观察者模式：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个主体对象，这个主题对象在状态发生变化时，会通知所有观察者。当一个对象改变需要同时改变其他对象，而且他不知道具体有多少对象需要改变的时候，应该考虑使用观察者模式。

Define a one-to-many dependency between objects so that when one objects changes state,all its dependents are notified and updated automatically.

观察者结构图：

代码：

public abstract class Subject    {        private IList
     
       observers = new List
      
       ();        public void Attach(Observer observer)        {            observers.Add(observer);        }        public void Detach(Observer observer)        {            observers.Remove(observer);        }        public void Notify()        {            foreach (Observer o in observers)            {                o.Update();            }        }    }    public abstract class Observer    {        public abstract void Update();    }    public class ConcreteSubject : Subject    {        private string subjectState;        public string SubjectState        {            get { return subjectState; }            set { subjectState = value; }        }    }    public class ConcreteObserver : Observer    {        private string name;        private string observerState;        private ConcreteSubject subject;        public ConcreteSubject Subject        {            get { return subject; }            set { subject = value; }        }        public ConcreteObserver(ConcreteSubject subject, string name)        {            this.subject = subject;            this.name = name;        }        public override void Update()        {            observerState = subject.SubjectState;            Console.WriteLine("观察者{0}的新状态是{1}",name,observerState);        }    }
      
     

引用：

ConcreteSubject c = new ConcreteSubject();            c.Attach(new ConcreteObserver(c, "A"));            c.Attach(new ConcreteObserver(c, "b"));            c.Attach(new ConcreteObserver(c, "z"));            c.SubjectState = "ABC";            c.Notify();

效果