委托和事件,委托的作用
分类:web前端

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 Framewo
rk 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b
oss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b
oss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un
iverse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univer
se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 Work
Progressing progressing = null;】
     public WorkCompleted completed; //【译注:这样写更规矩: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(,',',');
     }
}
/*【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univ
erse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(,',',');
     }
}
/*

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 Framewo
rk SDK Beta2。代码就是文章,请仔细阅读代码J】
取得所有结果
     现在,peter终于松了一口气。他已经设法满足了所有的监听者,而且不会和特定
实现紧密耦合。然而,他又注意到尽管boss和universe都为工作打了分,但他只得到了
一个打分。【译注:请参见上节例子代码及译注】他希望能得到每一个监听者的评分结
果。因此,他决定提取委托调用列表,以便手工分别调用它们:
public void DoWork()
{
//...
Console.WriteLine("Worker: work completed",',',');
     if( completed != null)
{
foreach( WorkCompleted wc in completed.GetInvocationList())
{
int grade = wc(,',',');
Console.WriteLine("Worker grade= " + grade,',',');
}
}
}
【译注:以下是本节描述之完整代码示例:
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)
        {
            foreach( WorkCompleted wc in completed.GetInvocationList())
            {
                int grade = wc(,',',');
                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,',',');
          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...
Worker grade = 4 【译注:boss打的4分也得到啦J】
Universe pleased with worker's work
Worker grade = 7
Main: worker completed work
*/

异步通知:触发和忽略
不料,boss和universe被别的什么事纠缠上了,这就意味着他们给peter打分的时间被延
迟了:
class Boss
{
public int WorkCompleted()
{
System.Threading.Thread.Sleep(3000,',',');
             Console.WriteLine("Better...",',',');
return 6; /* out of 10 */
}
}
class Universe
{
static int WorkerCompletedWork()
{
System.Threading.Thread.Sleep(4000,',',');
Console.WriteLine("Universe is pleased with worker's work",',',');
return 7;
    }
    //...
}
而不幸的是,由于peter是同时通知boss和universe并等待他们打分的,这些返回评分的
通知现在看来要占用他不少工作时间,因此,peter决定忽略评分并且异步触发事件:
public void DoWork()
{
//...
Console.WriteLine("Worker: work completed",',',');
         if( completed != null )
{
foreach( WorkCompleted wc in completed.GetInvocationList())
{
wc.BeginInvoke(null, null,',',');
}
}
}
【译注:下面给出本节例子完整代码:
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 )
        {
            foreach( WorkCompleted wc in completed.GetInvocationList())
            {
                wc.BeginInvoke(null, null,',',');
            }
        }
     }
     public event WorkStarted started ;
     public event WorkProgressing progressing;
     public event WorkCompleted completed;
}
class Boss
{
    public int WorkCompleted()
    {
        System.Threading.Thread.Sleep(3000,',',');
        Console.WriteLine("Better...",',',');
        return 6; /* out of 10 */
    }
}
class Universe
{
     static void WorkerStartedWork()
     {
          Console.WriteLine("Universe notices worker starting work",',',');
     }
    static int WorkerCompletedWork()
    {
        System.Threading.Thread.Sleep(4000,',',');
        Console.WriteLine("Universe is pleased with worker's work",',',');
        return 7;
    }
     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(,',',');
     }
}
/*
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
Main: worker completed work //【译注:由于是异步触发事件,因此这一行先输出啦
J】
Better... //【译注:评分已被忽略】
Universe pleased with worker's work //【译注:评分已被忽略】
*/

