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

万俟铭
2023-12-01
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.NET Delegates: A C# Bedtime Story中文版(上篇)

作者:Chris Sells

译者:荣耀

【译注:C#进阶文章。Chris Sells是《ATL Internals》一书作者之一。译文中所有程序调试环境均为Microsoft Visual Studio.NET 7.0 Beta2和 Microsoft .NET Framework SDK Beta2。代码就是文章,请仔细阅读代码J

类型耦合

从前,在南方的一个异国他乡,有一个叫peter的勤劳的工人。他对boss百依百顺,但他的boss却是个卑鄙无信的小人,他坚持要求peter不断汇报工作情况。由于peter不希望被boss盯着干活,于是他向boss承诺随时汇报工作进度。peter利用类型引用定期回调boss来实现这个承诺:

using System;//【译注:译者补充】

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不关心. */ }

     public void WorkProgressing() { /*boss不关心. */ }

     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();

     }

}

/*【译注:以下是上段程序输出结果:

Worker: work started

Worker: work progressing

Worker: work completed

It's about time!

Worker grade = 2

Main: worker completed work

*/

接口

     现在,peter成了一个特殊人物,他不但能够忍受卑鄙的boss,和universe也建立了紧密的联系。peter感到universe对他的工作进程同样感兴趣。不幸的是,除了保证boss能够被通知外,如果不为universe添加一个特殊的通知方法和回调,peter无法向universe通知其工作进程。Peter希望能从那些通知方法的实现中分离出潜在的通知约定,为此,他决定将方法剥离到接口中:

using System; //【译注:译者补充】

interface IWorkerEvents //【译注:这就是分离出来的接口】

{

     void WorkStarted();

     void WorkProgressing();

     int WorkCompleted();

}

class Worker

{

     public void Advise(IWorkerEvents events) //【译注:现在传递的参数类型为接口引用】

{

_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 //【译注:Boss实现该接口】

{

     public void WorkStarted(){ /*boss不关心. */ }

     public void WorkProgressing(){ /*boss不关心. */ }

     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); //【译注:或peter.Advise((IWorkerEvents)boss);】

          peter.DoWork();

          Console.WriteLine("Main: worker completed work");

          Console.ReadLine();

     }

}

/*【译注:以下是上段程序输出结果:

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,由于peter忙于通知boss实现这个接口,以至于没有顾得上通知universe也实现该接口,但他知道不久就需如此,至少,他已经抽象了对boss的引用,因此,别的实现了IworkerEvents接口的什么人都可以收到工作进度通知。【译注:请参见上一节代码示例及译注】

     然而,peter的boss依然极度不满,“Peter!”boss咆哮者,“你为什么要通知我什么时候开始工作、什么时候正在进行工作?我不关心这些事件,你不但强迫我实现这些方法,你还浪费了你的宝贵的工作时间等我从事件中返回。当我实现的方法需占用很长时间时,你等我的时间也要大大延长!你难道不能想想别的办法不要老是来烦我吗?”

     此时,peter意识到尽管在很多情况下接口很有用,但在处理事件时,接口的粒度还不够精细。他还要能做到仅仅通知监听者真正感兴趣的事件。因此,peter决定把接口里的方法肢解成若干个独立的委托函数,每一个都好象是只有一个方法的小接口。

using System; //【译注:译者补充】

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; //【译注:这样写更规矩:public WorkStarted started = null;】

     public WorkProgressing progressing; //【译注:这样写更规矩:public WorkProgressing progressing = null;】

     public WorkCompleted completed; //【译注:这样写更规矩:public WorkCompleted 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();

     }

}

/*【译注:以下是上段程序输出结果:

Worker: work started

Worker: work progressing

Worker: work completed

Better...

Worker grade = 4

Main: worker completed work

*/

【译注:对“但在处理事件时,接口的粒度还不够精细”的理解可用下例说明,请仔细观察一下程序,思考一下这样做的不利之处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();

     }

}

/*以下是上段程序输出结果:

Worker: work completed

Better...

Worker grade = 4

Main: worker completed work

*/

静态监听者

     这就达到了不用boss不关心的事件去烦他的目标。但是,peter还是不能够使universe成为其监听者。因为universe是一个全封闭的实体,所以将委托挂钩在universe的实例上不妥的(设想一下Universe的多个实例需要多少资源...)。peter意识到应将委托挂钩于universe的静态成员上,因为委托也完全适应于静态成员:

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 */

     }

}

//【译注:以上代码为译者补充】

class Universe

{

    static void WorkerStartedWork()

    {

        Console.WriteLine("Universe notices worker starting work");

    }

    static int WorkerCompletedWork()

    {

        Console.WriteLine("Universe pleased <script type="text/javascript"> </script> <script type="text/javascript" src="http://pagead2.googlesyndication.com/pagead/show_ads.js"> </script>

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