Design Pattern : Chain of Responsibility

Design Pattern : Chain of Responsibility

Chain of Responsibility 可將所有職責鍊在一起， 使用者可直接發動開頭的第一件職責， 第一件職責做完後就會去啟動第二件、第三件 … etc

底下的例子為去洗車的時候，通常有三個步驟：洗車 -> 打蠟 -> 曬曬陽光做日光浴

如果是開去給洗車場洗的話，其實我們就不需要知道太多細節，可以去吃個晚餐再回來就好。

這時候就可以使用Chain of Responsibility。

1.  Handler interface
public interface ICarWashHandler
{
   ICarWashHandler NextStep { set; get; }
    void Service<T>(T obj);
}

u  Handler可以看出職責鍊是如何運作的， 我們只要將下一個Handler存放在NextStep，在Service動作的時候做完本身的職責後，再呼叫NextStep.Service就可以一直往下做完整個職責鍊了。

2.  實作Handler

u   第一件職責： 洗車：CarShower

public class CarShower:ICarWashHandler
{
   public ICarWashHandler NextStep{ get; set;}

    public void Service<T>(T obj)
    {
       object client = obj;
       if (typeof(T).Name.ToString() == "Client")
       {
           String msg = String.Format(
                "Car washed! (Customer : {0} {1})",
                (client as Client).Name,
                (client as Client).PhoneNumber);
           Console.WriteLine(msg);
       }
       if(this.NextStep!=null)
       {
           this.NextStep.Service(obj);
       }
       else //Set the default NextStep
       {
            this.NextStep = new CarWaxing();
            this.NextStep.Service(obj);
       }
    }
}

u  第二件職責： 打蠟：CarWaxing
public class CarWaxing:ICarWashHandler
{
   public ICarWashHandler NextStep{get;set;}
   public void Service<T>(T obj)
   {
      object client = obj;
      if (typeof(T).Name.ToString() == "Client")
      {
          String msg = String.Format(
               "Car waxing! (Customer : {0} {1})",
               (client as Client).Name,
               (client as Client).PhoneNumber);
          Console.WriteLine(msg);
      }
      if (this.NextStep != null)
      {
          this.NextStep.Service(obj);
      }
      else //Set the default NextStep
      {
          this.NextStep = new CarSunBathing();
          this.NextStep.Service(obj);
      }
   }
}

u  第三件職責： 日光浴：CarSunBathing
public class CarSunBathing:ICarWashHandler
{
   public ICarWashHandler NextStep { get; set; }
   public void Service<T>(T obj)
   {
      object client = obj;
      if (typeof(T).Name.ToString() == "Client")
      {
          String msg = String.Format(
             "Car sunbathing! (Customer : {0} {1})",
             (client as Client).Name,
             (client as Client).PhoneNumber);
          Console.WriteLine(msg);
      }
      if (this.NextStep != null)
      {
          this.NextStep.Service(client);
      }
      else //Set the default NextStep
      {
          Console.WriteLine("Car is great now!");
      }
   }
}

3.      主程式
Client JB = new Client("JB", "0911-29X-XXX");
ICarWashHandler carWash1 = new CarShower();
carWash1.Service<Client>(JB);

結果：
Car washed! (Customer : JB 0911-29X-XXX)
Car waxing! (Customer : JB 0911-29X-XXX)
Car sunbathing! (Customer : JB 0911-29X-XXX)
Car is great now!

4.      我們也可以手動指定每個職責的下一個為哪個職責，例如我老婆看車子很髒想要洗兩次：
Client Lily = new Client("Lily", "0930-8XX-XXX");
ICarWashHandler carWash2 = new CarShower();
carWash2.NextStep = new CarShower(); //指定下一個責任為 (再洗車一次)
carWash2.Service<Client>(Lily);

結果：
Car washed! (Customer : Lily 0930-8XX-XXX)
Car washed! (Customer : Lily 0930-8XX-XXX) //洗了兩次
Car waxing! (Customer : Lily 0930-8XX-XXX)
Car sunbathing! (Customer : Lily 0930-8XX-XXX)
Car is great now!

5.      另外如果遇到某個無法處理目前要求的Handler， 該Handler(職責)也可以選擇不處理直接丟給下一個。
例如 消光的車洗車後不用打蠟，這時候只要在打蠟的Handler中加上判斷就行了。

6.      雖然Chain of Responsibility降低耦合，但是只要職責一多，Performance就會被拖累。

7.      延伸閱讀：
What's the difference between “Chain of responsibility” and “Strategy” patterns?