Lecture 6: Observer Pattern –or– Lindsay Lohan , CSC Genius

1. CSC 313 – Advanced Programming Topics Lecture 6:Observer Pattern –or– Lindsay Lohan, CSC Genius

2. Lindsay Lohan Economy • Studies investigated economy of celebrities • Direct earnings from movies, music, TV, ads • Profits earned by her label, handlers, etc. • Photographers’ income for pictures on TMZ • When placed on cover, increase in magazine sales • $150+ million/year industry for top celebrity

3. Something about Lindsay • Is it her singing?

4. Something about Lindsay • Is it her singing? • Acting key to her success?

5. Something about Lindsay • Is it her singing? • Acting key to her success? • Her keen fashion sense?

6. Something about Lindsay • Is it her singing? • Acting key to her success? • Her keen fashion sense? • Long, slow, very public total breakdown?

7. Something about Lindsay • Is it her singing? • Acting key to her success? • Her keen fashion sense? • Long, slow, very public total breakdown? • No! Really due to her mastery of Observer Pattern!

8. How Lindsay Does It • Millions interested in seeing this unfold • Each and every mistake, • Every outburst that she makes, • And, of course, what or whom she is dating • Could follow her to be around for each event • But may have job, school, or restraining orders • Lindsay uses Observer Pattern for attention & $$$

9. Observer Pattern • Observerswant update when subjectchanges • Must manage this 1:many relationship • Efficiently communicate all of these updates ConcreteSubject stateChanged() : booleansetChanged() : void

10. Foundation of an Industry • Lindsay notifies paparazzi before going out • With notice, photographers go to her destination • Take incriminating pictures of her doing her thing • With pictures takes, paparazzi notify press • Some published in magazines & used on Dr. Phil • Others will get posted to blogs • “Private use” of individuals for some pictures • Entire crisis then unfolds for enquiring minds

11. Textbook Observer Pattern • Readers sent Peoplewhen it is printed • Readersare observers • Subject is People magazine (in this case) • Whenever subject updated, observers notified • Magazines notified of picspaparazzi took • In this relationship, People is an observer • Subject is each photographer

12. Textbook Observer Pattern • Paparazzi alerted by Lindsay before leaving • Lindsay is ultimate subject (& pattern master) • Observers in this relationship are Paparazzi

13. True Genius • Very efficient way for Lindsay to get money • Also has benefit of limiting people following her • Readers notified of each and every change • In this adventure, readersobservers only • Lindsayonly a subject in this implementation • Magazines& photographersplay both roles • Observer Pattern mastered for this to work

14. True Genius Observer Pattern == $$$ in my bank!

15. Observer Is Hollywood • Hollywood Principle: Don’t Call Us, We’ll Call You • If observers were to ask constantly “Are we there yet? Are we there yet?” • Really slow when many observers exist • Principle says to have subject send alerts • Articles only after craziness on Lindsay’s part • When no DUIs, covers with Kate Gosselin instead

16. Co-Dependent Relationships • UML class diagrams with cycles are problem • Two classes inseparable no matter what • Classes completely dependent on each other • Almost impossible to add or extend the design • Since both classes needed, reusing one impossible

17. Replace Cycles With Observer • Cycle still in UML class diagram…

18. Replace Cycles With Observer • Cycle still in UML class diagram… • … but now classes depend only on interfaces • Classes can be replaced at any time • Only requires that new class implement interface • Far easier to add or modify classes now

19. For Next Lecture • Read pages 56 – 69 in book • Great… so how do we code the observer pattern? • Haven’t I seen this pattern before? • How is it actually written in the real world? • Is it really this simple and powerful?