Vue源码学习笔记（三）数据驱动

本小节将解析当我们使用new Vue的时候，Vue做了哪些事情

Vue.js一个核心思想就是数据驱动。视图是由数据驱动生成的，我们对视图的修改不会直接操作DOM，而是通过数据的修改。比我们传统的开发方式，大大简化了代码

这节主要分析$mount如何把数据渲染到DOM上

$mount定义

我们使用的是runtime+compiler版本，入口文件在web/entry-runtime-width-compiler.js中

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const mount = Vue.prototype.$mount
Vue.prototype.$mount = function (
  el?: string | Element,
  hydrating?: boolean
): Component {
  // el = el && query(el)首先拿到传入的DOM对象
  el = el && query(el)

  // 判断如果是body或者html标签，报警告，因为会覆盖标签
  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
  // 判断是否有render函数
  if (!options.render) {
    let template = options.template
    // 判断是否传入template，对template做处理
    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) {
      // 没有template的话拿到el的outerHTML作为template
      template = getOuterHTML(el)
    }
    if (template) {
      /* istanbul ignore if */
      if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
        mark('compile')
      }
	// 编译相关，拿到render函数
      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')
      }
    }
  }
   // 调用缓存的mount方法
  return mount.call(this, el, hydrating)
}

可以看出，在这个文件中，先缓存了Vue原型上的$mount方法，之后又重新定义了$mount方法，最终调用了之前的$mount方法

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

原始$mount方法，关键会执行到vm._update(vm._render(), hydrating)

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// core/instance/lifecycle.js
export function mountComponent (
  vm: Component,
  el: ?Element,
  hydrating?: boolean
): Component {
  // 定义$el
  vm.$el = el
  if (!vm.$options.render) {
    // 如果没有render函数，创建一个空的VNode
    vm.$options.render = createEmptyVNode
    if (process.env.NODE_ENV !== 'production') {
       // 警告，用runtime only版本时，使用template属性不能编译出render函数
      if ((vm.$options.template && vm.$options.template.charAt(0) !== '#') ||
        vm.$options.el || el) {
        warn(
          'You are using the runtime-only build of Vue where the template...',
          vm
        )
      } else {
        warn(
          'Failed to mount component: template or render function not defined.',
          vm
        )
      }
    }
  }
   // 触发beforMount钩子
  callHook(vm, 'beforeMount')

  let updateComponent
  // 性能埋点
  if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
    ...
  } else {
     // 最终都定义了updateComponent方法，调用_update方法，传入render函数生成的Vnode
    updateComponent = () => {
      // 真正会挂载的方法
      vm._update(vm._render(), hydrating)
    }
  }
  // 创建渲染watcher，在内部会执行到updateComponent方法
  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
}

_update参数1vm._render

生成vnode过程，调用createElement

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// core/instance/render.js
Vue.prototype._render = function (): VNode {
    const vm: Component = this
    // 拿到render函数，可以是编译生成的，也可以是用户自己写的
    const { render, _parentVnode } = vm.$options
	...
    // render self
    let vnode
    try {
      // 调用render函数，生产环境下renderproxy就是vm本身
      //  30行 编译生成的render方法使用 vm._c = (a, b, c, d) => createElement(vm, a, b, c, d, false)
      // 手写render方法使用的 vm.$createElement = (a, b, c, d) => createElement(vm, a, b, c, d, true)
      // 生成vnode
      vnode = render.call(vm._renderProxy, vm.$createElement)
    } catch (e) {
      handleError(e, vm, `render`)
      // return error render result,
      // or previous vnode to prevent render error causing blank component
      /* istanbul ignore else */
      if (process.env.NODE_ENV !== 'production') {
        if (vm.$options.renderError) {
          try {
            vnode = vm.$options.renderError.call(vm._renderProxy, vm.$createElement, e)
          } catch (e) {
            handleError(e, vm, `renderError`)
            vnode = vm._vnode
          }
        } else {
          vnode = vm._vnode
        }
      } else {
        vnode = vm._vnode
      }
    }
    // return empty vnode in case the render function errored out
    if (!(vnode instanceof VNode)) {
      if (process.env.NODE_ENV !== 'production' && Array.isArray(vnode)) {
        // 多个根节点警告
        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
  }

$createElement

执行vm._render时会执行vnode = render.call(vm._renderProxy, vm.$createElement)

