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Vue2的组件渲染

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示例HTML

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<!DOCTYPE html>
<html>

<head>
  <title>Vue.js</title>
  <!-- Delete ".min" for console warnings in development -->
  <script src="../../dist/vue.js"></script>
</head>

<body>
  <div id="app">
    <ol>
      <!-- Create an instance of the todo-item component -->
      <todo-item></todo-item>
    </ol>
    
  </div>

  <script>
    Vue.component('todo-item', {
      template: '<li>This is a todo</li>'
    })
    var app = new Vue({
      el: '#app',
      data: {
        message: 'Hello Vue!'
      },
    })
  </script>
</body>

</html>

Vue.component发生了什么

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export const ASSET_TYPES = [
  'component',
  'directive',
  'filter'
]

ASSET_TYPES.forEach(type => {
    Vue[type] = function (
      id: string,
      definition: Function | Object
    ): Function | Object | void {
      if (!definition) {
        return this.options[type + 's'][id]
      } else {
        /* istanbul ignore if */
        if (process.env.NODE_ENV !== 'production' && type === 'component') {
          validateComponentName(id)
        }
        if (type === 'component' && isPlainObject(definition)) {
          definition.name = definition.name || id
          definition = this.options._base.extend(definition)
        }
        if (type === 'directive' && typeof definition === 'function') {
          definition = { bind: definition, update: definition }
        }
        this.options[type + 's'][id] = definition
        return definition
      }
    }
})

首先我们验证组件的名称是否合格，然后从继承base VueComponent构造器并根据当前的defination构造一个新的构造函数，并将这个组件的定义赋值给当前实例的options的components对象中，键是定义的组件名称，值为这个extend得来的构造函数。通过这种方式，我们将组件todo-item注册到当前vue实例上。

如何渲染子组件

Vue通过vm._update(vm._render(), hydrating)这条语句进行渲染，用_render生成vdom对象，并用_update方法内部调用patch方法渲染vnode到页面上去。vm._render实际上调用生成的render函数生成vnode对象。这里的由模板编译之后的render函数是这样的：

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(function anonymous(
) {
with(this){return _c('div',{attrs:{"id":"app"}},[_c('ol',[_c('todo-item')],1)])}
})

最终执行完render function返回一个vnode对象，这个vnode以div为根并包含了所有子vnode信息。我们拿到这个vnode去调用_update进行渲染，_update内部会调用patch方法传入新旧vnode进行比对渲染。这里的patch方法如下：

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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.
          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
        const oldElm = oldVnode.elm
        const parentElm = nodeOps.parentNode(oldElm)

        // create new node
        // 这个是实际插入dom的地方
        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
        if (isDef(parentElm)) {
          removeVnodes([oldVnode], 0, 0)
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode)
        }
      }
    }

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

在patch中，最核心的部分依然是createElm

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function createElm (
   vnode,
   insertedVnodeQueue,
   parentElm,
   refElm,
   nested,
   ownerArray,
   index
 ) {
   if (isDef(vnode.elm) && isDef(ownerArray)) {
     // This vnode was used in a previous render!
     // now it's used as a new node, overwriting its elm would cause
     // potential patch errors down the road when it's used as an insertion
     // reference node. Instead, we clone the node on-demand before creating
     // associated DOM element for it.
     vnode = ownerArray[index] = cloneVNode(vnode)
   }

   vnode.isRootInsert = !nested // for transition enter check
   if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {
     return
   }

   const data = vnode.data
   const children = vnode.children
   const tag = vnode.tag
   if (isDef(tag)) {
     if (process.env.NODE_ENV !== 'production') {
       if (data && data.pre) {
         creatingElmInVPre++
       }
       if (isUnknownElement(vnode, creatingElmInVPre)) {
         warn(
           'Unknown custom element: <' + tag + '> - did you ' +
           'register the component correctly? For recursive components, ' +
           'make sure to provide the "name" option.',
           vnode.context
         )
       }
     }

     vnode.elm = vnode.ns
       ? nodeOps.createElementNS(vnode.ns, tag)
       : nodeOps.createElement(tag, vnode)
     setScope(vnode)

