Vue2.x - 子组件的实例化过程

Vue的其中一个核心思想为组件化，将页面拆分成不同的组件，独立了资源，利于开发和维护。前面讲了整个Vue的实例和挂载，但并没有详细记录子组件是怎么开始一轮生命周期的。

createComponent

回顾一下vnode的创建的过程：

1. createElement
2. _createElement
3. createComponent

// 在 src/core/vdom/create-component.js 中:
export function createComponent (
  Ctor: Class<Component> | Function | Object | void,
  data: ?VNodeData,
  context: Component,
  children: ?Array<VNode>,
  tag?: string
): VNode | Array<VNode> | void {

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

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

核心流程大致分为4步：

• 创建Ctor(Vue子类构造函数)
• 提取props
• 安装组件钩子函数
• 实例化vnode

Ctor

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

这里的关键在于弄清baseCtor是什么：

• 在 initGlobalAPI 中，我们定义了 Vue.options._base = Vue。

• 在 Vue.prototype._init 中，将Vue.options与options进行了合并。

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

所以这里的baseCtor就是Vue本身，而这里相当于执行了Vue.extend(Ctor)

• Vue.extend

  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
  }
}

这里用了经典原型继承的方式，构造了一个Vue的子类Sub，当实例化Sub的时候，就会调用_init方法，重新走到组件初始化创建的逻辑。

installComponentHooks

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
    }
  }
}

遍历hooksToMerge，不断向data.hook插入componentVNodeHooks对象中对应的钩子函数，包括init、prepatch、insert、destory。
这一步就是安装组件钩子函数，等待patch过程时去执行。

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 */)
      }
    }
  }
}

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

最终生成的vnode对象。

组件的patch

在createElm的实现中，有下面这个判断:

function createElm (
  vnode,
  insertedVnodeQueue,
  parentElm,
  refElm,
  nested,
  ownerArray,
  index
) {
  // ...
  if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {
    return
  }
  // ...
}

function createComponent (vnode, insertedVnodeQueue, parentElm, refElm) {
  let i = vnode.data
  if (isDef(i)) {
    const isReactivated = isDef(vnode.componentInstance) && i.keepAlive
    if (isDef(i = i.hook) && isDef(i = i.init)) {
      i(vnode, false /* hydrating */)
    }
    // after calling the init hook, if the vnode is a child component
    // it should've created a child instance and mounted it. the child
    // component also has set the placeholder vnode's elm.
    // in that case we can just return the element and be done.
    if (isDef(vnode.componentInstance)) {
      initComponent(vnode, insertedVnodeQueue)
      insert(parentElm, vnode.elm, refElm)
      if (isTrue(isReactivated)) {
        reactivateComponent(vnode, insertedVnodeQueue, parentElm, refElm)
      }
      return true
    }
  }
}

let i = vnode.data 如果 i 有定义，则说明vnode是一个组件，最后i经过一系列的赋值指向了data.hook.init，然后执行 i(vnode, false)，也就是执行了上面提到过的init钩子函数。

  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)
    }
  },

这里的逻辑就是通过 createComponentInstanceForVnode 创建子组件实例，然后通过$mount挂载子组件。

export function createComponentInstanceForVnode (
  vnode: any,
  parent: any,
): Component {
  const options: InternalComponentOptions = {
    _isComponent: true,
    _parentVnode: vnode,
    parent
  }
  // ...
  return new vnode.componentOptions.Ctor(options)
}

首先看vnode.componentOptions，它是在new VNode()实例化vnode时，将Ctor作为参数传入的，上面也提到了，它其实就是Vue的子类构造器Sub，所以这里相当于在new Sub()创建子组件实例。这里用_isComponent标识为一个组件，它的用处是在_init()的时候会采取不同方式处理options。

Vue.prototype._init = function (options?: Object) {
  const vm: Component = this
  // merge options
  if (options && options._isComponent) {
    initInternalComponent(vm, options)
  } else {
    vm.$options = mergeOptions(
      resolveConstructorOptions(vm.constructor),
      options || {},
      vm
    )
  }
}

在前面关于Vue实例化的文章也提到过，Vue的合并策略是很慢的，当注册一个内部组件的时候不需要做特殊的处理，所以可以直接初始化内部组件提升性能。

---

可以注意到 child.$mount(hydrating ? vnode.elm : undefined, hydrating) 接管了子组件的挂载，又开始新的一轮render、update、patch，不断的递归，只到整颗树挂载完毕为止。由于这种递归的关系，在进行 insert(parentElm, vnode.elm, refElm)的时候，插入的顺序是先子后父。所以其实也可以得知了父子的生命周期是
父beforeCreate -> 父created -> 父beforeMount -> 子beforeCreate -> 子created -> 子beforeMount -> 子mounted -> 父mounted 这样的顺序。