Virtual DOM

Virtual DOM這個(gè)概念相信大部分人都不會(huì)陌生，它產(chǎn)生的前提是瀏覽器中的DOM是很“昂貴"的监右，為了更直觀的感受抖僵，我們可以簡(jiǎn)單的把一個(gè)簡(jiǎn)單的div元素的屬性都打印出來(lái)反番，如圖所示：

可以看到灭红，真正的DOM元素是非常龐大的狗唉，因?yàn)闉g覽器的標(biāo)準(zhǔn)就把DOM設(shè)計(jì)的非常復(fù)雜初烘。當(dāng)我們頻繁的去做DOM更新，會(huì)產(chǎn)生一定的性能問(wèn)題。
而Virtual DOM就是用一個(gè)原生的JS對(duì)象去描述一個(gè)DOM節(jié)點(diǎn)肾筐，所以它比創(chuàng)建一個(gè)DOM的代價(jià)要小很多哆料。在Vue.js中，Virtual DOM是用VNode這么一個(gè)Class去描述吗铐，它是定義在src/core/vdom/vnode.js中的东亦。

export default class VNode {
  tag: string | void;
  data: VNodeData | void;
  children: ?Array<VNode>;
  text: string | void;
  elm: Node | void;
  ns: string | void;
  context: Component | void; // rendered in this component's scope
  key: string | number | void;
  componentOptions: VNodeComponentOptions | void;
  componentInstance: Component | void; // component instance
  parent: VNode | void; // component placeholder node

  // strictly internal
  raw: boolean; // contains raw HTML? (server only)
  isStatic: boolean; // hoisted static node
  isRootInsert: boolean; // necessary for enter transition check
  isComment: boolean; // empty comment placeholder?
  isCloned: boolean; // is a cloned node?
  isOnce: boolean; // is a v-once node?
  asyncFactory: Function | void; // async component factory function
  asyncMeta: Object | void;
  isAsyncPlaceholder: boolean;
  ssrContext: Object | void;
  fnContext: Component | void; // real context vm for functional nodes
  fnOptions: ?ComponentOptions; // for SSR caching
  fnScopeId: ?string; // functional scope id support

  constructor (
    tag?: string,
    data?: VNodeData,
    children?: ?Array<VNode>,
    text?: string,
    elm?: Node,
    context?: Component,
    componentOptions?: VNodeComponentOptions,
    asyncFactory?: Function
  ) {
    this.tag = tag
    this.data = data
    this.children = children
    this.text = text
    this.elm = elm
    this.ns = undefined
    this.context = context
    this.fnContext = undefined
    this.fnOptions = undefined
    this.fnScopeId = undefined
    this.key = data && data.key
    this.componentOptions = componentOptions
    this.componentInstance = undefined
    this.parent = undefined
    this.raw = false
    this.isStatic = false
    this.isRootInsert = true
    this.isComment = false
    this.isCloned = false
    this.isOnce = false
    this.asyncFactory = asyncFactory
    this.asyncMeta = undefined
    this.isAsyncPlaceholder = false
  }

  // DEPRECATED: alias for componentInstance for backwards compat.
  /* istanbul ignore next */
  get child (): Component | void {
    return this.componentInstance
  }
}

實(shí)際上Vue.js中Virtual DOM是借鑒了一個(gè)開(kāi)源庫(kù)snabbdom 的實(shí)現(xiàn)，然后加入了一些Vue.js特色的東西唬渗。這個(gè)庫(kù)更加簡(jiǎn)單和純粹典阵。
其實(shí)VNode是對(duì)真實(shí)DOM的一種抽象描述，它的核心定義無(wú)非就幾個(gè)關(guān)鍵屬性镊逝，標(biāo)簽名壮啊、數(shù)據(jù)、子節(jié)點(diǎn)撑蒜、鍵值等歹啼，其它屬性都是都是用來(lái)擴(kuò)展VNode的靈活性以及實(shí)現(xiàn)一些特殊feature的。由于VNode只是用來(lái)映射到真實(shí)DOM的渲染座菠，不需要包含操作DOM的方法狸眼，因此它是非常輕量和簡(jiǎn)單的。
Virtual DOM除了它的數(shù)據(jù)結(jié)構(gòu)的定義浴滴，映射到真實(shí)的DOM實(shí)際上要經(jīng)歷VNode的create拓萌、diff、patch等過(guò)程巡莹。那么在Vue.js中司志，VNode的create是通過(guò)之前提到的createElement方法創(chuàng)建的。

