Qt Design Patterns
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The document discusses design patterns in software development, particularly focusing on the strategy and proxy patterns in C++/Qt. It emphasizes the importance of reusable and tested solutions to improve code maintainability and reduce complexity. Additionally, it outlines the structure, intent, and applicability of the strategy and proxy patterns through examples and participant roles.
Qt Design Patterns
- 1. C++/Qt Design Patterns Ynon Perek ynon@ynonperek.com http://ynonperek.com Sunday, May 5, 13
- 2. Good Code Bad Code Sunday, May 5, 13
- 3. When OO Goes Wrong Sunday, May 5, 13
- 4. Java InputValidation • In the past, Java Swing provided only JTextField for input texts • JNumericTextField quickly appeared by the community Sunday, May 5, 13
- 5. Java InputValidation Sunday, May 5, 13
- 6. OO Fail • Too many classes • Hard to maintain • Frustrated Developers Sunday, May 5, 13
- 7. Enter Design Patterns • Reusable and tested solutions • Sustainable for the long run Sunday, May 5, 13
- 8. Enter Design Patterns • Started by Christopher Alexander in 1977 • And he was an Architect Sunday, May 5, 13
- 9. Example: Street Caffe “The street cafe provides a unique setting, special to cities: a place where people can sit lazily, legitimately, be on view, and watch the world go by...” Sunday, May 5, 13
- 10. Patterns Are Not • Not a detailed solution • Not a guide to help you build it Sunday, May 5, 13
- 11. Patterns Are • Describing existing and working solutions • Your job: Apply to your world Sunday, May 5, 13
- 12. Patterns In Software • Defined by GoF • Categorized: • Creational • Behavioral • Structural Sunday, May 5, 13
- 13. Using Patterns • Understand • Memorize • Apply Repeatedly Sunday, May 5, 13
- 14. Patterns In Qt • Plenty ! • We’ll show 2: • Strategy • Proxy Sunday, May 5, 13
- 15. Strategy Pattern Sunday, May 5, 13
- 16. The Problem • An input text provides different validation techniques • Only one is used at any given time Sunday, May 5, 13
- 17. Approach #1 Sunday, May 5, 13
- 18. Why Is It Bad • Class Explosion • Hard to reuse validation code Sunday, May 5, 13
- 19. The Better Way Sunday, May 5, 13
- 20. Why Is It Awesome • Can reuse validators code • Reduce class explosion Sunday, May 5, 13
- 21. The Pattern • A validator encapsulates the validation algorithm • The general pattern is called Strategy Sunday, May 5, 13
- 22. Intent • Define a family of algorithms, encapsulate each one, and make them interchangeable. Sunday, May 5, 13
- 23. Motivation • Client code is simpler if the algorithm is external to it • No need to support an algorithm we don’t use • We need to make it easy to add new algorithms Sunday, May 5, 13
- 24. Applicability • Configure a class with one of many behaviors • Use different variants of an algorithm • Eliminate Switch blocks Sunday, May 5, 13
- 25. Structure Sunday, May 5, 13
- 26. Participants • Strategy • ConcreteStrategy • Context Sunday, May 5, 13
- 27. Collaborations • Strategy + Context = Algorithm • Context forwards requests to Strategy Sunday, May 5, 13
- 28. Consequences • Families of related algorithms • Eliminate switch blocks • Allow different implementations • Strategy and Context interface can get complicated • Increased number of objects Sunday, May 5, 13
- 29. More Examples • Layout management • QAbstractNetworkCache Sunday, May 5, 13
- 30. Keep In Mind • A Default Strategy will make everyone happy • Allow clients to create their own strategies and pass as parameters Sunday, May 5, 13
- 31. Demo Sunday, May 5, 13
- 32. Q & A Sunday, May 5, 13
- 33. Proxy Pattern Sunday, May 5, 13
- 34. ______________ < Let's do COW > -------------- ^__^ (oo)_______ (__) )/ ||----w | || || Sunday, May 5, 13
- 35. Example Code QString s1 = getText(); QString s2 = getText(); QString s3 = s1; // "Hello World" is kept in memory only once s1.append(" And now it's copied"); Sunday, May 5, 13
- 36. The Proxy Way • A QString is just a proxy to the data • Implicitly shared Sunday, May 5, 13
- 37. class MyCow { public: MyCow &operator=(const MyCow &other) { m_data = other.m_data; } void write() { m_data = new Data( m_data ); m_data.write(); } private: Data *m_data; }; The Proxy Way Sunday, May 5, 13
- 38. Proxy Types • Virtual Proxy • Protection Proxy • Remote Proxy Sunday, May 5, 13
- 39. Proxy: Pattern Details Sunday, May 5, 13
- 40. Intent • Provide a surrogate or placeholder for another object Sunday, May 5, 13
- 41. Motivation • Save time by lazy loading heavy resources • Easy Cows Sunday, May 5, 13
- 42. Applicability • Access remote resources • Create resources on demand • Enforce per-object access control Sunday, May 5, 13
- 43. Structure Sunday, May 5, 13
- 44. Participants • Proxy • Subject • Real Subject Sunday, May 5, 13
- 45. Collaborations • Proxy forwards requests to Real Subject Sunday, May 5, 13
- 46. Consequences • Hide the fact that an object is remote • Optimize load times (virtual proxy) • Can use for Copy-On-Write Sunday, May 5, 13
- 47. Qt Cow • Not really “By The Book” • QString has a Data struct • Does all work by itself Sunday, May 5, 13
- 48. Qt Patterns • Patterns are part of any framework (Qt Included) • Understanding patterns helps us better understand the framework Sunday, May 5, 13
- 49. Thanks For Listening ! Slides at: http://ynonperek.com Questions / Comments: ynon@ynonperek.com Stock Photos from: http://123rf.com Sunday, May 5, 13
- 50. Thanks For Listening Sunday, May 5, 13