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);
}
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