createElement

Vue.js利用createElement方法創(chuàng)建VNode降宅，它定義在src/core/vdom/create-elemenet.js中：

// wrapper function for providing a more flexible interface
// without getting yelled at by flow
export function createElement (
  context: Component,
  tag: any,
  data: any,
  children: any,
  normalizationType: any,
  alwaysNormalize: boolean
): VNode | Array<VNode> {
  if (Array.isArray(data) || isPrimitive(data)) {
    normalizationType = children
    children = data
    data = undefined
  }
  if (isTrue(alwaysNormalize)) {
    normalizationType = ALWAYS_NORMALIZE
  }
  return _createElement(context, tag, data, children, normalizationType)
}

createElement方法實(shí)際上是對(duì)_createElement方法的封裝骂远，它允許傳入的參數(shù)更加靈活，在處理這些參數(shù)后腰根，調(diào)用真正創(chuàng)建VNode的函數(shù)_createElement：

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: ${JSON.stringify(data)}\n` +
      'Always create fresh vnode data objects in each render!',
      context
    )
    return createEmptyVNode()
  }
  // object syntax in v-bind
  if (isDef(data) && isDef(data.is)) {
    tag = data.is
  }
  if (!tag) {
    // in case of component :is set to falsy value
    return createEmptyVNode()
  }
  // warn against non-primitive key
  if (process.env.NODE_ENV !== 'production' &&
    isDef(data) && isDef(data.key) && !isPrimitive(data.key)
  ) {
    if (!__WEEX__ || !('@binding' in data.key)) {
      warn(
        'Avoid using non-primitive value as key, ' +
        'use string/number value instead.',
        context
      )
    }
  }
  // support single function children as default scoped slot
  if (Array.isArray(children) &&
    typeof children[0] === 'function'
  ) {
    data = data || {}
    data.scopedSlots = { default: children[0] }
    children.length = 0
  }
  if (normalizationType === ALWAYS_NORMALIZE) {
    children = normalizeChildren(children)
  } else if (normalizationType === SIMPLE_NORMALIZE) {
    children = simpleNormalizeChildren(children)
  }
  let vnode, ns
  if (typeof tag === 'string') {
    let Ctor
    ns = (context.$vnode && context.$vnode.ns) || config.getTagNamespace(tag)
    if (config.isReservedTag(tag)) {
      // platform built-in elements
      vnode = new VNode(
        config.parsePlatformTagName(tag), data, children,
        undefined, undefined, context
      )
    } else if (isDef(Ctor = resolveAsset(context.$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)
  }
  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()
  }
}

_createElement方法有5個(gè)參數(shù)激才，context表示VNode的上下文環(huán)境，它是Component類型额嘿；tag 表示標(biāo)簽瘸恼，它可以是一個(gè)字符串，也可以是一個(gè)Component册养；data表示VNode的數(shù)據(jù)东帅，它是一個(gè)VNodeData類型，可以在flow/vnode.js中找到它的定義球拦，這里先不展開(kāi)說(shuō)靠闭；children表示當(dāng)前VNode的子節(jié)點(diǎn)帐我，它是任意類型的，它接下來(lái)需要被規(guī)范為標(biāo)準(zhǔn)的VNode數(shù)組愧膀；normalizationType表示子節(jié)點(diǎn)規(guī)范的類型拦键，類型不同規(guī)范的方法也就不一樣，它主要是參考 render 函數(shù)是編譯生成的還是用戶手寫(xiě)的檩淋。
createElement函數(shù)的流程略微有點(diǎn)多芬为，我們接下來(lái)主要分析2個(gè)重點(diǎn)的流程——children的規(guī)范化以及VNode的創(chuàng)建。

