Time for action – creating a black, rectangular item

As a first approach, let's create an item that paints a black rectangle:

class BlackRectangle : public QGraphicsItem {
public:
  explicit BlackRectangle(QGraphicsItem *parent = 0)
    : QGraphicsItem(parent) {}
  virtual ~BlackRectangle() {}

  QRectF boundingRect() const {
    return QRectF(0, 0, 75, 25);
  }

  void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) {
    Q_UNUSED(option)
    Q_UNUSED(widget)
    painter->fillRect(boundingRect(), Qt::black);
  }
};

What just happened?

First, we subclass QGraphicItem and call the new class BlackRectangle. The class' constructor accepts a pointer to a QGraphicItem item. This pointer is then passed to the constructor of the QGraphicItem item. We do not have to worry about it; QGraphicItem will take care of it and establish the parent-child relationship for our item, among other things. Next, the virtual destructor makes sure that it gets called even if the...