text: /* @flow */
  
  import config from '../config'
  import VNode, { createEmptyVNode } from './vnode'
  import { createComponent } from './create-component'
  import { traverse } from '../observer/traverse'
  
  import {
    warn,
    isDef,
    isUndef,
    isTrue,
    isObject,
    isPrimitive,
    resolveAsset
  } from '../util/index'
  
  import {
    normalizeChildren,
    simpleNormalizeChildren
  } from './helpers/index'
  
  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<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)
  }
  
  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: ${JSON.stringify(data)}\n` +
        'Always create fresh vnode data objects in each render!',
        context
      )
      return createEmptyVNode()
    }
    // object syntax in v-bind
    if (isDef(data) && isDef(data.is)) {
      tag = data.is
    }
    if (!tag) {
      // in case of component :is set to falsy value
      return createEmptyVNode()
    }
    // warn against non-primitive key
    if (process.env.NODE_ENV !== 'production' &&
      isDef(data) && isDef(data.key) && !isPrimitive(data.key)
    ) {
      if (!__WEEX__ || !('@binding' in data.key)) {
        warn(
          'Avoid using non-primitive value as key, ' +
          'use string/number value instead.',
          context
        )
      }
    }
    // support single function children as default scoped slot
    if (Array.isArray(children) &&
      typeof children[0] === 'function'
    ) {
      data = data || {}
      data.scopedSlots = { default: children[0] }
      children.length = 0
    }
    // 去处理children
    if (normalizationType === ALWAYS_NORMALIZE) {
      /* 
      当手写render函数的时候调用
      判断children的类型，如果是原始值的话转换成VNode数组
      如果是数组的话，继续处理数组中的元素
      如果数组中的子元素
       */
      // 返回一维数组，处理用户手写的render
      children = normalizeChildren(children)
    } else if (normalizationType === SIMPLE_NORMALIZE) {
      // 把二维数组转换成一维数组
      children = simpleNormalizeChildren(children)
    }
    let vnode, ns
    if (typeof tag === 'string') {
      let Ctor
      ns = (context.$vnode && context.$vnode.ns) || config.getTagNamespace(tag)
      // 是否是html的保留标签
      if (config.isReservedTag(tag)) {
        // platform built-in elements
        if (process.env.NODE_ENV !== 'production' && isDef(data) && isDef(data.nativeOn)) {
          warn(
            `The .native modifier for v-on is only valid on components but it was used on <${tag}>.`,
            context
          )
        }
        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()
    }
  }
  
  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
  function registerDeepBindings (data) {
    if (isObject(data.style)) {
      traverse(data.style)
    }
    if (isObject(data.class)) {
      traverse(data.class)
    }
  }
  