function initData (vm: Component) {
  let data = vm.$options.data
  data = vm._data = typeof data === 'function'
    ? getData(data, vm)
    : data || {}
  // 如果返回的data 不是对象， 那么在开发环境 直接 抛出警告
  if (!isPlainObject(data)) {
    data = {}
    process.env.NODE_ENV !== 'production' && warn(
      'data functions should return an object:\n' +
      'https://vuejs.org/v2/guide/components.html#data-Must-Be-a-Function',
      vm
    )
  }
  // proxy data on instance
  const keys = Object.keys(data)
  const props = vm.$options.props
  const methods = vm.$options.methods
  let i = keys.length
  while (i--) {
    const key = keys[i]
    // 是否在 methods 中初始化过
    if (process.env.NODE_ENV !== 'production') {
      if (methods && hasOwn(methods, key)) {
        warn(
          `Method "${key}" has already been defined as a data property.`,
          vm
        )
      }
    }
    // 是否在 props 中初始化过
    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
      )
      // 如果不是 以 _ 或者 $ 开头
    } else if (!isReserved(key)) { 
      proxy(vm, `_data`, key)
    }
  }
  // observe data
  observe(data, true /* asRootData */)
}

`proxy(vm,` _data`, key)`

将所有的 data 中的元素通过 defineProperty存储到 _data中

const sharedPropertyDefinition = {
  enumerable: true,
  configurable: true,
  get: noop,
  set: noop
}

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)
}

`observe(data, true)`

只会对非Null的对象进行处理 typeof value === 'object'

function observe (value: any, asRootData: ?boolean): Observer | void {
   // 如果value不是 对象， 或者 是 VNode 直接返回
  if (!isObject(value) || value instanceof VNode) {
    return
  }
  let ob: Observer | void
  // 如果 value 已经是 是 Observer实例 
  if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
    ob = value.__ob__
  } else if (
    shouldObserve &&
    !isServerRendering() &&
    (Array.isArray(value) || isPlainObject(value)) &&
    Object.isExtensible(value) &&
    !value._isVue
  ) {
    // 初始化 Observer 观察者
    ob = new Observer(value)
  }
  if (asRootData && ob) {
    ob.vmCount++
  }
  return ob
}

Object.isExtensible(value) value 是否可扩展，所谓的扩展就是在其上增加心的属性

`Observer`

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() // 订阅者
    this.vmCount = 0
    def(value, '__ob__', this)
    // 数组
    if (Array.isArray(value)) {
      if (hasProto) { // '__proto__' in {} 
        protoAugment(value, arrayMethods) // 原型增强
      } 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])
    }
  }
}

如果 value 是一个数组，先对数组进行原型上的改造（知识拓展 ）

对数组进行遍历，调用 observe

如果是对象，则调用 defineReactive

`defineReactive(obj, keys[i])`

将传入的对象进行响应式

export function defineReactive (
  obj: Object, ..
  key: string,
  val: any,
  customSetter?: ?Function,
  shallow?: boolean
) {
  const dep = new Dep()

  const property = Object.getOwnPropertyDescriptor(obj, key)
  if (property && property.configurable === false) {
    return
  }

  // cater for pre-defined getter/setters
  const getter = property && property.get
  const setter = property && property.set
  if ((!getter || setter) && arguments.length === 2) {
    val = obj[key]
  }

  let childOb = !shallow && observe(val)
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter () {
      const value = getter ? getter.call(obj) : val
      if (Dep.target) {
        dep.depend()
        if (childOb) {
          childOb.dep.depend()
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      const value = getter ? getter.call(obj) : val
      /* eslint-disable no-self-compare */
      if (newVal === value || (newVal !== newVal && value !== value)) {
        return
      }
      /* eslint-enable no-self-compare */
      if (process.env.NODE_ENV !== 'production' && customSetter) {
        customSetter()
      }
      // #7981: for accessor properties without setter
      if (getter && !setter) return
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      dep.notify()
    }
  })
}

创建一个 new Dep() 依赖收集

Object.getOwnPropertyDescriptor 判断属性是否可以配置，如果不可以，直接返回，否则继续

定义 getter 和 setter 保存原本的get和get方法‘

如果没有 getter , 那么 val 值, 为后续定义的 get的返回值

childOb 对于属性值对象也进行订阅
data: {
    a: {
        b: ''
    }
}
Object.defineProperty 设置存取描述符

