AngularFire2 Child_added事件再次返回所有数据

Question

在firebase中child_added中angularfire中的child_added事件是否有所不同？

为什么angularfire返回所有列表，包括child_added之后的值，而在firebase中它只返回la

 this.userList = this.room.getAllUser();
    this.userList.snapshotChanges()
      .subscribe(actions => {
         console.log(actions);
        actions.forEach(action => {
          console.log(action.type);
          console.log(action.key);
          console.log(action.payload.val());
        })

我是否需要再次扫描响应并在数组中比较密钥？在推之前？