摘要：源碼漫游二描述在一中其實(shí)已經(jīng)把作為的框架中數(shù)據(jù)流相關(guān)跑了一遍。看上面兩排公共方法這個方法的調(diào)用在整個源碼中就兩處，和。過程，這也是導(dǎo)致我們在源碼運(yùn)行中總是看見在有無函數(shù)分支，的時候總是能看見函數(shù)，然后就進(jìn)入對組件。

描述：

    在（一）中其實(shí)已經(jīng)把Vue作為MVVM的框架中數(shù)據(jù)流相關(guān)跑了一遍。這一章我們先看mount這一步，這樣Vue大的主線就基本跑通了。然后我們再去看compile，v-bind等功能性模塊的處理。

path:
    platformswebentry-runtime-with-compiler.js
這里對原本的公用$mount方法進(jìn)行了代理.實(shí)際的直接方法是core/instance/lifecycle.js中的mountComponent方法。
根據(jù)組件模板的不同形式這里出現(xiàn)了兩個分支，一個核心：
    分支：
        1、組件參數(shù)中有render屬性：執(zhí)行mount.call(this, el, hydrating)
        2、組件參數(shù)中沒有render屬性：將template/el轉(zhuǎn)換為render方法
    核心：Vue.prototype._render公共方法

/* @flow */

import config from "core/config"
import { warn, cached } from "core/util/index"
import { mark, measure } from "core/util/perf"

import Vue from "./runtime/index"
import { query } from "./util/index"
import { compileToFunctions } from "./compiler/index"
import { shouldDecodeNewlines, shouldDecodeNewlinesForHref } from "./util/compat"

const idToTemplate = cached(id => {
  const el = query(id)
  return el && el.innerHTML
})
//這里代理了vue實(shí)例的$mount方法
const mount = Vue.prototype.$mount
Vue.prototype.$mount = function (
  el?: string | Element,
  hydrating?: boolean
): Component {
  el = el && query(el)

  /* istanbul ignore if */
  if (el === document.body || el === document.documentElement) {
    process.env.NODE_ENV !== "production" && warn(
      `Do not mount Vue to  or  - mount to normal elements instead.`
    )
    return this
  }

  const options = this.$options
  // resolve template/el and convert to render function
  //解析template/el轉(zhuǎn)化為render方法。這里就是一個大的分支，我們可以將它稱為render分支
  if (!options.render) {
    //如果沒有傳入render方法，且template參數(shù)存在，那么就開始解析模板，這就是compile的開始
    let template = options.template
    if (template) {
      if (typeof template === "string") {
        if (template.charAt(0) === "#") {
          template = idToTemplate(template)
          /* istanbul ignore if */
          if (process.env.NODE_ENV !== "production" && !template) {
            warn(
              `Template element not found or is empty: ${options.template}`,
              this
            )
          }
        }
      } else if (template.nodeType) {
        template = template.innerHTML
      } else {
        if (process.env.NODE_ENV !== "production") {
          warn("invalid template option:" + template, this)
        }
        return this
      }
    } else if (el) {
      template = getOuterHTML(el)
    }
    if (template) {
      /* istanbul ignore if */
      if (process.env.NODE_ENV !== "production" && config.performance && mark) {
        mark("compile")
      }

      const { render, staticRenderFns } = compileToFunctions(template, {
        shouldDecodeNewlines,
        shouldDecodeNewlinesForHref,
        delimiters: options.delimiters,
        comments: options.comments
      }, this)
      options.render = render
      options.staticRenderFns = staticRenderFns

      /* istanbul ignore if */
      if (process.env.NODE_ENV !== "production" && config.performance && mark) {
        mark("compile end")
        measure(`vue ${this._name} compile`, "compile", "compile end")
      }
    }
  }
  //調(diào)用公用mount方法
  return mount.call(this, el, hydrating)
}