对于 get 的时候， value 如果有原本有 getter则调用，否则使用保存的 val 值

收集依赖

对于 set的时候，如果新的值和原来的值一样，直接返回，否则继续

如果有 getter 但是没有setter函数直接返回，否则继续

调用 setter函数更新值或者直接赋值

对于新值进行响应式

触发更新，这样就可以更新视图

Dep

export default class Dep {
  static target: ?Watcher;
  id: number;
  subs: Array<Watcher>;

  constructor () {
    this.id = uid++
    this.subs = []
  }

  addSub (sub: Watcher) {
    this.subs.push(sub)
  }

  removeSub (sub: Watcher) {
    remove(this.subs, sub)
  }

  depend () {
    if (Dep.target) {
      Dep.target.addDep(this)
    }
  }

  notify () {
    // stabilize the subscriber list first
    const subs = this.subs.slice()
    if (process.env.NODE_ENV !== 'production' && !config.async) {
      // subs aren't sorted in scheduler if not running async
      // we need to sort them now to make sure they fire in correct
      // order
      subs.sort((a, b) => a.id - b.id)
    }
    for (let i = 0, l = subs.length; i < l; i++) {
      subs[i].update()
    }
  }
}

// The current target watcher being evaluated.
// This is globally unique because only one watcher
// can be evaluated at a time.
Dep.target = null
const targetStack = []

export function pushTarget (target: ?Watcher) {
  targetStack.push(target)
  Dep.target = target
}

export function popTarget () {
  targetStack.pop()
  Dep.target = targetStack[targetStack.length - 1]
}

依赖收集

 data: {
     a: 1
 }

当获取 this.a 的时候，会触发 depend 方法， Dep.target.addDep 将当前的dep，添加到 Watcher 中

当设置 this.a的时候， notify 执行，然后调用 Wather实例的 update方法 Dep 是 Observer 和 Watcher 的中间商，将两者链接起来

`Wather`

解析表达式，收集依赖，当表达式发生改变的时候，触发回调，也是 $watch 的实现方式

class Watcher {
  vm: Component; 
  expression: string; 
  cb: Function; 
  id: number;
  deep: boolean; 
  user: boolean;
  lazy: boolean; 
  sync: boolean;
  dirty: boolean;
  active: boolean;
  deps: Array<Dep>;
  newDeps: Array<Dep>;
  depIds: SimpleSet;
  newDepIds: SimpleSet;
  before: ?Function;
  getter: Function;
  value: any;

  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.deep = !!options.deep
      this.user = !!options.user
      this.lazy = !!options.lazy
      this.sync = !!options.sync
      this.before = options.before
    } else {
      this.deep = this.user = this.lazy = this.sync = false
    }
    this.cb = cb
    this.id = ++uid // uid for batching
    this.active = true
    this.dirty = this.lazy // for lazy watchers
    this.deps = []
    this.newDeps = []
    this.depIds = new Set()
    this.newDepIds = new Set()
    this.expression = process.env.NODE_ENV !== 'production'
      ? expOrFn.toString()
      : ''
    // parse expression for getter
    if (typeof expOrFn === 'function') {
      this.getter = expOrFn
    } else {
      this.getter = parsePath(expOrFn)
      if (!this.getter) {
        this.getter = noop
        process.env.NODE_ENV !== 'production' && warn(
          `Failed watching path: "${expOrFn}" ` +
          'Watcher only accepts simple dot-delimited paths. ' +
          'For full control, use a function instead.',
          vm
        )
      }
    }
    this.value = this.lazy
      ? undefined
      : this.get()
  }

  /**
   * Evaluate the getter, and re-collect dependencies.
   */
  get () {
    pushTarget(this)
    let value
    const vm = this.vm
    try {
      value = this.getter.call(vm, vm)
    } catch (e) {
      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
  }

  /**
   * Add a dependency to this directive.
   */
  addDep (dep: Dep) {
    const id = dep.id
    if (!this.newDepIds.has(id)) {
      this.newDepIds.add(id)
      this.newDeps.push(dep)
      if (!this.depIds.has(id)) {
        dep.addSub(this)
      }
    }
  }

  /**
   * Clean up for dependency collection.
   */
  cleanupDeps () {
    let i = this.deps.length
    while (i--) {
      const dep = this.deps[i]
      if (!this.newDepIds.has(dep.id)) {
        dep.removeSub(this)
      }
    }
    let tmp = this.depIds
    this.depIds = this.newDepIds
    this.newDepIds = tmp
    this.newDepIds.clear()
    tmp = this.deps
    this.deps = this.newDeps
    this.newDeps = tmp
    this.newDeps.length = 0
  }

