程式師世界 >> 編程語言 >> C語言 >> 關於C語言 >> .NET Delegates: A C# Bedtime Story中文版(上篇)

.NET Delegates: A C# Bedtime Story中文版(上篇)

編輯：關於C語言

【譯注：C#進階文章。Chris Sells是《ATL Internals》一書作者之一。譯文中所有程
序調試環境均為Microsoft Visual Studio.NET 7.0 Beta2和 Microsoft .Net Framewo
rk SDK Beta2。代碼就是文章，請仔細閱讀代碼J】
類型耦合
從前，在南方的一個異國他鄉，有一個叫peter的勤勞的工人。他對boss百依百順，但他
的boss卻是個卑鄙無信的小人，他堅持要求peter不斷匯報工作情況。由於peter不希望
被boss盯著干活，於是他向boss承諾隨時匯報工作進度。peter利用類型引用定期回調b
oss來實現這個承諾：
using System;//【譯注：譯者補充】
class Worker
{
     public void Advise(Boss boss)
{
_boss = boss;
}
     public void DoWork()
     {
          Console.WriteLine("Worker: work started");
          if( _boss != null ) _boss.WorkStarted();
          Console.WriteLine("Worker: work progressing");
          if( _boss != null ) _boss.WorkProgressing();
          Console.WriteLine("Worker: work completed");
          if( _boss != null )
         {
              int grade = _boss.WorkCompleted();
              Console.WriteLine("Worker grade = " + grade);
         }
     }
     private Boss _boss;
}
class Boss
{
     public void WorkStarted() { /*boss不關心. */ }
     public void WorkProgressing() { /*boss不關心. */ }
     public int WorkCompleted()
     {
          Console.WriteLine("It's about time!");
         return 2; /* out of 10 */
     }
}
class Universe
{
     static void Main()
     {
         Worker peter = new Worker();
         Boss boss = new Boss();
          peter.Advise(boss);
          peter.DoWork();
          Console.WriteLine("Main: worker completed work");
          Console.ReadLine();
     }
}
/*【譯注：以下是上段程序輸出結果：
Worker: work started
Worker: work progressing
Worker: work completed
It's about time!
Worker grade = 2
Main: worker completed work
】*/
接口
     現在，peter成了一個特殊人物，他不但能夠忍受卑鄙的boss，和universe也建立
了緊密的聯系。peter感到universe對他的工作進程同樣感興趣。不幸的是，除了保證b
oss能夠被通知外，如果不為universe添加一個特殊的通知方法和回調，peter無法向un
iverse通知其工作進程。Peter希望能從那些通知方法的實現中分離出潛在的通知約定，
為此，他決定將方法剝離到接口中：
using System; //【譯注：譯者補充】
interface IWorkerEvents //【譯注：這就是分離出來的接口】
{
     void WorkStarted();
     void WorkProgressing();
     int WorkCompleted();
}
class Worker
{
     public void Advise(IWorkerEvents events) //【譯注：現在傳遞的參數類型為
接口引用】
{
_events = events;
}
     public void DoWork()
     {
          Console.WriteLine("Worker: work started");
          if( _events != null ) _events.WorkStarted();
          Console.WriteLine("Worker: work progressing");
          if(_events != null ) _events.WorkProgressing();
          Console.WriteLine("Worker: work completed");
          if(_events != null )
         {
int grade = _events.WorkCompleted();
              Console.WriteLine("Worker grade = " + grade);
         }
     }
     private IWorkerEvents _events;
}
class Boss : IWorkerEvents //【譯注：Boss實現該接口】
{
     public void WorkStarted(){ /*boss不關心. */ }
     public void WorkProgressing(){ /*boss不關心. */ }
     public int WorkCompleted()
     {
          Console.WriteLine("It's about time!");
         return 3; /* out of 10 */
     }
}
class Universe
{
     static void Main()
     {
         Worker peter = new Worker();
         Boss boss = new Boss();
          peter.Advise(boss); //【譯注：或peter.Advise((IWorkerEvents)boss);
】
          peter.DoWork();
          Console.WriteLine("Main: worker completed work");
          Console.ReadLine();
     }
}
/*【譯注：以下是上段程序輸出結果：
Worker: work started
Worker: work progressing
Worker: work completed
It's about time!
