观察者模式

1. 当一个对象的改变需要同时改变其他对象的时候。而且不知道具体有多少对象有待改变时，应该考虑使用观察者模式。
2. 当一个抽象模型有两个方面，其中一方面依赖于另一方面，这时用观察者模式可以将这两者封装在独立的对象中使他们各自独立的改变和复用。

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <string>
using namespace std;

class Observer
{
public:
	virtual void Update() = 0;
	virtual ~Observer(){}
};
class Subject
{
public:
	virtual ~Subject(){}
	virtual void Attach(Observer* observer)
	{
		_observers.push_back(observer);
	}
	virtual void Detach(Observer* observer)
	{
		vector<Observer*>::iterator it = find(_observers.begin(), _observers.end(), observer);
		_observers.erase(it);
	}
	virtual void Notify()
	{
		for each (Observer* o in _observers)
		{
			o->Update();
		}
	}
private:
	vector<Observer*> _observers = vector<Observer*>();
};

class ConcreteSubject :public Subject
{
public :
	~ConcreteSubject(){}
	string SubjectState;
};

class ConcreteObserver :public Observer
{
private:
	string _name;
	string _ObserverState;
	ConcreteSubject *_pSubject;
public:
	ConcreteObserver( ConcreteSubject *subject, string name)
	{
		_pSubject = subject;
		_name = name;
	}
	virtual void Update()
	{
		_ObserverState = _pSubject->SubjectState;
		cout << "观察者："<<_name<<"  当前状态："<<_ObserverState << endl;
	}
};

// ObserverPattern.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include "Observer.h"

int _tmain(int argc, _TCHAR* argv[])
{
	ConcreteSubject *s = new ConcreteSubject();
	s->Attach(new ConcreteObserver(s, "x"));
	s->Attach(new ConcreteObserver(s, "y"));
	s->SubjectState = "老板回来了";
	s->Notify();
	getchar();
	return 0;
}