c++设计模式：观察者模式(Observer Pattern)

定义：

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

场景：

我们有一个气象站，我们通过WeatherData获取气象站的最新数据，并更新布告板上的数据。我们定义了两个布告板，一个是温度计，显示最新的温度，另一个布告板就是显示当前的所有温度、湿度和气压的最新数据。

类图：

c++代码如下：

#include <iostream>
#include <list>
#include <string>
#include <algorithm>
using namespace std;

class Observer;

// 主题
class Subject
{
public:
    virtual void registerObserver(Observer* o) = 0;
    virtual void removeObserver(Observer* o) = 0;
    virtual void notifyObserver() = 0;
    float getTemperature();
    float getHumidity();
    float getPressure();
protected:
    float m_temperature;
    float m_humidity;
    float m_pressure;
};

// 观察者
class Observer
{
public:
    Observer(Subject* pWeaterData);
    virtual ~Observer() {};
    virtual void update() = 0;
    Subject* getWeaterData();
protected:
    Subject* m_weaterData;
};

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

class WeatherData:public Subject
{
public:
    WeatherData();
    ~WeatherData();
    void registerObserver(Observer* o);
    void removeObserver(Observer* o);
    void notifyObserver();
    void measurementsChanged();
    void setMeasurements(float temperature, float humidity, float pressure);
private:
    list<Observer*> m_lObserver;
};

// 当前条件显示
class CurrentConditionsDisplay:public Observer,public DisplayElement
{
public:
    CurrentConditionsDisplay(Subject* pWeaterData);
    ~CurrentConditionsDisplay();
    void update();
    void display();
private:
    float m_template; // 温度
    float m_humidity; // 湿度
    float m_pressure; // 气压
};

// 温度计
class Thermometer:public Observer,public DisplayElement
{
public:
    Thermometer(Subject* pWeaterData);
    ~Thermometer();
    void update();
    void display();
private:
    float m_template; // 温度
};

float Subject::getTemperature()
{
    return m_temperature;
}

float Subject::getHumidity()
{
    return m_humidity;
}

float Subject::getPressure()
{
    return m_pressure;
}

Observer::Observer(Subject* pWeaterData)
{
    m_weaterData = pWeaterData;
}

Subject* Observer::getWeaterData()
{
    return m_weaterData;
}

WeatherData::WeatherData()
{
    m_temperature = 0;
    m_humidity = 0;
    m_pressure = 0;
}

WeatherData::~WeatherData()
{
    list<Observer*>::iterator iter, temp;
    for (iter=m_lObserver.begin(); iter!=m_lObserver.end(); ++iter)
    {
        delete (*iter);
    }
    m_lObserver.clear();
}

void WeatherData::registerObserver(Observer* o)
{
    m_lObserver.push_back(o);
}

void WeatherData::removeObserver(Observer* o)
{
    list<Observer*>::iterator iter;

    iter = std::find(m_lObserver.begin(), m_lObserver.end(), o);

    if (m_lObserver.end() != iter)
    {
        m_lObserver.erase(iter);
    }
}

void WeatherData::notifyObserver()
{
    list<Observer*>::iterator iter;
    for (iter=m_lObserver.begin(); iter!=m_lObserver.end(); ++iter)
    {
        (*iter)->update();
    }
}

void WeatherData::measurementsChanged()
{
    notifyObserver();
}

void WeatherData::setMeasurements(float temperature, float humidity, float pressure)
{
    m_temperature = temperature;
    m_humidity = humidity;
    m_pressure = pressure;
    measurementsChanged();
}

CurrentConditionsDisplay::CurrentConditionsDisplay(Subject* pWeaterData):Observer(pWeaterData)
{
    pWeaterData->registerObserver(this);
}
CurrentConditionsDisplay::~CurrentConditionsDisplay()
{
    m_weaterData = NULL;
}
void CurrentConditionsDisplay::update()
{
    m_template = m_weaterData->getTemperature();
    m_humidity = m_weaterData->getHumidity();
    m_pressure = m_weaterData->getPressure();
    display();
}
void CurrentConditionsDisplay::display()
{
    printf("Current conditions %f0.2F degrees and %0.2f%s humidity and pressure %0.2f\n",m_template,m_humidity,"%",m_pressure);
}

Thermometer::Thermometer(Subject* pWeaterData):Observer(pWeaterData)
{
    pWeaterData->registerObserver(this);
}

Thermometer::~Thermometer()
{
    m_weaterData = NULL;
}

void Thermometer::update()
{
    m_template = m_weaterData->getTemperature();
    display();
}

void Thermometer::display()
{
    printf("Current conditions %f0.2F degrees\n",m_template);
}

int main()
{
    WeatherData* pWeatherData = new WeatherData();
    CurrentConditionsDisplay* pCurrentConditionsDisplay = new CurrentConditionsDisplay(pWeatherData);
    Thermometer* pThermometer = new Thermometer(pWeatherData);
    pWeatherData->setMeasurements(123,456,789);

    // 当移除观察者后，需要手动释放内存
    pCurrentConditionsDisplay->getWeaterData()->removeObserver(pCurrentConditionsDisplay);
    pWeatherData->setMeasurements(789,456,123);

    // 当再次注册后，内存交由主题管理
    pCurrentConditionsDisplay->getWeaterData()->registerObserver(pCurrentConditionsDisplay);
    pWeatherData->setMeasurements(135,257,479);

    delete pWeatherData;
    return 0;
}

运行后结果如下：

Current conditions 123.000F degrees and 456.00% humidity and pressure 789.00
Current conditions 123.00F degrees
Current conditions 789.00F degrees
Current conditions 135.00F degrees
Current conditions 135.00F degrees and 257.00% humidity and pressure 479.00

参考资料：《Head First 设计模式》