     /* istanbul ignore if */
     if (__WEEX__) {
       // in Weex, the default insertion order is parent-first.
       // List items can be optimized to use children-first insertion
       // with append="tree".
       const appendAsTree = isDef(data) && isTrue(data.appendAsTree)
       if (!appendAsTree) {
         if (isDef(data)) {
           invokeCreateHooks(vnode, insertedVnodeQueue)
         }
         insert(parentElm, vnode.elm, refElm)
       }
       createChildren(vnode, children, insertedVnodeQueue)
       if (appendAsTree) {
         if (isDef(data)) {
           invokeCreateHooks(vnode, insertedVnodeQueue)
         }
         insert(parentElm, vnode.elm, refElm)
       }
     } else {
       // 关键所在
       createChildren(vnode, children, insertedVnodeQueue)
       if (isDef(data)) {
         invokeCreateHooks(vnode, insertedVnodeQueue)
       }
       insert(parentElm, vnode.elm, refElm)
     }

     if (process.env.NODE_ENV !== 'production' && data && data.pre) {
       creatingElmInVPre--
     }
   } else if (isTrue(vnode.isComment)) {
     vnode.elm = nodeOps.createComment(vnode.text)
     insert(parentElm, vnode.elm, refElm)
   } else {
     vnode.elm = nodeOps.createTextNode(vnode.text)
     insert(parentElm, vnode.elm, refElm)
   }
 }

这里我们调用createChildren对孩子vnode依次调用createElm创建dom节点，方法如下：

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function createChildren (vnode, children, insertedVnodeQueue) {
    if (Array.isArray(children)) {
      if (process.env.NODE_ENV !== 'production') {
        checkDuplicateKeys(children)
      }
      // 重点是对于每个子vnode，递归调用createElm插入dom节点
      for (let i = 0; i < children.length; ++i) {
        createElm(children[i], insertedVnodeQueue, vnode.elm, null, true, children, i)
      }
    } else if (isPrimitive(vnode.text)) {
      nodeOps.appendChild(vnode.elm, nodeOps.createTextNode(String(vnode.text)))
    }
  }

我们注意到createElm中有这样一段逻辑：

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if (createComponent(vnode, insertedVnodeQueue, parentElm, refElm)) {
  return
}

而createComponent定义如下：

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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
      }
    }
  }

对于这条语句:

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if (isDef(i = i.hook) && isDef(i = i.init)) {
    i(vnode, false /* hydrating */)
}

当当前的vnode为组件时，如这里的todo-item，vnode.data.hook.init有定义，故执行init函数即这里的i函数，并传入vnode，而vue component的init函数定义为：

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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
      )
      // 手动挂载
      child.$mount(hydrating ? vnode.elm : undefined, hydrating)
    }
  }
}

keepalive暂不考虑，关注else的情况，

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function createComponentInstanceForVnode (
  // we know it's MountedComponentVNode but flow doesn't
  vnode: any,
  // activeInstance in lifecycle state
  parent: any
): 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
  }
  // 实际上这个Ctor是VueComponent构造函数
  return new vnode.componentOptions.Ctor(options)
}

对于这个vnode我们通过createComponentInstanceForVnode方法创造vue component实例， 在实例化这个Vue component实力的过程中，我们调用_init方法。实际上因为VueComponent继承自Vue，所以这里调用的_init方法实际上定义在Vue.prototype上：

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function VueComponent (options) {
  this._init(options)
}

Vue.prototype._init = function (options?: Object) {
    const vm: Component = this
    // a uid
    // 每一个组件有一个全局唯一_uid, 从0开始递增，Uid为0的组件是应用的跟组件
    vm._uid = uid++

    let startTag, endTag
    /* istanbul ignore if */
    if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
      startTag = `vue-perf-start:${vm._uid}`
      endTag = `vue-perf-end:${vm._uid}`
      mark(startTag)
    }

    // a flag to avoid this being observed
    vm._isVue = true
    // merge options
    if (options && options._isComponent) {
      // optimize internal component instantiation
      // since dynamic options merging is pretty slow, and none of the
      // internal component options needs special treatment.
      initInternalComponent(vm, options)
    } else {
      vm.$options = mergeOptions(
        resolveConstructorOptions(vm.constructor),
        options || {},
        vm
      )
    }
    /* istanbul ignore else */
    if (process.env.NODE_ENV !== 'production') {
      initProxy(vm)
    } else {
      vm._renderProxy = vm
    }
    // expose real self
    vm._self = vm
    initLifecycle(vm)
    initEvents(vm)
    initRender(vm)
    callHook(vm, 'beforeCreate')
    initInjections(vm) // resolve injections before data/props
    initState(vm)
    initProvide(vm) // resolve provide after data/props
    callHook(vm, 'created')

