最近在學習設計模式，用的是 《Head First 設計模式》這本書。感覺這本書寫的還是很不錯的，深入淺出的介紹了各種常用的設計模式。唯一有點不方便的地方是這本書的例子全都是用的 Java 來實現的。而我主要是用 C++。所以就動手將書上的代碼用 C++ 來實現了一遍。

觀察者模式

首先是三個接口的代碼：

//observer.h #ifndef OBSERVER_H #define OBSERVER_Hclass Observer { public:Observer() {}virtual void update(double temp, double humidity, double pressure) = 0; }; #endif // OBSERVER_H //Subject.h #ifndef SUBJECT_H #define SUBJECT_H#include "observer.h" #include <QList> class Subject { public:Subject();void registerObserver(Observer *o);void removeObserver(Observer *o);void notifyObservers(); private:QList<Observer *> m_list; protected:double m_temp;double m_humidity;double m_pressure; };#endif // SUBJECT_H #include "subject.h"Subject::Subject() {}void Subject::registerObserver(Observer *o) {m_list.append(o); }void Subject::removeObserver(Observer *o) {int i = m_list.indexOf(o);if( i >= 0 ){m_list.removeAt(i);} }void Subject::notifyObservers() {foreach (Observer * o, m_list){o->update(m_temp, m_humidity, m_pressure);} } #ifndef DISPLAYELEMENT_H #define DISPLAYELEMENT_Hclass DisplayElement { public:DisplayElement() {}virtual void display() = 0; };#endif // DISPLAYELEMENT_H

下面是 WeatherData 類：

#ifndef WEATHERDATA_H #define WEATHERDATA_H#include "subject.h"class WeatherData : public Subject { public:WeatherData();void setMeasurements(double t, double h, double p);void measurementsChanged(); };#endif // WEATHERDATA_H #include "weatherdata.h"WeatherData::WeatherData() {}void WeatherData::setMeasurements(double t, double h, double p) {m_temp = t;m_humidity = h;m_pressure = p;measurementsChanged(); }void WeatherData::measurementsChanged() {notifyObservers(); }

最后是四種觀察者：

#include "observer.h" #include "displayelement.h"class CurrentConditionDisplay : public Observer, public DisplayElement { public:CurrentConditionDisplay();void display() override;void update(double temp, double humidity, double pressure) override; private:double m_temp;double m_humidity;double m_pressure; };class ForecastDisplay: public Observer, public DisplayElement { public:ForecastDisplay();void display() override;void update(double temp, double humidity, double pressure) override; private:double currentPressure = 29.92f;double lastPressure; };class HeatIndexDisplay : public Observer, public DisplayElement { public:HeatIndexDisplay();void display() override;void update(double temp, double humidity, double pressure) override; private:double m_heatIndex;double computeHeatIndex(double t, double rh); };class StatisticsDisplay: public Observer, public DisplayElement { public:StatisticsDisplay();void display() override;void update(double temp, double humidity, double pressure) override; private:int m_numReadings;double m_tempSum;double m_maxTemp;double m_minTemp; }; #include "display.h" #include <iostream>using std::cout; using std::endl;CurrentConditionDisplay::CurrentConditionDisplay() {}void CurrentConditionDisplay::display() {cout << "Current conditions:" << m_temp<< " F degrees and " << m_humidity<< "% humidity" << endl; }void CurrentConditionDisplay::update(double temp, double humidity, double pressure) {m_temp = temp;m_humidity = humidity;m_pressure = pressure;display(); }ForecastDisplay::ForecastDisplay() {}void ForecastDisplay::display() {cout << "Forecast: ";if (currentPressure > lastPressure){cout << "Improving weather on the way!";}else if (currentPressure == lastPressure){cout << "More of the same";}else if (currentPressure < lastPressure){cout << "Watch out for cooler, rainy weather";}cout << endl; }void ForecastDisplay::update(double temp, double humidity, double pressure) {lastPressure = currentPressure;currentPressure = pressure;display(); }HeatIndexDisplay::HeatIndexDisplay() {}void HeatIndexDisplay::display() {cout << "Heat index is " << m_heatIndex << endl; }void HeatIndexDisplay::update(double temp, double humidity, double pressure) {(void) pressure;m_heatIndex = computeHeatIndex(temp, humidity);display(); }double HeatIndexDisplay::computeHeatIndex(double t, double rh) {double index = (double)((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; }StatisticsDisplay::StatisticsDisplay():m_tempSum(0.0),m_maxTemp(0),m_minTemp(200),m_numReadings(0) {}void StatisticsDisplay::display() {cout << "Avg/Max/Min temperature = " << m_tempSum / m_numReadings<< "/" << m_maxTemp << "/" << m_minTemp << endl; }void StatisticsDisplay::update(double temp, double humidity, double pressure) {(void) humidity;(void) pressure;m_tempSum += temp;m_numReadings++;if (temp > m_maxTemp){m_maxTemp = temp;}if (temp < m_minTemp){m_minTemp = temp;}display(); }

最后是測試代碼：

int main(int argc, char *argv[]) {WeatherData weatherData;CurrentConditionDisplay currentDisplay;StatisticsDisplay staticDisplay;ForecastDisplay forcastDisplay;HeatIndexDisplay heatDisplay;weatherData.registerObserver(&currentDisplay);weatherData.registerObserver(&staticDisplay);weatherData.registerObserver(&heatDisplay);weatherData.registerObserver(&forcastDisplay);weatherData.setMeasurements(80, 65, 30.4);weatherData.setMeasurements(82, 70, 29.2f);weatherData.setMeasurements(78, 90, 29.2f); }

總結

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