【Head First Design Pattern】在对象之间定义一对多的依赖，这样以来，当一个对象改变状态，依赖它的对象都会收到通知，并自动更新。
通俗概念：
在被观察者中存放着观察者的信息，一旦有改变，马上调用观察者的方法。
比如，你手机(被观察者)的通讯录保存着"张三李四王五"(观察者)的手机号码，"张李王"也要保存你的手机号码。1.你一旦有什么事情，就会主动打电话通知"张李王"。2.如果张三不想收到你的通知，可以打电话给你，叫你把他从通讯录删除，下次他就收不到你的通知。3.如果张三还需要你的通知，可以打电话让你把他的手机号码添加到通讯录，下次就可以收到通知。这例子不一定恰当，但有一点是：被观察者必须手持观察者的联系方式，同时观察者最好也要有被观察者的联系方式。
C++实现代码:
第一个例子:
ObserverPattern.h:
#pragma once
#include <iostream>
#include <list>
using namespace std;
class CAbstractObserver
{
public:
virtual void update(float temp, float humidity, float pressure) = 0;
};
class CAbstractDisplayElement
{
public:
virtual void display() = 0;
};
class CAbstractSubject
{
public:
virtual void registerObserver(CAbstractObserver *pObserver) = 0;
virtual void removeObserver(CAbstractObserver *pObserver) = 0;
virtual void notifyObservers() = 0;
};
class CWeatherData : public CAbstractSubject
{
public:
CWeatherData()
{
}
virtual void registerObserver(CAbstractObserver *pObserver)
{
m_ObserverList.push_back(pObserver);
}
virtual void removeObserver(CAbstractObserver *pObserver)
{
m_ObserverList.remove(pObserver);
}
virtual void notifyObservers()
{
CAbstractObserver *pObserver;
for (list<CAbstractObserver *>::iterator it = m_ObserverList.begin(); it != m_ObserverList.end(); ++it)
{
pObserver = *it;
pObserver->update(m_fTemperature, m_fHumidity, m_fPressure);
}
}
void measurementsChanged()
{
notifyObservers();
}
void setMeasurements(float temp, float humidity, float pressure)
{
m_fTemperature = temp;
m_fHumidity = humidity;
m_fPressure = pressure;
measurementsChanged();
}
private:
list<CAbstractObserver *> m_ObserverList;
float m_fTemperature;
float m_fHumidity;
float m_fPressure;
};
class CCurrentConditionsDisplay : public CAbstractObserver, public CAbstractDisplayElement
{
public:
CCurrentConditionsDisplay(CAbstractSubject *pWeatherData)
{
m_pWeatherData = pWeatherData;
m_pWeatherData->registerObserver(this);
}
virtual void update(float temp,float humidity,float pressure)
{
m_fTemperature = temp;
m_fHumidity = humidity;
display();
}
virtual void display()
{
cout << "Current conditions: " << m_fTemperature << "F degrees and " << m_fHumidity << "% humidity" << endl;
}
private:
float m_fTemperature;
float m_fHumidity;
CAbstractSubject *m_pWeatherData;
};
class CStatisticsDisplay : public CAbstractObserver, public CAbstractDisplayElement
{
public:
CStatisticsDisplay(CWeatherData *pWeatherData)
{
m_fMaxTemp = 0.0f;
m_fMinTemp = 200;
m_fTempSum = 0.0f;
m_iNumReadings = 0;
pWeatherData->registerObserver(this);
}
virtual void update(float temp,float humidity,float pressure)
{
m_fTempSum += temp;
m_iNumReadings++;
if (temp > m_fMaxTemp)
{
m_fMaxTemp = temp;
}
if (temp < m_fMinTemp)
{
m_fMinTemp = temp;
}
display();
}
virtual void display()
{
cout << "Avg/Max/Min temperature = " << (m_fTempSum / m_iNumReadings) << '/' << m_fMaxTemp << '/' << m_fMinTemp << endl;
}
private:
float m_fMaxTemp;
float m_fMinTemp;
float m_fTempSum;
int m_iNumReadings;
};
class CForecastDisplay : public CAbstractObserver, public CAbstractDisplayElement
{
public:
CForecastDisplay(CWeatherData *pWeatherData)
{
m_fCurrentPressure = 29.92f;
pWeatherData->registerObserver(this);
}
virtual void update(float temp,float humidity,float pressure)
{
m_fLastPressure = m_fCurrentPressure;
m_fCurrentPressure = pressure;
display();
}
virtual void display()
{
cout << "Forecast: ";
if (m_fCurrentPressure > m_fLastPressure)
{
cout << "Improving weather on the way!" << endl;
}
else if (m_fCurrentPressure == m_fLastPressure)
{
cout << "More of the same" << endl;
}
else if (m_fCurrentPressure < m_fLastPressure)