    /* istanbul ignore if */
    if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
      vm._name = formatComponentName(vm, false)
      mark(endTag)
      measure(`vue ${vm._name} init`, startTag, endTag)
    }

    if (vm.$options.el) {
      vm.$mount(vm.$options.el)
    }
  }

在实例化vue component的过程中，我们调用如beforeCreate和created这样的生命周期钩子。但是init并没有将组件mount，mount过程是在componentVnodeHooks中的init hook中完成，通过手动调用child.$mount(hydrating ? vnode.elm : undefined, hydrating)。我们知道child就是一个vue component实例，也继承自vue， 所以也是一个vue 实例，这里调用的$mount仍然存在于Vue.prototype上：

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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 <html> or <body> - mount to normal elements instead.`
    )
    return this
  }

  const options = this.$options
  // resolve template/el and convert to render function
  if (!options.render) {
    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, {
        outputSourceRange: process.env.NODE_ENV !== 'production',
        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')
      }
    }
  }
  return mount.call(this, el, hydrating)
}

我们在注册组件的时候提供了template: Vue.component('todo-item', { template: '<li>This is a todo</li>' }), 那么这里将template编译为render function，形式如下：

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ƒ anonymous() {
    with(this){return _c('li',[_v("This is a todo")])}
}

然后兜兜转转又开始重复我们初始化跟组件的过程，通过加入在线编译修改过后的$mount调用到原始的$mount方法，原始的$mount方法又调用mountComponent方法：

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export function mountComponent (
  vm: Component,
  el: ?Element,
  hydrating?: boolean
): Component {
  vm.$el = el
 
  callHook(vm, 'beforeMount')

  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 = () => {
      vm._update(vm._render(), hydrating)
    }
  }

  // we set this to vm._watcher inside the watcher's constructor
  // since the watcher's initial patch may call $forceUpdate (e.g. inside child
  // component's mounted hook), which relies on vm._watcher being already defined
  new Watcher(vm, updateComponent, noop, {
    before () {
      if (vm._isMounted && !vm._isDestroyed) {
        callHook(vm, 'beforeUpdate')
      }
    }
  }, true /* isRenderWatcher */)
  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
}

在mountComponent的过程中又继续实例化watcher，watcher实例化的过程中又调用updateComponent。即调用该组件的render function返回该组件对应的vnode对象，然后调用_update方法，_update进而调用patch方法来createElm，createElm进而createChildren，createChildren进而又createElm，将这个文本节点append到父li dom节点。最后patch方法返回这个li htmlelement节点赋值给vm.$el属性。

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if (vm.$el) {
  vm.$el.__vue__ = vm
}

htmlelement也可以反向引用到vue component实例

此时updateComponent调用完毕，vm的$el上也已有了对应的dom对象。对于todo-item组件，mountComponent的watcher实例化完毕，最终返回这个vm对象。init hook执行完毕，回到createComponent方法中：

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if (isDef(vnode.componentInstance)) {
    initComponent(vnode, insertedVnodeQueue)
    // 将li的dom节点插入到父ol dom节点
    // parentElm对应ol
    // vnode.elm对应li
    insert(parentElm, vnode.elm, refElm)
    if (isTrue(isReactivated)) {
      reactivateComponent(vnode, insertedVnodeQueue, parentElm, refElm)
    }
    return true
  }

此时vnode的componentInstance是vue component实例，最终createComponent返回true，意味着createElm提前终止，所以对于这个组件的vnode，执行流程与其他原生html元素会有所不同。然后就是递归的返回阶段，依此类推，将ol dom节点插入到div dom节点，然后将div dom节点插入到body，这一步完成之后，我们的This is a todo就显示在页面上了, 上面的这一切都发生在createElm方法中。

下面我们将回到根组件的patch方法中做一些清理工作，将旧的dom节点移除掉，然后执行inserted hook

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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)
    )

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

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

最后patch还是返回这个dom节点div#app，然后返回到updateComponent，watcher实例化完成，回到mountComponent，对应于根组件的mountComponent也完成了，触发mounted钩子。

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new Watcher(vm, updateComponent, noop, {
before () {
  if (vm._isMounted && !vm._isDestroyed) {
    callHook(vm, 'beforeUpdate')
  }
}
}, true /* isRenderWatcher */)
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')
}

mounted结束之后，init过程也就结束了，这就是带有注册组件的Vue实例化的全过程了。

总结

可以看到，在实例化vue实例的时候，通过mountComponent->watcher->updateComponent->render function -> return vnode，然后update->patch -> crateElm -> createChildren，在这个过程中，不断createElm -> createChildren，如果孩子是Vuecomponent实例，则在createElm时提前通过createComponent初始化组件提前return。递归插入dom节点，最终赋给body，渲染到页面上。