vue组件初始化过程

　　之前文章有写到vue构造函数的实例化过程，只是对vue实例做了个粗略的描述，并没有说明vue组件实例化的过程。本文主要对vue组件的实例化过程做一些简要的描述。

　　组件的实例化与vue构造函数的实例化，大部分是类似的，vue的实例可以当做一个根组件，普通组件的实例化可以当做子组件。真实的DOM是一个树形结构，虚拟DOM本质只是真实DOM的抽象，也是一个树形结构。简单来说，整个vue工程的实例化过程如下：

　　

　　如上图所示，在调用render函数时，会依次调用createElement方法，createElement方法的代码如下，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: ${JSON.stringify(data)}\n` +
      'Always create fresh vnode data objects in each render!',
      context
    )
    return createEmptyVNode()
  }
  // object syntax in v-bind
  if (isDef(data) && isDef(data.is)) {
    tag = data.is
  }
  if (!tag) {
    // in case of component :is set to falsy value
    return createEmptyVNode()
  }
  // warn against non-primitive key
  if (process.env.NODE_ENV !== 'production' &&
    isDef(data) && isDef(data.key) && !isPrimitive(data.key)
  ) {
    if (!__WEEX__ || !('@binding' in data.key)) {
      warn(
        'Avoid using non-primitive value as key, ' +
        'use string/number value instead.',
        context
      )
    }
  }
  // support single function children as default scoped slot
  if (Array.isArray(children) &&
    typeof children[0] === 'function'
  ) {
    data = data || {}
    data.scopedSlots = { default: children[0] }
    children.length = 0
  }
  // 组件格式化
  if (normalizationType === ALWAYS_NORMALIZE) {
    children = normalizeChildren(children)
  } else if (normalizationType === SIMPLE_NORMALIZE) {
    children = simpleNormalizeChildren(children)
  }
  let vnode, ns
  if (typeof tag === 'string') {
    let Ctor
    ns = (context.$vnode && context.$vnode.ns) || config.getTagNamespace(tag)
    // 普通的HTML标签
    if (config.isReservedTag(tag)) {
      // platform built-in elements
      if (process.env.NODE_ENV !== 'production' && isDef(data) && isDef(data.nativeOn)) {
        warn(
          `The .native modifier for v-on is only valid on components but it was used on <${tag}>.`,
          context
        )
      }
      // 创建一个普通的DOM节点
      vnode = new VNode(
        config.parsePlatformTagName(tag), data, children,
        undefined, undefined, context
      )
    } else if ((!data || !data.pre) && isDef(Ctor = resolveAsset(context.$options, 'components', tag))) {
      // component
      // 创建组件
      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()
  }
}

　　从上述代码中可以看出，如果存在tag且tag的类型为string，会走一些判断逻辑，主要就是判断两类，一类是HTML标签，通过config.isReservedTag判断是否是HTML标签，另外一类就是在当前实例作用域options中的component中查找，是否存在对该类标签的声明，存在，即使组件，详细流程图如下图所示：

　　如上图所示，主流程与实例化Vue类似，只是在实例化Vue的过程中，额外走了一个创建组件的分支，其中createComponent方法实现如下：

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
  }
  // 获取Vue基础构造函数，在initGlobal中，将vue基础构造方法赋值给_base属性
  const baseCtor = context.$options._base

  // plain options object: turn it into a constructor
  if (isObject(Ctor)) {
    // 将组件的配置，合并到构造方法中，extend是定义在Vue构造方法中的
    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
  // 初始化组件的钩子函数
  installComponentHooks(data)

  // return a placeholder vnode
  // 体现了组件名称在这里面的作用
  const name = Ctor.options.name || tag
  // 创建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
}

　　从上述代码中可以看出，createComponent主要作用就是返回一个vnode，中间的流程主要作用有两点，一是组装组件的构造方法，用于实例化组件，另外一点就是调用installComponentHooks，初始化组件的生命周期入口。组件的声明周期钩子虽然与vue根实例一致，但是调用的位置还是有一定的差别，具体有以下几点：

