| layout | pattern | ||
|---|---|---|---|
| title | Observer | ||
| folder | observer | ||
| permalink | /patterns/observer/ | ||
| categories | Behavioral | ||
| tags |
|
Dependents, Publish-Subscribe
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
Real world example
In a land far away lives the races of hobbits and orcs. Both of them are mostly outdoors so they closely follow the changes in weather. One could say that they are constantly observing the weather.
In plain words
Register as an observer to receive state changes in the object.
Wikipedia says
The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
Programmatic Example
Let's first introduce the weather observer interface and our races, orcs and hobbits.
publicinterfaceWeatherObserver {
voidupdate(inttemperature, inthumidity, intpressure);
}
publicclassStatisticDisplayimplementsObserver {
@Overridepublicvoidupdate(inttemperature, inthumidity, intpressure) {
System.out.println("Update StatisticDisplay display with new Parameters");
}
}
publicclassGeneralDisplayimplementsObserver {
@Overridepublicvoidupdate(inttemperature, inthumidity, intpressure) {
System.out.println("Update general display with new Parameters");
}
}Then here's the weather that is constantly changing.
publicinterfaceSubject {
voidregisterObserver(Observerobserver);
voidremoveObserver(Observerobserver);
voidnotifyObserver();
}
publicclassWeatherDataimplementsSubject {
privateList<Observer> observers;
publicWeatherData() {
this.observers = newArrayList<>();
}
@OverridepublicvoidregisterObserver(Observerobserver) {
observers.add(observer);
}
@OverridepublicvoidremoveObserver(Observerobserver) {
observers.remove(observer);
}
@OverridepublicvoidnotifyObserver() {
observers.forEach(observer -> observer.update(getTemperature(), getHumidity(), getPressure()));
}
intgetTemperature(){
// some code here
}
intgetHumidity(){
// some code here
}
intgetPressure(){
// some code here
}
}Here's the full example in action.
WeatherDataweatherData = newWeatherData();
weatherData.registerObserver(newGeneralDisplay());
weatherData.registerObserver(newStatisticDisplay());
weatherData.notifyObserver();
Use the Observer pattern in any of the following situations
- When an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
- When a change to one object requires changing others, and you don't know how many objects need to be changed
- When an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled
- Changing in one object leads to a change in other objects