vue2数据渲染（4）- createElement

src/core/vdom/create-element.ts

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 (isArray(data) || isPrimitive(data)) {
    normalizationType = children
    children = data
    data = undefined
  }
  if (isTrue(alwaysNormalize)) {
    normalizationType = ALWAYS_NORMALIZE
  }
  return _createElement(context, tag, data, children, normalizationType)
}

children是子节点，if (isArray(data) || isPrimitive(data))说明data没有传，第3个参数是children，对参数个数不一致做了参数重载处理，然后调用 _createElement

src/core/vdom/create-element.ts

export function _createElement(
  context: Component,
  tag?: string | Component | Function | Object,
  data?: VNodeData,
  children?: any,
  normalizationType?: number
): VNode | Array<VNode> {
  if (isDef(data) && isDef((data as any).__ob__)) {
    __DEV__ &&
      warn(
        `Avoid using observed data object as vnode data: ${JSON.stringify(
          data
        )}\n` + 'Always create fresh vnode data objects in each render!',
        context
      )
    return createEmptyVNode()
  }
  // object syntax in v-bind
  if (isDef(data) && isDef(data.is)) {
    tag = data.is
  }
  if (!tag) {
    // in case of component :is set to falsy value
    return createEmptyVNode()
  }
  // warn against non-primitive key
  if (__DEV__ && isDef(data) && isDef(data.key) && !isPrimitive(data.key)) {
    warn(
      'Avoid using non-primitive value as key, ' +
        'use string/number value instead.',
      context
    )
  }
  // support single function children as default scoped slot
  if (isArray(children) && isFunction(children[0])) {
    data = data || {}
    data.scopedSlots = { default: children[0] }
    children.length = 0
  }
  //对children做处理
  if (normalizationType === ALWAYS_NORMALIZE) {
    children = normalizeChildren(children)
  } else if (normalizationType === SIMPLE_NORMALIZE) {
    children = simpleNormalizeChildren(children)
  }
  let vnode, ns
  if (typeof tag === 'string') {
    let Ctor
    ns = (context.$vnode && context.$vnode.ns) || config.getTagNamespace(tag)
    if (config.isReservedTag(tag)) {
      // platform built-in elements
      if (
        __DEV__ &&
        isDef(data) &&
        isDef(data.nativeOn) &&
        data.tag !== 'component'
      ) {
        warn(
          `The .native modifier for v-on is only valid on components but it was used on <${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 as any, data, context, children)
  }
  if (isArray(vnode)) {
    return vnode
  } else if (isDef(vnode)) {
    if (isDef(ns)) applyNS(vnode, ns)
    if (isDef(data)) registerDeepBindings(data)
    return vnode
  } else {
    return createEmptyVNode()
  }
}

重点在对children做处理

  if (normalizationType === ALWAYS_NORMALIZE) {
    children = normalizeChildren(children)
  } else if (normalizationType === SIMPLE_NORMALIZE) {
    children = simpleNormalizeChildren(children)
  }

src/core/vdom/helpers/normalzie-children.ts

// 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 (isArray(children[i])) {
      return Array.prototype.concat.apply([], children)
    }
  }
  return children
}

只考虑把children把一层拍平，不递归，如果开始

children = [vnode1,vnode2,[vnode3,vnode4,[vnode5]]]，

返回的children是[vnode1,vnode2,vnode3,vnode4,[vnode5]]

// 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> | undefined {
  return isPrimitive(children)
    ? [createTextVNode(children)]
    : isArray(children)
    ? normalizeArrayChildren(children)
    : undefined
}

如果children是一个基础类型，就用数组包一个简单的文本vnode

src/core/vdom/vnode.ts

export function createTextVNode(val: string | number) {
  return new VNode(undefined, undefined, undefined, String(val))
}

src/core/vdom/helpers/normalzie-children.ts

function normalizeArrayChildren(
  children: any,
  nestedIndex?: string
): Array<VNode> {
  const res: VNode[] = []
  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 (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].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
}

normalizeArrayChildren最终是要把数组变为一个一维的vnode数组

回到前面的_createElement,当前不考虑组件，只是普通的元素返回的vnode是

vnode = new VNode( config.parsePlatformTagName(tag), data, children, undefined, undefined, context )