presents Design Patterns in Qt 4 Alan Ezust

presents Design Patterns in Qt 4 • Alan Ezust • April 2008 Integrated Computer Solutions Incorporated

Agenda • What are design patterns for? – Qt uses at least 20! • What are anti-patterns? • Examples: – Factory Pattern in plugins – Command pattern – Flyweight pattern © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 2

What is a Design Pattern? • It is a named rule or convention – Solves a specific software design problem • What kinds of problems? – those that lead to unmaintainable or inflexible code – Affectionately called: anti-patterns • Gang of Four (Gamma, Helm, Johnson, Vlissides): – "Design Patterns are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context. " © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 3

3 Kinds of Design Patterns • Structural – how to organize objects • Behavioral – how to organize code • Creational – how to organize code that creates objects © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 4

What is an Anti-Pattern? – Copy and paste code – Reinventing the (square) wheel • reimplementing a function/class that already exists in your framework – Interface bloat • too many methods in a class • too many arguments in a function – Hard coding • embedding assumptions about the environment – God Object • Too many dependencies on a single object that “does everything” © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 5

Structural Patterns: Composite • Composite Pattern – Object trees – Parent - Child relationships – What Qt classes implement this? • [QObject, QWidget, QGraphics. Item, QTree. Widget. Item, QDom. Node, QHelp. Content. Item, QResource] © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 6

Composite Pattern UML © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 7

QObject: Composite and Component © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 8

Decorator Pattern • Decorator Pattern (aka Wrapper) – Attach additional responsibilities to an object dynamically – Instead of using inheritance, wrap around the object and delegate • QScroll. Area provides a decoration around its underlying widget, and scrollbars as necessary • QTab. Widget decorates itself around underlying widgets and allows user to select between them. • Fits into the larger QLayout as the underlying widget would. – Other instances of Decorator pattern? © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 9

Problems Decorator Solves • Easier to add scrollability to other existing classes (QGraphics. View) • Other ways to add “scrollable” to a Text. Area: – Multiple inheritance: Scrollable. Text. Area derives from Text. Area and Scroll. Component – Push scrollability up into one of the Text. Area's base classes, switched off except when necessary – Scroll. Component wraps around Text. Area and 'decorates' it. © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 10

Structural Patterns • Facade (aka Adapter, and sometimes Wrapper too) Pattern – Cover up that ugly pointer/array, or non-portable C code – Qt classes? • [QFile, QProcess, QThread, QWidget, QSql. Database, QString, etc] • Reflection Pattern – The ability to inspect an object for information about its members – Qt classes? • [QMeta. Object, QMeta. Property] © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 11

Behavioral Patterns • Iterator Pattern – type-independent class or mechanism for traversing a collection – What Qt structures implement this? • [iterator, Iterator, and foreach loops] • Visitor Pattern – class/structure that visits generic/polymorphic nodes (in a collection) and provides a way to plug in a different visiting action • QTree. Widget. Item. Iterator a type-restricted iterator, can be considered a Visitor • QAbstract. Item. Model provides the visiting action to a QTree. View, which visits nodes. © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 12

Creational Patterns • Singleton Pattern – Only one instance can be created – Qt classes: • [QApplication, QPlugin. Loader: : static. Instances()]] • Monostate Pattern – Each instance has the same value – Qt classes? • [QSettings] © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 13

Creational Patterns cont'd • Factory method – force instances to be created via the method – Qt functions? • [QFile. Icon. Provider QDom. Document: : create. Element()] • Abstract Factory Pattern – A class with a virtual Factory method that can be overridden in derived classes – Used to eliminate dependencies between libraries • [QImage. IOPlugin, QItem. Editor. Factory, QAbstract. Extension. Factory, etc ] © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 14

Plugins and Creational Patterns • Plugin architectures need to use Creational patterns in creative ways – applications must create instances of objects that are defined in plugins – exact types are not known to the application • Well designed interfaces are key: – they expose common features of plugin-supplied classes • Plugins supply concrete factories © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 15

Qt Plugins • Most Qt plugins implement two classes: – The “Plugin class” contains a Factory method – The other class (created by the factory) does all the 'real work' • QImage. IOPlugin is an Abstract Factory for QImage. IOHandler objects • To define your own image format plugin, you must extend both classes – reading and writing is done by the Handler © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 16

Qt Image Plugins • Qt creates one instance of each QImage. Plugin (singleton) • Plugin libs register 'plugin' class via the Q_EXPORT_PLUGIN 2 macro • When an image of type T needs to be loaded – each plugin is checked with its capabilities(T) method – if a plugin is found that can handle the file format and requested operation, its handler is created and used © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 17

Behavioral Patterns • Observer Pattern – publish-subscribe event model – Where in Qt can we find this? • signals/slots • Mediator Pattern – Promotes loose coupling between objects – Where in Qt? • [QAbstract. Item. Delegate] © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 18

Behavioral Patterns • Template Method pattern – Define the skeleton of an algorithm in a template function – Qt algorithms use template methods – Qt containers are template-based © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 19

