First Steps with Qt

Julien FinetKitware Inc.Jan. 05th 2010

Hello World!

#include <QApplication>#include <QPushButton>

int main(int argc, char * argv[]){ QApplication app(argc, argv);

QPushButton button(“Hello World”);

button.show();

return app.exec();}

The QApplication object must be created before any GUI-related features of Qt are used.

Enter the loop of events. Qt receives and processes user and system events and passes these on to the appropriate widgets.

Qt classesQObject

QWidget

QLabelQAbstractButton

QPushButtonQCheckBox

QAction

QPaintDevice

…

…

QObjects

• The QObject class is the base class of all Qt objects

• When deleted, the parent deletes its children.

class QObject{public: QObject(QObject * parent = 0);

virtual ~QObject();

const QObjectList & children () const;

void setParent(QObject * parent);};

Add itself to the parent’s children list.

qobject.h

QWidgets

• If a widget’s parent is 0, the widget is a window.

• When shown/hidden, the parent shows/hides its children.

• When enabled/disabled, the parent enables/disables its children (except for children explicitly hidden).

QObjects

• QObjects can be safely created on the heap.

• Be careful when instantiating on the stack.

…int main(int argc, char* argv[]){ … QWidget parentWidget; QPushButton* button = new QPushButton(“Hello World”, parentWidget); …}

parentWidget’s destructor deletes button

int main(int argc, char* argv[]){ … QWidget parentWidget; QPushButton button(“Hello World”, & parentWidget); …}

int main(int argc, char* argv[]){ … QPushButton button(“Hello World”); QWidget parentWidget; button.setParent(&parentWidget); …}

Error: button is deleted twiceThe destructor of button is called first, and it removes itself from its parent.

Non QObject

• May be instantiated on the heap or stack.

• It’s safer to create them on the stack.

…QVector<int>* ages;…ages = new QVector<int>(10);…delete ages;…

…QString firstName(“John”);QString lastName(“Doe”);Qvector<int> ages;…ages.resize(10);…

QLayout – Geometry Manager

• Instead of specifying widget coordinates for each QWidget, use QLayouts.

…QGroupBox* parameters = new QGroupBox(“Parameters”, this);QLabel* label = new QLabel(“Number of Iterations:”, parameters);QSpinBox* spinBox = new QSpinBox(parameters);…label->setGeometry(0, 0, 50, 20);spinBox->setGeometry(56, 0, 30, 20);…

…QGroupBox* parameters = new QGroupBox(“Parameters, this);QLabel* label = new QLabel(“Number of Iterations:”, parameters);QSpinBox* spinBox = new QSpinBox(parameters);…QHBoxLayout* layout = new QHBoxLayout;layout->addWidget(label);layout->addWidget(spinBox);parameters->setLayout(layout);…Without QLayout With QLayout

QLayout class diagram

QObject

QLayout

QStackedLayoutQBoxLayout

QVBoxLayoutQHBoxLayout

QGridLayoutQFormLayout

QLayout Example 1/2

• QHBoxLayout

• QVBoxLayout

…QHBoxLayout* layout = new QHBoxLayout;layout>addWidget(iterationNumberLabel);layout->addWidget(iterationNumberSpinBox);layout->addWidget(thresholdLabel);layout->addWidget(thresholdDoubleSpinBox);parameters->setLayout(layout);…

…QVBoxLayout* layout = new QVBoxLayout;layout>addWidget(iterationNumberLabel);layout->addWidget(iterationNumberSpinBox);layout->addWidget(thresholdLabel);layout->addWidget(thresholdDoubleSpinBox);parameters->setLayout(layout);…

Qlayout Example 2/2

• QGridLayout

• QFormLayout

QGridLayout* layout = new QGridLayout;layout>addWidget(iterationNumberLabel, 0, 0, 1, 1);layout->addWidget(iterationNumberSpinBox , 0, 1, 1, 1);layout->addWidget(thresholdLabel, 1, 0, 1, 1);layout->addWidget(thresholdDoubleSpinBox, 1, 1, 1, 1);parameters->setLayout(layout);…

QFormLayout* layout = new QFormLayout;layout>setWidget(0, QFormLayout::LabelRole, iterationNumberLabel);layout->setWidget(0, QFormLayout::FieldRole, iterationNumberSpinBox);layout->setWidget(1, QFormLayout::LabelRole, thresholdLabel);layout->addWidget(1, QFormLayout::FieldRole, thresholdDoubleSpinBox);parameters->setLayout(layout);…

Signals & Slots

• Qt uses signals and slots for high-level events (listerners) and virtual methods for low-level events