异步通知:轮询
     这就使得peter可以通知监听者的同时自己也能立即返回工作,让进程的线程池调
用委托。然而不久他就发现监听者对其工作的评分丢掉了。【译注:请参见上节例子代
码及译注】peter知道他做了一件明智的事并乐意universe作为一个整体(不单单是他的
boss)评判他。因此,peter异步触发事件,但定期轮询,以察看可以获得的评分:
public void DoWork()
{
//...
Console.WriteLine("Worker: work completed",',',');
if( completed != null )
{
foreach( WorkCompleted wc in completed.GetInvocationList() )
{
IAsyncResult res = wc.BeginInvoke(null, null,',',');
while( !res.IsCompleted ) System.Threading.Thread.Sleep(1,',',');
int grade = wc.EndInvoke(res,',',');
Console.WriteLine("Worker grade= " + grade,',',');
}
}
}
【译注:下面给出本节例子完整代码:
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 )
        {
            foreach( WorkCompleted wc in completed.GetInvocationList() )
            {
                IAsyncResult res = wc.BeginInvoke(null, null,',',');
                while( !res.IsCompleted ) System.Threading.Thread.Sleep(1,',',');
                int grade = wc.EndInvoke(res,',',');
                Console.WriteLine("Worker grade= " + grade,',',');
            }
        }
     }
     public event WorkStarted started ;
     public event WorkProgressing progressing;
     public event WorkCompleted completed;
}
class Boss
{
     public int WorkCompleted()
     {
          System.Threading.Thread.Sleep(3000,',',');
          Console.WriteLine("Better...",',',');
         return 6; /* out of 10 */
     }
}
class Universe
{
     static void WorkerStartedWork()
     {
          Console.WriteLine("Universe notices worker starting work",',',');
     }
     static int WorkerCompletedWork()
     {
          System.Threading.Thread.Sleep(4000,',',');
          Console.WriteLine("Universe is pleased with worker's work",',',');
         return 7;
     }
     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(,',',');
     }
}
/*
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...
Worker grade = 6
Universe pleased with worker's work
Worker grade = 7
Main: worker completed work //【译注:注意这个结果到最后才输出,下一节首句意
思即是如此】
*/

异步通知:委托
     不幸的是,peter又倒退了—就象他一开始想避免boss站在一旁边监视他一样。也
就是说,他现在要监看整个工作过程。【译注:请参见上节示例输出结果的注释】因此
,peter决定使用自己的委托作为异步委托完成时的通知方式,这样他就可以立即回去工
作,而当工作被打分时,仍然可以接到通知:
public void DoWork()
{
//...
Console.WriteLine("Worker: work completed",',',');
if( completed != null )
{
foreach( WorkCompleted wc in completed.GetInvocationList() )
{
wc.BeginInvoke(new AsyncCallback(WorkGraded), wc,',',');
}
}
}
private void WorkGraded(IAsyncResult res)
{
WorkCompleted wc = (WorkCompleted)res.AsyncState;
int grade = wc.EndInvoke(res,',',');
Console.WriteLine("Worker grade= " + grade,',',');
}
【译注:下面给出本节例子完整代码:
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 )
        {
            foreach( WorkCompleted wc in completed.GetInvocationList() )
            {
                wc.BeginInvoke(new AsyncCallback(WorkGraded), wc,',',');
            }
        }
     }
    private void WorkGraded(IAsyncResult res)
    {
        WorkCompleted wc = (WorkCompleted)res.AsyncState;
        int grade = wc.EndInvoke(res,',',');
        Console.WriteLine("Worker grade= " + grade,',',');
    }
     public event WorkStarted started ;
     public event WorkProgressing progressing;
     public event WorkCompleted completed;
}
class Boss
{
     public int WorkCompleted()
     {
          System.Threading.Thread.Sleep(3000,',',');
          Console.WriteLine("Better...",',',');
         return 6; /* out of 10 */
     }
}
class Universe
{
     static void WorkerStartedWork()
     {
          Console.WriteLine("Universe notices worker starting work",',',');
     }
     static int WorkerCompletedWork()
     {
          System.Threading.Thread.Sleep(4000,',',');
          Console.WriteLine("Universe is pleased with worker's work",',',');
         return 7;
     }
     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(,',',');
     }
}
/*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
Main: worker completed work //【译注:异步委托发生了效果,因此这一行先输出啦
J】
Better...
Worker grade = 6
Universe pleased with worker's work
Worker grade = 7
*/

