Vue.js另一个核心思想是组件化。所谓组件化，就是把页面拆分成多个组件，每个组件依赖的CSS、JavaScript、模板、图片等资源放在一起开发和维护。组件是资源独立的，组件在系统内部可复用，组件和组件之间可以嵌套。
我们在用Vue.js开发实际项目的时候，就是像搭积木一样，编写一堆组件拼装生成页面。
接下来我们会用Vue-cli初始化的代码为例，来分析一下Vue组件初始化的一个过程。

import Vue from 'vue'
import App from './App.vue'

var app = new Vue({
  el: '#app',
  // 这里的 h 是 createElement 方法
  render: h => h(App)
})

这段代码是通过render函数去渲染的，通过createElement传的参数是一个组件而不是一个原生的标签，那么接下来我们就开始分析这一过程。

createComponent

createElement最终会调用_createElement方法，其中有一段逻辑是对参数tag的判断，如果是一个普通的html标签，则会实例化一个普通VNode节点，否则通过createComponent方法创建一个组件VNode。

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)
}

在这里入的是一个App对象，它本质上是一个Component类型，那么它会走到上述代码的else逻辑，直接通过createComponent方法来创建vnode。所以接下来我们来看一下createComponent方法的实现，它定义在src/core/vdom/create-component.js文件中：

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)
  }

  // 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
  }

  // async component
  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
  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
    }
  }

  // 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
  )

  // Weex specific: invoke recycle-list optimized @render function for
  // extracting cell-slot template.
  // https://github.com/Hanks10100/weex-native-directive/tree/master/component
  /* istanbul ignore if */
  if (__WEEX__ && isRecyclableComponent(vnode)) {
    return renderRecyclableComponentTemplate(vnode)
  }

  return vnode
}

可以看到，createComponent的逻辑也会有一些复杂，但是分析源码比较推荐的是只分析核心流程，分支流程可以之后针对性的看，所以这里针对组件渲染这个case主要就3个关键步骤：构造子类构造函数，安装组件钩子函数和实例化vnode。

构造子类构造函数

const baseCtor = context.$options._base

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

我们在编写一个组件的时候，通常都是创建一个普通对象，还是以我们的App.vue为例，代码如下：

import HelloWorld from './components/HelloWorld'

export default {
  name: 'app',
  components: {
    HelloWorld
  }
}

这里export的是一个对象，所以createComponent里的代码逻辑会执行到baseCtor.extend(Ctor)，在这里baseCtor实际上就是Vue，这个的定义是在最开始初始化Vue的阶段，在src/core/global-api/index.js中的initGlobalAPI函数有这么一段逻辑：

// this is used to identify the "base" constructor to extend all plain-object
// components with in Weex's multi-instance scenarios.
Vue.options._base = Vue

这里定义的是Vue.options，而我们的createComponent取的是context.$options，实际上在src/core/instance/init.js里Vue原型上的_init函数中有这么一段逻辑：

vm.$options = mergeOptions(
  resolveConstructorOptions(vm.constructor),
  options || {},
  vm
)

这样就把Vue上的一些option扩展到了vm.$options上，所以我们也就能通过vm.$options._base拿到Vue这个构造函数了。mergeOptions的功能是把Vue构造函数的options和用户传入的options做一层合并，到vm.$options上。
在了解了baseCtor指向了Vue之后，我们来看一下Vue.extend函数的定义，在src/core/global-api/extend.js中。

/**
 * Class inheritance
 */
Vue.extend = function (extendOptions: Object): Function {
  extendOptions = extendOptions || {}
  const Super = this
  const SuperId = Super.cid
  const cachedCtors = extendOptions._Ctor || (extendOptions._Ctor = {})
  if (cachedCtors[SuperId]) {
    return cachedCtors[SuperId]
  }

  const name = extendOptions.name || Super.options.name
  if (process.env.NODE_ENV !== 'production' && name) {
    validateComponentName(name)
  }

  const Sub = function VueComponent (options) {
    this._init(options)
  }
  Sub.prototype = Object.create(Super.prototype)
  Sub.prototype.constructor = Sub
  Sub.cid = cid++
  Sub.options = mergeOptions(
    Super.options,
    extendOptions
  )
  Sub['super'] = Super

  // For props and computed properties, we define the proxy getters on
  // the Vue instances at extension time, on the extended prototype. This
  // avoids Object.defineProperty calls for each instance created.
  if (Sub.options.props) {
    initProps(Sub)
  }
  if (Sub.options.computed) {
    initComputed(Sub)
  }

  // allow further extension/mixin/plugin usage
  Sub.extend = Super.extend
  Sub.mixin = Super.mixin
  Sub.use = Super.use

