Vue2源码-初始化

源码能理解的问题

Q1. 如何让构造函数和class 一样，不能直接调用只能通过new 实例化？
Q2. 为什么在beforeCreate钩子中无法读取data，props等属性？
Q3. 为什么data | props 中的数据，我们可以直接通过this.xx访问？
Q4. computed 和watch的区别？

Vue构造函数

Vue其实就是构造函数，主要为构造函数的原型上添加方法，源码：vue/src/core/instance/index.js
Q1的答案就在Vue 构造函数的注释里。

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) {	
/**
* Vue是一个构造函数，必须使用new关键字进行实例化才能正常使用, 防止构造函数被当成普通函数调用，
* 因为只有通过实例化this才指向实例，this instanceof Vue 才会为真，否则this将指向window
*/ 
  if (process.env.NODE_ENV !== 'production' && !(this instanceof Vue)) {
    warn('Vue is a constructor and should be called with the `new` keyword')
  }
  // 实例化的时候进行初始化
  this._init(options)
}
// 为Vue 构造函数的原型绑定初始化函数_init
initMixin(Vue)
/* 为Vue构造函数的原型绑定 $data,$props,$set,$delete,$watch */
stateMixin(Vue)
/* 为Vue构造函数的原型绑定$on,$once,$off,$off,$emit */
eventsMixin(Vue)
/* 为Vue构造函数的原型上绑定_update,$forceUpdate,$destroy */
lifecycleMixin(Vue)
/* 为Vue构造函数的原型绑定$nextTick, _render */
renderMixin(Vue)

export default Vue

初始化_init()

源码：vue/src/core/instance/init.js
Q：为什么在beforeCreate钩子中无法读取data，props等属性？
A：我们不难发现callHook(vm, ‘beforeCreate’)->initState-> callHook(vm, ‘created’) 执行顺序，这就是为什么我们在beforeCreate钩子中无法获取到data的原因，但是我们能获取到this 对象。

export function initMixin (Vue: Class<Component>) {
  Vue.prototype._init = function (options?: Object) {
    const vm: Component = this
    // a uid
    vm._uid = uid++
	// ...
    // 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.$options = mergeOptions(
        resolveConstructorOptions(vm.constructor),
        options || {},
        vm
      )
    }
    /* istanbul ignore else */
    if (process.env.NODE_ENV !== 'production') {
      initProxy(vm)
    } else {
      vm._renderProxy = vm
    }
    // expose real self
    vm._self = vm
    initLifecycle(vm)
    initEvents(vm)
    initRender(vm)
    callHook(vm, 'beforeCreate')
    initInjections(vm) // resolve injections before data/props
    initState(vm) // 初始化props,methods,data,computed,watch
    initProvide(vm) // resolve provide after data/props
    callHook(vm, 'created')
    // 挂载
    if (vm.$options.el) {
      vm.$mount(vm.$options.el)
    }
  }
}

initState

初始化props，methods，data，computed，watch
源码：vue/src/core/instance/state.js

export function initState (vm: Component) {
  vm._watchers = []
  const opts = vm.$options
  if (opts.props) initProps(vm, opts.props)
  if (opts.methods) initMethods(vm, opts.methods)
  if (opts.data) {
    initData(vm)
  } else {
    /*该组件没有data的时候绑定一个空对象*/
    observe(vm._data = {}, true /* asRootData */)
  }
  if (opts.computed) initComputed(vm, opts.computed)
  if (opts.watch && opts.watch !== nativeWatch) {
    initWatch(vm, opts.watch)
  }
}

