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

时间:2006/7/19 6:56:29 作者:佚名 人气:49

.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()

 {

 Workerpeter = 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

*/


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