The Observer Design Pattern

1. The Observer Design Pattern Author :Erich Gamma, et al. Source :Elements of Reusable Object-Oriented Software Speaker : Chiao-Ping Chang Advisor : Ku-Yaw Chang

2. Outline • Intent • Motivation • Applicability • Structure • Consequence • Implementation Knowledge Engineering Lab

3. Intent • The Observer Pattern: • Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. Knowledge Engineering Lab

4. Motivation Knowledge Engineering Lab

5. Motivation • Data changes in one place, but many other components depend on this data • Separate the presentational aspects from the application data • Application data and presentations can be reused independently Knowledge Engineering Lab

6. Motivation • Describes how to establish these relationships • The key objects： • Subject and observer in the Observer pattern Knowledge Engineering Lab

7. Motivation • A subject may have any number of dependent observers • All observers are notified whenever the subject changed its state • Each observer will query the subject to make its state the same with the subject's state Knowledge Engineering Lab

8. Applicability • When an abstraction has two aspects, one dependent on the other. • 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 notify other objects , and you don’t know who these objects are . Knowledge Engineering Lab

9. Structure Knowledge Engineering Lab

10. Structure Knowledge Engineering Lab

11. Demo example Knowledge Engineering Lab

12. Consequences • Reuse subjects (observers) without reusing their observers (subjects) • Add observers without modifying the subject or other observers Knowledge Engineering Lab

13. Consequences • Further benefits and liabilities : • Abstract coupling between Subject and Observer. • Support for broadcast communication • The notification is broadcast automatically to all interested objects • Up to the observer to handle or ignore a notification • Unexpected updates • The simple update protocol provides no details on whatchanged in the subject Knowledge Engineering Lab

14. Implementation • Observing more than one subject. • Extend the Update interface to let the observer know which subject is sending the notification • The subject can pass itself as a parameter • Dangling references to deleted subjects • Notify its observers to reset their reference to it Knowledge Engineering Lab

15. Implementation • Who triggers the update? • Have state-setting operations on Subject call Notify after they change the subject's state • Advantage ：Clients don't have to remember to call Notify on the subject • Disadvantage ：Cause several consecutive updates, which may be inefficient Knowledge Engineering Lab

16. Implementation • Who triggers the update? • Make clients responsible for calling Notify at the right time • Advantage ：Avoiding needless intermediate updates • Disadvantage ：Clients might forget to call Notify • Avoiding observer-specific update protocols: the push and pull models Knowledge Engineering Lab

17. Implementation • The push model： • The subject sends all changed data when it notifies the observers • Assumes subjects know something about their observers' needs • The pull model： • The subject only sends minimal information when sending a change notification • Emphasizes the subject's ignorance of its observers Knowledge Engineering Lab

18. Implementation • The push model： • Less reusable • Assumptions might not always be true • When the observers need the subject information most of the time • The pull model： • Inefficient • Observer classes must ascertain what changed without help from the Subject. • When observers can decide if and when they need a specific piece of information Knowledge Engineering Lab

19. The Publisher-Subscriber Pattern • Source：Pattern-Oriented Software Architecture  • The Publisher-Subscriber Pattern: • One publisher notifies any number of subscribers about changes to its state. • The key objects： • Publisher and subscriber in the Publisher-Subscriber pattern Knowledge Engineering Lab

20. Thanks! Knowledge Engineering Lab