观察者模式

常用的设计模式之一

原理

被观察者保留了观察者实例的地址与函数名，当有事件触发或主动调用Notify()时，通过实例的this指针类型与函数名就可以回调观察者函数

实现

Observer.h:先定义观察者，因为观察者会被被观察者拿去用

#include <iostream>
#include <list>
using namespace std;
 
class Observer
{
public:
     virtual void Update(int) = 0;
};

class ConcreteObserver : public Observer    
{
public:
     void Update(int value){
          cout<<"ConcreteObserver get the update. New State:"<<value<<endl;
     }
};

class ConcreteObserver2 : public Observer
{
public:
     void Update(int value){
          cout<<"ConcreteObserver2 get the update. New State:"<<value<<endl;
     }
};

Subject.h: 定义一个观察者列表，有需要对被观察者做出响应的实例追加到该列表

#include<iostream>
#include"Observer.h"

class Subject
{
public:
     virtual void Attach(Observer *) = 0;
     virtual void Detach(Observer *) = 0;
     virtual void Notify() = 0;
};

class ConcreteSubject : public Subject
{
public:
     void Attach(Observer *pObserver){
        m_ObserverList.push_back(pObserver);
     }
     void Detach(Observer *pObserver){
        m_ObserverList.remove(pObserver);
     }
     void Notify(){
        std::list<Observer *>::iterator it = m_ObserverList.begin();
        while (it != m_ObserverList.end())
        {
            (*it)->Update(m_iState);
            ++it;
        }
     }
 
     void SetState(int state)
     {
          m_iState = state;
     }
 
private:
     std::list<Observer *> m_ObserverList;
     int m_iState;
};

测试：

#include <iostream>
#include <list>
#include"Subject.h"
using namespace std;
 
int main()
{
     // Create Subject
     ConcreteSubject *pSubject = new ConcreteSubject();
     // Create Observer
     Observer *pObserver = new ConcreteObserver();
     Observer *pObserver2 = new ConcreteObserver2();
 
     // Change the state
     pSubject->SetState(2);
     // Register the observer
     pSubject->Attach(pObserver);
     pSubject->Attach(pObserver2);
     pSubject->Notify();
 
     // Unregister the observer
     pSubject->Detach(pObserver);
 
     pSubject->SetState(3);
     pSubject->Notify();
 
     delete pObserver;
     delete pObserver2;
     delete pSubject;
}