内容乃本人学习Vue2源码的一点笔记，若有错误还望指正。
源码版本：
vue: 2.6
vue-loader: 13.x
vue-template-compiler: 2.6

相关学习笔记：

• 数据响应式改造
• render方法、模板解析和依赖收集概述

我们使用vue-cli搭建vue 2.x项目时，大致由如下代码来做一个vue应用的初始化：

import Vue from "vue";
import App from "./App.vue";

Vue.config.productionTip = false;

new Vue({
  render: (h) => h(App),
}).$mount("#app");

我们可以就从此处开始对Vue的认识。可以看到，这里表面上只做了一个简单的工作，就是通过new操作创建了一个vue的实例，并传递了一个配置项对象，该对象包含了一个render方法。

根据这个调用，我们找到src/core/instance/index.js文件，内容如下：

// src/core/instance/index.js
import { initMixin } from './init'
import { stateMixin } from './state'
import { renderMixin } from './render'
import { eventsMixin } from './events'
import { lifecycleMixin } from './lifecycle'
import { warn } from '../util/index'

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)
}

initMixin(Vue)
stateMixin(Vue)
eventsMixin(Vue)
lifecycleMixin(Vue)
renderMixin(Vue)

export default Vue

内容也很直观，这里定义了一个只接受new构造调用的Vue Function，并对Vue进行了一系列的混入操作。

再粗浅地看一下这些Mixin都做了什么，可以看到是往Vue的prototype对象上挂了一些属性和方法。

大致如下：

Vue.prototype
|- initMixin
    |- _init(options?: Object)
|- stateMixin
    |- $data
    |- $props
    |- $set(target: Array<any> | Object, key: any, val: any): any   <- ../observer/index
    |- $delete(target: Array<any> | Object, key: any)               <- ../observer/index
    |- $watch(expOrFn: string | Function, cb: any, options?: Object): Function
|- eventMixin
    |- $on(event: string | Array<string>, fn: Function): Component
    |- $once(event: string, fn: Function): Component
    |- $off(event?: string | Array<string>, fn?: Function): Component
    |- $emit(event: string): Component
|- lifecycleMixin
    |- $_update(vnode: VNode, hydrating?: boolean)
    |- $forceUpdate()
    |- $destrouy()
|- renderMixin
    |- $nextTick(fn: Function)
    |- _render(): VNode

Vue的函数体中，调用了一个_init的方法，并将参数传入，可以看到，_init方法是在initMixin中定义的。

继续看_init方法的定义：

// src/core/instance/init.js
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
    // 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)
    }
}

见名知意，这个函数是对vue实例做一系列的初始化操作。

1. 获取vue实例的构造器以及父级构造器（依次递归）上的配置项，以及参数传递进来的配置项，在加上实例自带的属性，都合并到一起，挂在实例的$option属性身上

2. 将vue实例自身挂在_renderProxy属性上

3. 初始化数据和方法前做一些准备工作

   1. initLifecycle：初始化生命周期
   2. initEvents：初始化事件
   3. initRender：初始化render
   4. 触发beforeCreate钩子

4. 初始化数据和方法

   1. initInjections：处理$options.inject，对注入的数据做响应式处理

   2. initState做的几件事

      1. initProps：对$options.props做响应式处理

      2. initMethods：对$options.methods对象做处理，将所有的方法直接挂在实例对象上，并将方法的this绑定到vue实例对象vm[key] = typeof methods[key] !== 'function' ? noop : bind(methods[key], vm)

      3. initData：对$options.data进行observeobserve(data, true /* asRootData */)，继续追踪可以看到observe方法是对data进行响应式处理，返回一个Observer实例

         // src/core/boserver/index.js
         export class Observer {
           value: any;
           dep: Dep;
           vmCount: number; // number of vms that have this object as root $data
         
           constructor (value: any) {
             this.value = value
             this.dep = new Dep()
             this.vmCount = 0
             def(value, '__ob__', this)
             if (Array.isArray(value)) {
               if (hasProto) {
                 protoAugment(value, arrayMethods)
               } else {
                 copyAugment(value, arrayMethods, arrayKeys)
               }
               this.observeArray(value)
             } else {
               this.walk(value)
             }
           }
         
           /**
            * Walk through all properties and convert them into
            * getter/setters. This method should only be called when
            * value type is Object.
            */
           walk (obj: Object) {
             const keys = Object.keys(obj)
             for (let i = 0; i < keys.length; i++) {
               defineReactive(obj, keys[i])
             }
           }
         
           /**
            * Observe a list of Array items.
            */
           observeArray (items: Array<any>) {
             for (let i = 0, l = items.length; i < l; i++) {
               observe(items[i])
             }
           }
         }
         

