vue源码3----render函数

render函数定义在src/core/instance/render.js下，用来将模板渲染成虚拟dom(VNode)

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

    if (_parentVnode) {
      vm.$scopedSlots = normalizeScopedSlots(
        _parentVnode.data.scopedSlots,
        vm.$slots,
        vm.$scopedSlots
      )
    }

    // set parent vnode. this allows render functions to have access
    // to the data on the placeholder node.
    vm.$vnode = _parentVnode
    // render self
    let vnode
    try {
      // There's no need to maintain a stack because all render fns are called
      // separately from one another. Nested component's render fns are called
      // when parent component is patched.
      currentRenderingInstance = vm
      vnode = render.call(vm._renderProxy, vm.$createElement) //在这里实际调用
    } 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' && 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
      }
    } finally {
      currentRenderingInstance = null
    }
    // if the returned array contains only a single node, allow it
    if (Array.isArray(vnode) && vnode.length === 1) {
      vnode = vnode[0]
    }
    // return empty vnode in case the render function errored out
    if (!(vnode instanceof VNode)) {
      if (process.env.NODE_ENV !== 'production' && Array.isArray(vnode)) { //如果产生了多个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
  }

render函数的核心就是

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

首先看一下vm._renderProxy,在开发环境下，来源于initProxy(vm)

if (process.env.NODE_ENV !== 'production') {
      initProxy(vm)
    } else {
      vm._renderProxy = vm
    }

initProxy函数定义在src/core/instance/proxy.js

initProxy = function initProxy (vm) {
    if (hasProxy) {//首先判断当前环境是否支持proxy
      // determine which proxy handler to use
      const options = vm.$options
      const handlers = options.render && options.render._withStripped
        ? getHandler
        : hasHandler
      vm._renderProxy = new Proxy(vm, handlers)//创建一个代理
    } else {
      vm._renderProxy = vm
    }
  }

options.render._withStripped这个只有在严格模式下不支持with时，手动设置为true才启用，所以一般都是使用hasHandler

const hasHandler = {
    has (target, key) {
      const has = key in target
      const isAllowed = allowedGlobals(key) ||
        (typeof key === 'string' && key.charAt(0) === '_' && !(key in target.$data))
      if (!has && !isAllowed) {
        if (key in target.$data) warnReservedPrefix(target, key)
        else warnNonPresent(target, key)
      }
      return has || !isAllowed
    }
  }

在这里对has进行了代理，allowedGlobals是一些关键字

const allowedGlobals = makeMap(
    'Infinity,undefined,NaN,isFinite,isNaN,' +
    'parseFloat,parseInt,decodeURI,decodeURIComponent,encodeURI,encodeURIComponent,' +
    'Math,Number,Date,Array,Object,Boolean,String,RegExp,Map,Set,JSON,Intl,' +
    'require' // for Webpack/Browserify
  )

warnReservedPrefix，访问$data上的属性会得到警告

const warnReservedPrefix = (target, key) => {
    warn(
      `Property "${key}" must be accessed with "$data.${key}" because ` +
      'properties starting with "$" or "_" are not proxied in the Vue instance to ' +
      'prevent conflicts with Vue internals. ' +
      'See: https://vuejs.org/v2/api/#data',
      target
    )
  }

warnNonPresent，这个就是我们常见的错误之一：属性未定义就使用了（建议复制粘贴）...

const warnNonPresent = (target, key) => {
    warn(
      `Property or method "${key}" is not defined on the instance but ` +
      'referenced during render. Make sure that this property is reactive, ' +
      'either in the data option, or for class-based components, by ' +
      'initializing the property. ' +
      'See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.',
      target
    )
  }

接下来是vm.$createElement，首先来源于src/core/instance/render.js下的

// 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)

首先这两个函数区别在于最后一个参数，第一个用于编译生成的render，第二个用于用户手写的render