design patterns in qt4

Integrated ComputerSolutions Incorporated

presents

Design Patterns in Qt4

• Alan Ezust• April 2008

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©2008 Integrated Computer Solutions, Inc. All Rights Reserved.

Agenda• What are design patterns for?

– Qt uses at least 20!• What are anti-patterns?• Examples:

– Factory Pattern in plugins– Command pattern– Flyweight pattern

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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."

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3 Kinds of Design Patterns• Structural

– how to organize objects• Behavioral

– how to organize code• Creational

– how to organize code that creates objects

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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”

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Structural Patterns: Composite

• Composite Pattern– Object trees – Parent - Child relationships– What Qt classes implement this?

• [QObject, QWidget, QGraphicsItem, QTreeWidgetItem, QDomNode, QHelpContentItem, QResource]

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Composite Pattern UML

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QObject: Composite and Component

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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?

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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.

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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]

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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.

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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]

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Creational Patterns cont'd• Factory method

– force instances to be created via the method– Qt functions?

• [QFileIconProvider QDomDocument::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 ]

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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

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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

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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

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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]

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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

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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]

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

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Command Pattern Exampleclass 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 };

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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); }

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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]

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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]

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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

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Bridge - Wrapper

(unpublished class)

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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]

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Flyweight exampleclass 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;};

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Flyweight Example cont'dvoid 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;}

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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


presents

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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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