同乐乐
     peter、boss和universe最终都满意了。boss和universe都可以仅被通知其感兴趣
的事件,并减少了实现上的负担和不必要的来回调用。peter可以通知他们每一个人,而
不必管需要多长时间才能从那些目标方法中返回,并仍然可以异步得到评分结果。pete
r知道做到这一点并不太容易,因为由于是异步触发事件,目标方法就有可能运行在另一
个线程里,就如上节示例一样。不过,peter[J]和mike[J]是好朋友,而mike精通线程问
题并可提供该领域的指导。
     从此,他们都很快乐J
-全文完- 

委托和事件— 一个虚构的故事 收藏此页到茶秀网摘
作者: 荣耀   www.ASPCool.com 时间:2004-11-16 下午 02:56:11  阅读次数:5570 

 

从前,在南方一块奇异的土地上,有个工人名叫彼得,他非常勤奋,对他的老板总是百依百顺。但是他的老板是个吝啬的人,从不信任别人,坚决要求随时知道彼得的工作进度,以防止他偷懒。但是彼得又不想让老板呆在他的办公室里站在背后盯着他,于是就对老板做出承诺:无论何时,只要我的工作取得了一点进展我都会及时让你知道。彼得通过周期性地使用“带类型的引用”(原文为:“typed reference” 也就是delegate??)“回调”他的老板来实现他的承诺,如下:

class Worker {     public void Advise(Boss boss) { _boss = boss; }     public void DoWork() {         Console.WriteLine(“工作: 工作开始”);         if( _boss != null ) _boss.WorkStarted();
        Console.WriteLine(“工作: 工作进行中”);         if( _boss != null ) _boss.WorkProgressing();
        Console.WriteLine("“工作: 工作完成”");         if( _boss != null ) {             int grade = _boss.WorkCompleted();             Console.WriteLine(“工人的工作得分=” + grade);         }     }     private Boss _boss; }
class Boss {     public void WorkStarted() { /* 老板不关心。 */ }     public void WorkProgressing() { /*老板不关心。 */ }     public int WorkCompleted() {         Console.WriteLine(“时间差不多!”);         return 2; /* 总分为10 */     } }
class Universe {     static void Main() {         Worker peter = new Worker();         Boss        boss = new Boss();         peter.Advise(boss);         peter.DoWork();
        Console.WriteLine(“Main: 工人工作完成”);         Console.ReadLine();     } }

接口

现在,彼得成了一个特殊的人,他不但能容忍吝啬的老板,而且和他周围的宇宙也有了密切的联系,以至于他认为宇宙对他的工作进度也感兴趣。不幸的是,他必须也给宇宙添加一个特殊的回调函数Advise来实现同时向他老板和宇宙报告工作进度。彼得想要把潜在的通知的列表和这些通知的实现方法分离开来,于是他决定把方法分离为一个接口:

interface IWorkerEvents {     void WorkStarted();     void WorkProgressing();     int WorkCompleted(); }
class Worker {     public void Advise(IWorkerEvents events) { _events = events; }     public void DoWork() {         Console.WriteLine(“工作: 工作开始”);         if( _events != null ) _events.WorkStarted();
        Console.WriteLine(“工作: 工作进行中”);         if(_events != null ) _events.WorkProgressing();
        Console.WriteLine("“工作: 工作完成”");         if(_events != null ) {             int grade = _events.WorkCompleted();
            Console.WriteLine(“工人的工作得分=” + grade);         }     }     private IWorkerEvents _events; }
class Boss : IWorkerEvents {     public void WorkStarted() { /* 老板不关心。 */ }     public void WorkProgressing() { /* 老板不关心。 */ }     public int WorkCompleted() {         Console.WriteLine(“时间差不多!”);         return 3; /* 总分为10 */     } }

委托

不幸的是,每当彼得忙于通过接口的实现和老板交流时,就没有机会及时通知宇宙了。至少他应该忽略身在远方的老板的引用,好让其他实现了IWorkerEvents的对象得到他的工作报告。(”At least he~d abstracted the reference of his boss far away from him so that others who implemented the IWorkerEvents interface could be notified of his work progress” 原话如此,不理解到底是什么意思:))

他的老板还是抱怨得很厉害。“彼得!”他老板吼道,“你为什么在工作一开始和工作进行中都来烦我?!我不关心这些事件。你不但强迫我实现了这些方法,而且还在浪费我宝贵的工作时间来处理你的事件,特别是当我外出的时候更是如此!你能不能不再来烦我?”

