C++设计模式之Observer(观察者)模式

作用：定义对象间的一种一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并被自动更新。比如，QQ中你的好友上线下，他们的头像亮起，熄灭。

 

以下是参考代码

  
    

   
     
   
     #include 
   
     <
   
     Windows.h
   
     >
   
     
#include 
   
     <
   
     list
   
     >
   
     

   
     using
   
      
   
     namespace
   
      std;


   
     class
   
      Subject;


   
     class
   
      Observer {

   
     public
   
     :
 
   
     virtual
   
      
   
     ~
   
     Observer();
 
   
     virtual
   
      
   
     void
   
      Update(Subject
   
     *
   
      theChangedSubject) 
   
     =
   
      
   
     0
   
     ;

   
     protected
   
     :
 Observer();
};


   
     class
   
      Subject {

   
     public
   
     :
 
   
     virtual
   
      
   
     ~
   
     Subject();
 
   
     virtual
   
      
   
     void
   
      Attach(Observer
   
     *
   
     );
 
   
     virtual
   
      
   
     void
   
      Detach(Observer
   
     *
   
     );
 
   
     virtual
   
      
   
     void
   
      Notify();

   
     protected
   
     :
 Subject();

   
     private
   
     :
 std::list
   
     <
   
     Observer
   
     *>
   
      
   
     *
   
     _observers;
};


   
     void
   
      Subject::Attach(Observer
   
     *
   
      obj)
{
 _observers
   
     ->
   
     push_back(obj);
}


   
     void
   
      Subject::Detach(Observer
   
     *
   
      obj)
{
 typedef std::list
   
     <
   
     Observer
   
     *>
   
     ::iterator LI;
 
   
     for
   
      (LI iter
   
     =
   
     _observers
   
     ->
   
     begin();
 iter
   
     !=
   
     _observers
   
     ->
   
     end();
 
   
     ++
   
     iter)
 {
 
   
     if
   
      (
   
     *
   
     iter 
   
     ==
   
      obj)
 {
 _observers
   
     ->
   
     erase(iter);
 
   
     break
   
     ;
 }
 }
}


   
     void
   
      Subject::Notify()
{
 typedef std::list
   
     <
   
     Observer
   
     *>
   
     ::iterator LI;
 std::list
   
     <
   
     Observer
   
     *>
   
      
   
     *
   
     i(_observers);
 
   
     for
   
      (LI it
   
     =
   
     i
   
     ->
   
     begin(); it
   
     !=
   
     i
   
     ->
   
     end(); 
   
     ++
   
     it)
 {
 (
   
     *
   
     it)
   
     ->
   
     Update(
   
     this
   
     );
 }
}


   
     //
   
     ClockTimer是一个用于存储和维护一天时间的具体目标。它每秒通知一次它的观察者。
   
     

   
     class
   
      ClockTimer : 
   
     public
   
      Subject {

   
     public
   
     :
 ClockTimer();
 
   
     virtual
   
      
   
     int
   
      GetHour();
 
   
     virtual
   
      
   
     int
   
      GetMinute();
 
   
     virtual
   
      
   
     int
   
      GetSecond();

 
   
     void
   
      Tick();
};


   
     void
   
      ClockTimer::Tick()
{
 Sleep(
   
     1000
   
     );
 Notify();
}



   
     class
   
      Widget;

   
     //
   
     DigitalClock类来显示时间。Widget类继承了它的图形功能
   
     

   
     class
   
      DigitalClock : 
   
     public
   
      Widget, 
   
     public
   
      Observer {

   
     public
   
     :
 DigitalClock(ClockTimer
   
     *
   
     );
 
   
     virtual
   
      
   
     ~
   
     DigitalClock();

 
   
     virtual
   
      
   
     void
   
      Update(Subject
   
     *
   
     ); 
   
     //
   
     overrides Observer operation
   
     

   
     
 
   
     virtual
   
      
   
     void
   
      Draw(); 
   
     //
   
     overrides Widget operation
   
     

   
     private
   
     :
 ClockTimer
   
     *
   
      _subject;
};

DigitalClock::DigitalClock(ClockTimer
   
     *
   
      time)
{
 _subject 
   
     =
   
      time;
 _subject
   
     ->
   
     Attach(
   
     this
   
     );
}

DigitalClock::
   
     ~
   
     DigitalClock()
{
 _subject
   
     ->
   
     Detach(
   
     this
   
     );
}


   
     void
   
      DigitalClock::Update(Subject
   
     *
   
      theChangeSubject)
{
 
   
     if
   
      (theChangeSubject 
   
     ==
   
      _subject)
 {
 Draw();
 }
}


   
     void
   
      DigitalClock::Draw() 
{
 
   
     int
   
      hour 
   
     =
   
      _subject
   
     ->
   
     GetHour();
 
   
     int
   
      minute 
   
     =
   
      _subject
   
     ->
   
     GetMinute();
 
   
     int
   
      second 
   
     =
   
      _subject
   
     ->
   
     GetSecond();

 
   
     //
   
     draw the digital clock;
   
     

   
     }




   
     int
   
      main()
{
 ClockTimer
   
     *
   
      timer 
   
     =
   
      
   
     new
   
      ClockTimer;
 DigitalClock
   
     *
   
      digitalClock 
   
     =
   
      
   
     new
   
      DigitalClock(timer);
}