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Qt Tutorial 6 - Building Blocks Galore!

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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.

The following code example is written in c++.
#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



The following code example is written in c++.
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.

The following code example is written in c++.
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.

The following code example is written in c++.
class MyWidget : public QWidget
{
public:
    MyWidget(QWidget *parent = 0);
};
 
MyWidget, too, contains no API except a constructor.

The following code example is written in c++.
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.

The following code example is written in c++.
    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.

The following code example is written in c++.

    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.

The following code example is written in c++.
    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 of 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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