/// <summary>
 /// 移除观察者
 /// </summary>
 /// <param name="observer"></param>
 public void Delete(AbstractObserver observer)
 {
     observerList.Remove(observer);
 }
 /// <summary>
 /// 增加观察者(增加一个关注人)
 /// </summary>
 /// <param name="observer"></param>
 public void Add(AbstractObserver observer)
 {
     observerList.Add(observer);
 }