Vue 2.x源码学习：应用初始化大致流程

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：

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。

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
将vue实例自身挂在_renderProxy属性上
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

初始化数据和方法

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

initState做的几件事

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

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])
    }
  }
}

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()
    }
}

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。

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
触发created钩子

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：

拿到el放在vm.$el上
确认是否有vm.$options.render，没有则赋值创建一个空的VNode实例的方法
触发beforeMount钩子
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
触发mounted钩子

Vue应用初始化大致就是这样一个流程

posted @ 2022-11-04 19:50 beckyye 阅读(59) 评论(0) 编辑收藏举报

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