// 为data的的所有属性添加getter 和 setter
function defineReactive( obj,key,val,customSetter,shallow
) {
  //
  var dep = new Dep();
  /*....省略部分....*/
  var childOb = !shallow && observe(val); //为对象添加备份依赖dep
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter() {
      var value = getter "development" !== 'production' && customSetter) {
        customSetter();
      }
      if (setter) {
        setter.call(obj, newVal);
      } else {
        val = newVal;
      }
      childOb = !shallow && observe(newVal);
      dep.notify();//有改变触发watcher进行更新
    }
  });
} 

Vue.prototype.$watch = function (
      expOrFn,
      cb,
      options
    ) {
      var vm = this;
      if (isPlainObject(cb)) {
        return createWatcher(vm, expOrFn, cb, options)
      }
      options = options || {};
      options.user = true;
      //为需要观察的 expOrFn 添加watcher ，expOrFn的值有改变时执行cb，
      //在watcher的实例化的过程中会对expOrFn进行解析，并为expOrFn涉及到的data数据下的def添加该watcher
      var watcher = new Watcher(vm, expOrFn, cb, options);
      //immediate==true 立即执行watch handler
      if (options.immediate) { 
        cb.call(vm, watcher.value);
      }
      //取消观察函数
      return function unwatchFn() {
        watcher.teardown();
      }
    };

var Watcher = function Watcher(
    vm,
    expOrFn,
    cb,
    options,
    isRenderWatcher
  ) {
    this.vm = vm;
    if (isRenderWatcher) {
      vm._watcher = this;
    }
    vm._watchers.push(this);
    // options
    if (options) {
      this.deep = !!options.deep; //是否观察对象内部值的变化
      this.user = !!options.user;
      this.lazy = !!options.lazy;
      this.sync = !!options.sync;
    } else {
      this.deep = this.user = this.lazy = this.sync = false;
    }
    this.cb = cb; // 观察属性改变时执行的函数
    this.id = ++uid$1; // 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 = expOrFn.toString();
    // parse expression for getter
    if (typeof expOrFn === 'function') {
      this.getter = expOrFn;
    } else {
      // 将需要观察的数据：string | Function | Object | Array等进行解析 如：a.b.c, 并返回访问该表达式的函数 
      this.getter = parsePath(expOrFn); 
      if (!this.getter) {
        this.getter = function () { };
        "development" !== 'production' && warn(
          "Failed watching path: \"" + expOrFn + "\" " +
          'Watcher only accepts simple dot-delimited paths. ' +
          'For full control, use a function instead.',
          vm
        );
      }
    }
    // this.get()将访问需要观察的数据 
    this.value = this.lazy
      "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(); //将之前的watcher赋给Dep.target
      this.cleanupDeps();
    }
    return value
  };<br><br><br><br> 
　　Watcher.prototype.run = function run() {

/*....省略部分....*/ 
　　　　var value = this.get(); //重新获取info的值
　　　　var oldValue = this.value; //保存老的值
　　　　this.value = value;
　　　　this.cb.call(this.vm, value, oldValue); //执行watch的回调
/*....省略部分....*/ 
　　};