深入理解Java设计模式之观察者模式

目录

• 一、什么是观察者模式
• 二、观察者模式的结构
• 三、观察者模式的使用场景

• 使用观察者模式也有两个重点问题要解决：

• 广播链的问题。
• 异步处理问题。

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

abstract class Subject
{
    private List<Observer> observers = new List<Observer>();
    //增加观察者
    public void Attach(Observer observer)
    {
        observers.Add(observer);
    }
    //移除观察者
    public void Detach(Observer observer)
    {
        observers.Remove(observer);
    }
    //通知
    public void Notify()
    {
        foreach (var item in observers)
        {
            item.Update();
        }
    }
}

class ConcreteSubject : Subject
{
    private string subjectState;
    //具体被观察者状态
    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)
    {
        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);
    }
    public ConcreteSubject Subject
    {
        get { return subject; }
        set { subject = value; }
    }
}

static void Main(string[] args)
{
    ConcreteSubject cs = new ConcreteSubject();
     cs.Attach(new ConcreteObserver(cs, "X"));
    cs.Attach(new ConcreteObserver(cs, "Y"));
    cs.Attach(new ConcreteObserver(cs, "Z"));
     cs.SubjectState = "ABC";
    cs.Notify();
     Console.Read();
}

//通知者接口
interface Subject
{
    void Notify();
    string SubjectState { get; set; }
}

//看股票的同事
class StockObserver
{
    private string name;
    private Subject sub;
     public StockObserver(string name, Subject sub)
    {
        this.sub = sub;
        this.name = name;
    }
    //关闭股票
    public void CloseStock()
    {
        Console.WriteLine("{0}{1}关闭股票，继续工作", sub.SubjectState, sub);
    }
}
//看NBA的同事
class NBAObserver
{
    private string name;
    private Subject sub;
     public NBAObserver(string name, Subject sub)
    {
        this.sub = sub;
        this.name = name;
    }
    //关闭NBA
    public void CloseNBA()
    {
        Console.WriteLine("{0}{1}关闭NBA，继续工作", sub.SubjectState, sub);
    }
}

//声明一个委托，无参数，无返回值
delegate void EventHandler();

//老板类
class Boss : Subject
{
    private string action;
    //声明委托事件Update
    public event EventHandler Update;
     public string SubjectState
    {
        get { return action; }
         set { action = value; }
    }
     public void Notify()
    {
        //在访问通知时，调用Update
        Update();
    }
}
//秘书类
class Secretary : Subject
{
    //与老板类类似，省略......
}

static void Main(string[] args)
{
    //老板张
    Boss Zhang = new Boss();
     StockObserver tongshi1 = new StockObserver("张三",Zhang);
    NBAObserver tongshi2 = new NBAObserver("李四",Zhang);
     Zhang.Update += new EventHandler(tongshi1.CloseStock);
    Zhang.Update += new EventHandler(tongshi2.CloseNBA);
     Zhang.SubjectState = "老板张驾到！";
    Zhang.Notify();
    Console.Read();
}

老板张驾到！张三关闭股票，继续工作
老板张驾到！李四关闭NBA，继续工作