DesignPattern-Observer

观察者模式

 

出版者+订阅者=观察者模式

 

定义：

    观察者模式定义了对象之间的一对多依赖，这样一来，当一个对象（主题）改变状态时，它的所有依赖者（观察者）都会收到通知并自动更新。

 

首先观察者对象要向主题对象注册，告诉主题对象它是一个观察者（即，它对主题对象感兴趣）。这样，当主题对象发生变化时，就会通知观察者。

 

如图：

主题与观察者是一对多关系

 

看一个真实的例子，模拟天气预报系统，当天气数据值（主题)温度、湿度和压强发上变化时，要通知Observer对象，再显示到DisplayElement中。

 

ClassDiagram：

    在这里，天气数据（WeatherData)的变化，由它的一个方法setMeasurements()来模拟实现的，每次set不同的温度（temperature)、湿度(humidity)和压强(pressure),这样来通知Observer对象，并显示在DisplayElement中的display上。

 

Subject.h

class Subject
{
public:
    virtual void registerObserver(Observer* o) = 0;
    virtual void removeObserver(Observer* o) = 0;
    virtual void notifyObserver() = 0;
};

#endif //_SUBJECT_H

WeatherData.h

#if !defined(_WEATHERDATA_H)
#define _WEATHERDATA_H

#include "WeatherData.h"
#include "Subject.h"
#include <string>
#include <vector>

class WeatherData : public Subject
{
public:
    void registerObserver(Observer* o);
    void removeObserver(Observer* o);
    void notifyObserver();

    void setMeasurements(int tem,int hum,int pres);
private:
    int temperature;
    int humidity;
    int pressure;

    std::vector<Observer*> vec_observers;

};

#endif //_WEATHERDATA_H

Observer.h

#if !defined(_OBSERVER_H)
#define _OBSERVER_H


class Observer
{
public:
    virtual void update(int tem,int hum,int pres) = 0;
};

#endif //_OBSERVER_H

CurrentConditionDisplay.h

#if !defined(_CURRENTCONDITIONSDISPLY_H)
#define _CURRENTCONDITIONSDISPLY_H

#include "Observer.h"
#include "DisplayElement.h"
#include "Subject.h"
class CurrentConditionsDisply : public Observer, public DisplayElement
{
public:
    CurrentConditionsDisply(Subject* sub);
    ~CurrentConditionsDisply();
    void update(int tem,int hum,int pres);
    void display();

private:
    int temperature;
    int humidity;
    int pressure;
    Subject* sub;
};

#endif //_CURRENTCONDITIONSDISPLY_H

DisplayElement.h

#if !defined(_DISPLAYELEMENT_H)
#define _DISPLAYELEMENT_H


class DisplayElement
{
public:
    virtual void display() = 0;
};

#endif //_DISPLAYELEMENT_H

Test.cpp

#include "stdafx.h"
#include "WeatherData.h"
#include "CurrentConditionsDisply.h"
#include "ForecastDisplay.h"
#include "StatisticsDisplay.h"
#include "ThirdPartyDisplay.h"
int main(int argc, char* argv[])
{
    printf("Hello World!\n");

    WeatherData* sub1 = new WeatherData();
    Observer* obr1 = new CurrentConditionsDisply(sub1);
    Observer* obr2 = new ForecastDisplay(sub1);
    Observer* obr3 = new StatisticsDisplay(sub1);
    Observer* obr4 = new ThirdPartyDisplay(sub1);
    sub1->setMeasurements(100,300,900);
    printf("\n-----------------------------------------------------------------------------\n");
    sub1->setMeasurements(200,400,800);
    //unregister ForecastDisplay
    sub1->removeObserver(obr2);
    printf("\n------------------------------------------------------------------------------\n");
    sub1->setMeasurements(2000,4000,8000);


    return 0;
}

OutPut:

Hello World!
The Current Condition Display: temperature = 100;humidity = 300;pressure = 900
The Forecast Display: temperature = 101;humidity = 301;pressure = 901
The Static Condition Display: temperature = 1000;humidity = 3000;pressure = 9000
The Third Party Condition Display: temperature = 10;humidity = 30;pressure = 90

-----------------------------------------------------------------------------
The Current Condition Display: temperature = 200;humidity = 400;pressure = 800
The Forecast Display: temperature = 201;humidity = 401;pressure = 801
The Static Condition Display: temperature = 2000;humidity = 4000;pressure = 8000
The Third Party Condition Display: temperature = 20;humidity = 40;pressure = 80

------------------------------------------------------------------------------
The Current Condition Display: temperature = 2000;humidity = 4000;pressure = 8000
The Static Condition Display: temperature = 20000;humidity = 40000;pressure = 80000
The Third Party Condition Display: temperature = 200;humidity = 400;pressure = 800
Press any key to continue

完整的项目代码，VC6.0工程。

 Observer.rar