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

.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 with worker's work");

        return 7;

    }

    static void Main()

    {

        Worker peter = new Worker();

        Boss boss = new Boss();

        peter.completed = new WorkCompleted(boss.WorkCompleted); //【译注：×】

        peter.started = new WorkStarted(Universe.WorkerStartedWork);

        peter.completed = new WorkCompleted(Universe.WorkerCompletedWork);//【译注：这一行代码使得“×”那一行代码白做了L】

        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

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现在变得太忙并且不习惯于注意某一个人—universe用自己的委托取代了peter的boss的委托，这显然是将Worker类的委托字段设为public的意外的副作用。【译注：请参见上节例子代码及译注】同样地，如果peter的boss不耐烦了，他自己就可以触发peter的委托（peter的boss可是有暴力倾向的）

// peter的boss自己动手了

if( peter.completed != null ) peter.completed();

peter希望确保不会发生这两种情况。他意识到必须为每一个委托加入注册和反注册函数，这样监听者就可以添加或移去它们，但谁都不能够清空整个事件列表。peter自己没去实现这些方法，相反，他使用event关键字让C#编译器帮他达到这个目的：

class Worker

{

//...

public event WorkStarted started;

    public event WorkProgressing progressing;

    public event WorkCompleted completed;

}

     peter懂得关键字event使得委托具有这样的特性：只允许C#客户用+=或-=操作符添加或移去它们自己，这样就迫使boss和universe举止文雅一些：

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();

}

【译注：以下是完整代码：

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); //【译注：√】

        peter.started += new WorkStarted(Universe.WorkerStartedWork);

        peter.completed += new WorkCompleted(Universe.WorkerCompletedWork);

          peter.DoWork();

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

          Console.ReadLine();

     }

}

/*

以下是上段程序输出结果：

Worker: work started

Universe notices worker starting work

Worker: work progressing

Worker: work completed

Better...// 【译注：boss也通知到啦J“√”那一行代码有用啦J，但是且慢，boss打的那4分没有得到，后面只得到了Universe给的7分L】

Universe pleased with worker's work

Worker grade = 7

Main: worker completed work

*/

】