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 painter.setBrush(Qt::HorPattern);
  painter.drawRect(10, 15, 90, 60);

We draw a rectangle with a specific pattern. The Qt::HorPattern is a constant used to
create a pattern of horizontal lines.

The "Donut" is an advanced geometrical shape resembling this kind of food.
We create it by drawing 72 rotated ellipses.

 painter.setRenderHint(QPainter::Antialiasing);

We will paint in antialiased mode. The rendering will be of higher quality.

 int h = height();
 int w = width();

 painter.translate(QPoint(w/2, h/2));

These lines move the beginning of the coordinate system into the middle of the window. By default, it
is positioned at 0, 0 point. In another words, at the upper left corner of the window.
By moving the coordinate system, the drawing will be much easier.

 for (qreal rot=0; rot < 360.0; rot+=5.0 ) {
     painter.drawEllipse(-125, -40, 250, 80);
     painter.rotate(5.0);
 }

In this for cycle, we draw 72 rotated ellipses.

 QPainterPath path1;

 path1.moveTo(5, 5);
 path1.cubicTo(40, 5,  50, 50,  99, 99);
 path1.cubicTo(5, 99,  50, 50,  5, 5);
 painter.drawPath(path1);

The QPainterPath is an object used to create complex shapes. We use it to draw bezier curves.

 painter.drawPie(130, 20, 90, 60, 30*16, 120*16);
 painter.drawChord(240, 30, 90, 60, 0, 16*180);
 painter.drawRoundRect(20, 120, 80, 50);

These code lines draw a pie, a chord and a rounded rectangle.

 QPolygon polygon;
 polygon << QPoint(130, 140) << QPoint(180, 170)
         << QPoint(180, 140) << QPoint(220, 110)
         << QPoint(140, 100);
 painter.drawPolygon(polygon);

Here we draw a polygon consisting of five points.

QPointF baseline(20, 250);
 QFont font("Georgia", 55);
 QPainterPath path2;
 path2.addText(baseline, font, "Q");
 painter.drawPath(path2);

The Qt4 programming library can be used to create a path based on a font character.

 painter.drawEllipse(140, 200, 60, 60);
 painter.drawEllipse(240, 200, 90, 60);

The drawEllipse() method can be used to draw an ellipse and a circle as well.
The circle is a special case of an ellipse.

QLinearGradient grad1(0, 20, 0, 110);

 grad1.setColorAt(0.1, Qt::black);
 grad1.setColorAt(0.5, Qt::yellow);
 grad1.setColorAt(0.9, Qt::black);

The colors in a gradient are defined using stop points. The setColorAt() creates a stop point at the given position with the given color.

 painter.fillRect(20, 20, 300, 90, grad1);

We fill the rectangle with the gradient.

Puff::Puff(QWidget *parent)
    : QWidget(parent)
{
  x = 1;
  opacity = 1.0;
  timerId = startTimer(15);
}

At the constructor, we start the timer. Each 15ms a timer event is generated.

void Puff::timerEvent(QTimerEvent *event)
{
  x += 1;
  repaint();
}

Inside the timerEvent() we increase the font size and repaint the widget.

 if (x > 10) {
   opacity -= 0.01;
   painter.setOpacity(opacity);
 }

If the font size is greater than 10 points, we gradually decrease the opacity. The text fades away.

if (opacity <= 0) {
   killTimer(timerId);
   std::cout << "timer stopped" << std::endl;
 }

If the text fades away, we kill the timer.

QPixmap pixmap("bojnice.jpg");

 QLabel *label = new QLabel(this);
 label->setPixmap(pixmap);

We create a pixmap and put it inside a label widget.

 QSplitter *splitter1 = new QSplitter(Qt::Horizontal, this);
 splitter1->addWidget(topleft);
 splitter1->addWidget(topright);

We create a splitter widget and add two frame widgets into the splitter.

 QSplitter *splitter2 = new QSplitter(Qt::Vertical, this);
 splitter2->addWidget(splitter1);

We can also add splitter to another splitter widget.

 statusBar();

To display a statusbar widget, we call the statusBar() method of the QMainWindow widget.

void Statusbar::OnOkPressed()
{
  statusBar()->showMessage("OK button pressed", 2000);
}

The showMessage() method shows the message on the statusbar.

void Disconnect::onCheck(int state)
{
  if (state == Qt::Checked) {
    connect(click, SIGNAL(clicked()), 
        this, SLOT(onClick()));
  } else {
    click->disconnect(SIGNAL(clicked()));
  }
}

Inside the onCheck() slot, we connect/disconnect an onClick() slot
from the click button.

 startTimer(1000);

We start the timer. Every 1000ms a timer event is generated.

void Timer::timerEvent(QTimerEvent *event)
{
  QTime qtime = QTime::currentTime();
  QString stime = qtime.toString(Qt::LocalDate);
  label->setText(stime);
}

To work with timer events, we must reimplement the timerEvent() method.