Vue2 源码漫游（二）

摘要：源码漫游二描述在一中其实已经把作为的框架中数据流相关跑了一遍。看上面两排公共方法这个方法的调用在整个源码中就两处，和。过程，这也是导致我们在源码运行中总是看见在有无函数分支，的时候总是能看见函数，然后就进入对组件。

描述：

    在（一）中其实已经把Vue作为MVVM的框架中数据流相关跑了一遍。这一章我们先看mount这一步，这样Vue大的主线就基本跑通了。然后我们再去看compile，v-bind等功能性模块的处理。

path:
    platformswebentry-runtime-with-compiler.js
这里对原本的公用$mount方法进行了代理.实际的直接方法是core/instance/lifecycle.js中的mountComponent方法。
根据组件模板的不同形式这里出现了两个分支，一个核心：
    分支：
        1、组件参数中有render属性：执行mount.call(this, el, hydrating)
        2、组件参数中没有render属性：将template/el转换为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
})
//这里代理了vue实例的$mount方法
const mount = Vue.prototype.$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转化为render方法。这里就是一个大的分支，我们可以将它称为render分支
  if (!options.render) {
    //如果没有传入render方法，且template参数存在，那么就开始解析模板，这就是compile的开始
    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")
      }
    }
  }
  //调用公用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

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

如果有render方法，那么就会调用公共mount方法，然后判断一下平台后直接调用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 {
  //因为是公用方法所以在这里有重新判断了一些el，其实如果有render属性的话，这里el就已经是DOM对象了
  el = el && inBrowser ? query(el) : undefined
  return mountComponent(this, el, hydrating)
}

接下来就是mountComponent。这里面有一个关键点 vm._watcher = new Watcher(vm, updateComponent, noop)，这个其实就是上篇中说到的依赖收集的一个触发点。你可以想想，组件在这个时候其实数据已经完成了响应式转换，就坐等收集依赖了，也就是坐等被第一次使用访问了。

export function mountComponent (
  vm: Component,
  el: ?Element,
  hydrating?: boolean
): Component {
  vm.$el = el
  //这个判断其实只是在配置默认render方法createEmptyVNode
  if (!vm.$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
        )
      }
    }
  }
  //执行beforeMount回调函数
  callHook(vm, "beforeMount")

  //这个updateComponent方法很重要，其实可以将它与Watcher中的参数expOrFn联系起来。他就是一个Watcher实例的值的获取过程，订阅者的一种真实身份。
  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 = () => {
      //实际方法,被Watcher.getter方法的执行给调用回来了，在这里先直接执行vm.render，这个就是compile的触发点
      vm._update(vm._render(), hydrating)
    }
  }
  //开始生产updateComponent这个动作的订阅者了，生产过程中调用Watcher.getter方法时又会回来执行这个updateComponent方法。看上面两排
  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()这个方法的调用在整个源码中就两处，vm._render()和child._render()。
从中可以理解到一个执行链条：

$mount -> new Watcher -> watcher.getter -> updateComponent -> vm._update -> vm._render -> vm.createElement -> createComponent(如果存在子组件，调用createElement，如果没有执行createElement)
在render的这一个层面上的出发点，都是来自于vm.$options.render函数，这也是为什么在Vue.prototype.$mount方法中会对vm.$options.render进行判断处理从而分出有render函数和没有render函数两种不同的处理方式

看一下vm._render源码：

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
    
    //如果父组件还没有更新，那么就先把子组件存在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.$vnode = _parentVnode
    // render self
    let vnode
    try {
      //执行$options.render,如果没传的就是一个默认的VNode实例。最后都会去掉用createElement公用方法corevdomcreate-element.js。这个就是大工程了。
      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 : 组件Module信息，最后与会被处理成vm实例对象
        data ： 组件数据
        context : 当前Vue组件
        children : 自组件
        tag ：组件名

const hooksToMerge = Object.keys(componentVNodeHooks)

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

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

  // plain options object: turn it into a constructor
  // 如果Ctor为对象，合并到vm数据，构建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
  }

  // 异步组件
  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
  //解析组件实例的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
  // 合并钩子函数 init 、 destroy  、 insert 、prepatch
  mergeHooks(data)

  // return a placeholder vnode
 // 最终目的生成一个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
}

过程线条：

$mount -> new Watcher -> watcher.getter -> updateComponent -> vm._update -> vm._render -> vm.createElement -> createComponent(如果存在子组件，调用createElement，如果没有执行createElement)

上面这个线条中其实都围绕着vm.$options进行render组件。现在大部分项目都是使用的.vue组件进行开发，所以使得对组件的配置对象不太敏感。
因为将.vue的内容转化为Vue组件配置模式的过程都被vue-loader处理（我们在require组件时处理的），其中就包括将template转换为render函数的关键。我们也可以定义一个配置型的组件，然后触发Vue$3.prototype.$mount中的mark("compile")进行处理。但是我觉得意义不是太大。
过程，这也是导致我们在源码运行中总是看见在有无render函数分支，的时候总是能看见render函数，然后就进入对组件 vm._update(vm._render(), hydrating)。
我们先记住这条主线，下一章我们进入到vue-loader中去看看