• Type-safe callbacks

• Precompiler (moc)

• A class which emits a signal neither knows nor cares which slots receive the signal

Signals & Slots

Signals & Slots - Example

...class QSlider: public QWidget{ Q_OBJECTpublic: …public slots: void setValue(int value); …signals: void valueChanged(int value); …};

…void QSlider::setValue(int value){ … if (value != old) { emit valueChanged(value); }}…

valueChanged() is not defined

…int main(…){ … QSpinBox* spinBox = new QSpinBox(panel); QSlider* slider = new QSlider(panel); … QObject::connect(slider, SIGNAL(valueChanged(int)), spinBox, SLOT(setValue (int))); QObject::connect(spinBox, SIGNAL(valueChanged(int)), slider, SLOT(setValue (int))); …}

...class QSpinBox: public QWidget{ Q_OBJECTpublic: …public slots: void setValue(int value); …signals: void valueChanged(int value); …};

…void QSlider::setValue(int value){ … if (value != old) { emit valueChanged(value); }}…

QSpinBox.h QSpinBox.cpp

Signals & Slots - ExamplePROJECT(MyProject)

FIND_PACKAGE(Qt4 REQUIRED)INCLUDE(${QT_USE_FILE})

SET(MyProject_SRCS MyWidget.cxx main.cxx)SET(MyProject_MOC_SRCS MyWidget.h)

QT4_WRAP_CPP(MyProject_SRCS ${MyProject_MOC_SRCS})

ADD_EXECUTABLE( MyProject ${MyProject_SRCS})

TARGET_LINK_LIBRARIES(MyProject ${QT_LIBRARIES})

#include <QWidget>class MyWidget: public QWidget{ Q_OBJECTpublic: MyWidget(QWidget* parent = 0);public slots: void setValue(int value);signals: void valueChanged(int value);private: int Value;};

#include “MyWidget.h”

void MyWidget::MyWidget(QWidget* parent) : QWidget(parent){}

void MyWidget::setValue(int value){ if (value != this->Value) { value = this->Value; emit this->valueChanged(this->Value); }}MyWidget.h

MyWidget.cxx

CMakeLists.cxx

#include <QApplication>#include “MyWidget.h”

int maint(int argc, char* argv[]){ QApplication app(argc, argv); QGroupBox container; MyWidget w1(&container); MyWidget w2(&container);

QObject::connect(w1, SIGNAL(valueChanged(int)), w2, SLOT(setValue(int))); container.show(); return app.exec();} main.cxx

Signals & Slots – Good to remember 1

• Every connection you make emits a signal– duplicate connections emit two signals.

– break a connection using disconnect()• You can connect a signal to a signal

• Signals can have default values

• _

…QObject::connect(loginPushbutton, SIGNAL(clicked()), this, SIGNAL(loginRequested()));…

class MyWidget: public QObject{ …signals: void mySignal(int value = 5); …};

… emit mySignal(); … emit mySignal(10); …

…QObject::connect(button, SIGNAL(clicked()), widget2, SLOT(updateScreen()));…QObject::connect(button, SIGNAL(clicked()), widget2, SLOT(updateScreen()));…

MyWidget.hMyWidget.cxx

Signals & Slots – Good to remember 2

• Slots are implemented as normal methods– Slots can be virtual, public, protected, private.

• Slot may have a shorter signature than the signal it receives– slots can ignore extra arguments.

class MyWidget: public QObject{ …protected slots: virtual void myAbstractSlot() = 0; …};

…QObject::connect(pushButton, SIGNAL(clicked()), this, SLOT(myAbstractSlot()));…

…QObject::connect(slider, SIGNAL(valueChanged(int)), this, SLOT(update()));…

MyWidget.hMyWidget.cxx

Qt Designer

QtDesigner- Example

…SET(MyProject_UI_SRCS Resources/UI/MyProject.ui)…QT4_WRAP_UI(MyProject_UI_CXX ${MyProject_UI_SRCS})…

#include <QWidget>#include “ui_MyWidget.h”

class MyWidget: public Qwidget, Ui_MyWidget{ …};

#include “MyWidget.h”#include “"ui_MyWidget.h”

void MyWidget::MyWidget(QWidget* parent) : QWidget(parent){ this->setupUi(this);}

MyWidget.h

MyWidget.cxx

CMakeLists.cxx

#include <QApplication>#include “MyWidget.h”

int maint(int argc, char* argv[]){ QApplication app(argc, argv); MyWidget w(0); w.show(); return app.exec();} main.cxx