观察者模式

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();
    }
}

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

class ConcreteSubject : Subject
{
    private string subjectState;

    public string SubjectState
    {
        get {return subjectState;}
        set {subjectState = value;}
    }
}
 

class ConcreteObserver : Observer
{
    private string name_;
    private string observerState_;
    private ConcreteSubject subject;

    public ConcreteObserver(ConcreteSubject subject, string name)
    {
        subject_ = subject;
        name_ = name;
    }
    public override void Update()
    {
        observerState_ = subject.SubjectState;
    }

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

static void Main(string[] args)
{
    ConcreteSubject s = new ConcreteSubject();

    s.Attach(new ConcreteObserver(s, "Jay"));
    s.Attach(new ConcreteObserver(s, "Lily"));

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

}
 

以上为通用情况下的观察者模型。但是，有部分缺陷，下面将介绍采用了事件委托的观察者模式(摘自《大话设计模式》)。

class StockObserver
{
    private string name;
    private Subject sub;
    public StockObserver(string name, Subject sub)
    {
        this.name = name;
        this.sub =sub;
    }

    //update
     public void CloseStockMarke()
    {
        Console.WriteLine("{0} {1} close stock, go to work!", sub.SubjectState, name);
    }
}

 class NBAObserver
{
    private string name;
    private Subject sub;
    public NBAObserver(string name, Subject sub)
    {
        this.name = name;
        this.sub = sub;
    }

    public void CloseNBALive()
    {
        Console.WriteLine("{0} {1}", sub.SubjectState, name);
    }
}
 

interface Subject
{
    void Notify();
    string SubjectState
    {
        get;
        set;
    }
}
 

class Boss : Subject
{
    public event EventHandler Update;

    private string action;

    public void Notify()
    {
        Update();
    }

    public string SubjectState
    {
        get { return action; }
        set { action = value; }
    }
}

 class Secretory : Subject
{
    ...//same as above
 }
 

Boss boss = new Boss();

StockObserver colleague = new StockObserver("Henry", boss);
NBAObserver Jordan = new NBAObserver("Kobe", boss);

boss.Update += new EventHandler(colleague.CloseStockMarket);
boss.Update += new EventHandler(Jordan.CloseNBALive);


boss.SubjectState = "I'm coming";

boss.Notify();