Worker grade = 3
Main: worker completed work
】*/
委托
     不幸的是，由於peter忙於通知boss實現這個接口，以至於沒有顧得上通知univer
se也實現該接口，但他知道不久就需如此，至少，他已經抽象了對boss的引用，因此，
別的實現了IworkerEvents接口的什麼人都可以收到工作進度通知。【譯注：請參見上一
節代碼示例及譯注】
     然而，peter的boss依然極度不滿，“Peter！”boss咆哮者，“你為什麼要通知我
什麼時候開始工作、什麼時候正在進行工作？我不關心這些事件，你不但強迫我實現這
些方法，你還浪費了你的寶貴的工作時間等我從事件中返回。當我實現的方法需占用很
長時間時，你等我的時間也要大大延長！你難道不能想想別的辦法不要老是來煩我嗎？
”
     此時，peter意識到盡管在很多情況下接口很有用，但在處理事件時，接口的粒度
還不夠精細。他還要能做到僅僅通知監聽者真正感興趣的事件。因此，peter決定把接口
裡的方法肢解成若干個獨立的委托函數，每一個都好象是只有一個方法的小接口。
using System; //【譯注：譯者補充】
delegate void WorkStarted();
delegate void WorkProgressing();
delegate int WorkCompleted();
class Worker
{
     public void DoWork()
     {
          Console.WriteLine("Worker: work started");
          if( started != null ) started();
          Console.WriteLine("Worker: work progressing");
          if( progressing != null ) progressing();
          Console.WriteLine("Worker: work completed");
          if( completed != null )
         {
              int grade = completed();
              Console.WriteLine("Worker grade = " + grade);
         }
     }
     public WorkStarted started; //【譯注：這樣寫更規矩：public WorkStarted
started = null;】
     public WorkProgressing progressing; //【譯注：這樣寫更規矩：public Work
Progressing progressing = null;】
     public WorkCompleted completed; //【譯注：這樣寫更規矩：public WorkComp
leted completed = null;】
}
class Boss
{
     public int WorkCompleted()
     {
          Console.WriteLine("Better...");
         return 4; /* out of 10 */
     }
}
class Universe
{
     static void Main()
     {
         Worker  peter = new Worker();
         Boss boss = new Boss();
          peter.completed = new WorkCompleted(boss.WorkCompleted);
          peter.DoWork();
          Console.WriteLine("Main: worker completed work");
          Console.ReadLine();
     }
}
/*【譯注：以下是上段程序輸出結果：
Worker: work started
Worker: work progressing
Worker: work completed
Better...
Worker grade = 4
Main: worker completed work
】
*/
【譯注：對“但在處理事件時，接口的粒度還不夠精細”的理解可用下例說明，請仔細
觀察一下程序，思考一下這樣做的不利之處J
using System;
interface IWorkStartedEvent
{
     void WorkStarted();
}
interface IWorkProgressingEvent
{
     void WorkProgressing();
}
interface IWorkCompletedEvent
{
     int WorkCompleted();
}
class Worker
{
public void Advise(IWorkCompletedEvent AEvent)
{
_event = AEvent;
}
    public void DoWork()
    {
        Console.WriteLine("Worker: work completed");
        if(_event != null )
         {
            int grade = _event.WorkCompleted();
            Console.WriteLine("Worker grade = " + grade);
        }
    }
    private IWorkCompletedEvent _event;
}
class Boss : IWorkCompletedEvent
{
public int WorkCompleted()
{
Console.WriteLine("Better...");
        return 4; /* out of 10 */
    }
}
class Universe
{
static void Main()
     {
         Worker peter = new Worker();
         Boss boss = new Boss();
          peter.Advise(boss);
          peter.DoWork();
          Console.WriteLine("Main: worker completed work");
          Console.ReadLine();
     }
}
/*以下是上段程序輸出結果：
Worker: work completed
Better...