children的規(guī)范化

由于Virtual DOM實(shí)際上是一個(gè)樹(shù)狀結(jié)構(gòu)蟀悦，每一個(gè)VNode可能會(huì)有若干個(gè)子節(jié)點(diǎn)媚朦，這些子節(jié)點(diǎn)應(yīng)該也是VNode的類型。_createElement接收的第 4個(gè)參數(shù)children是任意類型的熬芜，因此我們需要把它們規(guī)范成VNode類型莲镣。
這里根據(jù)normalizationType的不同，調(diào)用了normalizeChildren(children)和simpleNormalizeChildren(children)方法涎拉，它們的定義都在src/core/vdom/helpers/normalzie-children.js中：

// The template compiler attempts to minimize the need for normalization by
// statically analyzing the template at compile time.
//
// For plain HTML markup, normalization can be completely skipped because the
// generated render function is guaranteed to return Array<VNode>. There are
// two cases where extra normalization is needed:

// 1\. When the children contains components - because a functional component
// may return an Array instead of a single root. In this case, just a simple
// normalization is needed - if any child is an Array, we flatten the whole
// thing with Array.prototype.concat. It is guaranteed to be only 1-level deep
// because functional components already normalize their own children.
export function simpleNormalizeChildren (children: any) {
  for (let i = 0; i < children.length; i++) {
    if (Array.isArray(children[i])) {
      return Array.prototype.concat.apply([], children)
    }
  }
  return children
}

// 2\. When the children contains constructs that always generated nested Arrays,
// e.g. <template>, <slot>, v-for, or when the children is provided by user
// with hand-written render functions / JSX. In such cases a full normalization
// is needed to cater to all possible types of children values.
export function normalizeChildren (children: any): ?Array<VNode> {
  return isPrimitive(children)
    ? [createTextVNode(children)]
    : Array.isArray(children)
      ? normalizeArrayChildren(children)
      : undefined
}

simpleNormalizeChildren方法調(diào)用場(chǎng)景是render函數(shù)是編譯生成的瑞侮。理論上編譯生成的children都已經(jīng)是VNode類型的，但這里有一個(gè)例外鼓拧，就是functional component函數(shù)式組件返回的是一個(gè)數(shù)組而不是一個(gè)根節(jié)點(diǎn)半火，所以會(huì)通過(guò)Array.prototype.concat方法把整個(gè)children數(shù)組打平，讓它的深度只有一層季俩。

normalizeChildren方法的調(diào)用場(chǎng)景有2種钮糖，一個(gè)場(chǎng)景是render函數(shù)是用戶手寫(xiě)的，當(dāng)children只有一個(gè)節(jié)點(diǎn)的時(shí)候酌住，Vue.js從接口層面允許用戶把children寫(xiě)成基礎(chǔ)類型用來(lái)創(chuàng)建單個(gè)簡(jiǎn)單的文本節(jié)點(diǎn)店归，這種情況會(huì)調(diào)用createTextVNode創(chuàng)建一個(gè)文本節(jié)點(diǎn)的VNode；另一個(gè)場(chǎng)景是當(dāng)編譯slot酪我、v-for 的時(shí)候會(huì)產(chǎn)生嵌套數(shù)組的情況消痛，會(huì)調(diào)用normalizeArrayChildren方法，接下來(lái)看一下它的實(shí)現(xiàn)：

function normalizeArrayChildren (children: any, nestedIndex?: string): Array<VNode> {
  const res = []
  let i, c, lastIndex, last
  for (i = 0; i < children.length; i++) {
    c = children[i]
    if (isUndef(c) || typeof c === 'boolean') continue
    lastIndex = res.length - 1
    last = res[lastIndex]
    //  nested
    if (Array.isArray(c)) {
      if (c.length > 0) {
        c = normalizeArrayChildren(c, `${nestedIndex || ''}_${i}`)
        // merge adjacent text nodes
        if (isTextNode(c[0]) && isTextNode(last)) {
          res[lastIndex] = createTextVNode(last.text + (c[0]: any).text)
          c.shift()
        }
        res.push.apply(res, c)
      }
    } else if (isPrimitive(c)) {
      if (isTextNode(last)) {
        // merge adjacent text nodes
        // this is necessary for SSR hydration because text nodes are
        // essentially merged when rendered to HTML strings
        res[lastIndex] = createTextVNode(last.text + c)
      } else if (c !== '') {
        // convert primitive to vnode
        res.push(createTextVNode(c))
      }
    } else {
      if (isTextNode(c) && isTextNode(last)) {
        // merge adjacent text nodes
        res[lastIndex] = createTextVNode(last.text + c.text)
      } else {
        // default key for nested array children (likely generated by v-for)
        if (isTrue(children._isVList) &&
          isDef(c.tag) &&
          isUndef(c.key) &&
          isDef(nestedIndex)) {
          c.key = `__vlist${nestedIndex}_${i}__`
        }
        res.push(c)
      }
    }
  }
  return res
}