/**
 * Get outerHTML of elements, taking care
 * of SVG elements in IE as well.
 */
function getOuterHTML (el: Element): string {
  if (el.outerHTML) {
    return el.outerHTML
  } else {
    const container = document.createElement("div")
    container.appendChild(el.cloneNode(true))
    return container.innerHTML
  }
}

Vue.compile = compileToFunctions

export default Vue

//最常見
new Vue({
    el: "#app",
    router,
    render: h => h(App),
});

如果有render方法，那么就會調(diào)用公共mount方法，然后判斷一下平臺后直接調(diào)用mountComponent方法

// public mount method
//入口中被代理的公用方法就是它，path : platformsweb
untimeindex.js
Vue.prototype.$mount = function (
  el?: string | Element,
  hydrating?: boolean
): Component {
  //因?yàn)槭枪梅椒ㄋ栽谶@里有重新判斷了一些el，其實(shí)如果有render屬性的話，這里el就已經(jīng)是DOM對象了
  el = el && inBrowser ? query(el) : undefined
  return mountComponent(this, el, hydrating)
}

接下來就是mountComponent。這里面有一個關(guān)鍵點(diǎn) vm._watcher = new Watcher(vm, updateComponent, noop)，這個其實(shí)就是上篇中說到的依賴收集的一個觸發(fā)點(diǎn)。你可以想想，組件在這個時候其實(shí)數(shù)據(jù)已經(jīng)完成了響應(yīng)式轉(zhuǎn)換，就坐等收集依賴了，也就是坐等被第一次使用訪問了。

export function mountComponent (
  vm: Component,
  el: ?Element,
  hydrating?: boolean
): Component {
  vm.$el = el
  //這個判斷其實(shí)只是在配置默認(rèn)render方法createEmptyVNode
  if (!vm.$options.render) {
    vm.$options.render = createEmptyVNode
    if (process.env.NODE_ENV !== "production") {
      /* istanbul ignore if */
      if ((vm.$options.template && vm.$options.template.charAt(0) !== "#") ||
        vm.$options.el || el) {
        warn(
          "You are using the runtime-only build of Vue where the template " +
          "compiler is not available. Either pre-compile the templates into " +
          "render functions, or use the compiler-included build.",
          vm
        )
      } else {
        warn(
          "Failed to mount component: template or render function not defined.",
          vm
        )
      }
    }
  }
  //執(zhí)行beforeMount回調(diào)函數(shù)
  callHook(vm, "beforeMount")

  //這個updateComponent方法很重要，其實(shí)可以將它與Watcher中的參數(shù)expOrFn聯(lián)系起來。他就是一個Watcher實(shí)例的值的獲取過程，訂閱者的一種真實(shí)身份。
  let updateComponent
  /* istanbul ignore if */
  if (process.env.NODE_ENV !== "production" && config.performance && mark) {
    updateComponent = () => {
      const name = vm._name
      const id = vm._uid
      const startTag = `vue-perf-start:${id}`
      const endTag = `vue-perf-end:${id}`

      mark(startTag)
      const vnode = vm._render()
      mark(endTag)
      measure(`vue ${name} render`, startTag, endTag)

      mark(startTag)
      vm._update(vnode, hydrating)
      mark(endTag)
      measure(`vue ${name} patch`, startTag, endTag)
    }
  } else {
    updateComponent = () => {
      //實(shí)際方法,被Watcher.getter方法的執(zhí)行給調(diào)用回來了，在這里先直接執(zhí)行vm.render，這個就是compile的觸發(fā)點(diǎn)
      vm._update(vm._render(), hydrating)
    }
  }
  //開始生產(chǎn)updateComponent這個動作的訂閱者了，生產(chǎn)過程中調(diào)用Watcher.getter方法時又會回來執(zhí)行這個updateComponent方法。看上面兩排
  vm._watcher = new Watcher(vm, updateComponent, noop)
  hydrating = false

