vue中VirtualDom被创建的方法

Vue.prototype._render = function (): VNode {
 const vm: Component = this
 const { render, _parentVnode } = vm.$options
 // ... 
 // set parent vnode. this allows render functions to have access
 // to the data on the placeholder node.
 vm.$vnode = _parentVnode
 // render self
 let vnode
 try {
  vnode = render.call(vm._renderProxy, vm.$createElement)
 } catch (e) {
  handleError(e, vm, `render`)
  // return error render result,
  // or previous vnode to prevent render error causing blank component
  /* istanbul ignore else */
  if (process.env.NODE_ENV !== 'production' && vm.$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
  }
 }
 // if the returned array contains only a single node, allow it
 if (Array.isArray(vnode) && vnode.length === 1) {
  vnode = vnode[0]
 }
 // 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
 }

/* istanbul ignore else */
if (process.env.NODE_ENV !== 'production') {
 initProxy(vm)
} else {
 vm._renderProxy = vm
}

let initProxy
// ...
initProxy = function initProxy (vm) {
 if (hasProxy) {
 // determine which proxy handler to use
 const options = vm.$options
 const handlers = options.render && options.render._withStripped
  ? getHandler
  : hasHandler
 vm._renderProxy = new Proxy(vm, handlers)
 } else {
 vm._renderProxy = vm
 }
}

const hasProxy =
 typeof Proxy !== 'undefined' && isNative(Proxy)

const hasHandler = {
 has (target, key) {
 const has = key in target
 const isAllowed = allowedGlobals(key) ||
  (typeof key === 'string' && key.charAt(0) === '_' && !(key in target.$data))
 if (!has && !isAllowed) {
  if (key in target.$data) warnReservedPrefix(target, key)
  else warnNonPresent(target, key)
 }
 return has || !isAllowed
 }
}

const getHandler = {
 get (target, key) {
 if (typeof key === 'string' && !(key in target)) {
  if (key in target.$data) warnReservedPrefix(target, key)
  else warnNonPresent(target, key)
 }
 return target[key]
 }
}

export function initRender (vm: Component) {
 // ...
 // bind the createElement fn to this instance
 // so that we get proper render context inside it.
 // args order: tag, data, children, normalizationType, alwaysNormalize
 // internal version is used by render functions compiled from templates
 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)
 // ...
}

const SIMPLE_NORMALIZE = 1
const ALWAYS_NORMALIZE = 2
// wrapper function for providing a more flexible interface
// without getting yelled at by flow
export function createElement (
 context: Component,
 tag: any,
 data: any,
 children: any,
 normalizationType: any,
 alwaysNormalize: boolean
): VNode | Array {
 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)
}

// @returns {VNode}
createElement(
 // {String | Object | Function}
 // 一个 HTML 标签字符串，组件选项对象，或者
 // 解析上述任何一种的一个 async 异步函数。必需参数。
 'div',

 // {Object}
 // 一个包含模板相关属性的数据对象
 // 你可以在 template 中使用这些特性。可选参数。
 {
 },

 // {String | Array}
 // 子虚拟节点 (VNodes)，由 `createElement()` 构建而成，
 // 也可以使用字符串来生成“文本虚拟节点”。可选参数。
 [
 '先写一些文字',
 createElement('h2', '一则头条'),
 createElement(MyComponent, {
  props: {
  someProp: 'foobar'
  }
 })
 ]
)

if (Array.isArray(data) || isPrimitive(data)) {
 normalizationType = children
 children = data
 data = undefined
}

export function _createElement (
 context: Component,
 tag?: string | Class | Function | Object,
 data?: VNodeData,
 children?: any,
 normalizationType?: number
): VNode | Array {
 // ...
 if (normalizationType === ALWAYS_NORMALIZE) {
  children = normalizeChildren(children)
 } else if (normalizationType === SIMPLE_NORMALIZE) {
  children = simpleNormalizeChildren(children)
 }
 let vnode, ns
 if (typeof tag === 'string') {
  let Ctor
  // ...
  if (config.isReservedTag(tag)) {
   // platform built-in elements
   vnode = new VNode(
    config.parsePlatformTagName(tag), data, children,
    undefined, undefined, context
   )
  } else if ((!data || !data.pre) && 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()
 }
}

// 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.