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重学Vue源码，根据黄轶大佬的vue技术揭秘，逐个过一遍，巩固一下vue源码知识点，毕竟嚼碎了才是自己的，所有文章都同步在 公众号(道道里的前端栈) 和 github 上。
正文
上一篇说到挂载是用 mountComponent 函数来进行的，而 mountCompnent 函数里面最主要的是一个updateComponent 函数执行：
updateComponent = () => { vm._update(vm._render(), hydrating) }
这个函数在初次挂载和之后的更新都用到了，而 _render() 方法最终会调用 $option.render 函数， 函数主要的目的就是创建 VNode，除了 Virtual DOM 的定义之外，还有一个 createElement 函数，本篇就来说下 createElement 都做了什么事情，它在 src/core/vdom/create-element.js 中。
const SIMPLE_NORMALIZE = 1 const ALWAYS_NORMALIZE = 2 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) }
createElement 方法支持六个参数， context 其实就是vm实例， tag 是 VNode 的标签，data 就是 VNode 的数据，children 就是一些子节点，后两个就是两个标识符，下面的：
if (Array.isArray(data) || isPrimitive(data)) { normalizationType = children children = data data = undefined }
是对传入参数的一种重载，简单一点就是对于传入的参数不一致的话，会做一点处理，把所有的参数都往后移一个，然后把 data 改成 null。后面赋值一个 normalizationType，紧接着返回一个 _createElement 函数，然后把刚才的 normalizationType 放进去，也就是说 createElement 方法最终调用的是 _createElement，而 createElement 本身其实只对参数做了一层处理，而真正创建 VNode 的是 _createElement，那来看下这个函数的内容：
export function _createElement ( context: Component, tag?: string | Class<Component> | Function | Object, data?: VNodeData, children?: any, normalizationType?: number ): VNode | Array<VNode> { if (isDef(data) && isDef((data: any).__ob__)) { process.env.NODE_ENV !== 'production' && warn( `Avoid using observed data object as vnode data: ${JSON.stringify(data)}\n` + 'Always create fresh vnode data objects in each render!', context ) return createEmptyVNode() } // object syntax in v-bind if (isDef(data) && isDef(data.is)) { tag = data.is } if (!tag) { // in case of component :is set to falsy value return createEmptyVNode() } // warn against non-primitive key if (process.env.NODE_ENV !== 'production' && isDef(data) && isDef(data.key) && !isPrimitive(data.key) ) { if (!__WEEX__ || !('@binding' in data.key)) { warn( 'Avoid using non-primitive value as key, ' + 'use string/number value instead.', context ) } } // support single function children as default scoped slot if (Array.isArray(children) && typeof children[0] === 'function' ) { data = data || {} data.scopedSlots = { default: children[0] } children.length = 0 } if (normalizationType === ALWAYS_NORMALIZE) { children = normalizeChildren(children) } else if (normalizationType === SIMPLE_NORMALIZE) { children = simpleNormalizeChildren(children) } let vnode, ns if (typeof tag === 'string') { let Ctor ns = (context.$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() } }
最上面：
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()
它的意思是判断 data 是不是一个响应式的对象，如果 data 有 __ob__ 属性，那么它就是一个响应式的，在开发环境中就会报错，然后返回创建一个空节点，就是说不允许 VNode 的 data 是响应式的。创建空 VNode 代码在 src/core/vdom/vnode.js 中：
export const createEmptyVNode = (text: string = '') => { const node = new VNode() node.text = text node.isComment = true return node }
接着它判断了 data.is，如果不为 true，也返回一个空 VNode ，后面也是对 data 做了一层判定，继续又简单的判断了 children。
到后面：
if (normalizationType === ALWAYS_NORMALIZE) { children = normalizeChildren(children) } else if (normalizationType === SIMPLE_NORMALIZE) { children = simpleNormalizeChildren(children) }
对 children 做了一层处理，首先 children 是一个数组，这个处理是判断 children 数组是单层嵌套还是多层，然后把它拍平，把 children 里的每一项都变成 VNode，代码如下：
// The template compiler attempts to minimize the need for normalization by // statically analyzing the template at compile time. // // For plain HTML markup, normalization can be completely skipped because the // generated render function is guaranteed to return Array<VNode>. There are // two cases where extra normalization is needed: // 1. When the children contains components - because a functional component // may return an Array instead of a single root. In this case, just a simple // normalization is needed - if any child is an Array, we flatten the whole // thing with Array.prototype.concat. It is guaranteed to be only 1-level deep // because functional components already normalize their own 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 } // 2. When the children contains constructs that always generated nested Arrays, // e.g. <template>, <slot>, v-for, or when the children is provided by user // with hand-written render functions / JSX. In such cases a full normalization // is needed to cater to all possible types of children values. export function normalizeChildren (children: any): ?Array<VNode> { return isPrimitive(children) ? [createTextVNode(children)] : Array.isArray(children) ? normalizeArrayChildren(children) : undefined } function normalizeArrayChildren (children: any, nestedIndex?: string): Array<VNode> { const res = [] let i, c, lastIndex, last for (i = 0; i < children.length; i++) { c = children[i] if (isUndef(c) || typeof c === 'boolean') continue lastIndex = res.length - 1 last = res[lastIndex] // nested if (Array.isArray(c)) { if (c.length > 0) { c = normalizeArrayChildren(c, `${nestedIndex || ''}_${i}`) // merge adjacent text nodes if (isTextNode(c[0]) && isTextNode(last)) { res[lastIndex] = createTextVNode(last.text + (c[0]: any).text) c.shift() } res.push.apply(res, c) } } else if (isPrimitive(c)) { if (isTextNode(last)) { // merge adjacent text nodes // this is necessary for SSR hydration because text nodes are // essentially merged when rendered to HTML strings res[lastIndex] = createTextVNode(last.text + c) } else if (c !== '') { // convert primitive to vnode res.push(createTextVNode(c)) } } else { if (isTextNode(c) && isTextNode(last)) { // merge adjacent text nodes res[lastIndex] = createTextVNode(last.text + c.text) } else { // default key for nested array children (likely generated by v-for) if (isTrue(children._isVList) && isDef(c.tag) && isUndef(c.key) && isDef(nestedIndex)) { c.key = `__vlist${nestedIndex}_${i}__` } res.push(c) } } } return res }
以上就是 _createElement 做的第一件事情：拍平 children，第二件事就是创建 vnode，判定分别是：html元素，组件，或者无法识别的节点。最终 vnode 就是 render.js 里面 vnode = render.call(vm._renderProxy, vm.$createElement) 的第二个参数，而 render.call 的结果就是 vm._c = (a, b, c, d) => createElement(vm, a, b, c, d, false) 中的 createElement 的返回值，所以 vnode 最终就是 createElement 的结果，接着 createElement 是 vm._render 的返回值，再回到之前的 updateComponent：
updateComponent = () => { vm._update(vm._render(), hydrating) }
vm._update 的第一个参数就是刚才的 createElement 的返回值，到此，拿到了一个完整的 vnode ，下篇说一下 _update 是将 vnode 如何变成真实的 DOM 节点。
