设计模式之四-观察者模式

一 意图

它定义了一种一对多的依赖关系，当目标的状态发生改变时，所有的观察者都会得到通知而自动更新，不用实时的监控目标。

二 别名

observer，依赖（depends），发布-订阅（Publish-Subsciribe）

三 适用性

1 当一个抽象模型有两个方面，其中一个方面依赖于另一个方面。将这二者封装在独立的对象中，以使得它们可以各自相互独立的改变和复用。

2 当对一个对象的改变需要同时改变其他对象，而不知道具体有多少对象需要被改变时。

3 当一个对象必须通知其它对象，而它又不能假定其它对象是谁。换言之，你不希望这些对象是紧密耦合的。

四 结构

   

五 实现

.h

#ifndef __CLASS__H#define __CLASS__H#include <string>#include <list>using std::list;using std::string;class CSubject;class CObserver{public:virtual void Update(CSubject* subject)=0;virtual ~CObserver(){};};class CSubject{public:void Attach(CObserver* observer);void Detach(CObserver* observer);void Modify();virtual ~CSubject(){};virtual string GetState()=0;virtual void SetState(const string&  state)=0;private:list<CObserver* > m_observerList;};class CConcreteSubject:public CSubject{public:string GetState();void   SetState(const string&  state);private:string m_state;};class CConcreteObserverA:public CObserver{public:void Update(CSubject* subject);};class CConcreteObserverB:public CObserver{;public:void Update(CSubject* subject);};class CConcreteObserverC:public CObserver{public:void Update(CSubject* subject);};#endif

 

.cpp

 

#include "class.h"#include <iostream>using  namespace std;void CSubject::Attach(CObserver* observer){m_observerList.push_back(observer);}void CSubject::Detach(CObserver* observer){if(! m_observerList.empty() ){m_observerList.remove(observer);}}void CSubject::Modify(){list<CObserver *>::iterator ite=m_observerList.begin();for(;ite!=m_observerList.end();ite++){(*ite)->Update(this);}}string CConcreteSubject::GetState(){return m_state;}void CConcreteSubject::SetState(const string& state){m_state=state;}void CConcreteObserverA::Update(CSubject* subject){string state=subject->GetState();if(state == "yes"){cout<<"CConcreteObserverA is ready!\n";}}void CConcreteObserverB::Update(CSubject* subject){string state=subject->GetState();if(state == "yes"){cout<<"CConcreteObserverB is ready!\n";}}void CConcreteObserverC::Update(CSubject* subject){string state=subject->GetState();if(state == "yes"){cout<<"CConcreteObserverC is ready!\n";}}

 

client:

#include "class.h"void main(){CSubject* pSubject=new CConcreteSubject();CObserver* pA= new CConcreteObserverA();CObserver* pB=new CConcreteObserverB();CObserver* pC=new CConcreteObserverC();pSubject->Attach(pA);pSubject->Attach(pB);pSubject->Attach(pC);pSubject->Detach(pB);pSubject->SetState("yes");pSubject->Modify();delete pSubject;delete pA;delete pB;delete pC;}