vue組件初始化過程

　　之前文章有寫到vue構造函數的實例化過程，只是對vue實例做了個粗略的描述，並沒有說明vue組件實例化的過程。本文主要對vue組件的實例化過程做一些簡要的描述。

　　組件的實例化與vue構造函數的實例化，大部分是類似的，vue的實例可以當做一個根組件，普通組件的實例化可以當做子組件。真實的DOM是一個樹形結構，虛擬DOM本質只是真實DOM的抽象，也是一個樹形結構。簡單來說，整個vue工程的實例化過程如下：

 　　如上圖所示，在調用render函數時，會依次調用createElement方法，createElement方法的代碼如下，主要作用就是生成vnode。

export function _createElement (
  context: Component,
  tag?: string | Class<Component> | Function | Object,
  data?: VNodeData,
  children?: any,
  normalizationType?: number
): VNode | Array<VNode> {
  if (isDef(data) && isDef((data: any).__ob__)) {
    process.env.NODE_ENV !== 'production' && warn(
      `Avoid using observed data object as vnode data: ${JSON.stringify(data)}\n` +
      'Always create fresh vnode data objects in each render!',
      context
    )
    return createEmptyVNode()
  }
  // object syntax in v-bind
  if (isDef(data) && isDef(data.is)) {
    tag = data.is
  }
  if (!tag) {
    // in case of component :is set to falsy value
    return createEmptyVNode()
  }
  // warn against non-primitive key
  if (process.env.NODE_ENV !== 'production' &&
    isDef(data) && isDef(data.key) && !isPrimitive(data.key)
  ) {
    if (!__WEEX__ || !('@binding' in data.key)) {
      warn(
        'Avoid using non-primitive value as key, ' +
        'use string/number value instead.',
        context
      )
    }
  }
  // support single function children as default scoped slot
  if (Array.isArray(children) &&
    typeof children[0] === 'function'
  ) {
    data = data || {}
    data.scopedSlots = { default: children[0] }
    children.length = 0
  }
  // 組件格式化
  if (normalizationType === ALWAYS_NORMALIZE) {
    children = normalizeChildren(children)
  } else if (normalizationType === SIMPLE_NORMALIZE) {
    children = simpleNormalizeChildren(children)
  }
  let vnode, ns
  if (typeof tag === 'string') {
    let Ctor
    ns = (context.$vnode && context.$vnode.ns) || config.getTagNamespace(tag)
    // 普通的HTML標簽
    if (config.isReservedTag(tag)) {
      // platform built-in elements
      if (process.env.NODE_ENV !== 'production' && isDef(data) && isDef(data.nativeOn)) {
        warn(
          `The .native modifier for v-on is only valid on components but it was used on <${tag}>.`,
          context
        )
      }
      // 創建一個普通的DOM節點
      vnode = new VNode(
        config.parsePlatformTagName(tag), data, children,
        undefined, undefined, context
      )
    } else if ((!data || !data.pre) && isDef(Ctor = resolveAsset(context.$options, 'components', tag))) {
      // component
      // 創建組件
      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 (Array.isArray(vnode)) {
    return vnode
  } else if (isDef(vnode)) {
    if (isDef(ns)) applyNS(vnode, ns)
    if (isDef(data)) registerDeepBindings(data)
    return vnode
  } else {
    return createEmptyVNode()
  }
}

 　　如上圖所示，主流程與實例化Vue類似，只是在實例化Vue的過程中，額外走了一個創建組件的分支，其中createComponent方法實現如下：

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
  }
  // 獲取Vue基礎構造函數，在initGlobal中，將vue基礎構造方法賦值給_base屬性
  const baseCtor = context.$options._base

  // plain options object: turn it into a constructor
  if (isObject(Ctor)) {
    // 將組件的配置，合併到構造方法中，extend是定義在Vue構造方法中的
    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)
    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
  // 初始化組件的鉤子函數
  installComponentHooks(data)

  // return a placeholder vnode
  // 體現了組件名稱在這裡面的作用
  const name = Ctor.options.name || tag
  // 創建vnode
  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
}

