Vue的_render方法是实例的一个私有方法，用来把实例渲染成一个虚拟DOM
定义在 src/core/instance/render.js 文件中

Vue.prototype._render

  1. Vue.prototype._render = function (): VNode {
  2.   const vm: Component = this
  3.   const { render, _parentVnode } = vm.$options
  5.   if (_parentVnode) {
  6.     vm.$scopedSlots = normalizeScopedSlots(
  7.       _parentVnode.data.scopedSlots,
  8.       vm.$slots,
  9.       vm.$scopedSlots
  10.     )
  11.   }
  13.   // set parent vnode. this allows render functions to have access
  14.   // to the data on the placeholder node.
  15.   vm.$vnode = _parentVnode
  16.   // render self
  17.   let vnode
  18.   try {
  19.     // There's no need to maintain a stack because all render fns are called
  20.     // separately from one another. Nested component's render fns are called
  21.     // when parent component is patched.
  22.     currentRenderingInstance = vm
  23.     vnode = render.call(vm._renderProxy, vm.$createElement)
  24.   } catch (e) {
  25.     handleError(e, vm, `render`)
  26.     // return error render result,
  27.     // or previous vnode to prevent render error causing blank component
  28.     /* istanbul ignore else */
  29.     if (process.env.NODE_ENV !== 'production' && vm.$options.renderError) {
  30.       try {
  31.         vnode = vm.$options.renderError.call(vm._renderProxy, vm.$createElement, e)
  32.       } catch (e) {
  33.         handleError(e, vm, `renderError`)
  34.         vnode = vm._vnode
  35.       }
  36.     } else {
  37.       vnode = vm._vnode
  38.     }
  39.   } finally {
  40.     currentRenderingInstance = null
  41.   }
  42.   // if the returned array contains only a single node, allow it
  43.   if (Array.isArray(vnode) && vnode.length === 1) {
  44.     vnode = vnode[0]
  45.   }
  46.   // return empty vnode in case the render function errored out
  47.   if (!(vnode instanceof VNode)) {
  48.     if (process.env.NODE_ENV !== 'production' && Array.isArray(vnode)) {
  49.       warn(
  50.         'Multiple root nodes returned from render function. Render function ' +
  51.         'should return a single root node.',
  52.         vm
  53.       )
  54.     }
  55.     vnode = createEmptyVNode()
  56.   }
  57.   // set parent
  58.   vnode.parent = _parentVnode
  59.   return vnode
  60. }

开发中手写render方法的场景比较少，而写的比较多的是template模板
在mounted方法实现中会把template编译成render方法

  1. <div id="app">
  2.   {{ message }}
  3. </div>

相当于编写如下render函数

  1. render: function (createElement) {
  2.   return createElement('div', {
  3.      attrs: {
  4.         id: 'app'
  5.       },
  6.   }, this.message)
  7. }

再回到_render函数中的render方法的调用

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

render函数中的createElement方法就是vm.$createElement方法

initRender

  1. export function initRender (vm: Component) {
  2.   vm._vnode = null // the root of the child tree
  3.   vm._staticTrees = null // v-once cached trees
  4.   const options = vm.$options
  5.   const parentVnode = vm.$vnode = options._parentVnode // the placeholder node in parent tree
  6.   const renderContext = parentVnode && parentVnode.context
  7.   vm.$slots = resolveSlots(options._renderChildren, renderContext)
  8.   vm.$scopedSlots = emptyObject
  9.   // bind the createElement fn to this instance
  10.   // so that we get proper render context inside it.
  11.   // args order: tag, data, children, normalizationType, alwaysNormalize
  12.   // internal version is used by render functions compiled from templates
  13.   vm._c = (a, b, c, d) => createElement(vm, a, b, c, d, false)
  14.   // normalization is always applied for the public version, used in
  15.   // user-written render functions.
  16.   vm.$createElement = (a, b, c, d) => createElement(vm, a, b, c, d, true)
  18.   // $attrs & $listeners are exposed for easier HOC creation.
  19.   // they need to be reactive so that HOCs using them are always updated
  20.   const parentData = parentVnode && parentVnode.data
  22.   /* istanbul ignore else */
  23.   if (process.env.NODE_ENV !== 'production') {
  24.     defineReactive(vm, '$attrs', parentData && parentData.attrs || emptyObject, () => {
  25.       !isUpdatingChildComponent && warn(`$attrs is readonly.`, vm)
  26.     }, true)
  27.     defineReactive(vm, '$listeners', options._parentListeners || emptyObject, () => {
  28.       !isUpdatingChildComponent && warn(`$listeners is readonly.`, vm)
  29.     }, true)
  30.   } else {
  31.     defineReactive(vm, '$attrs', parentData && parentData.attrs || emptyObject, null, true)
  32.     defineReactive(vm, '$listeners', options._parentListeners || emptyObject, null, true)
  33.   }
  34. }

vm.$createElement方法是在initRender方法中定义的
vm._c方法是在template编译成render函数时使用，vm.$createElement是用户手写render方法时使用；这两个方法支持的参数相同，并且内部都调用了createElement方法