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上文我们聊到了响应式的核心函数observe，接下来我们分析一下:

observe

observe 的功能就是用来监测数据的变化，它是vue2.x 实现响应式的核心函数，它定义在vue源码的 src/core/observer/index.js 中：

/**
 * Attempt to create an observer instance for a value,
 * returns the new observer if successfully observed,
 * or the existing observer if the value already has one.
*/

export function observe (value: any, asRootData: ?boolean): Observer | void {
  if (!isObject(value) || value instanceof VNode) {
    return
  }
  let ob: Observer | void
  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
  ) {
    ob = new Observer(value)
  }
  if (asRootData && ob) {
    ob.vmCount++
  }
  return ob
}

可以看到，这里实例化了一个Observer类，并返回这个实例化后的值，接着看下Observer这个类的源码

/**
 * Observer class that is attached to each observed
 * object. Once attached, the observer converts the target
 * object's property keys into getter/setters that
 * collect dependencies and dispatch updates.
 */
export class Observer {
  value: any;
  dep: Dep;
  vmCount: number; // number of vms that has 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)) {
      const augment = hasProto
        ? protoAugment
        : copyAugment
      augment(value, arrayMethods, arrayKeys)
      this.observeArray(value)
    } else {
      this.walk(value)
    }
  }

  /**
   * Walk through each property 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])
    }
  }
}

这个构造函数首先实例化了Dep对象（这个类跟依赖收集相关，这里不做分析，以后的文章会讲清。）接着执行def函数，把自身的实例添加到数据对象value的__ob__属性上

export function def (obj: Object, key: string, val: any, enumerable?: boolean) {

Object.defineProperty(obj, key, {
    value: val,
    enumerable: !!enumerable,
    writable: true,
    configurable: true
    })
}

接下来对这个value值做判断，数组的话会调用observeArray方法，否则调用walk方法。而observe最终也会调用observe方法，最终都会执行defineReactive。

/**
 * Define a reactive property on an Object.
 */
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()
      }
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      dep.notify()
    }
  })
}

defineReactive目的就是定义一个响应式对象，首先实例化了一个Dep对象，接着拿到属性描述符，然后对子对象递归调用observe，这样就实现了对象的深度响应式，保证无论访问多深的obj的属性，最后都能触发属性描述符的getter和setter。最后使用Object.defineProperty给对应的属性添加了getter和setter。这个作用是为了访问属性的时候触发getter，收集属性的依赖，改变属性的时候触发setter，派发对应的更新，从而通知页面做渲染，这就涉及到了观察者模式，在之后的文章我们再细说。