vue源碼分析（二）-從new Vue到渲染到頁面

_init方法

new Vue()會執行Vue構造函數的_init方法，_init方法被initMixin中擴展的，src\core\instance\init.js

export function initMixin (Vue: Class<Component>) {
  Vue.prototype._init = function (options?: Object) {
    const vm: Component = this
    // a uid
    vm._uid = uid++

    let startTag, endTag
    /* istanbul ignore if */
    if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
      startTag = `vue-perf-start:${vm._uid}`
      endTag = `vue-perf-end:${vm._uid}`
      mark(startTag)
    }

    // a flag to avoid this being observed
    vm._isVue = true
    // 合併配置
    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
    // vue實例vm擴展
    // 生命週期
    initLifecycle(vm)
    // 事件
    initEvents(vm)
    // 渲染
    // createElement也就是咱們手寫render函數的參數h
    // slots
    initRender(vm)
    // 第一個生命週期
    // data,props尚未
    callHook(vm, 'beforeCreate')
    // inject 的實現，在provide以前
    initInjections(vm) // resolve injections before data/props
    // props，data,computed,methods都在這裏，須要重點看，因此beforCreate鉤子函數中沒法讀取props和data的變量
    initState(vm)
    // provide 的實現
    initProvide(vm) // resolve provide after data/props
    // 第二個生命週期，這是能夠獲取到data,props,computed,methods
    // 全部的生命週期函數都是調用的callHook函數
    callHook(vm, 'created')

    /* istanbul ignore if */
    if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
      vm._name = formatComponentName(vm, false)
      mark(endTag)
      measure(`vue ${vm._name} init`, startTag, endTag)
    }
    // 若是有el，則調用$mount
    if (vm.$options.el) {
      vm.$mount(vm.$options.el)
    }
  }
}

_init方法主要作的就是合併配置，初始化生命週期，初始化事件中心，初始化渲染，初始化 data、props、computed、watcher 等等

掛載$mount

若是實例化的時候提供了el屬性，就執行vm.$mount(vm.$options.el)，若是沒有提供就執行new Vue().$mount('#app'),$mount方法定義在src\platforms\web\runtime\index.js

// 這是web環境，若是是服務端渲染爲noop(空函數)，不然爲patch
Vue.prototype.__patch__ = inBrowser ? patch : noop

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

最終調用的是mountComponent，定義在src\core\instance\lifecycle.js

// 此方法核心就是先實例化一個渲染Watcher，在它的回調函數中會調用 updateComponent 方法，
// 在此方法中調用 vm._render 方法先生成虛擬 Node，最終調用 vm._update 更新 DOM
export function mountComponent (
  vm: Component,
  el: ?Element,
  hydrating?: boolean
): Component {
  vm.$el = el
  if (!vm.$options.render) {
    vm.$options.render = createEmptyVNode
    if (process.env.NODE_ENV !== 'production') {
      /* istanbul ignore if */
      if ((vm.$options.template && vm.$options.template.charAt(0) !== '#') ||
        vm.$options.el || el) {
        warn(
          'You are using the runtime-only build of Vue where the template ' +
          'compiler is not available. Either pre-compile the templates into ' +
          'render functions, or use the compiler-included build.',
          vm
        )
      } else {
        warn(
          'Failed to mount component: template or render function not defined.',
          vm
        )
      }
    }
  }
  // 調用beforeMount生命週期函數
  callHook(vm, 'beforeMount')

  let updateComponent
  /* istanbul ignore if */
  if (process.env.NODE_ENV !== 'production' && config.performance && mark) {
    updateComponent = () => {
      const name = vm._name
      const id = vm._uid
      const startTag = `vue-perf-start:${id}`
      const endTag = `vue-perf-end:${id}`

      mark(startTag)
      // vm._render 最終是經過執行 createElement 方法並返回的是 vnode
      // 調用vm._render()生成虛擬dom，vm._render()調用vm.$options.render函數
      const vnode = vm._render()
      mark(endTag)
      measure(`vue ${name} render`, startTag, endTag)

      mark(startTag)
      // 調用vm._update更新dom
      vm._update(vnode, hydrating)
      mark(endTag)
      measure(`vue ${name} patch`, startTag, endTag)
    }
  } 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's mounted hook), which relies on vm._watcher being already defined
  // Watcher作什麼用？
  // Watcher 在這裏起到兩個做用，一個是初始化的時候會執行回調函數updateComponent
  // 另外一個是當 vm 實例中的監測的數據發生變化的時候執行回調函數
  new Watcher(vm, updateComponent, noop, {
    before () {
      if (vm._isMounted && !vm._isDestroyed) {
        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
  // vm.$vnode指的是父組件虛擬dom
  if (vm.$vnode == null) {
    vm._isMounted = true
    // 調用mounted生命週期函數
    callHook(vm, 'mounted')
  }
  return vm
}

mountComponent方法有三個核心：vm._render()， vm._update()，new Watcher（）。renderMixin函數在Vue的原型上掛載了_render方法，src\core\instance\render.js

export function renderMixin (Vue: Class<Component>) {
  // install runtime convenience helpers
  installRenderHelpers(Vue.prototype)

  Vue.prototype.$nextTick = function (fn: Function) {
    return nextTick(fn, this)
  }

  // 虛擬dom
  Vue.prototype._render = function (): VNode {
    const vm: Component = this
    const { render, _parentVnode } = vm.$options
    ...
    let vnode
      // 渲染函數是實例私有的，有一個參數createElement，createElement就是vm.$createElement,它的定義是在執行initRender時
      // render就是vm.option.render,接受vm.$createElement(別名爲h)做爲參數：render: h => h(App)
      // vm.$createElement在initRender中定義
      vnode = render.call(vm._renderProxy, vm.$createElement)
   ..
      vnode = createEmptyVNode()
    }
    // set parent
    // _parentVnode就是vm.$vnode
    vnode.parent = _parentVnode
    return vnode
  }
}

vm_render()最終調用vm.$options.render,而且傳遞了vm.$createElemnt,定義在initRender()中。render函數的返回是createElemnt方法生成的vnode，經過new VNode()生產vnode。
vm._update放在執行lifecycleMixin()是掛載到Vue原型上的，src\core\instance\lifecycle.js

// 此方法核心時調用了vm.__patch__方法（src/platforms/web/runtime/index.js）
  Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {
    const vm: Component = this
    const prevEl = vm.$el
    const prevVnode = vm._vnode
    const restoreActiveInstance = setActiveInstance(vm)
    vm._vnode = vnode
    // Vue.prototype.__patch__ is injected in entry points
    // based on the rendering backend used.
    if (!prevVnode) {
      // initial render
      vm.$el = vm.__patch__(vm.$el, vnode, hydrating, false /* removeOnly */)
    } else {
      // updates
      vm.$el = vm.__patch__(prevVnode, vnode)
    }
    restoreActiveInstance()
    // 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.
  }

最終調用vm.__patch__方法，定義在__patch__方法調用了createPatchFunction，src\core\vdom\patch.js

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

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

path方法核心是調用createElm方法，createElm調用insert，insert方法將vnode經過原生方法appendChild插入到app標籤中，實際上整個過程就是遞歸建立了一個完整的 DOM 樹並插入到 #app父級標籤上
有這樣一個列子

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

new Vue的完整流程