在这里定义了vm.$createElement方法

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// 在init时执行 core/instance/render.js
export function initRender (vm: Component) {
  vm._vnode = null // the root of the child tree
  ...
  vm._c = (a, b, c, d) => createElement(vm, a, b, c, d, false)
  // normalization is always applied for the public version, used in
  // user-written render functions.
  vm.$createElement = (a, b, c, d) => createElement(vm, a, b, c, d, true)

  const parentData = parentVnode && parentVnode.data
  if (process.env.NODE_ENV !== 'production') {
    defineReactive(vm, '$attrs', parentData && parentData.attrs || emptyObject, () => {
      !isUpdatingChildComponent && warn(`$attrs is readonly.`, vm)
    }, true)
    defineReactive(vm, '$listeners', options._parentListeners || emptyObject, () => {
      !isUpdatingChildComponent && warn(`$listeners is readonly.`, vm)
    }, true)
  } else {
    defineReactive(vm, '$attrs', parentData && parentData.attrs || emptyObject, null, true)
    defineReactive(vm, '$listeners', options._parentListeners || emptyObject, null, true)
  }
}
createElement
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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
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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__)) {
    // 不允许data是响应式
    process.env.NODE_ENV !== 'production' && warn(
      。。。
    )
    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
      )
    }
  }
// 校验
  if (Array.isArray(children) &&
    typeof children[0] === 'function'
  ) {
    data = data || {}
    data.scopedSlots = { default: children[0] }
    children.length = 0
  }
  // 对chidren做normalize 拍平数组 每个元素是一个vnode
  if (normalizationType === ALWAYS_NORMALIZE) {
    children = normalizeChildren(children)
  } else if (normalizationType === SIMPLE_NORMALIZE) {
    children = simpleNormalizeChildren(children)
  }
  // 创建vnode
  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()
  }
}

vm._update

有两个调用时机，初始化时，渲染到真实dom；数据改变时，驱动视图变化，调用

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Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {
    const vm: Component = this
    // 数据改变时
    const prevEl = vm.$el
    const prevVnode = vm._vnode
    const prevActiveInstance = activeInstance
    activeInstance = vm
    vm._vnode = vnode
    // Vue.prototype.__patch__ is injected in entry points
    // based on the rendering backend used.
    if (!prevVnode) {
      // initial render
      // 初始渲染，本节情况
      // 真实dom和vdom参数
      vm.$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.
  }

__patch__

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// runtime/index.js
Vue.prototype.__patch__ = inBrowser ? patch : noop

在浏览器环境下是patch

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// runtime/patch.js
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'

const modules = platformModules.concat(baseModules)
// nodeOps 操作dom的方法
// modules 一些在patch中会调用到的钩子函数
export const patch: Function = createPatchFunction({ nodeOps, modules })
createPatchFunction

最终调用了createElm，真正把vnode渲染到dom上

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// core/vdom/patch
const hooks = ['create', 'activate', 'update', 'remove', 'destroy']
export function createPatchFunction (backend) {
  let i, j
  const cbs = {}

  const { modules, nodeOps } = backend
  // 初始化钩子函数
  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]])
      }
    }
  }
    createElm 。。。。
  // 定义了很多很多函数。。。
  // 本节传入的是vm.$el和vnode
  return function patch (oldVnode, vnode, hydrating, removeOnly) {
    if (isUndef(vnode)) {
      if (isDef(oldVnode)) invokeDestroyHook(oldVnode)
      return
    }
	
    let isInitialPatch = false
    const insertedVnodeQueue = []
	// 是否是undefined
    if (isUndef(oldVnode)) {
      // empty mount (likely as component), create new root element
      isInitialPatch = true
      createElm(vnode, insertedVnodeQueue)
    } else {
      // oldValue是不是一个真实dom true
      const isRealElement = isDef(oldVnode.nodeType)
      if (!isRealElement && sameVnode(oldVnode, vnode)) {
        // patch existing root node
        patchVnode(oldVnode, vnode, insertedVnodeQueue, removeOnly)
      } else {
         // 是真实dom
        if (isRealElement) {
          if (oldVnode.nodeType === 1 && oldVnode.hasAttribute(SSR_ATTR)) {
             // x
            oldVnode.removeAttribute(SSR_ATTR)
            hydrating = true
          }
          if (isTrue(hydrating)) {
              // x
            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.'
              )
            }
          }
          // 到这里 把真实的dom转化成vnode
          oldVnode = emptyNodeAt(oldVnode)
        }

        // replacing existing element
          // 真实dom
        const oldElm = oldVnode.elm
        // 父元素body
        const parentElm = nodeOps.parentNode(oldElm)

        // create new node
        // 把vnode挂载到真实的dom上
        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
          }
        }

        // 销毁老节点
        if (isDef(parentElm)) {
          removeVnodes(parentElm, [oldVnode], 0, 0)
        } else if (isDef(oldVnode.tag)) {
          invokeDestroyHook(oldVnode)
        }
      }
    }

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

tag:

    缺失模块。
    1、请确保node版本大于6.2
    2、在博客根目录（注意不是yilia根目录）执行以下命令：
    npm i hexo-generator-json-content --save

    3、在根目录_config.yml里添加配置： 

      jsonContent:
    meta: false
    pages: false
    posts:
      title: true
      date: true
      path: true
      text: false
      raw: false
      content: false
      slug: false
      updated: false
      comments: false
      link: false
      permalink: false
      excerpt: false
      categories: false
      tags: true

瞎折腾的<br>前端开发工程师...