关于前端:Vue-2x源码学习应用初始化大致流程

咱们应用 vue-cli 搭建 vue 2.x 我的项目时，大抵由如下代码来做一个 vue 利用的初始化：

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

Vue.config.productionTip = false;

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

咱们能够就从此处开始对 Vue 的意识。能够看到，这里外表上只做了一个简略的工作，就是通过 new 操作创立了一个 vue 的实例，并传递了一个配置项对象，该对象蕴含了一个 render 办法。

依据这个调用，咱们找到 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，这里定义了一个只承受 new 结构调用的 Vue Function，并对 Vue 进行了一系列的混入操作。

再浅显地看一下这些 Mixin 都做了什么，能够看到是往 Vue 的 prototype 对象上挂了一些属性和办法。

大抵如下：

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 的办法，并将参数传入，能够看到，_init办法是在 initMixin 中定义的。

持续看 _init 办法的定义：

// 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 实例的结构器以及父级结构器（顺次递归）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 实例本身挂在_renderProxy 属性上

3. 初始化数据和办法前做一些筹备工作

   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. 初始化数据和办法

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

   2. initState 做的几件事

      1. initProps：对 $options.props 做响应式解决
      2. initMethods：对 $options.methods 对象做解决，将所有的办法间接挂在实例对象上，并将办法的 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 */)，持续追踪能够看到 observe 办法是对 data 进行响应式解决，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：解决计算属性 $options.computed

         给每个计算属性创立 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
             }
           }
         }

         能够看到创立 Watcher 实例时传入一个配置项 {lazy: true}，再看Watcher 的结构器中的代码，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() 办法，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：解决自定义监听 $options.watch

         执行了 $watch 办法，能够先看下它的定义：

         // 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()
             }
          }

         能够看到也是创立了一个 Watcher 实例对象。

   3. initProvide：解决 $options.provide，将 provide 的数据（或者 provide 执行后的数据）挂在实例的 _provided 属性上
   4. 触发 created 钩子

5. 最初执行 vm.$mount 办法，执行挂载流程，因为挂载的形式由平台决定，所以 $mount 的办法并未定义在 src/core 中；web 端的 $mount 办法定义在 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，是对挂载的解决：

   1. 拿到 el 放在 vm.$el 上
   2. 确认是否有vm.$options.render，没有则赋值创立一个空的 VNode 实例的办法
   3. 触发 beforeMount 钩子

   4. 创立一个新的 Watcher 实例，用于实例更新后触发从新渲染

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

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

   5. 触发 mounted 钩子

Vue 利用初始化大抵就是这样一个流程