于是,彼得意识到接口虽然在很多情况都很有用,但是当用作事件时,“粒度”不够好。他希望能够仅在别人想要时才通知他们,于是他决定把接口的方法分离为单独的委托,每个委托都像一个小的接口方法:

delegate void WorkStarted(); delegate void WorkProgressing(); delegate int WorkCompleted();
class Worker {     public void DoWork() {         Console.WriteLine(“工作: 工作开始”);         if( started != null ) started();
        Console.WriteLine(“工作: 工作进行中”);         if( progressing != null ) progressing();
        Console.WriteLine("“工作: 工作完成”");         if( completed != null ) {             int grade = completed();             Console.WriteLine(“工人的工作得分=” + grade);         }     }     public WorkStarted started;     public WorkProgressing progressing;     public WorkCompleted completed; }
class Boss {     public int WorkCompleted() {         Console.WriteLine("Better...");         return 4; /* 总分为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: 工人工作完成”);         Console.ReadLine();     } }

静态监听者

这样,彼得不会再拿他老板不想要的事件来烦他老板了,但是他还没有把宇宙放到他的监听者列表中。因为宇宙是个包涵一切的实体,看来不适合使用实例方法的委托(想像一下,实例化一个“宇宙”要花费多少资源…..),于是彼得就需要能够对静态委托进行挂钩,委托对这一点支持得很好:

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);         peter.started = new WorkStarted(Universe.WorkerStartedWork);         peter.completed = new WorkCompleted(Universe.WorkerCompletedWork);         peter.DoWork();
        Console.WriteLine(“Main: 工人工作完成”);         Console.ReadLine();     } }

事件

不幸的是,宇宙太忙了,也不习惯时刻关注它里面的个体,它可以用自己的委托替换了彼得老板的委托。这是把彼得的Worker类的的委托字段做成public的一个无意识的副作用。同样,如果彼得的老板不耐烦了,也可以决定自己来激发彼得的委托(真是一个粗鲁的老板):

        // Peter~s boss taking matters into his own hands         if( peter.completed != null ) peter.completed(); 彼得不想让这些事发生,他意识到需要给每个委托提供“注册”和“反注册”功能,这样监听者就可以自己添加和移除委托,但同时又不能清空整个列表也不能随意激发彼得的事件了。彼得并没有来自己实现这些功能,相反,他使用了event关键字让C#编译器为他构建这些方法:

class Worker { ...     public event WorkStarted started;     public event WorkProgressing progressing;     public event WorkCompleted completed; }

彼得知道event关键字在委托的外边包装了一个property,仅让C#客户通过+= 和 -=操作符来添加和移除,强迫他的老板和宇宙正确地使用事件。

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: 工人工作完成”);         Console.ReadLine();     }

“收获”所有结果

到这时,彼得终于可以送一口气了,他成功地满足了所有监听者的需求,同时避免了与特定实现的紧耦合。但是他注意到他的老板和宇宙都为它的工作打了分,但是他仅仅接收了一个分数。面对多个监听者,他想要“收获”所有的结果,于是他深入到代理里面,轮询监听者列表,手工一个个调用:

public void DoWork() {         ...         Console.WriteLine("“工作: 工作完成”");         if( completed != null ) {             foreach( WorkCompleted wc in completed.GetInvocationList() ) {                 int grade = wc();                 Console.WriteLine(“工人的工作得分=” + grade);             }         }     }

