观察者模式（Observer Pattern）

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<Observer> observers = new List<Observer>();

        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