設計模式：觀察者模式

觀察者模式（Observer）：定義了一種一對多的依賴關系，讓多個觀察者對象同時監聽某一主題對象。這個主題對象在狀態發生變化時，會通知所有觀察者對象，使它們能夠自動更新自己。

{
    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 observer in observers)
            {
                observer.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 ConcreteObserver(ConcreteSubject subject,string name)
        {
            this.subject = subject;
            this.name = name;
        }
        public override void Update()
        {
            observerState = subject.SubjectState;
            Console.WriteLine("觀察者{0}的新狀態是{1}", name, observerState);
        }
        public ConcreteSubject Subject
        {
            get { return subject; }
            set { subject = value; }
        }
    }
}

測試代碼：

            ConcreteSubject subject = new ConcreteSubject();
            subject.Attach(new ConcreteObserver(subject, "X"));
            subject.Attach(new ConcreteObserver(subject, "Y"));
            subject.Attach(new ConcreteObserver(subject, "Z"));
            subject.SubjectState = "ABC";
            subject.Notify();

 事件委托：

namespace ObserverDesign
{
    public interface ISubject
    {
        string SubjectState { get; set; }
        void Notify();
    }
    public delegate void HandleMessage();
    public class ConcreteSubjectA : ISubject
    {
        public string SubjectState
        {
            get
            {
                return action;
            }

            set
            {
                action = value ;
            }
        }
        private string action;
        public event HandleMessage Update;
        public void Notify()
        {
            Update();
        }
    }
    public class ConcreteSubjectB : ISubject
    {
        public string SubjectState
        {
            get
            {
                return action;
            }

            set
            {
                action = value;
            }
        }
        private string action;
        public event EventHandler Update;
        public void Notify()
        {
            Update();
        }
    }
   public class ObserverA
    {
        private string name;
        private ISubject sub;
        public ObserverA(string name,ISubject sub)
        {
            this.name = name;
            this.sub = sub;
        }
        public void MethodA()
        {
            Console.WriteLine("A doing...");
        }
    }
    public class ObserverB
    {
        private string name;
        private ISubject sub;
        public ObserverB(string name, ISubject sub)
        {
            this.name = name;
            this.sub = sub;
        }
        public void MethodB()
        {
            Console.WriteLine("B doing...");
        }
    }
}

委托測試代碼：

            ConcreteSubjectA subject = new ConcreteSubjectA();
            ObserverA oa = new ObserverA("A", subject);
            ObserverB ob = new ObserverB("B", subject);
            subject.Update += new HandleMessage(oa.MethodA);
            subject.Update += new HandleMessage(ob.MethodB);
            subject.SubjectState = "start...";
            subject.Notify();