 　　從上述代碼中可以看出，createComponent主要作用就是返回一個vnode，中間的流程主要作用有兩點，一是組裝組件的構造方法，用於實例化組件，另外一點就是調用installComponentHooks，初始化組件的生命周期入口。組件的聲明周期鉤子雖然與vue根實例一致，但是調用的位置還是有一定的差別，具體有以下幾點：

　　1. Vue構造方法是在src\core\instance\index.js中，而組件的構造方法是基於Vue根構造方法，在上述createComponet中調用Vue.extend方法進行組裝而成，本質上都是調用Vue實例上的_init方法，但是組件的構造方法VueComponent聲明瞭一些屬於自己的自定義屬性，具體實現代碼如下：

 Vue.extend = function (extendOptions: Object): Function {
    extendOptions = extendOptions || {}
    const Super = this
    // 父級實例cid
    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)
    }
    // 定義vue初始化方法，和實例化Vue走同一個路線
    const Sub = function VueComponent (options) {
      this._init(options)
    }
    // super -> this -> Vue 繼承Vue構造方法中的屬性
    Sub.prototype = Object.create(Super.prototype)
    // 指定子組件的構造方法為Sub -> VueComponent
    Sub.prototype.constructor = Sub
    Sub.cid = cid++
    // 合併組件屬性
    Sub.options = mergeOptions(
      Super.options,
      extendOptions
    )
    // 定義父級作用域
    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
    // 子組件的實例，保持對vue構造方法的引用
    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
  }
}

　　2. Vue根實例的模板解析與DOM掛載入口不一致，在_init方法中，提供了對根實例的模板解析與DOM掛載，而組件沒有。在創建組件時，調用了installComponentHooks，componet hooks主要包含init、prepatch、insert、destory，init在實例化組件時調用，insert是插入DOM時調用，destory是在銷毀組件時調用，而prepatch是在更新組件時調用，具體如下：

const componentVNodeHooks = {
  // 組件初始化方法
  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 {
      // 實例化組件
      const child = vnode.componentInstance = createComponentInstanceForVnode(
        vnode,
        activeInstance
      )
      //掛載組件
      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 */)
      }
    }
  }
}

　　如上述代碼所示，實例化組件調用的是createComponentInstanceForVnode,createComponentInstanceForVnode代碼如下，調用在Vue.extend中組裝的組件構造方法VueComponent，初始化調用的還是Vue原型上的_init方法，大致流程與Vue初始化類似，只是解析模板有所區別，組件解析模板調用的是child.$mount。

// 創建組件的作用域，執行組件的_init方法，同vue實例化過程
export function createComponentInstanceForVnode (
  vnode: any, // we know it's MountedComponentVNode but flow doesn't
  parent: any, // activeInstance in lifecycle state
): Component {
  const options: InternalComponentOptions = {
    _isComponent: true,
    _parentVnode: vnode,
    parent
  }
  // check inline-template render functions
  const inlineTemplate = vnode.data.inlineTemplate
  if (isDef(inlineTemplate)) {
    options.render = inlineTemplate.render
    options.staticRenderFns = inlineTemplate.staticRenderFns
  }
  // 實例化組件的構造方法
  return new vnode.componentOptions.Ctor(options)
}

　　在installComponentHooks中，在vnode的data屬性中初始化了hooks,後面在_patch__中，會調用patch.js中聲明的createComponent -> init -> 實例化組件。組件實例化完成後，會將真實DOM元素，插入到上一級元素。patch.js中的createComponent方法如下：

  // 創建組件，如果節點類型是組件，則直接走創建組件的方法
  function createComponent (vnode, insertedVnodeQueue, parentElm, refElm) {
    let i = vnode.data
    // 判斷是否存在組件的生命周期，存在，即需要走創建組件的流程
    if (isDef(i)) {
      const isReactivated = isDef(vnode.componentInstance) && i.keepAlive
      if (isDef(i = i.hook) && isDef(i = i.init)) {
        // 執行component的init方法，獲取組件的實例
        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.
      // 組件的vnode對象中存在當前組件的作用域
      if (isDef(vnode.componentInstance)) {
        initComponent(vnode, insertedVnodeQueue)
        // 將子組件插入到父節點中
        insert(parentElm, vnode.elm, refElm)
        if (isTrue(isReactivated)) {
          reactivateComponent(vnode, insertedVnodeQueue, parentElm, refElm)
        }
        return true
      }
    }
  }

