QT ProgrammingQT Programming

Ruku Ruku RoychowdhuryRoychowdhury

Background• QT is a cross platform application framework.

• Widely used to develop GUI applications.

• Originally developed by Norwegian company Trolltech.

• After Nokia's acquisition of TrollTech, now Nokia's Qt Development Frameworks division is enhancing QT.

Some Applications with QT

• Google Earth

• KDE

• Adobe Photoshop Album

• Skype

• VLC media player

• VirtualBox

Features• Cross-platform application framework.

• Qt uses standard C++.

• Makes extensive use of Meta Object Compiler together with several macros to enrich the language.

• Can be used with many other languages by Language Binding.

C++ Features• Procedural as well as Object oriented language

• Abstraction

• Inheritance– Allows Multiple inheritance

• Polymorphism– Runtime Polymorphism (Virtual functions)– Static Polymorphism (Function Overloading)

QT Class Hierarchy

Multiple Inheritance

Event Handling• Event handling is done by Signals and Slots

• Signal– Emitted when a particular event occurs (e.g., clicked())– Qt widgets: predefined signals– Developer can create custom signals

• Slot– Function called in response to a signal– Qt widgets: predefined slots– Developer can create custom slots

• Connection: – connects signals to slots – established by developer– handled by Qt framework

Connections

Meta Object System• Meta information not supported by standard C++

– QObject class• Base class for objects that use meta-object system

– Q_OBJECT macro• Enables meta-object features

– “moc” tool• –Parses Q_OBJECT macro• –Extends source code with additional functions (extra

files)

Used for: getting list of signals & slots, properties and text translation

Meta-Object FeaturesFeatures provided by meta-object code:• Signals and slots

• metaObject() –returns associated meta-object for the class

• className() –returns class name as a string, without requiring native run-time type information (RTTI) support through the C++ compiler

• inherits() –returns whether this instance inherits the specified QObjectclass

• tr() –translate strings for internationalization

• setProperty() and property() –set and get properties of object by Property name

Signals and Slots (in details)

• QT signals and slots are TYPE SAFE– Signals & slots have to have exactly same parameters– It does not give any compilation error / warning– But gives runtime error when executing connect():

– No connection if either sender or receiver doesn’t exist– Or if signatures of signal and slot do not match

• connect() returns boolean value indicating success

Multiple Signal-Slot Connections

• One signal can be connected to multiple slots and vice versa also holds good.

Demonstration

• Every QWidget has a set of predefined signals and slots.

• A QPushButton and QRadioButton has predefined signals such as clicked() which they inherit from QAbstractButton.

• Slots can be written for these predefined signals and can be called directly without associating through connect().

Custom Signals and Slots

• A custom signal and slot can also be created.

• Custom signals and slots can be created in user’s own classes.– E.g. mysignal signal in the code sample– E.g. myslot slot in the code sample

• These signals and slot need to be explicitly connected with the connect() statement.– E.g.

QObject::connect(this,SIGNAL(mysignal()),this,SLOT(myslot())); in code sample

Example• Converter Constructor• converter2::converter2(QWidget *parent) : QWidget(parent), ui(new

Ui::converter2) { ui->setupUi(this); ui->label->setText("Unit1"); ui->label_2->setText("Unit2"); QObject::connect(this,SIGNAL(mysignal()),this,SLOT(myslot())); input=""; }

• Signal • Void converter2::on_pushButton_2_clicked() { emit mysignal(); }

• Slot Declaration• void converter2::myslot()

{ time_t now; time(&now); QString time=ctime(&now); ui->textEdit->setText(time); }

Creating custom signals and slots in the QWidgets

• User can create his own custom signals and slots for a QWidget.

• User has to extend any widget to create his custom subclass of that widget.– E.g. customButton is extended from QPushButton.

• It has custom signal clicked(int id) and slot click() in addition to the predefined signals and slots in QPushButton class.

Example• Custom Button Class

• class customButton: public QPushButton{ Q_OBJECT public: customButton(const QString& text, int id,QWidget* parent = NULL); customButton(const QString& text, QWidget* parent = NULL); private: static int nextId; int m_id; public slots: void click(); signals:void clicked (int id); };//Custom signal for CustomButton

Compilation

• Every class is declared in the header file.• Every class is defined in the .cpp file.

– For example, converter_2.h contains the class declarations

– Converter_2.cpp contains the class definitions.

converter_2.h

moc_converter_2.cpp

Moc parses .h file change the QT specific code to plain c++ code

+ converter_2.cpp

Linker links both of these .cpp file

Meta Object Compilation

• const QMetaObject QObject::staticMetaObject method is used to have meta object of the class defined in .h files.

• Moc generated code places data into two arrays

– qt_meta_data_<Classname> array• In the qt_meta_data array moc creates a own line for every QMeta

related item

– qt_meta_stringdata_<Classname> array• In the qt_meta_stringdata array moc puts all the method,

parameter, slot, signal.

Contd.

• when a signal is called, QMetaObject::activate() is called with the signal’s name and the signal’s arguments.

• The activate() function then figures out which connections have been established and fetches the names for the appropriate slots.

• Then it calls qt_metacall with the slot names and the arguments given to the signal.

• the metacall function delegates this with the help of a large switch—case statement to the real slots.

Thank You