>watcher订阅者对象 usage: new Watcher(vm,getter,cb,options); vm: component实例 getter: 取值函数； cb: 回调函数；dep.notify => watcher.update() => watcher.run() => cb.call(watcher); options: 配置参数 >属性 vm: Component; 实例化时传入 expression: string; getter.toString() cb: Function; 实例化时传入 id: number; deep: boolean; //--是否深度依赖 user: boolean; //---是否用户$watch lazy: boolean; sync: boolean; dirty: boolean; active: boolean; deps: Array<Dep>; newDeps: Array<Dep>; depIds: Set; newDepIds: Set; getter: Function; value: any; //--this.getter()得到