normalizeArrayChildren接收2個(gè)參數(shù)都哭，children表示要規(guī)范的子節(jié)點(diǎn)秩伞，nestedIndex表示嵌套的索引，因?yàn)閱蝹€(gè)child可能是一個(gè)數(shù)組類型欺矫。 normalizeArrayChildren主要的邏輯就是遍歷children纱新，獲得單個(gè)節(jié)點(diǎn)c，然后對(duì)c的類型判斷穆趴，如果是一個(gè)數(shù)組類型脸爱，則遞歸調(diào)用normalizeArrayChildren; 如果是基礎(chǔ)類型，則通過(guò)createTextVNode方法轉(zhuǎn)換成VNode類型未妹；否則就已經(jīng)是VNode類型了阅羹，如果children是一個(gè)列表并且列表還存在嵌套的情況勺疼，則根據(jù)nestedIndex去更新它的key教寂。這里需要注意一點(diǎn)捏鱼，在遍歷的過(guò)程中，對(duì)這3種情況都做了如下處理：如果存在兩個(gè)連續(xù)的text節(jié)點(diǎn)酪耕，會(huì)把它們合并成一個(gè)text節(jié)點(diǎn)导梆。
經(jīng)過(guò)對(duì)children的規(guī)范化，children變成了一個(gè)類型為VNode的Array迂烁。

VNode的創(chuàng)建

回到createElement函數(shù)看尼，規(guī)范化children后，接下來(lái)會(huì)去創(chuàng)建一個(gè)VNode的實(shí)例：

let vnode, ns
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)
}

這里先對(duì)tag做判斷盟步，如果是string類型藏斩，則接著判斷如果是內(nèi)置的一些節(jié)點(diǎn)，則直接創(chuàng)建一個(gè)普通VNode却盘，如果是為已注冊(cè)的組件名狰域，則通過(guò)createComponent創(chuàng)建一個(gè)組件類型的VNode，否則創(chuàng)建一個(gè)未知的標(biāo)簽的VNode黄橘。 如果是tag一個(gè)Component類型兆览，則直接調(diào)用createComponent創(chuàng)建一個(gè)組件類型的VNode節(jié)點(diǎn)。對(duì)于createComponent創(chuàng)建組件類型的VNode的過(guò)程塞关，本質(zhì)上它還是返回了一個(gè)VNode抬探。

那么至此，我們大致了解了createElement 創(chuàng)建VNode的過(guò)程帆赢，每個(gè)VNode有children小压，children每個(gè)元素也是一個(gè)VNode，這樣就形成了一個(gè)VNode Tree椰于，它很好的描述了我們的 DOM Tree怠益。

回到mountComponent函數(shù)的過(guò)程，我們已經(jīng)知道vm._render是如何創(chuàng)建了一個(gè)VNode廉羔，接下來(lái)就是要把這個(gè)VNode渲染成一個(gè)真實(shí)的DOM并渲染出來(lái)溉痢，這個(gè)過(guò)程是通過(guò)vm._update完成的，接下來(lái)分析一下這個(gè)過(guò)程憋他。

update

Vue的_update是實(shí)例的一個(gè)私有方法孩饼，它被調(diào)用的時(shí)機(jī)有2個(gè)，一個(gè)是首次渲染竹挡，一個(gè)是數(shù)據(jù)更新的時(shí)候镀娶。_update方法的作用是把VNode渲染成真實(shí)的DOM，它的定義在src/core/instance/lifecycle.js中：

Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {
  const vm: Component = this
  const prevEl = vm.$el
  const prevVnode = vm._vnode
  const prevActiveInstance = activeInstance
  activeInstance = vm
  vm._vnode = vnode
  // Vue.prototype.__patch__ is injected in entry points
  // based on the rendering backend used.
  if (!prevVnode) {
    // initial render
    vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)
  } else {
    // updates
    vm.$el = vm.__patch__(prevVnode, vnode)
  }
  activeInstance = prevActiveInstance
  // update __vue__ reference
  if (prevEl) {
    prevEl.__vue__ = null
  }
  if (vm.$el) {
    vm.$el.__vue__ = vm
  }
  // if parent is an HOC, update its $el as well
  if (vm.$vnode && vm.$parent && vm.$vnode === vm.$parent._vnode) {
    vm.$parent.$el = vm.$el
  }
  // updated hook is called by the scheduler to ensure that children are
  // updated in a parent's updated hook.
}

_update的核心就是調(diào)用vm.__patch__方法揪罕，這個(gè)方法實(shí)際上在不同的平臺(tái)梯码，比如web和weex上的定義是不一樣的宝泵，因此在web平臺(tái)中它的定義在src/platforms/web/runtime/index.js中：

Vue.prototype.__patch__ = inBrowser ? patch : noop

可以看到，甚至在web平臺(tái)上轩娶，是否是服務(wù)端渲染也會(huì)對(duì)這個(gè)方法產(chǎn)生影響儿奶。因?yàn)樵诜?wù)端渲染中，沒(méi)有真實(shí)的瀏覽器DOM環(huán)境鳄抒，所以不需要把VNode最終轉(zhuǎn)換成DOM闯捎，因此是一個(gè)空函數(shù)，而在瀏覽器端渲染中许溅，它指向了patch方法瓤鼻，它的定義在src/platforms/web/runtime/patch.js中：

import * as nodeOps from 'web/runtime/node-ops'
import { createPatchFunction } from 'core/vdom/patch'
import baseModules from 'core/vdom/modules/index'
import platformModules from 'web/runtime/modules/index'

// the directive module should be applied last, after all
// built-in modules have been applied.
const modules = platformModules.concat(baseModules)

export const patch: Function = createPatchFunction({ nodeOps, modules })

該方法的定義是調(diào)用createPatchFunction方法的返回值，這里傳入了一個(gè)對(duì)象贤重，包含nodeOps參數(shù)和modules參數(shù)茬祷。其中，nodeOps封裝了一系列DOM操作的方法并蝗，modules定義了一些模塊的鉤子函數(shù)的實(shí)現(xiàn)祭犯，來(lái)看一下createPatchFunction的實(shí)現(xiàn)，它定義在src/core/vdom/patch.js中：

const hooks = ['create', 'activate', 'update', 'remove', 'destroy']

export function createPatchFunction (backend) {
  let i, j
  const cbs = {}

  const { modules, nodeOps } = backend

  for (i = 0; i < hooks.length; ++i) {
    cbs[hooks[i]] = []
    for (j = 0; j < modules.length; ++j) {
      if (isDef(modules[j][hooks[i]])) {
        cbs[hooks[i]].push(modules[j][hooks[i]])
      }
    }
  }

  // ...

  return 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, 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
        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(parentElm, [oldVnode], 0, 0)
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode)
        }
      }
    }

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

createPatchFunction內(nèi)部定義了一系列的輔助方法借卧，最終返回了一個(gè)patch方法盹憎，這個(gè)方法就賦值給了vm._update函數(shù)里調(diào)用的vm.__patch__。

在介紹patch的方法實(shí)現(xiàn)之前铐刘，我們可以思考一下為何Vue.js源碼繞了這么一大圈陪每，把相關(guān)代碼分散到各個(gè)目錄。因?yàn)?code>patch是平臺(tái)相關(guān)的镰吵，在Web和Weex環(huán)境檩禾，它們把虛擬DOM映射到“平臺(tái)DOM”的方法是不同的，并且對(duì)“DOM”包括的屬性模塊創(chuàng)建和更新也不盡相同疤祭。因此每個(gè)平臺(tái)都有各自的nodeOps和modules盼产，它們的代碼需要托管在src/platforms這個(gè)大目錄下。

