Head first笔记二：观察者模式

Headfirst Java代码

public interface Subject {
	public void registerObserver(Observer o);
	public void removeObserver(Observer o);
	public void notifyObservers();
}
public interface Observer {
	public void update(float temp, float humidity, float pressure);
}
public interface DisplayElement {
	public void display();
}

public class WeatherData implements Subject {
	private ArrayList observers;
	private float temperature;
	private float humidity;
	private float pressure;
	public WeatherData() {
		observers = new ArrayList();
	}
	public void registerObserver(Observer o) {
		observers.add(o);
	}
	public void removeObserver(Observer o) {
		int i = observers.indexOf(o);
		if (i >= 0) {
			observers.remove(i);
		}
	}
	public void notifyObservers() {
		for (int i = 0; i < observers.size(); i++) {
			Observer observer = (Observer)observers.get(i);
			observer.update(temperature, humidity, pressure);
		}
	}
	public void measurementsChanged() {
		notifyObservers();
	}
	public void setMeasurements(float temperature, float humidity, float pressure) {
		this.temperature = temperature;
		this.humidity = humidity;
		this.pressure = pressure;
		measurementsChanged();
	}
	// other WeatherData methods here
}

public class CurrentConditionsDisplay implements Observer, DisplayElement {
	private float temperature;
	private float humidity;
	private Subject weatherData;
	public CurrentConditionsDisplay(Subject weatherData) {
		this.weatherData = weatherData;
		weatherData.registerObserver(this);
	}
	public void update(float temperature, float humidity, float pressure) {
		this.temperature = temperature;
		this.humidity = humidity;
		display();
	}
	public void display() {
		System.out.println(“Current conditions: “ + temperature
			+ “F degrees and “ + humidity + “% humidity”);
	}
}

public class WeatherStation {
	public static void main(String[] args) {
		WeatherData weatherData = new WeatherData();
		CurrentConditionsDisplay currentDisplay =
			new CurrentConditionsDisplay(weatherData);
		StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
		ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);
		weatherData.setMeasurements(80, 65, 30.4f);
		weatherData.setMeasurements(82, 70, 29.2f);
		weatherData.setMeasurements(78, 90, 29.2f);
	}
}

C++ 例子：

#include 
#include 
using namespace std;
 
class Observer
{
public:
     virtual void Update(int) = 0;
};
 
class Subject
{
public:
     virtual void Attach(Observer *) = 0;
     virtual void Detach(Observer *) = 0;
     virtual void Notify() = 0;
};
 
class ConcreteObserver : public Observer
{
public:
     ConcreteObserver(Subject *pSubject) : m_pSubject(pSubject){}
 
     void Update(int value)
     {
          cout<<"ConcreteObserver get the update. New State:"< m_ObserverList;
     int m_iState;
};
 
void ConcreteSubject::Attach(Observer *pObserver)
{
     m_ObserverList.push_back(pObserver);
}
 
void ConcreteSubject::Detach(Observer *pObserver)
{
     m_ObserverList.remove(pObserver);
}
 
void ConcreteSubject::Notify()
{
     std::list::iterator it = m_ObserverList.begin();
     while (it != m_ObserverList.end())
     {
          (*it)->Update(m_iState);
          ++it;
     }
}
 
int main()
{
     // Create Subject
     ConcreteSubject *pSubject = new ConcreteSubject();
 
     // Create Observer
     Observer *pObserver = new ConcreteObserver(pSubject);
     Observer *pObserver2 = new ConcreteObserver2(pSubject);
 
     // Change the state
     pSubject->SetState(2);
 
     // Register the observer
     pSubject->Attach(pObserver);
     pSubject->Attach(pObserver2);
 
     pSubject->Notify();
 
     // Unregister the observer
     pSubject->Detach(pObserver);
 
     pSubject->SetState(3);
     pSubject->Notify();
 
     delete pObserver;
     delete pObserver2;
     delete pSubject;
}