1. 1 Observer Design Pattern By Eric Perret Pages 372-380 in Applying UML and Patterns

2. 2 What we are covering. Definition Example Example using code Why it is a good idea to use the pattern. Summary

3. 3 What is an Observer? The Observer design pattern, or more accurately some implementation of this pattern, is used to assure consistency between different objects. AKA Publish-Subscribe

4. 4 What does this really mean. When an object changes state, all of the objects that need to know about it are notified and updated automatically.

5. 5 Example… Let the state of an object "B" depend on the state of an object "A". Whenever the state of object "A" changes, "B" has to recompute its state in order to remain consistent with "A". The observer design pattern supports such dependencies and at the same time tries to reduce the coupling between the object that changes (the publisher) and the object that needs a change notification (the subscriber).

6. 6 Here's an example of the interfaces in java-like code: public abstract interface Subject { public abstract void addObserver(Observer observer); public abstract void removeObserver(Observer observer); public abstract void notify(); } public abstract class Observer { public abstract void update(); } The subject calls its "notify" operation whenever it changes. "Notify" calls the "update" operation on each observer. The observers then call back to the subject and update themselves accordingly.

7. 7 Why Use the Pattern? State dependencies between objects are unfavorable for several reasons. If we could model our problem domain as a set of independent classes, we would get a software system without state dependencies. Coding and especially maintaining such a system would be much easier. The programmer could focus on single classes only.

8. 8 Summary Loosely couple objects in terms of communication Publishers know about subscribers only through an interface and subscribers can register or de-register dynamically with the publisher

9. 9 Question, comments, theories, or conundrums? Slides can be found here: http://www.rpi.edu/~perree/sdd http://www.rpi.edu/~perree/sdd

10. 10 References Applying UML and Patterns by Craig Larman, 352-380 On Using the Observer Design Pattern by Constantin Szallies, http://www.wohnklo.de/patterns/observer.ht ml http://www.wohnklo.de/patterns/observer.ht ml Observer Design Pattern by Stephen Lam, http://sern.ucalgary.ca/courses/SENG/609.0 4/W98/lamsh/observerLib.html http://sern.ucalgary.ca/courses/SENG/609.0 4/W98/lamsh/observerLib.html