设计模式：观察者模式（Observer）

定  义：定义了一种一对多的依赖关系，让多个观察者对象同时监听某一主题对象。这个主题对象在状态发生

        变化时，会通知所有观察者对象，使他们能够自动更新自己。

结构图：

抽象主题类：

abstract class Subject
    {
        private IList<Observer> observers = new List<Observer>();

        /// <summary>
        /// 增加观察者
        /// </summary>
        /// <param name="observer"></param>
        public void Attach(Observer observer)
        {
            observers.Add(observer);
        }

        /// <summary>
        /// 移除观察者
        /// </summary>
        /// <param name="observer"></param>
        public void Detach(Observer observer)
        {
            observers.Remove(observer);
        }

        //通知
        public void Notify()
        {
            foreach (Observer observer in observers)
            {
                observer.Update();
            }
        }
    }

抽象观察者类：

 abstract class Observer
    {
        public abstract void Update();
    }

具体主题类：

 class ConcreteSubject : Subject
    {
        private string subjectState;

        //具体被观察者状态
        public string SubjectState
        {
            get { return subjectState; }
            set { subjectState = value; }
        }
    }

具体观察者类：

 class ConcreteObserver : Observer
    {
        private string name;
        private string observerState;
        private ConcreteSubject subject;

        public ConcreteObserver(ConcreteSubject subject, string name)
        {
            this.name = name;
            this.subject = subject;
        }

        public override void Update()
        {
            observerState = subject.SubjectState;
            Console.WriteLine("观察者{0}的新状态为{1}", name, observerState);
        }

        public ConcreteSubject Subject
        {
            get { return subject; }
            set { subject = value; }
        }
    }

客户端代码：

            ConcreteSubject s = new ConcreteSubject();

            s.Attach(new ConcreteObserver(s, "X"));
            s.Attach(new ConcreteObserver(s, "Y"));
            s.Attach(new ConcreteObserver(s, "Z"));

            s.SubjectState = "ABC";
            s.Notify();