牛奶与报纸

上次听俞晖老师讲委托与事件的时候引用了一个牛奶与报纸的故事，对理解委托比较有用， 分享一下。  

先来看看利用接口的实现：

定义两个接口用于报纸、牛奶，然后订阅者继承这个接口这里我们看看利用接口实现存在的问题：不能达到箱子的共享，每个接口都只能实现特定功能。
 

public interface IMilkBox
......{
    void PutMilkIn(string s);
}
 
public interface INewsPaperBox
......{
    void PutNewsPaperIn(string s);
}
 
public class Subscriber : INewsPaperBox, IMilkBox
......{
    public string name;
 
    public Subscriber(string name)
    ......{ 
        this.name = name; 
    }
 
    public void PutNewsPaperIn(string s)
    ......{ 
        Console.WriteLine("{0}在报箱里收到送来的{1}", name, s); 
    }
 
    public void PutMilkIn(string s)
    ......{ 
        Console.WriteLine("{0}在奶箱里收到送来的{1}", name, s); 
    }
}
 
public class NewsPaperSender
......{
    //接口实现
    protected ArrayList subscribers = new ArrayList();
 
    public void SendNewsPaper()
    ......{
        foreach (INewsPaperBox newsPaperBox in subscribers)
            newsPaperBox.PutNewsPaperIn("报纸");
    }
 
    public void Subscribe(INewsPaperBox newsPaperBox)
    ......{ 
        subscribers.Add(newsPaperBox); 
    }
}
 
public class MilkSender
......{
    //接口实现
    protected ArrayList subscribers = new ArrayList();
 
    public void SendMilk()
    ......{
        foreach (IMilkBox milkBox in subscribers)
            milkBox.PutMilkIn("牛奶");
    }
    public void Subscribe(IMilkBox milkBox)
    ......{
        subscribers.Add(milkBox);
}
}
//功能的实现
public static void Main( string[] args )
......{
    //接口实现
    MilkSender ms = new MilkSender();
    NewsPaperSender nps = new NewsPaperSender();
    Subscriber s1 = new Subscriber("张三");
    Subscriber s2 = new Subscriber("李四");
    Subscriber s3 = new Subscriber("王五");
 
    ms.Subscribe(s1); 
    ms.Subscribe(s2);
    nps.Subscribe(s2);
    nps.Subscribe(s3);
    ms.SendMilk(); 
    nps.SendNewsPaper();
}

再看看利用委托实现牛奶和报纸的订阅来解决上面的问题：使用委托让牛奶报纸共享箱子。

namespace myDelegate
...{
    public delegate void SendableDelegate(string s);
 
    public class Subscriber
    ...{
        public string name;
 
        public Subscriber(string name)
        ...{ 
            this.name = name; 
        }
 
        public void PutNewsPaperIn(string s)
        ...{ 
            Console.WriteLine("{0}在报箱里收到送来的{1}", name, s); 
        }
 
        public void PutMilkIn(string s)
        ...{ 
            Console.WriteLine("{0}在奶箱里收到送来的{1}", name, s); 
        }
    }
 
    public class NewsPaperSender
    ...{
        //委托实现
        public event SendableDelegate SendNewsPaperHandler;
 
        public void SendNewsPaper()
        ...{
            if (SendNewsPaperHandler != null)
                SendNewsPaperHandler("报纸");
        }
 
        public void Subscribe(SendableDelegate sd)
        ...{ 
            SendNewsPaperHandler += sd; 
        }
    }
 
    public class MilkSender
    ...{
        //委托实现
        public event SendableDelegate SendMilkHandler;
 
        public void SendMilk()
        ...{
            if (SendMilkHandler != null)
                SendMilkHandler("牛奶");          //事件调用时带参数
        }
 
        public void Subscribe(SendableDelegate sd)
        ...{
            SendMilkHandler += sd;
        }
    }
    //功能的实现
    public class Client
    ...{
        
        public static void Main( string[] args )
        ...{
            //委托实现
            MilkSender ms = new MilkSender();
            NewsPaperSender nps = new NewsPaperSender();
            Subscriber s1 = new Subscriber("张三");
            Subscriber s2 = new Subscriber("李四");
            Subscriber s3 = new Subscriber("王五");
 
            ms.Subscribe(new SendableDelegate(s1.PutMilkIn));
            ms.Subscribe(new SendableDelegate(s2.PutMilkIn));
            nps.Subscribe(new SendableDelegate(s2.PutNewsPaperIn));
            nps.Subscribe(new SendableDelegate(s3.PutMilkIn));
            ms.SendMilk();
            nps.SendNewsPaper();        
        }
    }
}