异步通知:激发 & 忘掉

同时,他的老板和宇宙还要忙于处理其他事情,也就是说他们给彼得打分所花费的事件变得非常长:

class Boss {     public int WorkCompleted() {         System.Threading.Thread.Sleep(3000);         Console.WriteLine("Better..."); return 6; /* 总分为10 */     } }
class Universe {     static int WorkerCompletedWork() {         System.Threading.Thread.Sleep(4000);         Console.WriteLine("Universe is pleased with worker~s work");         return 7;     }     ... } 很不幸,彼得每次通知一个监听者后必须等待它给自己打分,现在这些通知花费了他太多的工作事件。于是他决定忘掉分数,仅仅异步激发事件:

public void DoWork() {         ...         Console.WriteLine("“工作: 工作完成”");         if( completed != null ) {             foreach( WorkCompleted wc in completed.GetInvocationList() )             {                 wc.BeginInvoke(null, null);             }         }     }

异步通知:轮询

这使得彼得可以通知他的监听者,然后立即返回工作,让进程的线程池来调用这些代理。随着时间的过去,彼得发现他丢失了他工作的反馈,他知道听取别人的赞扬和努力工作一样重要,于是他异步激发事件,但是周期性地轮询,取得可用的分数。

public void DoWork() {         ...         Console.WriteLine("“工作: 工作完成”");         if( completed != null ) {             foreach( WorkCompleted wc in completed.GetInvocationList() ) {                 IAsyncResult res = wc.BeginInvoke(null, null);                 while( !res.IsCompleted ) System.Threading.Thread.Sleep(1);                 int grade = wc.EndInvoke(res);                 Console.WriteLine(“工人的工作得分=” + grade);             }         }     }

异步通知:委托

不幸地,彼得有回到了一开始就想避免的情况中来,比如,老板站在背后盯着他工作。于是,他决定使用自己的委托作为他调用的异步委托完成的通知,让他自己立即回到工作,但是仍可以在别人给他的工作打分后得到通知:

public void DoWork() {         ...         Console.WriteLine("“工作: 工作完成”");         if( completed != null ) {             foreach( WorkCompleted wc in completed.GetInvocationList() ) {                 wc.BeginInvoke(new AsyncCallback(WorkGraded), wc);             }         }     }
    private void WorkGraded(IAsyncResult res) {         WorkCompleted wc = (WorkCompleted)res.AsyncState;         int grade = wc.EndInvoke(res);         Console.WriteLine(“工人的工作得分=” + grade);     }

宇宙中的幸福

彼得、他的老板和宇宙最终都满足了。彼得的老板和宇宙可以收到他们感兴趣的事件通知,减少了实现的负担和非必需的往返“差旅费”。彼得可以通知他们,而不管他们要花多长时间来从目的方法中返回,同时又可以异步地得到他的结果。彼得知道,这并不*十分*简单,因为当他异步激发事件时,方法要在另外一个线程中执行,彼得的目的方法完成的通知也是一样的道理。但是,迈克和彼得是好朋友,他很熟悉线程的事情,可以在这个领域提供指导。

 

他们永远幸福地生活下去……<完>

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
*/
】 

 
  
   
 
     
  本文摘自人民邮电出版社出版的《Windows Forms程序设计》(Chris Sells著,荣耀、蒋贤哲译)。通过一个栩栩如生的虚构故事解释了C#/.NET中委托和事件的机制和应用。
  
  1 委托
  
  从前,在南方的一个异国他乡,有一个叫Peter的勤劳的工人,他对老板(boss)百依百顺,然而他的boss却是个卑鄙多疑的家伙,他坚持要求Peter不断汇报工作进展。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通过如下所示的类型化的引用(typed reference)定期回调boss来实现这个承诺:
  
  class Worker {
   public void Advise(Boss boss) {this.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);
   }
   }
   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; /* 10分以内 */
   }
  }
  
  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();
   }
  }
  
  1.1 接口
  
  现在,Peter成了一个特殊人物,他不但能够忍受卑鄙的boss,和周围的世界(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,如果不为universe添加一个特殊的Advise方法和特殊的回调,除了保证boss能够被通知外,Peter并不能向universe通知工作进度。Peter希望能从那些通知方法的实现中分离出潜在的通知列表,为此,他决定将方法分离到一个接口中:
  
  interface IWorkerEvents {
   void WorkStarted();
   void WorkProgressing();
   int WorkCompleted();
  }
  
  class Worker {
   public void Advise(IWorkerEvents events) {this.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);
   }
   }
   IWorkerEvents events;
  }
  
  class Boss : IWorkerEvents {
   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; /* 10分以内 */
   }
  }
  
