设计模式--观察者模式

using System;
using System.Collections.Generic;

// 定义被观察者接口
interface ISubject
{
    void AddObserver(IObserver observer);    //添加观察者
    void RemoveObserver(IObserver observer); //移除观察者
    void NotifyObservers();                  //通知所有观察者
}

// 定义观察者接口
interface IObserver
{
    void Update(ISubject subject);           //更新方法
}

// 具体被观察者类
class ConcreteSubject : ISubject
{
    List<IObserver> observers = new List<IObserver>();   //维护观察者列表
    string state;                                      //被观察对象状态

    public void AddObserver(IObserver observer)
    {
        observers.Add(observer);
        Console.WriteLine("Observer added.");
    }

    public void RemoveObserver(IObserver observer)
    {
        observers.Remove(observer);
        Console.WriteLine("Observer removed.");
    }

    public void NotifyObservers()
    {
        Console.WriteLine("Subject state changed.");
        foreach (IObserver observer in observers)
        {
            observer.Update(this);
        }
    }

    // 设置被观察对象状态
    public void SetState(string state)
    {
        this.state = state;
        NotifyObservers();   //状态改变时通知所有观察者
    }

    // 获取被观察对象状态
    public string GetState()
    {
        return state;
    }
}

// 具体观察者类
class ConcreteObserver : IObserver
{
    string name;

    public ConcreteObserver(string name)
    {
        this.name = name;
    }

    public void Update(ISubject subject)
    {
        Console.WriteLine("{0} is notified. Subject state is {1}", 
            name, ((ConcreteSubject)subject).GetState());
    }
}

// 测试代码
class Program
{
    static void Main(string[] args)
    {
        //创建被观察者对象
        ConcreteSubject subject = new ConcreteSubject();

        //创建观察者对象
        ConcreteObserver observer1 = new ConcreteObserver("Observer1");
        ConcreteObserver observer2 = new ConcreteObserver("Observer2");

        //将观察者对象添加到被观察者对象的观察者列表
        subject.AddObserver(observer1);
        subject.AddObserver(observer2);

        //改变被观察对象状态，触发通知
        subject.SetState("new state");
    }
}

Observer added.
Observer added.
Subject state changed.
Observer1 is notified. Subject state is new state
Observer2 is notified. Subject state is new state