  /**
   * Subscriber interface.
   * Will be called when a dependency changes.
   */
  update () {
    /* istanbul ignore else */
    if (this.lazy) {
      this.dirty = true
    } else if (this.sync) {
      this.run()
    } else {
      queueWatcher(this)
    }
  }

  /**
   * Scheduler job interface.
   * Will be called by the scheduler.
   */
  run () {
    if (this.active) {
      const value = this.get()
      if (
        value !== this.value ||
        // Deep watchers and watchers on Object/Arrays should fire even
        // when the value is the same, because the value may
        // have mutated.
        isObject(value) ||
        this.deep
      ) {
        // set new value
        const oldValue = this.value
        this.value = value
        if (this.user) {
          const info = `callback for watcher "${this.expression}"`
          invokeWithErrorHandling(this.cb, this.vm, [value, oldValue], this.vm, info)
        } else {
          this.cb.call(this.vm, value, oldValue)
        }
      }
    }
  }

  /**
   * Evaluate the value of the watcher.
   * This only gets called for lazy watchers.
   */
  evaluate () {
    this.value = this.get()
    this.dirty = false
  }

  /**
   * Depend on all deps collected by this watcher.
   */
  depend () {
    let i = this.deps.length
    while (i--) {
      this.deps[i].depend()
    }
  }

  /**
   * Remove self from all dependencies' subscriber list.
   */
  teardown () {
    if (this.active) {
      // remove self from vm's watcher list
      // this is a somewhat expensive operation so we skip it
      // if the vm is being destroyed.
      if (!this.vm._isBeingDestroyed) {
        remove(this.vm._watchers, this)
      }
      let i = this.deps.length
      while (i--) {
        this.deps[i].removeSub(this)
      }
      this.active = false
    }
  }
}

第一次使用的时候在挂载组件 core/instance/lifecycle.js

// 更新组件
updateComponent = () => {
  vm._update(vm._render(), hydrating)
}
// 渲染Wacther
new Watcher(vm, updateComponent, noop, {
    before () {
      if (vm._isMounted && !vm._isDestroyed) {
        callHook(vm, 'beforeUpdate')
      }
    }
  }, true /* isRenderWatcher */)

constructor

constructor ( vm: Component, expOrFn: string | Function, cb: Function, options?: ?Object, isRenderWatcher?: boolean )

vm: Vue实例

expOrFn： updateComponent 函数

cb: noop

options: before

isRenderWatcher: true

this.getter: expOrFn

this.value = this.get() 是就是调用了 this.getter 方法，触发组件更新

此时的Dep.target 指向该渲染Wacther, 并且将当前的渲染Wacther 追加到 Dep中

get

设置 Dep.target 为当前的 Watcher

执行 this.getter 方法

设置 Dep.target 为之前的 Watcher

addDep

如果 dep 已经存在则不做任何操作，否则，将当前的watcher 添加到 dep 中

cleanupDeps

根据 newDepIds 和 newDeps去更新 depIds 和 deps

update

如果是 lazy = true 则标记 dirty= true

如果是 sync = true 则运行 run 方法

否则就进入队列，等待 nextTick 执行

run

对比新旧值，执行回调函数

evaluate

计算 watcher 值，只针对 lazy = true

depend

调用 deps 数组元素的 depend 方法

teardown

将当前watcher从所有依赖中删除

知识拓展

1. arrayMethods

为了能够对数组的变化进行拦截，所以Vue 对数组的原型方法 ['push', 'pop', 'shift', 'unshift', 'splice', 'sort', 'reverse' ] 进行了调整，通过名称可以看出，这些方法都会改变 原数组，所以对于 无法改变原数组的，比如arr[0] = xxx 请使用 $set, 对于 push， unshift, splice 对原数组新增的元素 增加观察订阅模式

const arrayProto = Array.prototype

export const arrayMethods = Object.create(arrayProto)

const methodsToPatch =['push', 'pop', 'shift', 'unshift', 'splice', 'sort', 'reverse' ]

methodsToPatch.forEach(function (method) {
  // cache original method
  const original = arrayProto[method]
  def(arrayMethods, method, function mutator (...args) {
    const result = original.apply(this, args)
    const ob = this.__ob__
    let inserted
    switch (method) {
      case 'push':
      case 'unshift':
        inserted = args
        break
      case 'splice':
        inserted = args.slice(2)
        break
    }
    // 新增的元素 进行 观察订阅
    if (inserted) ob.observeArray(inserted)
    // notify change
    ob.dep.notify()
    return result
  })
})

Vue2源码阅读——initData

proxy(vm, _data, key)

observe(data, true)

Observer

defineReactive(obj, keys[i])

Dep

Wather