不同, 后续解决children的形式也不同, 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曾经是响应式, 即已有__ob__属性, 间接返回空节点并收回正告 if (isDef(data) && isDef((data: any).__ob__)) { process.env.NODE_ENV !== 'production' && warn( 'Avoid using observed data object as vnode data:' + ` ${JSON.stringify(data)}\n` + 'Always create fresh vnode data objects in each render!', context ) return createEmptyVNode() } // 解决动静组件 // object syntax in v-bind if (isDef(data) && isDef(data.is)) { tag = data.is } // 没有tag字段的组件间接返回一个空节点 if (!tag) { // in case of component :is set to falsy value return createEmptyVNode() } // warn against non-primitive key // 查看key是否是string or number if (process.env.NODE_ENV !== 'production' && isDef(data) && isDef(data.key) && !isPrimitive(data.key) ) { if (!__WEEX__ || !('@binding' in data.key)) { warn( 'Avoid using non-primitive value as key, ' + 'use string/number value instead.', context ) } } // support single function children as default scoped slot // 对于children数组的两种解决形式, 详见helper/normalize-children.js if (Array.isArray(children) && typeof children[0] === 'function' ) { data = data || {} data.scopedSlots = { default: children[0] } children.length = 0 } if (normalizationType === ALWAYS_NORMALIZE) { children = normalizeChildren(children) } else if (normalizationType === SIMPLE_NORMALIZE) { children = simpleNormalizeChildren(children) } let vnode, ns /** * 先对tag进行判断 如果是 string ，接着判断是否是dom内置的节点，如果是则间接创立一个一般 VNode * 如果是为已注册的组件名，则通过 createComponent 创立一个组件类型的 VNode * 否则创立一个未知的标签的 VNode * * 如果tag是Component类型, 通过createComponent创立一个节点 */ if (typeof tag === 'string') { let Ctor ns = (context.$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进行标准化解决, 保障每次都返回一个规范格局的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节点利用命名空间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) }}其中在判断ALWAYS_NORMALIZE与normalizationType时候，用户手写的ALWAYS_NORMALIZE始终是true，所以就会调用normalizeChildren（children）
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}- 当
childre是子组件的时候就会扁平化 - 当
children是根底数据类型的时候，间接调用createTextNode函数变成文本节点 - 当
children是数组的时候，会调用normalizeArrayChildren进行格式化
最初调用vnode = new VNode()生成一个vnode。
这一章解说了generate解析AST对象生成render function code在到虚构VNode是怎么生成进去的，下一章讲一个外围概念diff算法