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Qt Tutorial 6 - Building Blocks Galore!

Files:

• tutorial/t6/main.cpp

This example shows how to encapsulate two widgets into a new component and how easy it is to use many widgets. For the first time, we use a custom widget as a child widget.

 #include <QApplication>
 #include <QFont>
 #include <QGridLayout>
 #include <QLCDNumber>
 #include <QPushButton>
 #include <QSlider>
 #include <QVBoxLayout>
 #include <QWidget>

 class LCDRange : public QWidget
 {
 public:
     LCDRange(QWidget *parent = 0);
 };

 LCDRange::LCDRange(QWidget *parent)
     : QWidget(parent)
 {
     QLCDNumber *lcd = new QLCDNumber(2);
     lcd->setSegmentStyle(QLCDNumber::Filled);

     QSlider *slider = new QSlider(Qt::Horizontal);
     slider->setRange(0, 99);
     slider->setValue(0);
     connect(slider, SIGNAL(valueChanged(int)),
             lcd, SLOT(display(int)));

     QVBoxLayout *layout = new QVBoxLayout;
     layout->addWidget(lcd);
     layout->addWidget(slider);
     setLayout(layout);
 }

 class MyWidget : public QWidget
 {
 public:
     MyWidget(QWidget *parent = 0);
 };

 MyWidget::MyWidget(QWidget *parent)
     : QWidget(parent)
 {
     QPushButton *quit = new QPushButton(tr("Quit"));
     quit->setFont(QFont("Times", 18, QFont::Bold));
     connect(quit, SIGNAL(clicked()), qApp, SLOT(quit()));

     QGridLayout *grid = new QGridLayout;
     for (int row = 0; row < 3; ++row) {
         for (int column = 0; column < 3; ++column) {
             LCDRange *lcdRange = new LCDRange;
             grid->addWidget(lcdRange, row, column);
         }
     }

     QVBoxLayout *layout = new QVBoxLayout;
     layout->addWidget(quit);
     layout->addLayout(grid);
     setLayout(layout);
 }

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

Line by Line Walkthrough

 class LCDRange : public QWidget
 {
 public:
     LCDRange(QWidget *parent = 0);
 };

The LCDRange widget is a widget without any API. It just has a constructor. This sort of widget is not very useful, so we'll add some API later.

 LCDRange::LCDRange(QWidget *parent)
     : QWidget(parent)
 {
     QLCDNumber *lcd = new QLCDNumber(2);
     lcd->setSegmentStyle(QLCDNumber::Filled);

     QSlider *slider = new QSlider(Qt::Horizontal);
     slider->setRange(0, 99);
     slider->setValue(0);
     connect(slider, SIGNAL(valueChanged(int)),
             lcd, SLOT(display(int)));

     QVBoxLayout *layout = new QVBoxLayout;
     layout->addWidget(lcd);
     layout->addWidget(slider);
     setLayout(layout);
 }

This is lifted straight from the MyWidget constructor in Chapter 5. The only differences are that the Quit button is left out and the class is renamed.

 class MyWidget : public QWidget
 {
 public:
     MyWidget(QWidget *parent = 0);
 };

MyWidget, too, contains no API except a constructor.

 MyWidget::MyWidget(QWidget *parent)
     : QWidget(parent)
 {
     QPushButton *quit = new QPushButton(tr("Quit"));
     quit->setFont(QFont("Times", 18, QFont::Bold));
     connect(quit, SIGNAL(clicked()), qApp, SLOT(quit()));

The push button that used to be in what is now LCDRange has been separated so that we can have one Quit button and many LCDRange objects.

     QGridLayout *grid = new QGridLayout;

We create a QWidget with a QGridLayout that will contain three columns.

The QGridLayout automatically arranges its widgets in rows and columns; you can specify the row and column numbers when adding widgets to the layout, and QGridLayout will fit them into the grid.

     for (int row = 0; row < 3; ++row) {
         for (int column = 0; column < 3; ++column) {
             LCDRange *lcdRange = new LCDRange;
             grid->addWidget(lcdRange, row, column);
         }
     }

We create nine LCDRange widgets, all of which are children of the grid object, and we arrange them in three rows and three columns.

     QVBoxLayout *layout = new QVBoxLayout;
     layout->addWidget(quit);
     layout->addLayout(grid);
     setLayout(layout);
 }

Finally, we add the Quit button and the grid layout containing LCDRange widgets, to the main layout. The QWidget::addLayout() function is similar to the QWidget::addWidget() function, making the given layout a child og the main layout.

That's all.

Running the Application

This program shows how easy it is to use many widgets at a time. Each one behaves like the slider and LCD number in the previous chapter. Again, the difference lies in the implementation.

Exercises

Initialize each slider with a different/random value on startup.

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