Design Patterns in Qt4

1. Alan Ezust • April 2008 Design Patterns in Qt4

2. Agenda What are design patterns for? Qt uses at least 20! What are anti-patterns? Examples: Factory Pattern in plugins Command pattern Flyweight pattern

3. 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."

4. 3 Kinds of Design Patterns Structural how to organize objects Behavioral how to organize code Creational how to organize code that creates objects

5. 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”

6. Structural Patterns: Composite Composite Pattern Object trees Parent - Child relationships What Qt classes implement this? [QObject, QWidget, QGraphicsItem, QTreeWidgetItem, QDomNode, QHelpContentItem, QResource]

7. Composite Pattern UML

8. QObject: Composite and Component

9. Decorator Pattern Decorator Pattern (aka Wrapper)‏ Attach additional responsibilities to an object dynamically Instead of using inheritance, wrap around the object and delegate QScrollArea provides a decoration around its underlying widget, and scrollbars as necessary QTabWidget 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?

10. Problems Decorator Solves Easier to add scrollability to other existing classes (QGraphicsView)‏ Other ways to add “scrollable” to a TextArea: Multiple inheritance: ScrollableTextArea derives from TextArea and ScrollComponent Push scrollability up into one of the TextArea's base classes, switched off except when necessary ScrollComponent wraps around TextArea and 'decorates' it.

11. 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, QSqlDatabase, QString,etc] Reflection Pattern The ability to inspect an object for information about its members Qt classes? [QMetaObject, QMetaProperty]

12. 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 QTreeWidgetItemIterator a type-restricted iterator, can be considered a Visitor QAbstractItemModel provides the visiting action to a QTreeView, which visits nodes.

13. Creational Patterns Singleton Pattern Only one instance can be created Qt classes: [QApplication, QPluginLoader::staticInstances()]] Monostate Pattern Each instance has the same value Qt classes? [QSettings]

14. Creational Patterns cont'd Factory method force instances to be created via the method Qt functions? [QFileIconProviderQDomDocument::createElement()] Abstract Factory Pattern A class with a virtual Factory method that can be overridden in derived classes Used to eliminate dependencies between libraries [QImageIOPlugin, QItemEditorFactory, QAbstractExtensionFactory, etc ]

15. 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

16. 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' QImageIOPlugin is an Abstract Factory for QImageIOHandler objects To define your own image format plugin, you must extend both classes reading and writing is done by the Handler

17. Qt Image Plugins Qt creates one instance of each QImagePlugin (singleton)‏ Plugin libs register 'plugin' class via the Q_EXPORT_PLUGIN2 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

18. 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? [QAbstractItemDelegate]

19. 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

20. Behavioral Patterns Memento Pattern Capture an object's state so it can be restored later In Qt? [QDataStream, QMainWindow::saveState()] Command Pattern Abstract an executable 'unit of work' switch statement vs virtual method call Transaction journaling, undo, rollback, etc What Qt classes implement this? [QUndoCommand, QRunnable, QProcess, QAction, QThread, QtConcurrentRun]

21. Command Pattern Example Long running threads with Progress Bar A uniform solution would be nice

23. Command Pattern Example class Command : public QObject { Q_OBJECT public: Command() : m_aborted(false) {} virtual void run() = 0; void setAborted(bool a); bool isAborted() {return m_aborted;} signals: void newRow(QString, QString); void newProgressValue(int); void newProgressRange(int, int); protected: volatile bool m_aborted; 1 };

24. void MainWindow::runInThread(Command* cmd) { if (!canStartWorkThread()) return; workThread = new WorkThread(cmd); connect (workThread,SIGNAL(finished()), this, SLOT(stopWorking())); connect (cmd, SIGNAL(newRow(QString, QString)), &resultsModel, SLOT(addRow(QString,QString))); connect (cmd, SIGNAL(newProgressRange(int, int)), &progressBar, SLOT(setRange(int, int))); connect (cmd, SIGNAL(newProgressValue(int)), &progressBar, SLOT(setValue(int))); resultsModel.clear(); stopButton.setEnabled(true); workThread->start(); } void MainWindow::tag() { runInThread(new TagCommand); } void MainWindow::import() { runInThread(new ImportCommand); }

25. Behavioral Patterns Strategy Pattern Make algorithms interchangable as virtual functions on objects QXmlReader uses the strategy pattern plug in a QXmlContentHandler 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? [QtAlgorithms, QSqlDriverPlugin]

26. 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 [QRegExp, QScriptEngine]

27. 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

28. Bridge - Wrapper (unpublished class)‏

29. 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, QDomNode, implicitly shared classes]

30. Flyweight example class MyString { public: MyString() : m_Impl(new MyStringPrivate) {} MyString(const char* p)‏ : m_Impl(new MyStringPrivate(p)) {} MyString(const MyString& str); ~MyString(); void operator=(const MyString& str); void display() const ; int length() const; operator const char*() const; private: MyStringPrivate* m_Impl; };

31. Flyweight Example cont'd void MyString::operator=(const MyString& str) { if (str.m_Impl != m_Impl) { if (--m_Impl -> m_RefCount == 0)‏ delete m_Impl; m_Impl = str.m_Impl; ++(m_Impl->m_RefCount); } } MyString::MyString(const MyString& str) : m_Impl(str.m_Impl) { m_Impl -> m_RefCount++; } MyString::~MyString() { if (--m_Impl -> m_RefCount == 0) delete m_Impl; }

32. Summary What are design patterns for? Help you avoid common pitfalls What are anti-patterns? Qt4 uses at least 20 of them! How many can you name? Where are they used in Qt4? Why should I use them in my code? Code examples Command pattern Flyweight pattern

34. Bibliography [Ezust07] Introduction to Design Patterns in C++ and Qt4. Alan Ezust and Paul Ezust. 2007, Prentice Hall. 0-13-187905-7. [Fowler04] UML Distilled. Third Edition. Martin Fowler. 2004. Addison Wesley. 0-321-19368-7 . [Gamma95] Design Patterns. Elements of Reusable Object-Oriented Software. Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides. 1995. Addison-Wesley. 0-201-63361-2. [Thelin07] Foundations of Qt Development. Johan Thelin. 2007. Apress. 978-1-59059-831-3 [Blanchette08] C++ GUI Programming with Qt 4, 2/E. Jasmin Blanchette and Mark Summerfield. 2008, Prentice Hall. 0-13-235416

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