      4. initComputed：处理计算属性$options.computed

         给每个计算属性创建Watcher实例

         // src/core/instance/state.js
         const computedWatcherOptions = { lazy: true }
         
         function initComputed(vm: Component, computed: Object) {
           // ...
           const watchers = (vm._computedWatchers = Object.create(null))
           // ...
           const isSSR = isServerRendering()
           
           for (const key in computed) {
             const userDef = computed[key]
             const getter = isFunction(userDef) ? userDef : userDef.get
             if (__DEV__ && getter == null) {
               warn(`Getter is missing for computed property "${key}".`, vm)
             }
         
             if (!isSSR) {
               // create internal watcher for the computed property.
               watchers[key] = new Watcher(
                 vm,
                 getter || noop,
                 noop,
                 computedWatcherOptions
               )
             }
             
             if (!(key in vm)) {
               defineComputed(vm, key, userDef)
             }
             // ...
           }
           
           // ...
         }
         
         export function defineComputed (
           target: any,
           key: string,
           userDef: Object | Function
         ) {
           const shouldCache = !isServerRendering()
           if (typeof userDef === 'function') {
             sharedPropertyDefinition.get = shouldCache
               ? createComputedGetter(key)
               : createGetterInvoker(userDef)
             sharedPropertyDefinition.set = noop
           } else {
             // ...
           }
           // ...
           Object.defineProperty(target, key, sharedPropertyDefinition)
         }
         
         function createComputedGetter (key) {
           return function computedGetter () {
             const watcher = this._computedWatchers && this._computedWatchers[key]
             if (watcher) {
               if (watcher.dirty) {
                 watcher.evaluate()
               }
               if (Dep.target) {
                 watcher.depend()
               }
               return watcher.value
             }
           }
         }
         

         可以看到创建Watcher实例时传入一个配置项{ lazy: true }，再看Watcher的构造器中的代码，即默认watcher.dirty为true，所以执行watcher.evaluate()，watcher.get()。

         watcher.get()会去执行计算方法或者计算属性的get()方法，即this.getter.call(vm, vm)。

         // src/core/observer/watcher.js
         constructor (
             vm: Component,
             expOrFn: string | Function,
             cb: Function,
             options?: ?Object,
             isRenderWatcher?: boolean
           ) {
             this.vm = vm
             if (isRenderWatcher) {
               vm._watcher = this
             }
             vm._watchers.push(this)
             // options
             if (options) {
               // ...
               this.lazy = !!options.lazy
               // ...
             } else {
               // ...
             }
             // ...
             this.dirty = this.lazy // for lazy watchers
             // ...
         }
         
         evaluate () {
             this.value = this.get()
             this.dirty = false
         }
         
         get() {
             pushTarget(this)
             let value
             const vm = this.vm
             try {
               value = this.getter.call(vm, vm)
             } catch (e: any) {
               if (this.user) {
                 handleError(e, vm, `getter for watcher "${this.expression}"`)
               } else {
                 throw e
               }
             } finally {
               // "touch" every property so they are all tracked as
               // dependencies for deep watching
               if (this.deep) {
                 traverse(value)
               }
               popTarget()
               this.cleanupDeps()
             }
             return value
         }
         
         depend() {
             let i = this.deps.length
             while (i--) {
               this.deps[i].depend()
             }
         }
         

      5. initWatch：处理自定义监听$options.watch

         执行了$watch方法，可以先看下它的定义：

         // src/core/instance/state.js
         Vue.prototype.$watch = function (
             expOrFn: string | (() => any),
             cb: any,
             options?: Record<string, any>
           ): Function {
             const vm: Component = this
             if (isPlainObject(cb)) {
               return createWatcher(vm, expOrFn, cb, options)
             }
             options = options || {}
             options.user = true
             const watcher = new Watcher(vm, expOrFn, cb, options)
             if (options.immediate) {
               const info = `callback for immediate watcher "${watcher.expression}"`
               pushTarget()
               invokeWithErrorHandling(cb, vm, [watcher.value], vm, info)
               popTarget()
             }
             return function unwatchFn() {
               watcher.teardown()
             }
          }
         

         可以看到也是创建了一个Watcher实例对象。

   3. initProvide：处理$options.provide，将provide的数据（或者provide执行后的数据）挂在实例的_provide属性上

   4. 触发created钩子

5. 最后执行vm.$mount方法，执行挂载流程，由于挂载的方式由平台决定，所以$mount的方法并未定义在src/core中；web端的$mount方法定义在src/platforms/web/runtime/index.js中。

   // src/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)
   }
   

   调用的mountComponent(this, el, hydrating)定义在src/core/instance/lifecycle.js中。

   // src/core/instance/lifecycle.js
   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
           )
         }
       }
     }
     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)
         const vnode = vm._render()
         mark(endTag)
         measure(`vue ${name} render`, startTag, endTag)
   
         mark(startTag)
         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
     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
     if (vm.$vnode == null) {
       vm._isMounted = true
       callHook(vm, 'mounted')
     }
     return vm
   }
   

   见名知意，是对挂载的处理：

   1. 拿到el放在vm.$el上

   2. 确认是否有vm.$options.render，没有则赋值创建一个空的VNode实例的方法

   3. 触发beforeMount钩子

   4. 创建一个新的Watcher实例，用于实例更新后触发重新渲染

      updateComponent = () => {
        vm._update(vm._render(), hydrating)
      }
      

      并传递一个before方法，用于在组件更新前触发beforeUpdate钩子

   5. 触发mounted钩子

Vue应用初始化大致就是这样一个流程