我与C++设计模式（十五）——观察者模式

毋庸置疑，observer模式的地位是十分重要的，此处省略一些套话，那么什么是observer模式，他在帮我们解决什么问题或者说他在面对什么情形进行设计的。

observer模式要解决的问题是，建立一个一对多的依赖关系，并且做到当“一”变化的时候，“多”能够及时作出相应的响应，书中举的例子很好，即，对一组数据进行统计分析，我们希望可以用多种形式进行展现，有表格，柱状图，百分比图，或者变化曲线等，当数据发生改变时（说不定我们正在说的数据正是股票的指数呢？），各种展现形式也应该做出反应。这就是观察者模式。

#ifndef _SUBJECT_H__#define _SUBJECT_H__#include <list>#include <string>using namespace std;class observer;typedef string state;class subject{public:virtual ~subject();virtual void attach(observer *p_obs);virtual void detach(observer *p_obs);virtual void notify();virtual void set_state(const state&) = 0;virtual state get_state() = 0;protected:subject();private:list<observer *> *_pl_obs;};class concrete_subject:public subject{public:concrete_subject();~concrete_subject();state get_state();void set_state(const state&);private:state _st;};#endif

//subject.cpp#include "subject.h"#include "observer.h"subject::subject():_pl_obs(new list<observer *>){}subject::~subject(){}void subject::attach(observer *p_obs){_pl_obs->push_front(p_obs);}void subject::detach(observer *p_obs){if (p_obs)_pl_obs->remove(p_obs);}void subject::notify(){list<observer *>::iterator ite = _pl_obs->begin();for ( ; ite != _pl_obs->end(); ++ite){(*ite)->update(this);}}concrete_subject::concrete_subject(){}concrete_subject::~concrete_subject(){}state concrete_subject::get_state(){return _st;}void concrete_subject::set_state(const state &st){_st = st;}

#ifndef _OBSERVER_H__#define _OBSERVER_H__#include <string>using namespace std;class subject;typedef string state;class observer{public:~observer();virtual void update(subject *) = 0;virtual void print_info() = 0;protected:observer();state _st;};class concrete_observer_A:public observer{public:concrete_observer_A(subject *);~concrete_observer_A();virtual subject *get_subject();void update(subject *);void print_info();private:subject *_p_sub;};class concrete_observer_B:public observer{public:concrete_observer_B(subject *);~concrete_observer_B();virtual subject *get_subject();void update(subject *);void print_info();private:subject *_p_sub;};#endif

//observer.cpp#include "observer.h"#include "subject.h"#include <iostream>using namespace std;observer::observer(){}observer::~observer(){}//-----------------A------------------concrete_observer_A::concrete_observer_A(subject *p_sub):_p_sub(p_sub){_p_sub->attach(this);}concrete_observer_A::~concrete_observer_A(){_p_sub->detach(this);if (_p_sub){delete _p_sub;_p_sub = NULL;}}void concrete_observer_A::update(subject * p_sub){_st = _p_sub->get_state();print_info();}subject *concrete_observer_A::get_subject(){return _p_sub;}void concrete_observer_A::print_info(){cout<<"concrete_observer_A observer..."<<_p_sub->get_state()<<endl;}//-----------------B------------------concrete_observer_B::concrete_observer_B(subject *p_sub):_p_sub(p_sub){_p_sub->attach(this);}concrete_observer_B::~concrete_observer_B(){_p_sub->detach(this);if (_p_sub){delete _p_sub;_p_sub = NULL;}}void concrete_observer_B::update(subject * p_sub){_st = _p_sub->get_state();print_info();}subject *concrete_observer_B::get_subject(){return _p_sub;}void concrete_observer_B::print_info(){cout<<"concrete_observer_B observer..."<<_p_sub->get_state()<<endl;}

//main.cpp#include "observer.h"#include "subject.h"int main(int argc,char **argv){//concrete_subject *p_con_sub = new concrete_subject();subject *p_con_sub = new concrete_subject();observer *p_obs_1 = new concrete_observer_A(p_con_sub);observer *p_obs_2 = new concrete_observer_B(p_con_sub);p_con_sub->set_state("old");p_con_sub->notify();p_con_sub->set_state("new");p_con_sub->notify();return 0;}

如例所示，subject正是我们的数据，设有两个观察者，nodify函数实现数据同步，observer类的设计使用了类的组合。