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Vue源码分析—数据驱动(二)

Virtual DOM

Virtual DOM这个概念相信大部分人都不会陌生，它产生的前提是浏览器中的DOM是很“昂贵"的，为了更直观的感受，我们可以简单的把一个简单的div元素的属性都打印出来，如图所示：

Vue源码分析—数据驱动(二)_第1张图片

可以看到，真正的DOM元素是非常庞大的，因为浏览器的标准就把DOM设计的非常复杂。当我们频繁的去做DOM更新，会产生一定的性能问题。
而Virtual DOM就是用一个原生的JS对象去描述一个DOM节点，所以它比创建一个DOM的代价要小很多。在Vue.js中，Virtual DOM是用VNode这么一个Class去描述，它是定义在src/core/vdom/vnode.js中的。

export default class VNode {
  tag: string | void;
  data: VNodeData | void;
  children: ?Array;
  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,
    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
  }
}

实际上Vue.js中Virtual DOM是借鉴了一个开源库snabbdom 的实现，然后加入了一些Vue.js特色的东西。这个库更加简单和纯粹。
其实VNode是对真实DOM的一种抽象描述，它的核心定义无非就几个关键属性，标签名、数据、子节点、键值等，其它属性都是都是用来扩展VNode的灵活性以及实现一些特殊feature的。由于VNode只是用来映射到真实DOM的渲染，不需要包含操作DOM的方法，因此它是非常轻量和简单的。
Virtual DOM除了它的数据结构的定义，映射到真实的DOM实际上要经历VNode的create、diff、patch等过程。那么在Vue.js中，VNode的create是通过之前提到的createElement方法创建的。

createElement

Vue.js利用createElement方法创建VNode，它定义在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 {
  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方法实际上是对_createElement方法的封装，它允许传入的参数更加灵活，在处理这些参数后，调用真正创建VNode的函数_createElement：

export function _createElement (
  context: Component,
  tag?: string | Class | Function | Object,
  data?: VNodeData,
  children?: any,
  normalizationType?: number
): VNode | Array {
  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个参数，context表示VNode的上下文环境，它是Component类型；tag 表示标签，它可以是一个字符串，也可以是一个Component；data表示VNode的数据，它是一个VNodeData类型，可以在flow/vnode.js中找到它的定义，这里先不展开说；children表示当前VNode的子节点，它是任意类型的，它接下来需要被规范为标准的VNode数组；normalizationType表示子节点规范的类型，类型不同规范的方法也就不一样，它主要是参考 render 函数是编译生成的还是用户手写的。
createElement函数的流程略微有点多，我们接下来主要分析2个重点的流程——children的规范化以及VNode的创建。

children的规范化

由于Virtual DOM实际上是一个树状结构，每一个VNode可能会有若干个子节点，这些子节点应该也是VNode的类型。_createElement接收的第 4个参数children是任意类型的，因此我们需要把它们规范成VNode类型。
这里根据normalizationType的不同，调用了normalizeChildren(children)和simpleNormalizeChildren(children)方法，它们的定义都在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. 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.