Design Patterns help us solve problems in tried and tested ways. Turns out they also help us understand our framework better, for framework developers also use patterns. In these slides you'll see how Design Patterns are implemented by Qt framework, to better both understand patterns and Qt

1. C++/Qt Design
Patterns
Ynon Perek
ynon@ynonperek.com
http://ynonperek.com
Sunday, May 5, 13

2. Good Code Bad Code
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3. When OO
Goes Wrong
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4. Java InputValidation
• In the past, Java Swing
provided only JTextField for
input texts
• JNumericTextField quickly
appeared by the community
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5. Java InputValidation
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6. OO Fail
• Too many classes
• Hard to maintain
• Frustrated Developers
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7. Enter Design Patterns
• Reusable and tested solutions
• Sustainable for the long run
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8. Enter Design Patterns
• Started by Christopher
Alexander in 1977
• And he was an Architect
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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...”
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10. Patterns Are Not
• Not a detailed solution
• Not a guide to help you build it
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11. Patterns Are
• Describing existing and working solutions
• Your job: Apply to your
world
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12. Patterns In Software
• Defined by GoF
• Categorized:
• Creational
• Behavioral
• Structural
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13. Using Patterns
• Understand
• Memorize
• Apply Repeatedly
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14. Patterns In Qt
• Plenty !
• We’ll show 2:
• Strategy
• Proxy
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15. Strategy Pattern
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16. The Problem
• An input text provides different validation
techniques
• Only one is used at any given time
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17. Approach #1
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18. Why Is It Bad
• Class Explosion
• Hard to reuse validation code
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19. The Better Way
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20. Why Is It Awesome
• Can reuse validators code
• Reduce class explosion
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21. The Pattern
• A validator encapsulates the validation
algorithm
• The general pattern is called Strategy
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22. Intent
• Define a family of algorithms, encapsulate each
one, and make them interchangeable.
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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
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24. Applicability
• Configure a class with one of many
behaviors
• Use different variants of an algorithm
• Eliminate Switch blocks
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25. Structure
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26. Participants
• Strategy
• ConcreteStrategy
• Context
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27. Collaborations
• Strategy + Context = Algorithm
• Context forwards requests to Strategy
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28. Consequences
• Families of related algorithms
• Eliminate switch blocks
• Allow different implementations
• Strategy and Context interface can get
complicated
• Increased number of objects
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29. More Examples
• Layout management
• QAbstractNetworkCache
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30. Keep In Mind
• A Default Strategy will make everyone
happy
• Allow clients to create their own strategies
and pass as parameters
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31. Demo
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32. Q & A
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33. Proxy Pattern
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34. ______________
< Let's do COW >
--------------
^__^
(oo)_______
(__) )/
||----w |
|| ||
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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");
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36. The Proxy Way
• A QString is just a proxy to the data
• Implicitly shared
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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
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38. Proxy Types
• Virtual Proxy
• Protection Proxy
• Remote Proxy
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39. Proxy: Pattern Details
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40. Intent
• Provide a surrogate or placeholder for another
object
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41. Motivation
• Save time by lazy loading heavy resources
• Easy Cows
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42. Applicability
• Access remote resources
• Create resources on demand
• Enforce per-object access control
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43. Structure
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44. Participants
• Proxy
• Subject
• Real Subject
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45. Collaborations
• Proxy forwards requests to Real Subject
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46. Consequences
• Hide the fact that an object is remote
• Optimize load times (virtual proxy)
• Can use for Copy-On-Write
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47. Qt Cow
• Not really “By The Book”
• QString has a Data struct
• Does all work by itself
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48. Qt Patterns
• Patterns are part of any framework (Qt
Included)
• Understanding patterns helps us better
understand the framework
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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
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