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<vector>
#include<string>
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<Observer*> 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<Observer*>::iterator it = m_pObs.begin();
    for ( ;it!=m_pObs.end();++it)
    {
        if (*it == pObs)
        {
            m_pObs.erase(it);
            return;
        }
    }
}

void Subject::Notify()
{
    vector<Observer*>::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<iostream>
#include<string>
#include<vector>
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A
class 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B
class 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<<m_name<<" update state :"<<m_observerState<<endl;
}

void ConcreteObserverA::update(Subject* sub)
{
    m_observerState = sub->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