  1.2 委托
  
  不幸的是,由于Peter忙于说服boss实现这个接口,以至于没有顾得上通知universe也实现该接口,但他希望尽可能做到这一点,至少他已经抽象了对boss的引用,因此,别的实现了IWorkerEvents接口的什么人也可以得到工作进度通知。
  
  然而, Peter的boss仍然极其不满。“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。当我的实现需要占用很长时间时,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?”
  
  因此,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。他希望能做到仅仅通知监听者真正感兴趣的事件。为此,Peter决定把接口中的方法分解为若干个独立的委托函数,每一个都好象是只包含一个方法的微型接口:
  
  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;
   public WorkProgressing progressing;
   public WorkCompleted completed;
  }
  
  class Boss {
   public int WorkCompleted() {
   Console.WriteLine("Better...");
   return 4; /* 10分以内 */
   }
  }
  
  class Universe {
   static void Main() {
   Worker peter = new Worker();
   Boss boss = new Boss();
  
   // 注意:我们已将Advise方法替换为赋值运算符
   peter.completed = new WorkCompleted(boss.WorkCompleted);
   peter.DoWork();
   Console.WriteLine("Main: worker completed work");
   Console.ReadLine();
   }
  }
  
  1.3 静态订阅者
  
  利用委托,Peter达到了不拿boss不关心的事件去烦他的目标,然而Peter还是不能够使universe成为其订阅者之一。因为universe是一个全封闭的实体,所以将委托挂钩在实例成员上不妥的(设想一下Universe的多个实例需要多少资源)。相反,Peter需要将委托挂钩到静态成员上,因为委托也完全支持静态成员:
  
  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();
  
   // 注意:在下面的三行代码中,
   // 使用赋值运算符不是一个好习惯,
   // 请接着读下去,以便了解添加委托的正确方式。
   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();
   }
  }
  
  2 事件
  
  不幸的是,由于universe现在变得太忙并且不习惯于注意某一个人,universe已经设法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而造成的意外的副作用。同样,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自己控制一切
  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,这样订阅者就可以添加或移去它们自个儿,但谁都不能够清空整个事件列表或者触发它的事件。peter自己没去实现这些方法,相反,他使用event关键字让C#编译器帮他构建这些方法:
  
  class Worker {
  ...
   public event WorkStarted started;
   public event WorkProgressing progressing;
   public event WorkCompleted completed;
  }
  
  Peter晓得event关键字使委托具有这样的属性: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();
  }
  
  2.1 获取所有结果
  
  至此,Peter终于松了一口气。他已经设法满足了所有订阅者的需求,而且不会和特定实现紧密耦合。然而,他又注意到尽管boss和universe都为他的工作打了分,但他只得到了一个打分。在有多个订阅者的情形下,Peter希望能得到所有订阅者的评分结果。因此,他决定“进入委托”,提取订阅者列表,以便手工分别调用它们:
  
  public void DoWork() {
   ...
   Console.WriteLine("Worker: work completed");
   if( completed != null ) {
   foreach( WorkCompleted wc in completed.GetInvocationList() ) {
   int grade = wc();
   Console.WriteLine("Worker grade= " + grade);
   }
   }
  }
  
  2.2 异步通知:触发和忽略
  
  不料,在此期间,boss和universe被别的什么事纠缠上了,这就意味着他们给Peter的工作打分的时间被大大延长了:
  
  class Boss {
   public int WorkCompleted() {
   System.Threading.Thread.Sleep(3000);
   Console.WriteLine("Better..."); return 6; /* 10分以内 */
   }
  }
  
  class Universe {
   static int WorkerCompletedWork() {
   System.Threading.Thread.Sleep(4000);
   Console.WriteLine("Universe is pleased with worker's work");
   return 7;
   }
   ...
  }
  
