Observer模式

Observer模式是开发中，使用比较多的模式之一。Observer模式叫做观察者模式，它定义了一种一对多的依赖关系，让多个观察者同时监听某一对象状态，当这个对象状态发生变化时，这些观察者都得到通知。

下图即为Observer模式的类图：

实现如下：
//Subject.h

//Subject.h#ifndef _SUBJECT_H_#define _SUBJECT_H_#include <string>#include <boost/ptr_container/ptr_container.hpp>typedef std::string State;class Observer;class Subject{public:    virtual ~Subject();    virtual void Attach(Observer* obv);    virtual void Detach(Observer* obv);    virtual void Notify();    virtual void SetState(const State& st) = 0;    virtual State GetState() = 0;protected:    Subject();private:    boost::ptr_vector<Observer> obvs_;};class ConcreteSubject : public Subject{public:    ConcreteSubject();    ~ConcreteSubject();    State GetState();    void SetState(const State& st);private:    State st_;};#endif

//Subject.cpp

//Subject.cpp#include "Subject.h"#include "Observer.h"#include <iostream>Subject::Subject(){}Subject::~Subject(){}void Subject::Attach(Observer* obv){    obvs_.push_back(obv);}void Subject::Detach(Observer* obv){    for(boost::ptr_vector<Observer>::iterator iter=obvs_.begin(); iter!=obvs_.end(); ++iter)    {        if(obv==&(*iter))        {            obvs_.erase(iter);//dead loop????            break;        }    }    return ;}void Subject::Notify(){    for(boost::ptr_vector<Observer>::iterator iter=obvs_.begin(); iter!=obvs_.end(); ++iter)    {        iter->Update(this);    }}ConcreteSubject::ConcreteSubject(){    st_='\0';}ConcreteSubject::~ConcreteSubject(){}void ConcreteSubject::SetState(const State& st){    st_=st;}State ConcreteSubject::GetState(){    return st_;}

//Observer.h

#ifndef _OBSERVER_H_#define _OBSERVER_H_#include "Subject.h"#include <string>typedef std::string State;class Observer{public:    virtual ~Observer();    virtual void Update(Subject* sub)=0;    virtual void PrintInfo()=0;protected:    Observer();    State st_;};class ConcreteObserverA:public Observer{public:    virtual ~ConcreteObserverA();    ConcreteObserverA(Subject* sub);    virtual Subject* GetSubject();    void Update(Subject* sub);    void PrintInfo();private:    Subject* sub_;};class ConcreteObserverB:public Observer{public:    virtual ~ConcreteObserverB();    ConcreteObserverB(Subject* sub);    virtual Subject* GetSubject();    void Update(Subject* sub);    void PrintInfo();private:    Subject* sub_;};#endif //_OBSERVER_H_

//Observer.cpp

//Observer.cpp#include "Observer.h"#include "Subject.h"#include <iostream>#include <string>Observer::Observer(){    st_='\0';}Observer::~Observer(){}ConcreteObserverA::ConcreteObserverA(Subject* sub){    sub_=sub;    sub_->Attach(this);}ConcreteObserverA::~ConcreteObserverA(){    sub_->Detach(this);    //if(sub_!=0)        //delete sub_;}Subject* ConcreteObserverA::GetSubject(){    return sub_;}void ConcreteObserverA::PrintInfo(){    std::cout<<"ConcreteObserverA observer "<<st_<<std::endl;}void ConcreteObserverA::Update(Subject* sub){    st_=sub->GetState();    PrintInfo();}ConcreteObserverB::ConcreteObserverB(Subject* sub){    sub_=sub;    sub_->Attach(this);}ConcreteObserverB::~ConcreteObserverB(){    sub_->Detach(this);    //if(sub_!=0)        //delete sub_;}Subject* ConcreteObserverB::GetSubject(){    return sub_;}void  ConcreteObserverB::PrintInfo(){    std::cout<<"ConcreteObserverB observer "<<st_<<std::endl;}void ConcreteObserverB::Update(Subject* sub){    st_=sub->GetState();    PrintInfo();}

//main.cpp

#include "Subject.h"#include "Observer.h"int main(){    ConcreteSubject* sub=new ConcreteSubject();    Observer* o1=new ConcreteObserverA(sub);    Observer* o2=new ConcreteObserverB(sub);    sub->SetState("old");    sub->Notify();    sub->Detach(o1);    sub->SetState("new");    sub->Notify();    return 0;}