initProps

props 上的属性调用defineReactive 进行劫持监听。

function initProps (vm: Component, propsOptions: Object) {
  const propsData = vm.$options.propsData || {}
  // 引用类型的应用
  const props = vm._props = {}
  // cache prop keys so that future props updates can iterate using Array
  // instead of dynamic object key enumeration.
  const keys = vm.$options._propKeys = []
  const isRoot = !vm.$parent
  // root instance props should be converted
  if (!isRoot) {
    toggleObserving(false)
  }
  for (const key in propsOptions) {
    keys.push(key)
    const value = validateProp(key, propsOptions, propsData, vm)
    /* istanbul ignore else */
    if (process.env.NODE_ENV !== 'production') {
      const hyphenatedKey = hyphenate(key)
      if (isReservedAttribute(hyphenatedKey) ||
          config.isReservedAttr(hyphenatedKey)) {
        warn(
          `"${hyphenatedKey}" is a reserved attribute and cannot be used as component prop.`,
          vm
        )
      }
       // 进行数据劫持改写,回调检测props的异常overwritten
      defineReactive(props, key, value, () => {
        if (!isRoot && !isUpdatingChildComponent) {
          warn(
            `Avoid mutating a prop directly since the value will be ` +
            `overwritten whenever the parent component re-renders. ` +
            `Instead, use a data or computed property based on the prop's ` +
            `value. Prop being mutated: "${key}"`,
            vm
          )
        }
      })
    } else {
      defineReactive(props, key, value)
    }
    // static props are already proxied on the component's prototype
    // during Vue.extend(). We only need to proxy props defined at
    // instantiation here.
    if (!(key in vm)) {
      proxy(vm, `_props`, key)
    }
  }
  // 是否进行劫持改写的变量控制
  toggleObserving(true)
}

initMethods

• 验证methods的名字是否重名
• 通过bind函数将事件绑定到vm；

function initMethods (vm: Component, methods: Object) {
  const props = vm.$options.props
  for (const key in methods) {
    if (process.env.NODE_ENV !== 'production') {
      if (typeof methods[key] !== 'function') {
        warn(
          `Method "${key}" has type "${typeof methods[key]}" in the component definition. ` +
          `Did you reference the function correctly?`,
          vm
        )
      }
      if (props && hasOwn(props, key)) {
        warn(
          `Method "${key}" has already been defined as a prop.`,
          vm
        )
      }
      if ((key in vm) && isReserved(key)) {
        warn(
          `Method "${key}" conflicts with an existing Vue instance method. ` +
          `Avoid defining component methods that start with _ or $.`
        )
      }
    }
    vm[key] = typeof methods[key] !== 'function' ? noop : bind(methods[key], vm)
  }
}

initData

• 判断data是否为函数，并且赋值给vm._data上；
• 判断data 内的属性与props，methods 是否重名；
• 对data 的属性进行proxy代理，this.xx === this._data.xx。这就是为什么data | props 中的数据，我们可以直接通过this.xx访问；
• data属性进行劫持监听observe；

function initData (vm: Component) {
  let data = vm.$options.data
  data = vm._data = typeof data === 'function'
    ? getData(data, vm)
    : data || {}
  if (!isPlainObject(data)) {
    data = {}
    process.env.NODE_ENV !== 'production' && warn(
      'data functions should return an object:\n' +
      'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
      vm
    )
  }
  // proxy data on instance
  const keys = Object.keys(data)
  const props = vm.$options.props
  const methods = vm.$options.methods
  let i = keys.length
  while (i--) {
    const key = keys[i]
    if (process.env.NODE_ENV !== 'production') {
      if (methods && hasOwn(methods, key)) {
        warn(
          `Method "${key}" has already been defined as a data property.`,
          vm
        )
      }
    }
    if (props && hasOwn(props, key)) {
      process.env.NODE_ENV !== 'production' && warn(
        `The data property "${key}" is already declared as a prop. ` +
        `Use prop default value instead.`,
        vm
      )
    } else if (!isReserved(key)) {// 非服务端渲染data内的属性同步到vm 并且同步到vm._data
      proxy(vm, `_data`, key)
    }
  }
  // observe data 数据监听
  observe(data, true /* asRootData */)
}

export function proxy (target: Object, sourceKey: string, key: string) {
  sharedPropertyDefinition.get = function proxyGetter () {
    return this[sourceKey][key]
  }
  sharedPropertyDefinition.set = function proxySetter (val) {
    this[sourceKey][key] = val
  }
  Object.defineProperty(target, key, sharedPropertyDefinition)
}

