1. Design Patterns and MVC
Sway Wang
2014.12.25

2. Outline
• What is Design Pattern?
– Singleton pattern
– Strategy pattern
– Observer pattern
• What is MVC?
– C# example
– Ruby on Rails
• Conclusion

3. What is Design Pattern?
• Patterns are formalized best practices that the
programmer can use to solve common problems when
designing an application or system

4. The History of Design Pattern
• Christopher Alexander (1977)
• Kent Beck and Ward Cunningham (1987)
• Gamma, Erich; Richard Helm, Ralph Johnson,
and John Vlissides
– Design Patterns: Elements of Reusable Object-
Oriented Software (1995)

5. History

6. Gang of Four (GoF) Patterns
• Creational Patterns
(abstracting the object-instantiation process)
– Factory Method Abstract Factory
– Singleton Builder
– Prototype
• Structural Patterns
(how objects/classes can be combined to form larger structures)
– Adapter Bridge
– Composite Decorator
– Facade Flyweight
– Proxy

7. Gang of Four (GoF) patterns
• Behavioral Patterns
(communication between objects)
– Command Interpreter Iterator
– Mediator Observer State
– Strategy Chain of Responsibility Visitor
– Template Method

8. Singleton Pattern
• It restricts the instantiation of a class to
one object

9. Singleton Pattern(C#)

10. Is Your Singleton Broken?
• The singleton pattern must be carefully
constructed in multi-threaded applications
• [OSDC 2014] fr3@K - Is Your Singleton
Broken? - YouTube

11. Is Your Singleton Broken?
• The singleton pattern must be carefully
constructed in multi-threaded applications
• [OSDC 2014] fr3@K - Is Your Singleton
Broken? – YouTube
• Global variable issue

12. Strategy Pattern
• It enables an algorithm's behavior to be
selected at runtime

13. Strategy Pattern: Duck Example

14. Now We Need the Ducks to Fly

15. Now We Need the Ducks to Fly

16. Decoy Duck

17. How About a Interface?
Duplicate code

18. Strategy Pattern: Duck Example

19. Strategy Pattern: Duck Example

20. Strategy Pattern: Duck Example

21. Strategy Pattern: Duck Example

22. Strategy Pattern: Duck Example

23. Setting Behavior Dynamically

24. Strategy Pattern: Duck Example

25. Design Principle
多用合成，少用繼承

26. Open-Closed Principle(OCP)
Software entities (classes, modules,
functions, etc.) should be open for
extension, but closed for modification
一個軟體應該要夠開放使得它可以被擴充，
但是也要夠封閉以避免不必要的修改

27. Strategy Pattern
• It enables an algorithm's behavior to be
selected at runtime

28. Observer Pattern
• Newspaper subscription
• Publishers + Subscribers = Observer Pattern

29. Observer Pattern
• Newspaper subscription
• Publishers + Subscribers = Observer Pattern

30. Observer Pattern

31. Observer Pattern

32. Observer Example (C#)

33. Subject Example (C#)

34. Subject Example (C#)

35. The History of MVC
• MVC is really old and was first described in
1979 by Trygve Reenskaug
• Smalltalk at Xerox PARC.
• Today MVC is used in most modern web and
GUI frameworks

36. The History of MVC
• Ruby On Rails: A popular Ruby web-
framework
• Apple Cocoa: Apple's framework for
developing Mac OS and iOS applications
• ASP.Net Framework: Microsoft's web-
framework for implementing web applications
• ... And many many more languages and
frameworks

37. MVC

38. View

39. Controller

40. Model

41. Perl CGI – 計數器

42. PHP計數器

43. Ruby on Rails

44. 缺點
• HTML和邏輯綁在一起
• 網站規模變大後不易修改
• 難以分工
• Web Framework
– 導入OO概念
– 制定好了一套規範和慣例，讓開發者在該架構下來進行
開發

45. Web Framework
• 導入OO概念
• 制定好了一套規範和慣例，讓開發者在該架
構下來進行開發

46. Conclusion
• 要精通OOP一定要會Design Pattern
• Design Pattern三種境界
– 什麼東西都套pattern
– 知道什麼時候該用pattern
– KISS principle
• MVC架構的AP後續維護較容易
• 設計良好的程式碼當未來需求變更時，較容易改
變