Course UML and Design Patterns of module Software Engineering and Design, version November 18, 2013.

Transcription

1 The Observer Pattern Prof. Dr. Eric Dubuis Berner Fachhochschule, Engineering and Information Biel Course UML and Design Patterns of module Software Engineering and Design, version November 8, 203. Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page /

2 The Problem An object (the subject) needs to update various client objects (observers): The subject needs to inform other interested objects about its state changes The subject does not know its observers in advance The set of observers may change dynamically Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page 2/

3 A Solution: The Classical Observer Pattern The class structure of the classical (GoF) observer pattern (complete): Subject {abstract} +attach(observer o) +detach(observer o) #notifyobservers() ConcreteSubject -subjectstate +getstate() +setstate() -mysubject setstate(newstate) { subjectstate = newstate; } for (Observer o: observers) { } o.update(); notifyobservers(); * -observers +update() «interface» Observer ConcreteObserver -observerstate +update() observerstate = mysubject.getstate(); Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page 3/

4 Collaboration :Client asubject: Subject observer: Observer observer2: Observer setstate notifyobservers() update() getstate() update() getstate() Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page 4/

5 Weakness of Above Solution (from a J ava Perspective) subjects must extend a common base class observers are not typed subjects cannot send observer-specific data observers must use getstate() overhead Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page 5/

6 The J ava Solution: Typed Observers (a.k.a. Listeners) Object «interface» EventListener * EventObject ConcreteSubject * «interface» SomeListener SomeEvent -state +addsomelistener(s.l.) +removesomelistener(s.l.) -notifylisteners() +setstate(newstate)..* * -counts +somechange(s.e.) SomeObserver «create» +SomeObserver(Con.S.).....* «create» +SomeEvent(Con.S.,..)... «inner class» SomeListenerImpl +somechange(s.e.) Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page 6/

7 The J ava Code Sketch for Typed Observers (a.k.a. Listeners) Provide an interface and an event class for each type of observer. For example: public interface SomeListener extends EventListener { 2 public void somechange(someevent e); 3 } 4 5 public class SomeEvent extends EventObject { 6 // optional: additional instance variables go here... 7 public SomeEvent(ConcreteSubject s,...) { 8 super(s); 9 // optional: initialize instance variables 0 }. public ConcreteSubject getconcretesubject() { 2. return (ConcreteSubject) getsource(); 3 } 4. // optional: other getters... 5 } update method contains the sender + optionally, data optional: add additional parameters defined in class EventObject Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page 7/

8 Clients (observers) can register themselves at objects (concrete subjects) generating SomeEvent: // in class ConcreteSubject acting as subject: 2 private Set<SomeListener> listeners = 3 new HashSet<SomeListener>(); 4 5 public void addsomelistener(somelistener l) { 6 this.listeners.add(l); 7 } 8 9 public boolean removesomelistener(somelistener l) { 0 return this.listeners.remove(l); } 2 Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page 8/

9 The subject informs any listener whenever its state changes: // in class ConcreteSubject acting as a subject: 2 private void notifylisteners() { 3 SomeEvent e = new SomeEvent(this,...); 4 for (SomeListener l: this.listeners) { 5 l.somechange(e); 6 } 7 } 8 9 private State state; 0 public void setstate(state newstate) { 2 this.state = newstate; 3 notifylisteners(); 4 } Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page 9/

10 Any object can act as listener: public class SomeObserver { // extends Object implicitly 2 private ConcreteSubject s; 3 private SomeListener mylistener; SomeListenerImpl 4 private int counts = 0; is an inner class 5 private class SomeListenerImpl implements SomeListener { 6 public void somechange(someevent e) { 7 SomeObserver.this.counts++; 8 } 9 } 0 public SomeObserver(ConcreteSubject s) { this.s = s; 2 this.mylistener = new SomeListenerImpl(); 3 this.s.addsomelistener(this.mylistener); 4 } 5 } Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page 0/

11 Remarks Java observers (listeners) are typed! subjects need not extend a common base class subjects send events that contain the source (= the sender) optional: event objects can contain additional data, too if needed: SomeObserver instance can remove its listener from ConcreteSubject later Berner Fachhochschule Haute école spécialisée bernoise Bern University of Applied Sciences Page /