  // manually mounted instance, call mounted on self
  // mounted is called for render-created child components in its inserted hook
  if (vm.$vnode == null) {
    vm._isMounted = true
    callHook(vm, "mounted")
  }
  return vm
}

path : coreinstance ender.js
Vue.prototype._render()這個方法的調(diào)用在整個源碼中就兩處，vm._render()和child._render()。
從中可以理解到一個執(zhí)行鏈條：

$mount -> new Watcher -> watcher.getter -> updateComponent -> vm._update -> vm._render -> vm.createElement -> createComponent(如果存在子組件，調(diào)用createElement，如果沒有執(zhí)行createElement)
在render的這一個層面上的出發(fā)點(diǎn)，都是來自于vm.$options.render函數(shù)，這也是為什么在Vue.prototype.$mount方法中會對vm.$options.render進(jìn)行判斷處理從而分出有render函數(shù)和沒有render函數(shù)兩種不同的處理方式

看一下vm._render源碼：

export function renderMixin (Vue: Class) {
  // 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
    
    //如果父組件還沒有更新，那么就先把子組件存在vm.$slots中
    if (vm._isMounted) {
      // if the parent didn"t update, the slot nodes will be the ones from
      // last render. They need to be cloned to ensure "freshness" for this render.
      for (const key in vm.$slots) {
        const slot = vm.$slots[key]
        if (slot._rendered) {
          vm.$slots[key] = cloneVNodes(slot, true /* deep */)
        }
      }
    }
    //作用域插槽
    vm.$scopedSlots = (_parentVnode && _parentVnode.data.scopedSlots) || emptyObject

    // set parent vnode. this allows render functions to have access
    // to the data on the placeholder node.
    vm.$vnode = _parentVnode
    // render self
    let vnode
    try {
      //執(zhí)行$options.render,如果沒傳的就是一個默認(rèn)的VNode實(shí)例。最后都會去掉用createElement公用方法corevdomcreate-element.js。這個就是大工程了。
      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
  }
}

3、_createElement, createComponent

path : corevdomcreate-element.js
_createElement(context, tag, data, children, normalizationType)

export function _createElement (
  context: Component,
  tag?: string | Class | Function | Object,
  data?: VNodeData,
  children?: any,
  normalizationType?: number
): VNode {
  //可以使用
  if (isDef(data) && isDef((data: any).__ob__)) {
    process.env.NODE_ENV !== "production" && warn(
      `Avoid using observed data object as vnode data: ${JSON.stringify(data)}
` +
      "Always create fresh vnode data objects in each render!",
      context
    )
    return createEmptyVNode()
  }
  // object syntax in v-bind
  if (isDef(data) && isDef(data.is)) {
    tag = data.is
  }
  if (!tag) {
    // in case of component :is set to falsy value
    return createEmptyVNode()
  }
  // warn against non-primitive key
  if (process.env.NODE_ENV !== "production" &&
    isDef(data) && isDef(data.key) && !isPrimitive(data.key)
  ) {
    warn(
      "Avoid using non-primitive value as key, " +
      "use string/number value instead.",
      context
    )
  }
  // support single function children as default scoped slot
  if (Array.isArray(children) &&
    typeof children[0] === "function"
  ) {
    data = data || {}
    data.scopedSlots = { default: children[0] }
    children.length = 0
  }
  if (normalizationType === ALWAYS_NORMALIZE) {
    children = normalizeChildren(children)
  } else if (normalizationType === SIMPLE_NORMALIZE) {
    children = simpleNormalizeChildren(children)
  }
  let vnode, ns
  if (typeof tag === "string") {
    let Ctor
    ns = (context.$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 (isDef(vnode)) {
    if (ns) applyNS(vnode, ns)
    return vnode
  } else {
    return createEmptyVNode()
  }
}

    path : corevdomcreate-component.js
    createComponent (Ctor, data, context, children, tag)
        Ctor : 組件Module信息，最后與會被處理成vm實(shí)例對象
        data ： 組件數(shù)據(jù)
        context : 當(dāng)前Vue組件
        children : 自組件
        tag ：組件名

const hooksToMerge = Object.keys(componentVNodeHooks)

export function createComponent (
  Ctor: Class | Function | Object | void,
  data: ?VNodeData,
  context: Component,
  children: ?Array,
  tag?: string
): VNode | void {