Behavioral Patterns • Memento Pattern – Capture an object's state so it can be restored later – In Qt? • [QData. Stream, QMain. Window: : save. State()] • Command Pattern – – Abstract an executable 'unit of work' switch statement vs virtual method call Transaction journaling, undo, rollback, etc What Qt classes implement this? • [QUndo. Command, QRunnable, QProcess, QAction, QThread, Qt. Concurrent. Run] © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 20

Command Pattern Example • Long running threads with Progress Bar • A uniform solution would be nice © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 21

© 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 22

Command Pattern Example class Command : public QObject { Q_OBJECT public: Command() : m_aborted(false) {} virtual void run() = 0; void set. Aborted(bool a); bool is. Aborted() {return m_aborted; } signals: void new. Row(QString, QString); void new. Progress. Value(int); void new. Progress. Range(int, int); protected: volatile bool m_aborted; 1 }; © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 23

void Main. Window: : run. In. Thread(Command* cmd) { if (!can. Start. Work. Thread()) return; work. Thread = new Work. Thread(cmd); connect (work. Thread, SIGNAL(finished()), this, SLOT(stop. Working())); connect (cmd, SIGNAL(new. Row(QString, QString)), &results. Model, SLOT(add. Row(QString, QString))); connect (cmd, SIGNAL(new. Progress. Range(int, int)), &progress. Bar, SLOT(set. Range(int, int))); connect (cmd, SIGNAL(new. Progress. Value(int)), &progress. Bar, SLOT(set. Value(int))); results. Model. clear(); stop. Button. set. Enabled(true); work. Thread->start(); } void Main. Window: : tag() { run. In. Thread(new Tag. Command); } void Main. Window: : import() { run. In. Thread(new Import. Command); } © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 24

Behavioral Patterns • Strategy Pattern – Make algorithms interchangable as virtual functions on objects • QXml. Reader uses the strategy pattern – plug in a QXml. Content. Handler for handling parse events in different ways • QWidget: : render() – different render technique for each kind of widget – Similar to the command pattern • use virtual method call instead of big switch statement – Qt Classes? • [Qt. Algorithms, QSql. Driver. Plugin] © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 25

Behavioral/Structural • Interpreter Pattern – A 'word' (a function, or operator) in the language is a class in the application • Instances of these 'words' each have an evaluate method (similar to command pattern). • Can be joined together with “AND”, “OR”, or constraint objects to form more complex expressions – This pattern is used to implement an interpreter for a language • [QReg. Exp, QScript. Engine] © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 26

Structural Patterns • Bridge – Decouple abstraction from implementation • so they can vary independently – Many Qt classes have a public API class and a corresponding private 'impl' class. This helps Trolltech: • achieve platform independence • achieve sharing, and lazy copy on write • make major changes to implementation without breaking binary compatibility with linked applications • implement the next two design patterns © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 27

Bridge - Wrapper (unpublished class) © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 28

Structural Patterns • Proxy Pattern – Provide a surrogate or placeholder for another object to control access to it • Flyweight Pattern – Use lightweight objects as wrappers which point to shared actual values • Qt Classes? – [QString, QList, QMap, QDom. Node, implicitly shared classes] © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 29

Flyweight example class My. String { public: My. String() : m_Impl(new My. String. Private) {} My. String(const char* p) : m_Impl(new My. String. Private(p)) {} My. String(const My. String& str); ~My. String(); void operator=(const My. String& str); void display() const ; int length() const; operator const char*() const; private: My. String. Private* m_Impl; }; © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 30

Flyweight Example cont'd void My. String: : operator=(const My. String& str) { if (str. m_Impl != m_Impl) { if (--m_Impl -> m_Ref. Count == 0) delete m_Impl; m_Impl = str. m_Impl; ++(m_Impl->m_Ref. Count); } } My. String: : My. String(const My. String& str) : m_Impl(str. m_Impl) { m_Impl -> m_Ref. Count++; } My. String: : ~My. String() { if (--m_Impl -> m_Ref. Count == 0) delete m_Impl; } © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 31

Summary • What are design patterns for? – Help you avoid common pitfalls – What are anti-patterns? • Qt 4 uses at least 20 of them! – How many can you name? – Where are they used in Qt 4? • Why should I use them in my code? • Code examples – Command pattern – Flyweight pattern © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 32

presents Integrated Computer Solutions Incorporated

Bibliography • [Ezust 07] Introduction to Design Patterns in C++ and Qt 4. Alan Ezust and Paul Ezust. 2007, Prentice Hall. 0 -13 -187905 -7. • [Fowler 04] UML Distilled. Third Edition. Martin Fowler. 2004. Addison Wesley. 0 -321 -19368 -7. • [Gamma 95] Design Patterns. Elements of Reusable Object-Oriented Software. Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides. 1995. Addison-Wesley. 0 -201 -63361 -2. • [Thelin 07] Foundations of Qt Development. Johan Thelin. 2007. Apress. 978 -1 -59059 -831 -3 • [Blanchette 08] C++ GUI Programming with Qt 4, 2/E. Jasmin Blanchette and Mark Summerfield. 2008, Prentice Hall. 0 -13 -235416 © 2008 Integrated Computer Solutions, Inc. All Rights Reserved. www. ics. com 34

presents Thank you! For more information please visit us at www. ics. com Integrated Computer Solutions Incorporated