initComputed

• 遍历computed的key，实例化订阅器wactcher进行数据监听；
• 通过Object.defineProperty方法进行劫持改写；

computed 会对结果进行缓存，只有依赖改变才会触发更新。watch 是函数，只要依赖项改变就会再次执行，对象形式的watch也可以设置immediate参数立即执行。其实computed 就是惰性的watch, computed 创建就是watch的实例化，只是额外添加computedWatcherOptions = { lazy: true }参数。

const computedWatcherOptions = { lazy: true }

function initComputed (vm: Component, computed: Object) {
  // $flow-disable-line
  const watchers = vm._computedWatchers = Object.create(null)
  // computed properties are just getters during SSR
  const isSSR = isServerRendering()

  for (const key in computed) {
    const userDef = computed[key]
    const getter = typeof userDef === 'function' ? userDef : userDef.get
    if (process.env.NODE_ENV !== 'production' && 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
      )
    }

    // component-defined computed properties are already defined on the
    // component prototype. We only need to define computed properties defined
    // at instantiation here.
    if (!(key in vm)) {
    // 定义计算属性
      defineComputed(vm, key, userDef)
    } else if (process.env.NODE_ENV !== 'production') {
      if (key in vm.$data) {
        warn(`The computed property "${key}" is already defined in data.`, vm)
      } else if (vm.$options.props && key in vm.$options.props) {
        warn(`The computed property "${key}" is already defined as a prop.`, vm)
      }
    }
  }
}

定义计算属性有函数和对象两种方式，函数的形式只有get，没有set；对象形式可以自定义get，set。最后需要将计算属性绑定到vm

export function defineComputed (
  target: any,
  key: string,
  userDef: Object | Function
) {
  // 是否为服务端渲染判断是否需要缓存
  const shouldCache = !isServerRendering()
  // 计算属性默认没有set,但是对象类型的计算属性可以自定义
  if (typeof userDef === 'function') {
   // 判断创建监听还是使用自定义的方法
    sharedPropertyDefinition.get = shouldCache
      ? createComputedGetter(key)
      : createGetterInvoker(userDef)
    sharedPropertyDefinition.set = noop
  } else {
    sharedPropertyDefinition.get = userDef.get
      ? shouldCache && userDef.cache !== false
        ? createComputedGetter(key)
        : createGetterInvoker(userDef.get)
      : noop
    sharedPropertyDefinition.set = userDef.set || noop
  }
  if (process.env.NODE_ENV !== 'production' &&
      sharedPropertyDefinition.set === noop) {
    sharedPropertyDefinition.set = function () {
      warn(
        `Computed property "${key}" was assigned to but it has no setter.`,
        this
      )
    }
  }
  // 将计算属性key挂到vm上
  Object.defineProperty(target, key, sharedPropertyDefinition)
}

计算属性的getter方法

function createComputedGetter (key) {
  return function computedGetter () {
    const watcher = this._computedWatchers && this._computedWatchers[key]
    if (watcher) {
      //再创建watcher是存在lazy:true,即dirty:true
      if (watcher.dirty) {
        watcher.evaluate()// 返回计算属性的初始值,将dirty赋值false
      }
      if (Dep.target) {
        watcher.depend() // dep收集依赖
      }
      return watcher.value
    }
  }
}

initWatch

watch：{ [key: string]: string | Function | Object | Array }
将监听全部同步到vm.$watch

function initWatch (vm: Component, watch: Object) {
  for (const key in watch) {
    const handler = watch[key]
    if (Array.isArray(handler)) {
      for (let i = 0; i < handler.length; i++) {
        createWatcher(vm, key, handler[i])
      }
    } else {
      createWatcher(vm, key, handler)
    }
  }
}

function createWatcher (
  vm: Component,
  expOrFn: string | Function,
  handler: any,
  options?: Object
) {
  if (isPlainObject(handler)) {
    options = handler
    handler = handler.handler
  }
  if (typeof handler === 'string') {
    handler = vm[handler]
  }
  return vm.$watch(expOrFn, handler, options)
}