  //Ctor不能為undefined || null
  if (isUndef(Ctor)) {
    return
  }
  //
  const baseCtor = context.$options._base

  // plain options object: turn it into a constructor
  // 如果Ctor為對象，合并到vm數(shù)據(jù)，構(gòu)建Ctor
  if (isObject(Ctor)) {
    Ctor = baseCtor.extend(Ctor)
  }

  // if at this stage it"s not a constructor or an async component factory,
  // reject.
  if (typeof Ctor !== "function") {
    if (process.env.NODE_ENV !== "production") {
      warn(`Invalid Component definition: ${String(Ctor)}`, context)
    }
    return
  }

  // 異步組件
  let asyncFactory
  if (isUndef(Ctor.cid)) {
    asyncFactory = Ctor
    Ctor = resolveAsyncComponent(asyncFactory, baseCtor, context)
    if (Ctor === undefined) {
      // return a placeholder node for async component, which is rendered
      // as a comment node but preserves all the raw information for the node.
      // the information will be used for async server-rendering and hydration.
      return createAsyncPlaceholder(
        asyncFactory,
        data,
        context,
        children,
        tag
      )
    }
  }

  data = data || {}

  // resolve constructor options in case global mixins are applied after
  // component constructor creation
  //解析組件實(shí)例的options
  resolveConstructorOptions(Ctor)

  // transform component v-model data into props & events
  if (isDef(data.model)) {
    transformModel(Ctor.options, data)
  }

  // extract props
  const propsData = extractPropsFromVNodeData(data, Ctor, tag)

  // functional component
  if (isTrue(Ctor.options.functional)) {
    return createFunctionalComponent(Ctor, propsData, data, context, children)
  }

  // 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

  if (isTrue(Ctor.options.abstract)) {
    // abstract components do not keep anything
    // other than props & listeners & slot

    // work around flow
    const slot = data.slot
    data = {}
    if (slot) {
      data.slot = slot
    }
  }

  // merge component management hooks onto the placeholder node
  // 合并鉤子函數(shù) init 、 destroy  、 insert 、prepatch
  mergeHooks(data)

  // return a placeholder vnode
 // 最終目的生成一個vnode，然后就是一路的return出去
  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
}

過程線條：

$mount -> new Watcher -> watcher.getter -> updateComponent -> vm._update -> vm._render -> vm.createElement -> createComponent(如果存在子組件，調(diào)用createElement，如果沒有執(zhí)行createElement)

上面這個線條中其實(shí)都圍繞著vm.$options進(jìn)行render組件。現(xiàn)在大部分項(xiàng)目都是使用的.vue組件進(jìn)行開發(fā)，所以使得對組件的配置對象不太敏感。
因?yàn)閷?vue的內(nèi)容轉(zhuǎn)化為Vue組件配置模式的過程都被vue-loader處理（我們在require組件時處理的），其中就包括將template轉(zhuǎn)換為render函數(shù)的關(guān)鍵。我們也可以定義一個配置型的組件，然后觸發(fā)Vue$3.prototype.$mount中的mark("compile")進(jìn)行處理。但是我覺得意義不是太大。
過程，這也是導(dǎo)致我們在源碼運(yùn)行中總是看見在有無render函數(shù)分支，的時候總是能看見render函數(shù)，然后就進(jìn)入對組件 vm._update(vm._render(), hydrating)。
我們先記住這條主線，下一章我們進(jìn)入到vue-loader中去看看