Head First Design Patterns - Observer Pattern
The Observer Pattern defines a one-to-many dependency between objects so that when one
object changes state, all of its dependents are notified and updated automatically.
The Observer Pattern defines a one-to-many relatioship between objects.
Design Principle
- Strive for loosely coupled designs between objects that interact
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();
}
}
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(){
//do something
}
}