{
cout << "Watch out for cooler, rainy weather" << endl;
}
}
private:
float m_fCurrentPressure;
float m_fLastPressure;
};
class CHeatIndexDisplay : public CAbstractObserver, public CAbstractDisplayElement
{
public:
CHeatIndexDisplay(CWeatherData *pWeatherData)
{
m_fHeatIndex = 0.0f;
pWeatherData->registerObserver(this);
}
virtual void update(float temp,float humidity,float pressure)
{
m_fHeatIndex = computerHeatIndex(temp, humidity);
display();
}
virtual void display()
{
cout << "Heat index is " << m_fHeatIndex << endl;
}
private:
float computerHeatIndex(float t, float rh)
{
float index = (float) ((16.923 + (0.185212 * t) + (5.37941 * rh)
- (0.100254 * t * rh) + (0.00941695 * (t * t))
+ (0.00728898 * (rh * rh)) + (0.000345372 * (t * t * rh))
- (0.000814971 * (t * rh * rh))
+ (0.0000102102 * (t * t * rh * rh))
- (0.000038646 * (t * t * t)) + (0.0000291583 * (rh * rh * rh))
+ (0.00000142721 * (t * t * t * rh))
+ (0.000000197483 * (t * rh * rh * rh))
- (0.0000000218429 * (t * t * t * rh * rh)) + 0.000000000843296 * (t
* t * rh * rh * rh)) - (0.0000000000481975 * (t * t * t * rh
* rh * rh)));
return index;
}
private:
float m_fHeatIndex;
};
ObserverPattern.cpp:
#include "ObserverPattern.h"
int main()
{
CWeatherData weatherData;
CCurrentConditionsDisplay currentDisplay(&weatherData);
CStatisticsDisplay statisticsDisplay(&weatherData);
CForecastDisplay forecastDisplay(&weatherData);
CHeatIndexDisplay heatIndexDisplay(&weatherData);
weatherData.setMeasurements(80, 65, 30.4f);
weatherData.setMeasurements(82, 70, 29.2f);
weatherData.setMeasurements(78, 90, 29.2f);
return 0;
}
第二个例子(模仿Java本身的观察者模式):
ObserverPatternJ.h:
#pragma once
#include <iostream>
#include <list>
using namespace std;
class CAbstractObserver
{
public:
virtual void update(CAbstractObservable *pObservable, void *arg) = 0;
};
class CAbstractDisplayElement
{
public:
virtual void display() = 0;
};
class CAbstractObservable
{
public:
virtual void addObserver(CAbstractObserver *pObserver) = 0;
virtual void deleteObserver(CAbstractObserver *pObserver) = 0;
virtual void notifyObservers() = 0;
};
class CWeatherData : public CAbstractObservable
{
public:
CWeatherData()
{
m_bChanged = false;
}
virtual void addObserver(CAbstractObserver *pObserver)
{
m_ObserverList.push_back(pObserver);
}
virtual void deleteObserver(CAbstractObserver *pObserver)
{
m_ObserverList.remove(pObserver);
}
virtual void notifyObservers()
{
if (m_bChanged)
{
CAbstractObserver *pObserver;
for (list<CAbstractObserver *>::iterator it = m_ObserverList.begin(); it != m_ObserverList.end(); ++it)
{
pObserver = *it;
pObserver->update(m_fTemperature, m_fHumidity, m_fPressure);
}
}
m_bChanged = false;
}
void measurementsChanged()
{
setChanged();
notifyObservers();
}
void setMeasurements(float temp, float humidity, float pressure)
{
m_fTemperature = temp;
m_fHumidity = humidity;
m_fPressure = pressure;
measurementsChanged();
}
float getTemperature()
{
return m_fTemperature;
}
float getHumidity()
{
return m_fHumidity;
}
float getPressure()
{
return m_fPressure;
}
void setChanged()
{
m_bChanged = true;
}
void clearChanged()
{
m_bChanged = false;
}
bool hasChanged()
{
return m_bChanged;
}
private:
list<CAbstractObserver *> m_ObserverList;
float m_fTemperature;
float m_fHumidity;
float m_fPressure;
bool m_bChanged;
};
class CCurrentConditionsDisplay : public CAbstractObserver, public CAbstractDisplayElement
{
public:
CCurrentConditionsDisplay(CAbstractObservable *pObservable)
{
m_pObservable = pObservable;
m_pObservable->addObserver(this);
}