　　1. Vue构造方法是在src\core\instance\index.js中，而组件的构造方法是基于Vue根构造方法，在上述createComponet中调用Vue.extend方法进行组装而成，本质上都是调用Vue实例上的_init方法，但是组件的构造方法VueComponent声明了一些属于自己的自定义属性，具体实现代码如下：

 Vue.extend = function (extendOptions: Object): Function {
    extendOptions = extendOptions || {}
    const Super = this
    // 父级实例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)
    }
    // 定义vue初始化方法，和实例化Vue走同一个路线
    const Sub = function VueComponent (options) {
      this._init(options)
    }
    // super -> this -> Vue 继承Vue构造方法中的属性
    Sub.prototype = Object.create(Super.prototype)
    // 指定子组件的构造方法为Sub -> VueComponent
    Sub.prototype.constructor = Sub
    Sub.cid = cid++
    // 合并组件属性
    Sub.options = mergeOptions(
      Super.options,
      extendOptions
    )
    // 定义父级作用域
    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
    // 子组件的实例，保持对vue构造方法的引用
    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根实例的模板解析与DOM挂载入口不一致，在_init方法中，提供了对根实例的模板解析与DOM挂载，而组件没有。在创建组件时，调用了installComponentHooks，componet hooks主要包含init、prepatch、insert、destory，init在实例化组件时调用，insert是插入DOM时调用，destory是在销毁组件时调用，而prepatch是在更新组件时调用，具体如下：

const componentVNodeHooks = {
  // 组件初始化方法
  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 {
      // 实例化组件
      const child = vnode.componentInstance = createComponentInstanceForVnode(
        vnode,
        activeInstance
      )
      //挂载组件
      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 */)
      }
    }
  }
}

　　如上述代码所示，实例化组件调用的是createComponentInstanceForVnode,createComponentInstanceForVnode代码如下，调用在Vue.extend中组装的组件构造方法VueComponent，初始化调用的还是Vue原型上的_init方法，大致流程与Vue初始化类似，只是解析模板有所区别，组件解析模板调用的是child.$mount。

// 创建组件的作用域，执行组件的_init方法，同vue实例化过程
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
  }
  // 实例化组件的构造方法
  return new vnode.componentOptions.Ctor(options)
}

　　在installComponentHooks中，在vnode的data属性中初始化了hooks,后面在_patch__中，会调用patch.js中声明的createComponent -> init -> 实例化组件。组件实例化完成后，会将真实DOM元素，插入到上一级元素。patch.js中的createComponent方法如下：

  // 创建组件，如果节点类型是组件，则直接走创建组件的方法
  function createComponent (vnode, insertedVnodeQueue, parentElm, refElm) {
    let i = vnode.data
    // 判断是否存在组件的生命周期，存在，即需要走创建组件的流程
    if (isDef(i)) {
      const isReactivated = isDef(vnode.componentInstance) && i.keepAlive
      if (isDef(i = i.hook) && isDef(i = i.init)) {
        // 执行component的init方法，获取组件的实例
        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.
      // 组件的vnode对象中存在当前组件的作用域
      if (isDef(vnode.componentInstance)) {
        initComponent(vnode, insertedVnodeQueue)
        // 将子组件插入到父节点中
        insert(parentElm, vnode.elm, refElm)
        if (isTrue(isReactivated)) {
          reactivateComponent(vnode, insertedVnodeQueue, parentElm, refElm)
        }
        return true
      }
    }
  }

　　在实例化完成后，会将生成的真实DOM元素插入到上级元素中，vue在获取真实DOM时，是从低往上，一级级添加，最终将渲染的元素添加到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
        // 获取老旧节点
        const oldElm = oldVnode.elm
        // 获取老旧节点的父节点
        const parentElm = nodeOps.parentNode(oldElm)

        // create new node
        // 将虚拟DOM转换成真实DOM
        // 传入父级节点，一级级添加
        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
        // 移除老旧节点
        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，然后再讲老旧节点移除。