Good Code Bad Code
Sunday, May 5, 13
When OO Goes Wrong
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Java Input Validation
• In the past, Java Swing provided only JTextField for input texts
• JNumericTextField quickly appeared by the community
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Java Input Validation
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OO Fail
• Too many classes
• Hard to maintain
• Frustrated Developers
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Enter Design Patterns
• Reusable and tested solutions
• Sustainable for the long run
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Enter Design Patterns
• Started by Christopher Alexander in 1977
• And he was an Architect
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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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Patterns Are Not
• Not a detailed solution
• Not a guide to help you build it
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Patterns Are
• Describing existing and working solutions
• Your job: Apply to your world
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Patterns In Software
• Defined by GoF
• Categorized:
• Creational
• Behavioral
• Structural
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Using Patterns
• Understand
• Memorize
• Apply Repeatedly
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Patterns In Qt
• Plenty !
• We’ll show 2:
• Strategy
• Proxy
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Strategy Pattern
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The Problem
• An input text provides different validation techniques
• Only one is used at any given time
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Approach #1
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Why Is It Bad
• Class Explosion
• Hard to reuse validation code
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The Better Way
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Why Is It Awesome
• Can reuse validators code
• Reduce class explosion
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The Pattern
• A validator encapsulates the validation algorithm
• The general pattern is called Strategy
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Intent
• Define a family of algorithms, encapsulate each one, and make them interchangeable.
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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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Applicability
• Configure a class with one of many behaviors
• Use different variants of an algorithm
• Eliminate Switch blocks
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Structure
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Participants
• Strategy
• ConcreteStrategy
• Context
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Collaborations
• Strategy + Context = Algorithm
• Context forwards requests to Strategy
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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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More Examples
• Layout management
• QAbstractNetworkCache
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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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Demo
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Q & A
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Proxy Pattern
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______________ < Let's do COW > -------------- \ ^__^ \ (oo)\_______ (__)\ )\/\ ||----w | || ||
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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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The Proxy Way
• A QString is just a proxy to the data
• Implicitly shared
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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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Proxy Types
• Virtual Proxy
• Protection Proxy
• Remote Proxy
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Proxy: Pattern Details
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Intent
• Provide a surrogate or placeholder for another object
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Motivation
• Save time by lazy loading heavy resources
• Easy Cows
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Applicability
• Access remote resources
• Create resources on demand
• Enforce per-object access control
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Structure
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Participants
• Proxy
• Subject
• Real Subject
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Collaborations
• Proxy forwards requests to Real Subject
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Consequences
• Hide the fact that an object is remote
• Optimize load times (virtual proxy)
• Can use for Copy-On-Write
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Qt Cow
• Not really “By The Book”
• QString has a Data struct
• Does all work by itself
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Qt Patterns
• Patterns are part of any framework (Qt Included)
• Understanding patterns helps us better understand the framework
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Thanks For Listening !
Slides at: http://ynonperek.com
Questions / Comments:[email protected]
Stock Photos from:http://123rf.com
Sunday, May 5, 13
Thanks For Listening
Sunday, May 5, 13
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