Vue源码简析之视图生成（上）

野性酷女 2022-03-14 11:06 240阅读 0赞

#### 一个例子 ####

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。