观察者模式（Observer Pattern） —— 定义对象间的一种一对多的依赖关系，以便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自动刷新。

观察者模式可以理解为发布-订阅模式，即多个订阅者（观察者）向发布者（被观察者）订阅状态信息，当发布者更新状态时会将状态信息向它的订阅者发布信息。发布者需要自己维护订阅者列表，可以注册或者注销对状态信息感兴趣或不感兴趣的订阅者。

// subject.h#include
    
     #include
     
      using namespace std;typedef int State;class Observer;// 被观察者(发布者)抽象class Subject {public:    Subject(){}    virtual ~Subject(){}    virtual void Attach(Observer* obv); // 注册观察者    virtual void Detach(Observer* obv); // 注销观察者    virtual void Notify();  // 通知观察者    virtual void SetState(const State& st) = 0;    virtual State GetState() = 0;private:    //观察者列表    vector
      
        m_pObs;};// 被观察者（实际发布者）class ConcreteSubject: public Subject{public:    ConcreteSubject(){}    ~ConcreteSubject(){}    State GetState();    void SetState(const State& st);private:    State m_state;};
      
     
    

// subject.cpp#include "subject.h"#include "observer.h"void Subject::Attach(Observer* pObs){    m_pObs.push_back(pObs);}void Subject::Detach(Observer* pObs){    vector
    
     ::iterator it = m_pObs.begin();    for ( ;it!=m_pObs.end();++it)    {        if (*it == pObs)        {            m_pObs.erase(it);            return;        }    }}void Subject::Notify(){    vector
     
      ::iterator it = m_pObs.begin();    for ( ; it!=m_pObs.end(); ++it)    {         (*it)->update(this);    }}void ConcreteSubject::SetState(const State& st){     m_state = st;}State ConcreteSubject::GetState(){     return m_state;}
     
    

// observer.h#include
    
     #include
     
      #include
      
       using namespace std;typedef int State;class Subject;// 观察者抽象class Observer {public:    Observer(){}    virtual ~Observer(){}    virtual void update(Subject* sub) = 0;    virtual void outputState();protected:    string m_name;    State m_observerState;};// 实际观察者Aclass ConcreteObserverA: public Observer {public:    ConcreteObserverA(string name, State init_state){        m_name = name;        m_observerState = init_state;    }    ~ConcreteObserverA(){}    void update(Subject* sub);private://    Subject* m_sub;};// 实际观察者Bclass ConcreteObserverB: public Observer {public:    ConcreteObserverB(string name, State init_state){        m_name = name;        m_observerState = init_state;    }    ~ConcreteObserverB(){}    void update(Subject* sub);private://    Subject* m_sub;};
      
     
    

// observer.cpp#include "observer.h"#include "subject.h"void Observer::outputState(){    cout<
    
     <<" update state :"<
     
      <
      
       GetState();    outputState();}void ConcreteObserverB::update(Subject* sub){    m_observerState = sub->GetState();    outputState();}
      
     
    

// main.cpp#include "observer.h"#include "subject.h"int main(){    ConcreteSubject* subject = new ConcreteSubject();    Observer* obA = new ConcreteObserverA("observerA", -1);    Observer* obB = new ConcreteObserverB("observerB", -1);    subject->Attach(obA);    subject->Attach(obB);    subject->SetState(0);    subject->Notify();    subject->Detach(obA);    subject->SetState(1);    subject->Notify();    delete obA;    delete obB;    delete subject;    return 0;}// 输出结果：//observerA update state :0//observerB update state :0//observerB update state :1