  // create asset registers, so extended classes
  // can have their private assets too.
  ASSET_TYPES.forEach(function (type) {
    Sub[type] = Super[type]
  })
  // enable recursive self-lookup
  if (name) {
    Sub.options.components[name] = Sub
  }

  // keep a reference to the super options at extension time.
  // later at instantiation we can check if Super's options have
  // been updated.
  Sub.superOptions = Super.options
  Sub.extendOptions = extendOptions
  Sub.sealedOptions = extend({}, Sub.options)

  // cache constructor
  cachedCtors[SuperId] = Sub
  return Sub
}

Vue.extend的作用就是构造一个Vue的子类，它使用一种非常经典的原型继承的方式把一个纯对象转换一个继承于Vue的构造器Sub并返回，然后对Sub这个对象本身扩展了一些属性，如扩展options、添加全局 API等；并且对配置中的props和computed做了初始化工作；最后对于这个Sub构造函数做了缓存，避免多次执行Vue.extend的时候对同一个子组件重复构造。

这样当我们去实例化Sub的时候，就会执行this._init逻辑再次走到了Vue实例的初始化逻辑。

const Sub = function VueComponent (options) {
  this._init(options)
}

安装组件钩子函数

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

Vue.js使用的Virtual DOM参考的是开源库snabbdom，它的一个特点是在VNode的patch流程中对外暴露了各种时机的钩子函数，方便我们做一些额外的事情，Vue.js也是充分利用这一点，在初始化一个Component类型的VNode的过程中实现了几个钩子函数：

const componentVNodeHooks = {
  init (vnode: VNodeWithData, hydrating: boolean): ?boolean {
    if (
      vnode.componentInstance &&
      !vnode.componentInstance._isDestroyed &&
      vnode.data.keepAlive
    ) {
      // kept-alive components, treat as a patch
      const mountedNode: any = vnode // work around flow
      componentVNodeHooks.prepatch(mountedNode, mountedNode)
    } else {
      const child = vnode.componentInstance = createComponentInstanceForVnode(
        vnode,
        activeInstance
      )
      child.$mount(hydrating ? vnode.elm : undefined, hydrating)
    }
  },

  prepatch (oldVnode: MountedComponentVNode, vnode: MountedComponentVNode) {
    const options = vnode.componentOptions
    const child = vnode.componentInstance = oldVnode.componentInstance
    updateChildComponent(
      child,
      options.propsData, // updated props
      options.listeners, // updated listeners
      vnode, // new parent vnode
      options.children // new children
    )
  },

  insert (vnode: MountedComponentVNode) {
    const { context, componentInstance } = vnode
    if (!componentInstance._isMounted) {
      componentInstance._isMounted = true
      callHook(componentInstance, 'mounted')
    }
    if (vnode.data.keepAlive) {
      if (context._isMounted) {
        // vue-router#1212
        // During updates, a kept-alive component's child components may
        // change, so directly walking the tree here may call activated hooks
        // on incorrect children. Instead we push them into a queue which will
        // be processed after the whole patch process ended.
        queueActivatedComponent(componentInstance)
      } else {
        activateChildComponent(componentInstance, true /* direct */)
      }
    }
  },

  destroy (vnode: MountedComponentVNode) {
    const { componentInstance } = vnode
    if (!componentInstance._isDestroyed) {
      if (!vnode.data.keepAlive) {
        componentInstance.$destroy()
      } else {
        deactivateChildComponent(componentInstance, true /* direct */)
      }
    }
  }
}

const hooksToMerge = Object.keys(componentVNodeHooks)

function installComponentHooks (data: VNodeData) {
  const hooks = data.hook || (data.hook = {})
  for (let i = 0; i < hooksToMerge.length; i++) {
    const key = hooksToMerge[i]
    const existing = hooks[key]
    const toMerge = componentVNodeHooks[key]
    if (existing !== toMerge && !(existing && existing._merged)) {
      hooks[key] = existing ? mergeHook(toMerge, existing) : toMerge
    }
  }
}

function mergeHook (f1: any, f2: any): Function {
  const merged = (a, b) => {
    // flow complains about extra args which is why we use any
    f1(a, b)
    f2(a, b)
  }
  merged._merged = true
  return merged
}

整个installComponentHooks的过程就是把componentVNodeHooks的钩子函数合并到data.hook中，在VNode执行 patch的过程中执行相关的钩子函数。这里要注意的是合并策略，在合并过程中，如果某个时机的钩子已经存在data.hook中，那么通过执行mergeHook函数做合并，这个逻辑很简单，就是在最终执行的时候，依次执行这两个钩子函数即可。

实例化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

最后一步非常简单，通过new VNode实例化一个vnode并返回。需要注意的是和普通元素节点的vnode不同，组件的vnode是没有children的，这点很关键，在之后的patch过程中我们会再提。