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。
