十、觀察者模式

觀察者模式（Observer）定義了一種一對多的依賴關係，讓多個觀察者對象同時監聽某一個主體對象。這個主題對象在狀態發生變化時，會通知所有觀察着對象，使他們能夠自動更新自己。

//Subject.h
#ifndef _SUBJECT_H_
#define _SUBJECT_H_
#include <list>
#include <string>
#include "Observer.h"
class Subject
{
public:
	Subject();
	virtual ~Subject();
	virtual void Attach(Observer* observer);
	virtual void Detach(Observer* observer);
	virtual void Notify();
	virtual void SetState(const string& st)=0;
	virtual string GetState()=0;
private:
	list<Observer*>* _obvs;
};
class ConcreteSubject:public Subject
{
public:
	ConcreteSubject();
	void SetState(const string& st);
	string GetState();
private:
	string _st;
};
#endif

//Subject.cpp
#include "Subject.h"
#include <iostream>
using namespace std;
Subject::Subject()
{
	_obvs = new list<Observer*>;
}
Subject::~Subject()
{

}
void Subject::Attach(Observer* observer)
{
	_obvs->push_back(observer);
}
void Subject::Detach(Observer* observer)
{
	if (!observer)
	{
		_obvs->remove(observer);
	}
}
void Subject::Notify()
{
	list<Observer*>::iterator it;
	for (it=_obvs->begin();it!=_obvs->end();it++)
	{
		(*it)->Update();
	}
}

ConcreteSubject::ConcreteSubject()
{
	_st='\0';
}
void ConcreteSubject::SetState(const string& st)
{
	_st = st;
}
string ConcreteSubject::GetState()
{
	return _st;
}

//Observer.h
#ifndef _OBSERVER_H_
#define _OBSERVER_H_
#include <string>
using namespace std;
class Subject;
class Observer
{
public:
	Observer();
	virtual ~Observer();
	virtual void Update()=0;
protected:
	string _st;
};
class ConcreteObserver1:public Observer
{
public:
	ConcreteObserver1(Subject* sub,string name);
	~ConcreteObserver1();
	Subject* GetSubject();
	void Update();
protected:
	Subject* _sub;
	string _name;
};
class ConcreteObserver2:public Observer
{
public:
	ConcreteObserver2(Subject* sub,string name);
	~ConcreteObserver2();
	Subject* GetSubject();
	void Update();
protected:
	Subject* _sub;
	string _name;
};
#endif

//Observer.cpp
#include "Observer.h"
#include "Subject.h"
#include <iostream>
using namespace std;

Observer::Observer()
{
	_st = '\0';
}
Observer::~Observer()
{

}
ConcreteObserver1::ConcreteObserver1(Subject* sub,string name)
{
	_sub = sub;
	_name = name;
	_sub->Attach(this);
}
ConcreteObserver1::~ConcreteObserver1()
{
	_sub->Detach(this);
	delete _sub;
}
Subject* ConcreteObserver1::GetSubject()
{
	return _sub;
}
void ConcreteObserver1::Update()
{
	_st = _sub->GetState();
	cout<<"this is observer "<<_name<<" my update state is: "<<_st<<endl;
}
ConcreteObserver2::ConcreteObserver2(Subject* sub,string name)
{
	_sub = sub;
	_name = name;
	_sub->Attach(this);
}
ConcreteObserver2::~ConcreteObserver2()
{
	_sub->Detach(this);
	delete _sub;
}
Subject* ConcreteObserver2::GetSubject()
{
	return _sub;
}
void ConcreteObserver2::Update()
{
	_st = _sub->GetState();
	cout<<"this is observer "<<_name<<" my update state is: "<<_st<<endl;
}

//main.cpp
#include "Observer.h"
#include "Subject.h"

int main()
{
	ConcreteSubject* pSub = new ConcreteSubject();
	ConcreteObserver1* obs1 = new ConcreteObserver1(pSub,"No1");
	ConcreteObserver2* obs2 = new ConcreteObserver2(pSub,"No2");
	pSub->SetState("old");
	pSub->Notify();
	pSub->SetState("new");
	pSub->Notify();
	return 0;
}