设计模式：观察者模式

观察着人模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象，这个主题对象在状态发生变化时，会通知所有观察者对象，使它们能够自动更新自己。

UML类图如下：

 

一个例子：上课时，有些人在打牌，但是又怕老师看见，所以他们就叫一个人看着老师，只有老师一过来就告诉他们。

具体代码如下：

 //Subject基类

#ifndef OBSERVER_H#define OBSERVER_H#include <string>class Subject;using std::string;class Observer{protected:Observer(string na,Subject* sub=0):name(na),subject(sub){}virtual ~Observer(){}public:virtual void Update()=0;protected:string name;Subject* subject;};#endif

 

具体Subject类

#include "Subject.h"#include <vector>#include <algorithm>using std::vector;class TeacherSubject:public Subject{public:TeacherSubject(){}~TeacherSubject(){}public:virtual void Attach(Observer* observer){vObserver.push_back (observer);};virtual void Detach(Observer* observer);virtual void Notify();virtual string GetSubjectState()const{return action;}virtual void SetSubjectState(string strState){action=strState;};private:string action; vector<Observer*> vObserver;};void TeacherSubject::Detach(Observer* observer){vector<Observer*>::iterator iter;iter=find(vObserver.begin (),vObserver.end (),observer);if(iter!=vObserver.end ())vObserver.erase (iter);}void TeacherSubject::Notify(){vector<Observer*>::iterator iter=vObserver.begin ();while(iter!=vObserver.end()){(*iter)->Update ();++iter;}}还有一个StudentSubject类于此类类似

 

Observer基类：

#ifndef OBSERVER_H#define OBSERVER_H#include <string>class Subject;using std::string;class Observer{protected:Observer(string na,Subject* sub=0):name(na),subject(sub){}virtual ~Observer(){}public:virtual void Update()=0;protected:string name;Subject* subject;//为了获得subject的状态};#endif

 

//具体Observer类
class StudentObserver:public Observer{public:StudentObserver(string na,Subject* sub=0):Observer(na,sub){}~StudentObserver(){}public:virtual void Update(){cout<<subject->GetSubjectState ()<<endl;cout<<name<<" Stop Playing Cards,Continue Listening to Teacher/n";}};

 

//客户端

#include "StudentSubject.h"#include "TeacherSubject.h"#include "StudentObserver.h"int main(){Subject* subject1=new StudentSubject(); StudentObserver StudentA("huangbing",subject1);StudentObserver StudentB("haozi",subject1);subject1->Attach (&StudentA);subject1->Attach (&StudentB);subject1->SetSubjectState ("Teacher is coming");subject1->Notify ();delete subject1;return 0;}结果：Teacher is cominghuangbing Stop Playing Cards,Continue Listening to TeacherTeacher is cominghaozi Stop Playing Cards,Continue Listening to Teacher