render

vm._render是在core/instance/render.js中定义的

/* core/instance/render.js */
export function renderMixin (Vue: Class<Component>) {
  // install runtime convenience helpers
  installRenderHelpers(Vue.prototype)

  Vue.prototype.$nextTick = function (fn: Function) {
    return nextTick(fn, this)
  }

  Vue.prototype._render = function (): VNode {
    const vm: Component = this
    const { render, _parentVnode } = vm.$options

    // reset _rendered flag on slots for duplicate slot check
    if (process.env.NODE_ENV !== 'production') {
      for (const key in vm.$slots) {
        // $flow-disable-line
        vm.$slots[key]._rendered = false
      }
    }

    if (_parentVnode) {
      vm.$scopedSlots = _parentVnode.data.scopedSlots || emptyObject
    }

    // 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') {
        if (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
        }
      } else {
        vnode = vm._vnode
      }
    }
    // 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
  }
}

这段代码的核心是render方法的调用:

vnode = render.call(vm._renderProxy, vm.$createElement)

根据Vue的文档，如下的模版：

<div id="app">
  {{ message }}
</div>

相当于如下的render函数：

render: function (createElement) {
  return createElement('div', {
     attrs: {
        id: 'app'
      },
  }, this.message)
}

render函数的第一个参数是createElement，也就是调用时传入的vm.$createElement方法。
vm.$createElement是在initRender时被加入到vm实例上的：

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)
}

可以看到，实际上调用了定义在core/vdom/create-element.js中定义的createElement函数。内部版本vm._c是用于从模版编译得到的render函数，vm.$createElement是用于用户手写的render函数，它们的区别就是最后一个参数alwaysNormalize

createElement

createElement函数参数是：

• tag: 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，必要参数。
• data: 一个包含模板相关属性的数据对象, 可选参数
• children: 子节点（VNodes）或子节点的Array, 或使用字符串来生成“文本节点”。可选参数
  由于可选参数的存在，createElement函数先对参数做了处理，然后调用真正的_createElement函数

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()
  }
}

这段代码的核心逻辑一个是childeren的normalize, 另一个是VNode的创建。

Normalize

根据render函数是编译生成还是用户手写，分别调用了simpleNormalizeChildren和normalizeChildren
这两个函数定义在core/vdom/helpers/normalize-children.js中

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
}

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 functional component 函数式组件返回的是一个数组而不是一个根节点，所以simpleNormalizeChildren会将children数组flatten到只有一层

export function normalizeChildren (children: any): ?Array<VNode> {
  return isPrimitive(children)
    ? [createTextVNode(children)]
    : Array.isArray(children)
      ? normalizeArrayChildren(children)
      : undefined
}

如果children是基础类型，会创建一个文本节点，返回只包含这个文本节点的Array，否则调用normalizeArrayChildren。
一些情况下children会包含嵌套数组，比如模版里有<template>, <slot>, v-for或者用户手写的render函数。 normalizeArrayChildren会展开children中的嵌套数组，使children只有一层。
同时如果遇到连续的多个文本节点，normalizeArrayChildren会把它们合并成一个文本节点。

创建VNode

如果根节点tag是HTML标签，那么用

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

创建一个VNode实例
如果根节点是一个组件那么调用createComponent创建组件的VNode

/* core/vdom/create-component.js */
export function createComponent (
  Ctor: Class<Component> | Function | Object | void,
  data: ?VNodeData,
  context: Component,
  children: ?Array<VNode>,
  tag?: string
): VNode | Array<VNode> | void {
  if (isUndef(Ctor)) {
    return
  }

  const baseCtor = context.$options._base

  // plain options object: turn it into a constructor
  if (isObject(Ctor)) {
    Ctor = baseCtor.extend(Ctor)
  }

  ...

  data = data || {}
  ...
  // extract listeners, since these needs to be treated as
  // child component listeners instead of DOM listeners
  const listeners = data.on
  // replace with listeners with .native modifier
  // so it gets processed during parent component patch.
  data.on = data.nativeOn

  ...
  // install component management hooks onto the placeholder node
  installComponentHooks(data)

  // return a placeholder vnode
  const name = Ctor.options.name || tag
  const vnode = new VNode(
    `vue-component-${Ctor.cid}${name ? `-${name}` : ''}`,
    data, undefined, undefined, undefined, context,
    { Ctor, propsData, listeners, tag, children },
    asyncFactory
  )
  ...
  return vnode
}

首先创建组件的构造函数，Ctor = baseCtor.extend(Ctor)，这里的baseCtor就是Vue的构造函数，然后提取data里的事件监听器。通过installComponentHooks安装init,prepatch,insert,destroy这些钩子函数，添加到data.hook。最后用

  const vnode = new VNode(
    `vue-component-${Ctor.cid}${name ? `-${name}` : ''}`,
    data, undefined, undefined, undefined, context,
    { Ctor, propsData, listeners, tag, children },
    asyncFactory
  )

创建组件的VNode，{ Ctor, propsData, listeners, tag, children }是作为组件的componentOpotions，那么组件的VNode与普通元素的VNode相比大致上就是多了componentOptions这个属性和data中的hook属性