　　在實例化完成後，會將生成的真實DOM元素插入到上級元素中，vue在獲取真實DOM時，是從低往上，一級級添加，最終將渲染的元素添加到DOM body中，__patch__主流程如下：

function patch (oldVnode, vnode, hydrating, removeOnly) {
    if (isUndef(vnode)) {
      if (isDef(oldVnode)) invokeDestroyHook(oldVnode)
      return
    }

    let isInitialPatch = false
    const insertedVnodeQueue = []

    if (isUndef(oldVnode)) {
      // empty mount (likely as component), create new root element
      isInitialPatch = true
      createElm(vnode, insertedVnodeQueue)
    } else {
      const isRealElement = isDef(oldVnode.nodeType)
      if (!isRealElement && sameVnode(oldVnode, vnode)) {
        // patch existing root node
        patchVnode(oldVnode, vnode, insertedVnodeQueue, null, null, removeOnly)
      } else {
        if (isRealElement) {
          // mounting to a real element
          // check if this is server-rendered content and if we can perform
          // a successful hydration.
          // nodeType 1 元素 3 文字
          if (oldVnode.nodeType === 1 && oldVnode.hasAttribute(SSR_ATTR)) {
            oldVnode.removeAttribute(SSR_ATTR)
            hydrating = true
          }
          if (isTrue(hydrating)) {
            if (hydrate(oldVnode, vnode, insertedVnodeQueue)) {
              invokeInsertHook(vnode, insertedVnodeQueue, true)
              return oldVnode
            } else if (process.env.NODE_ENV !== 'production') {
              warn(
                'The client-side rendered virtual DOM tree is not matching ' +
                'server-rendered content. This is likely caused by incorrect ' +
                'HTML markup, for example nesting block-level elements inside ' +
                '<p>, or missing <tbody>. Bailing hydration and performing ' +
                'full client-side render.'
              )
            }
          }
          // either not server-rendered, or hydration failed.
          // create an empty node and replace it
          oldVnode = emptyNodeAt(oldVnode)
        }

        // replacing existing element
        // 獲取老舊節點
        const oldElm = oldVnode.elm
        // 獲取老舊節點的父節點
        const parentElm = nodeOps.parentNode(oldElm)

        // create new node
        // 將虛擬DOM轉換成真實DOM
        // 傳入父級節點，一級級添加
        createElm(
          vnode,
          insertedVnodeQueue,
          // extremely rare edge case: do not insert if old element is in a
          // leaving transition. Only happens when combining transition +
          // keep-alive + HOCs. (#4590)
          oldElm._leaveCb ? null : parentElm,
          nodeOps.nextSibling(oldElm)
        )

        // update parent placeholder node element, recursively
        if (isDef(vnode.parent)) {
          let ancestor = vnode.parent
          const patchable = isPatchable(vnode)
          while (ancestor) {
            for (let i = 0; i < cbs.destroy.length; ++i) {
              cbs.destroy[i](ancestor)
            }
            ancestor.elm = vnode.elm
            if (patchable) {
              for (let i = 0; i < cbs.create.length; ++i) {
                cbs.create[i](emptyNode, ancestor)
              }
              // #6513
              // invoke insert hooks that may have been merged by create hooks.
              // e.g. for directives that uses the "inserted" hook.
              const insert = ancestor.data.hook.insert
              if (insert.merged) {
                // start at index 1 to avoid re-invoking component mounted hook
                for (let i = 1; i < insert.fns.length; i++) {
                  insert.fns[i]()
                }
              }
            } else {
              registerRef(ancestor)
            }
            ancestor = ancestor.parent
          }
        }

        // destroy old node
        // 移除老舊節點
        if (isDef(parentElm)) {
          removeVnodes([oldVnode], 0, 0)
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode)
        }
      }
    }

    invokeInsertHook(vnode, insertedVnodeQueue, isInitialPatch)
    return vnode.elm
  }

　　模板的解析，是先把模板解析成HTML，然後再講老舊節點移除。