  不幸的是,由于Peter是同时通知每一个订阅者并等待他们打分的,这些需要返回评分的通知现在看来要占用他不少工作时间,因此,Peter决定忽略评分并且异步触发事件:
  
  public void DoWork() {
   ...
   Console.WriteLine("Worker: work completed");
   if( completed != null ) {
   foreach( WorkCompleted wc in completed.GetInvocationList() ) {
   wc.BeginInvoke(null, null);
   }
   }
  }
  
  2.3 异步通知:轮询
  
  这个聪明的小把戏允许Peter在通知订阅者的同时能立即返回工作,让进程的线程池调用委托。然而没过多久Peter就发现订阅者给他的打分被搞丢了。他知道自己工作做得不错,并乐意universe作为一个整体(而不仅仅是他的boss)表扬他。因此,Peter异步触发事件,但定期轮询,以便察看可以获得的评分:
  
  public void DoWork() {
   ...
   Console.WriteLine("Worker: work completed");
   if( completed != null ) {
   foreach( WorkCompleted wc in completed.GetInvocationList() ) {
   IAsyncResult res = wc.BeginInvoke(null, null);
   while( !res.IsCompleted ) System.Threading.Thread.Sleep(1);
   int grade = wc.EndInvoke(res);
   Console.WriteLine("Worker grade= " + grade);
   }
   }
  }
  
  2.4 异步通知:委托
  
  不幸的是,Peter又回到了问题的起点,就像他一开始希望避免boss站在一旁边监视他工作一样。因此,Peter决定使用另一个委托作为异步工作完成时的通知方式,这样他就可以立即回去工作,而当工作被打分时,仍然可以接到通知:
  
  public void DoWork() {
   ...
   Console.WriteLine("Worker: work completed");
   if( completed != null ) {
   foreach( WorkCompleted wc in completed.GetInvocationList() ) {
   wc.BeginInvoke(new AsyncCallback(WorkGraded), wc);
   }
   }
  }
  
  void WorkGraded(IAsyncResult res) {
   WorkCompleted wc = (WorkCompleted)res.AsyncState;
   int grade = wc.EndInvoke(res);
   Console.WriteLine("Worker grade= " + grade);
  }
  
  3 普天同乐
  
  Peter、boss和universe最终都满意了。boss和universe都可以仅被通知其感兴趣的事件,并减少了实现的负担和不必要的来回调用。Peter可以通知他们每一个人,而不必管需要多长时间才能从那些目标方法中返回,并仍然可以异步得到评分结果。结果得到如下完整的解决方案:
  
  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 ) {
  
   foreach( WorkCompleted wc in completed.GetInvocationList() ) {
   wc.BeginInvoke(new AsyncCallback(WorkGraded), wc);
   }
   }
   }
  
   void WorkGraded(IAsyncResult res) {
   WorkCompleted wc = (WorkCompleted)res.AsyncState;
   int grade = wc.EndInvoke(res);
   Console.WriteLine("Worker grade= " + grade);
   }
  
   public event WorkStarted started;
   public event WorkProgressing progressing;
   public event WorkCompleted completed;
  }
  
  class Boss {
   public int WorkCompleted() {
   System.Threading.Thread.Sleep(3000);
   Console.WriteLine("Better..."); return 6; /* 10分以内 */
   }
  }
  
  class Universe {
   static void WorkerStartedWork() {
   Console.WriteLine("Universe notices worker starting work");
   }
  
   static int WorkerCompletedWork() {
   System.Threading.Thread.Sleep(4000);
   Console.WriteLine("Universe is pleased with worker's work");
   return 7;
   }
  
   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();
   }
  }
  
  Peter知道异步获取结果会带来一些问题。由于异步触发事件,所以目标方法有可能执行于另一个线程中,就像Peter的“目标方法何时完成”的通知那样。然而,Peter熟悉第14章“多线程用户界面”,因此,他知道在构建WinForms应用程序时如何去处理此类问题。
  
  从此,他们一直过得都很快乐。  

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