Worker grade = 4
Main: worker completed work
*/
】
靜態監聽者
     這就達到了不用boss不關心的事件去煩他的目標。但是，peter還是不能夠使univ
erse成為其監聽者。因為universe是一個全封閉的實體，所以將委托掛鉤在universe的
實例上不妥的（設想一下Universe的多個實例需要多少資源...）。peter意識到應將委
托掛鉤於universe的靜態成員上，因為委托也完全適應於靜態成員：
using System;
delegate void WorkStarted();
delegate void WorkProgressing();
delegate int WorkCompleted();
class Worker
{
     public void DoWork()
     {
          Console.WriteLine("Worker: work started");
          if( started != null ) started();
          Console.WriteLine("Worker: work progressing");
          if( progressing != null ) progressing();
          Console.WriteLine("Worker: work completed");
          if( completed != null )
         {
              int grade = completed();
              Console.WriteLine("Worker grade= " + grade);
         }
     }
     public WorkStarted started = null;
     public WorkProgressing progressing = null;
     public WorkCompleted completed = null;
}
class Boss
{
     public int WorkCompleted()
     {
          Console.WriteLine("Better...");
         return 4; /* out of 10 */
     }
}
//【譯注：以上代碼為譯者補充】
class Universe
{
    static void WorkerStartedWork()
    {
        Console.WriteLine("Universe notices worker starting work");
    }
    static int WorkerCompletedWork()
    {
        Console.WriteLine("Universe pleased with worker's work");
        return 7;
    }
    static void Main()
    {
        Worker peter = new Worker();
        Boss boss = new Boss();
        peter.completed = new WorkCompleted(boss.WorkCompleted); //【譯注：×
】
        peter.started = new WorkStarted(Universe.WorkerStartedWork);
        peter.completed = new WorkCompleted(Universe.WorkerCompletedWork);//
【譯注：這一行代碼使得“×”那一行代碼白做了L】
        peter.DoWork();
        Console.WriteLine("Main: worker completed work");
        Console.ReadLine();
    }
}
/*【譯注：以下是上段程序輸出結果：
Worker: work started
Universe notices worker starting work
Worker: work progressing
Worker: work completed
Universe pleased with worker's work
Worker grade = 7
Main: worker completed work
】*/
事件
     不幸的是，universe現在變得太忙並且不習慣於注意某一個人—universe用自己的
委托取代了peter的boss的委托，這顯然是將Worker類的委托字段設為public的意外的副
作用。【譯注：請參見上節例子代碼及譯注】同樣地，如果peter的boss不耐煩了，他自
己就可以觸發peter的委托（peter的boss可是有暴力傾向的）
// peter的boss自己動手了
if( peter.completed != null ) peter.completed();
peter希望確保不會發生這兩種情況。他意識到必須為每一個委托加入注冊和反注冊函數
，這樣監聽者就可以添加或移去它們，但誰都不能夠清空整個事件列表。peter自己沒去
實現這些方法，相反，他使用event關鍵字讓C#編譯器幫他達到這個目的：
class Worker
{
//...
public event WorkStarted started;
    public event WorkProgressing progressing;
    public event WorkCompleted completed;
}
     peter懂得關鍵字event使得委托具有這樣的特性：只允許C#客戶用+=或-=操作符添
加或移去它們自己，這樣就迫使boss和universe舉止文雅一些：
static void Main()
{
Worker peter = new Worker();
     Boss boss = new Boss();
     peter.completed += new WorkCompleted(boss.WorkCompleted);
     peter.started += new WorkStarted(Universe.WorkerStartedWork);
     peter.completed += new WorkCompleted(Universe.WorkerCompletedWork);
peter.DoWork();
Console.WriteLine("Main: worker completed work");
Console.ReadLine();
}
【譯注：以下是完整代碼：
using System;
delegate void WorkStarted();
delegate void WorkProgressing();
delegate int WorkCompleted();
class Worker
{
     public void DoWork()
     {
          Console.WriteLine("Worker: work started");
          if( started != null ) started();
          Console.WriteLine("Worker: work progressing");
          if( progressing != null ) progressing();
          Console.WriteLine("Worker: work completed");
          if( completed != null )
         {
              int grade = completed();
              Console.WriteLine("Worker grade = " + grade);
         }
     }
    public event WorkStarted started ;
    public event WorkProgressing progressing;
    public event WorkCompleted completed;
}
class Boss
{
     public int WorkCompleted()
     {
          Console.WriteLine("Better...");
         return 4; /* out of 10 */
     }
}
class Universe
{
     static void WorkerStartedWork()
     {
          Console.WriteLine("Universe notices worker starting work");
     }
     static int WorkerCompletedWork()
     {
          Console.WriteLine("Universe pleased with worker's work");
         return 7;
     }
     static void Main()
     {
         Worker peter = new Worker();
         Boss boss = new Boss();
        peter.completed += new WorkCompleted(boss.WorkCompleted); //【譯注：
√】
        peter.started += new WorkStarted(Universe.WorkerStartedWork);
        peter.completed += new WorkCompleted(Universe.WorkerCompletedWork);
          peter.DoWork();
          Console.WriteLine("Main: worker completed work");
          Console.ReadLine();
     }
}
/*

以下是上段程序輸出結果：
Worker: work started
Universe notices worker starting work
Worker: work progressing
Worker: work completed
Better...// 【譯注：boss也通知到啦J“√”那一行代碼有用啦J，但是且慢，boss打
的那4分沒有得到，後面只得到了Universe給的7分L】
Universe pleased with worker's work
Worker grade = 7
Main: worker completed work
*/
】