createElement

Vue.js利用createElement方法创建Vnode，它定义在src/core/vdom/create-element.js 中：

// 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)
}

createElement方法实际上是对_createElement方法的封装，它允许传入的参数更加灵活，在处理完这些参数后，调用真正创建Vnode的函数_createElement：

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()
  }
}

_createElement方法有5个参数，context表示Vnode的上下文环境，它是Component类型；tag表示标签，它可以是一个字符串，也可以是一个Component；data表示Vnode的数据，它是一个VnodeData类型，可以在flow/vnode.js中找到他的定义；children表示当前Vnode的子节点，它是任意类型的，它需要被规范为标准的Vnode数组；normalizationType表示子节点规范的类型，类型不同规范的方法不同，它主要参考render函数是编译生成的还是用户手写的。

children的规范化

由于虚拟DOM实际上是一个树状结构，每一个虚拟DOM可能有若干个子节点，这些子节点应该也是Vnode的类型。_createElement接收的第四个参数children是任意类型的，因此我们需要把它们规范成Vnode类型。

这里根据normalizationType的不同，调用了normalizeChildren(children)和simpleNormalizeChildren(children) 方法，它们的定义都在 src/core/vdom/helpers/normalzie-children.js 中：

// 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
}

simpleNormalizeChildren方法调用场景是render函数是编译生成的。理论上编译生成的children都已经是Vnode类型的，但这里有一个例外，就是function component函数式组件返回的是一个数字而不是一个根节点，所以会通过Array.prototypr.concat方法把children数组打平让让它的深度只有一成。

normalizeChildren方法调用的场景有2种。一种render函数是用户手写的，当children只有一个节点的时候，Vie.js从接口层面允许用户把children写成基础类型用来创建单个简单的文本节点，这种情况会调用createTextVNode创建一个文本节点的VNode；另一个场景是当编译slot、v-for的时候会产生嵌套数组的情况，会调用normalizeArrayChildren方法，接下来看看他的实现：

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
}

normalizeArrayChildren方法接收2个参数，children表示要规范的子节点，nestedIndex表示嵌套的索引，因为单个child可能是一个数组类型。normalizeArrayChildren主要逻辑就是遍历children，获得单个节点c，然后对c的类型判断，如果是一个数组类型则递归调用normalizeArrayChildren；如果是基础类型，则通过createTextVNode方法转换成VNode类型，否则就已经是VNode类型了。如果children是一个列表并且还存在嵌套的情况，则根据nestedIndex去更新它的key，这里需要注意一点是在遍历的过程中对这3中情况都做了处理，如果存在两个连续的text节点，会把他们合并成一个text节点。

经过对children的规范化，children就变成了一个类型为VNode的Array。

VNode的创建

回到createElement函数，规范完children之后接下来回去创建一个VNode实例：

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)
}

这里先对tag做判断，如果是string类型则接着判断如果是内置的一些节点则直接创建一个普通VNode，如果是已注册的组件名，则通过createElement创建一个组件类型的VNode，否则创建一个未知的标签的VNode。如果tag是一个component类型则直接调用createComponent创建一个组件类型的VNode节点，本质上它还是返回一个VNode。