而不同平臺(tái)的patch的主要邏輯部分是相同的勺馆，所以這部分公共的部分托管在core這個(gè)大目錄下戏售。差異化部分只需要通過(guò)參數(shù)來(lái)區(qū)別，這里用到了一個(gè)函數(shù)柯里化的技巧草穆，通過(guò)createPatchFunction把差異化參數(shù)提前固化灌灾，這樣不用每次調(diào)用patch的時(shí)候都傳遞nodeOps和modules了，這種編程技巧也非常值得學(xué)習(xí)悲柱。

在這里锋喜，nodeOps表示對(duì)“平臺(tái) DOM”的一些操作方法，modules表示平臺(tái)的一些模塊，它們會(huì)在整個(gè)patch過(guò)程的不同階段執(zhí)行相應(yīng)的鉤子函數(shù)嘿般。
回到patch方法本身段标，它接收4個(gè)參數(shù)，oldVnode表示舊的VNode節(jié)點(diǎn)炉奴，它也可以不存在或者是一個(gè)DOM對(duì)象逼庞；vnode表示執(zhí)行_render后返回的VNode的節(jié)點(diǎn)；hydrating表示是否是服務(wù)端渲染盆佣；removeOnly是給transition-group用的往堡。
先來(lái)回顧我們的例子：

var app = new Vue({
  el: '#app',
  render: function (createElement) {
    return createElement('div', {
      attrs: {
        id: 'app'
      },
    }, this.message)
  },
  data: {
    message: 'Hello Vue!'
  }
})

然后我們?cè)?code>vm._update的方法里是這么調(diào)用patch方法的：

// initial render
vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)

結(jié)合我們的例子，我們的場(chǎng)景是首次渲染共耍，所以在執(zhí)行patch函數(shù)的時(shí)候，傳入的vm.$el對(duì)應(yīng)的是例子中id為app的DOM對(duì)象吨瞎，這個(gè)也就是我們?cè)?code>index.html模板中寫(xiě)的<div id="app">痹兜， vm.$el的賦值是在之前mountComponent函數(shù)做的，vnode對(duì)應(yīng)的是調(diào)用render函數(shù)的返回值颤诀，hydrating在非服務(wù)端渲染情況下為false字旭，removeOnly為false。
確定了這些入?yún)⒑笱陆校覀兓氐?code>patch函數(shù)的執(zhí)行過(guò)程遗淳，看幾個(gè)關(guān)鍵步驟。

const isRealElement = isDef(oldVnode.nodeType)
if (!isRealElement && sameVnode(oldVnode, vnode)) {
  // patch existing root node
  patchVnode(oldVnode, vnode, insertedVnodeQueue, 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
  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)
  )
}

由于我們傳入的oldVnode實(shí)際上是一個(gè)DOM container心傀，所以isRealElement為true屈暗，接下來(lái)又通過(guò)emptyNodeAt方法把oldVnode轉(zhuǎn)換成VNode對(duì)象，然后再調(diào)用createElm方法脂男，這個(gè)方法在這里非常重要养叛，來(lái)看一下它的實(shí)現(xiàn)：

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__) {
      // ...
    } else {
      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)
  }
}

createElm的作用是通過(guò)虛擬節(jié)點(diǎn)創(chuàng)建真實(shí)的DOM并插入到它的父節(jié)點(diǎn)中。 我們來(lái)看一下它的一些關(guān)鍵邏輯宰翅，createComponent方法目的是嘗試創(chuàng)建子組件弃甥，在當(dāng)前這個(gè)case下它的返回值為false；接下來(lái)判斷vnode是否包含tag汁讼，如果包含淆攻，先簡(jiǎn)單對(duì)tag的合法性在非生產(chǎn)環(huán)境下做校驗(yàn)，看是否是一個(gè)合法標(biāo)簽嘿架；然后再去調(diào)用平臺(tái)DOM的操作去創(chuàng)建一個(gè)占位符元素瓶珊。

