Qt 4.3: Qt Tutorial 6 - Building Blocks Galore!

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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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