virtual void update(CAbstractObservable * pObservable,void * arg)
{
CWeatherData *pWeatherData = dynamic_cast<CWeatherData *>(pObservable);
if (pWeatherData)
{
m_fTemperature = pWeatherData->getTemperature();
m_fHumidity = pWeatherData->getHumidity();
display();
}
}
virtual void display()
{
cout << "Current conditions: " << m_fTemperature << "F degrees and " << m_fHumidity << "% humidity" << endl;
}
private:
float m_fTemperature;
float m_fHumidity;
CAbstractObservable *m_pObservable;
};
class CStatisticsDisplay : public CAbstractObserver, public CAbstractDisplayElement
{
public:
CStatisticsDisplay(CWeatherData *pObservable)
{
m_fMaxTemp = 0.0f;
m_fMinTemp = 200;
m_fTempSum = 0.0f;
m_iNumReadings = 0;
m_pObservable = pObservable;
m_pObservable->addObserver(this);
}
virtual void update(CAbstractObservable * pObservable,void * arg)
{
CWeatherData *pWeatherData = dynamic_cast<CWeatherData *>(pObservable);
if (pWeatherData)
{
int temp = pWeatherData->getTemperature();
m_fTempSum += temp;
m_iNumReadings++;
if (temp > m_fMaxTemp)
{
m_fMaxTemp = temp;
}
if (temp < m_fMinTemp)
{
m_fMinTemp = temp;
}
display();
}
}
virtual void display()
{
cout << "Avg/Max/Min temperature = " << (m_fTempSum / m_iNumReadings) << '/' << m_fMaxTemp << '/' << m_fMinTemp << endl;
}
private:
float m_fMaxTemp;
float m_fMinTemp;
float m_fTempSum;
int m_iNumReadings;
CAbstractObservable *m_pObservable;
};
class CForecastDisplay : public CAbstractObserver, public CAbstractDisplayElement
{
public:
CForecastDisplay(CWeatherData *pObservable)
{
m_fCurrentPressure = 29.92f;
m_pObservable = pObservable;
m_pObservable->addObserver(this);
}
virtual void update(CAbstractObservable * pObservable,void * arg)
{
CWeatherData *pWeatherData = dynamic_cast<CWeatherData *>(pObservable);
if (pWeatherData)
{
m_fLastPressure = m_fCurrentPressure;
m_fCurrentPressure = pWeatherData->getPressure();
}
}
virtual void display()
{
cout << "Forecast: ";
if (m_fCurrentPressure > m_fLastPressure)
{
cout << "Improving weather on the way!" << endl;
}
else if (m_fCurrentPressure == m_fLastPressure)
{
cout << "More of the same" << endl;
}
else if (m_fCurrentPressure < m_fLastPressure)
{
cout << "Watch out for cooler, rainy weather" << endl;
}
}
private:
float m_fCurrentPressure;
float m_fLastPressure;
CAbstractObservable *m_pObservable;
};
class CHeatIndexDisplay : public CAbstractObserver, public CAbstractDisplayElement
{
public:
CHeatIndexDisplay(CWeatherData *pObservable)
{
m_fHeatIndex = 0.0f;
m_pObservable = pObservable;
m_pObservable->addObserver(this);
}
virtual void update(CAbstractObservable * pObservable,void * arg)
{
CWeatherData *pWeatherData = dynamic_cast<CWeatherData *>(pObservable);
if (pWeatherData)
{
m_fHeatIndex = computerHeatIndex(pWeatherData->getTemperature(), pWeatherData->getHumidity());
display();
}
}
virtual void display()
{
cout << "Heat index is " << m_fHeatIndex << endl;
}
private:
float computerHeatIndex(float t, float rh)
{
float index = (float) ((16.923 + (0.185212 * t) + (5.37941 * rh)
- (0.100254 * t * rh) + (0.00941695 * (t * t))
+ (0.00728898 * (rh * rh)) + (0.000345372 * (t * t * rh))
- (0.000814971 * (t * rh * rh))
+ (0.0000102102 * (t * t * rh * rh))
- (0.000038646 * (t * t * t)) + (0.0000291583 * (rh * rh * rh))
+ (0.00000142721 * (t * t * t * rh))
+ (0.000000197483 * (t * rh * rh * rh))
- (0.0000000218429 * (t * t * t * rh * rh)) + 0.000000000843296 * (t
* t * rh * rh * rh)) - (0.0000000000481975 * (t * t * t * rh
* rh * rh)));
return index;
}
private:
float m_fHeatIndex;
CAbstractObservable *m_pObservable;
};
ObserverPatternJ.cpp:
与ObserverPattern.cpp一样