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

接下來(lái)調(diào)用createChildren方法去創(chuàng)建子元素：

createChildren(vnode, children, insertedVnodeQueue)

function createChildren (vnode, children, insertedVnodeQueue) {
  if (Array.isArray(children)) {
    if (process.env.NODE_ENV !== 'production') {
      checkDuplicateKeys(children)
    }
    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)))
  }
}

createChildren的邏輯很簡(jiǎn)單，實(shí)際上是遍歷子虛擬節(jié)點(diǎn)眶明，遞歸調(diào)用createElm艰毒，這是一種常用的深度優(yōu)先的遍歷算法，這里要注意的一點(diǎn)是在遍歷過(guò)程中會(huì)把vnode.elm作為父容器的DOM節(jié)點(diǎn)占位符傳入搜囱。
接著再調(diào)用invokeCreateHooks方法執(zhí)行所有的create的鉤子并把vnodev push到insertedVnodeQueue中丑瞧。

 if (isDef(data)) {
  invokeCreateHooks(vnode, insertedVnodeQueue)
}

function invokeCreateHooks (vnode, insertedVnodeQueue) {
  for (let i = 0; i < cbs.create.length; ++i) {
    cbs.create[i](emptyNode, vnode)
  }
  i = vnode.data.hook // Reuse variable
  if (isDef(i)) {
    if (isDef(i.create)) i.create(emptyNode, vnode)
    if (isDef(i.insert)) insertedVnodeQueue.push(vnode)
  }
}

最后調(diào)用insert方法把DOM插入到父節(jié)點(diǎn)中柑土，因?yàn)槭沁f歸調(diào)用，子元素會(huì)優(yōu)先調(diào)用insert绊汹，所以整個(gè)vnode樹(shù)節(jié)點(diǎn)的插入順序是先子后父稽屏。來(lái)看一下insert方法，它的定義在src/core/vdom/patch.js上西乖。

insert(parentElm, vnode.elm, refElm)

function insert (parent, elm, ref) {
  if (isDef(parent)) {
    if (isDef(ref)) {
      if (ref.parentNode === parent) {
        nodeOps.insertBefore(parent, elm, ref)
      }
    } else {
      nodeOps.appendChild(parent, elm)
    }
  }
}

insert邏輯很簡(jiǎn)單狐榔，調(diào)用一些nodeOps把子節(jié)點(diǎn)插入到父節(jié)點(diǎn)中，這些輔助方法定義在src/platforms/web/runtime/node-ops.js中：

export function insertBefore (parentNode: Node, newNode: Node, referenceNode: Node) {
  parentNode.insertBefore(newNode, referenceNode)
}

export function appendChild (node: Node, child: Node) {
  node.appendChild(child)
}

其實(shí)就是調(diào)用原生DOM的API進(jìn)行DOM操作获雕。
在createElm過(guò)程中薄腻，如果vnode節(jié)點(diǎn)不包含tag，則它有可能是一個(gè)注釋或者純文本節(jié)點(diǎn)届案，可以直接插入到父元素中庵楷。在我們這個(gè)例子中，最內(nèi)層就是一個(gè)文本 vnode楣颠，它的text值取的就是之前的this.message的值Hello Vue!尽纽。

再回到patch方法，首次渲染我們調(diào)用了createElm方法童漩，這里傳入的parentElm是oldVnode.elm的父元素弄贿，在我們的例子是id為#app div的父元素，也就是body矫膨；實(shí)際上整個(gè)過(guò)程就是遞歸創(chuàng)建了一個(gè)完整的DOM樹(shù)并插入到body上差凹。
最后，我們根據(jù)之前遞歸createElm生成的vnode插入順序隊(duì)列豆拨，執(zhí)行相關(guān)的insert鉤子函數(shù)直奋。

總結(jié)

那么至此我們從主線上把模板和數(shù)據(jù)如何渲染成最終的DOM的過(guò)程分析完畢了，我們可以通過(guò)下圖更直觀地看到從初始化Vue到最終渲染的整個(gè)過(guò)程施禾。