Vue源码简析之视图生成（上）

new Vue({
  el: '#app',
  template: '<div>{{msg}}</div>',
  data: {
    msg: 'hello vue'
  }
})
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最终会生成 <div>hello vue</div> 的视图，下面来分析一下流程。

new Vue()

function Vue (options) {
  if (process.env.NODE_ENV !== 'production' &&
    !(this instanceof Vue)
  ) {
    warn('Vue is a constructor and should be called with the `new` keyword')
  }
  this._init(options)
}
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_init

// core/instance/inti.js
Vue.prototype._init = function (options?: Object) {
    const vm: Component = this
    // a uid
    vm._uid = uid++
    ....
    ...
    // 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.$options.el)
    }
  }
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new Vue()的时候会执行_init方法，做一些初始化工作后执行vm.$mount,在这之前会判断有没有el属性，没有的话需要手动调用。就像这样:

new Vue({
  template: '<div>{{msg}}</div>',
  data: {
    msg: 'hello vue'
  }
}).$mount('#app')
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$mount

// platforms/web/entry-runtime-with-compilers.js
const mount = Vue.prototype.$mount  // 缓存$mount方法
Vue.prototype.$mount = function (
  el?: string | Element,
  hydrating?: boolean
): Component {
  el = el && query(el)
  ...
  ...
  const options = this.$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, {
        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 ${this._name} compile`, 'compile', 'compile end')
      }
    }
  }
  return mount.call(this, el, hydrating)
}
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这里先缓存了$mount方法，然后又重新定义了$mount方法，新方法中主要加了template转render函数的部分，这就对应了vue的 only-runtime 版本和 runtime-with-compiler 版本，compileToFunction的实现暂不深究，就是用来生成render函数，例子中的template转成的render函数如下：

缓存的mount方法：

//  platforms/web/runtime/index.js
Vue.prototype.$mount = function (
  el?: string | Element,
  hydrating?: boolean
): Component {
  el = el && inBrowser ? query(el) : undefined
  return mountComponent(this, el, hydrating)
}
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就是执行了mountComponent方法

mountComponent

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) {
    ...
  } 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 mounted hook), which relies on vm._watcher being already defined
  new Watcher(vm, updateComponent, noop, {
    before () {
      if (vm._isMounted) {
        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
}
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这里主要是定义了 updateComponent 函数，实例化一个watcher并将updateComponent作为参数传入，在watcher的构造函数中会执行。也就执行了下面这句：

vm._update(vm._render(), hydrating)
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整个DOm的生成和挂载都是由这一句代码完成的，可以分成两部分：

• render函数执行返回Vnode
• _update函数执行生成对应的DOM并挂载到页面上

render

// core/instance/render.js
Vue.prototype._render = function (): VNode {
    const vm: Component = this
    const { render, _parentVnode } = vm.$options
    ...
    let vnode
    try {
      vnode = render.call(vm._renderProxy, vm.$createElement)
    } catch (e) {
      ...
    }
    // return empty vnode in case the render function errored out
    if (!(vnode instanceof VNode)) {
      if (process.env.NODE_ENV !== 'production' && Array.isArray(vnode)) {
        warn(
          'Multiple root nodes returned from render function. Render function ' +
          'should return a single root node.',
          vm
        )
      }
      vnode = createEmptyVNode()
    }
    // set parent
    vnode.parent = _parentVnode
    return vnode
  }
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核心代码是：

vnode = render.call(vm._renderProxy, vm.$createElement)
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Vue是支持手写render函数的，接受一个参数，其实就是这里的vm.$createElement, 该函数也定义在同一个文件中。

vm.$createElement = (a, b, c, d) => createElement(vm, a, b, c, d, true)
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最终还是调用了createElement函数

createElement

// core/vdom/render.js
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)
}

export function _createElement (
  context: Component,
  tag?: string | Class<Component> | Function | Object,
  data?: VNodeData,
  children?: any,
  normalizationType?: number
): VNode | Array<VNode> {
 ...
  // 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()
  }
}
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createElement内部又调用了_createElemnet，这里就是生成Vnode的部分了，Vnode可以理解为用来描述dom节点的js对象，整个VirtualDOM就是由Vnode构成的Tree型结构。_createElement方法中有很多判断，是web还是weex，是组件还是简单的节点，对children格式化等，例子比较简单就不讨论这些了。最终会生成Vnode并返回，看下例子中的Vnode长啥样：

注意几个属性，tag表示标签名，context表示当前的Vue实例，children表示子节点，这里是我们的hello vue文本节点，见下图：

到这里render函数执行完毕，生成了Vnode并传入_update方法，最终由_update生成DOM并完成挂载，这部分下篇再说。