Qt Tutorial 10 - Smooth as Silk

t10/cannonfield.cpp

 CannonField::CannonField(QWidget *parent)
     : QWidget(parent)
 {
     currentAngle = 45;
     currentForce = 0;
     setPalette(QPalette(QColor(250, 250, 200)));
     setAutoFillBackground(true);
 }

The force is initialized to zero.

 void CannonField::setAngle(int angle)
 {
     if (angle < 5)
         angle = 5;
     if (angle > 70)
         angle = 70;
     if (currentAngle == angle)
         return;
     currentAngle = angle;
     update(cannonRect());
     emit angleChanged(currentAngle);
 }

We have made a slight change in the setAngle() function. It repaints only the portion of the widget that contains the cannon.

 void CannonField::setForce(int force)
 {
     if (force < 0)
         force = 0;
     if (currentForce == force)
         return;
     currentForce = force;
     emit forceChanged(currentForce);
 }

The implementation of setForce() is quite similar to that of setAngle(). The only difference is that because we don't show the force value, we don't need to repaint the widget.

 void CannonField::paintEvent(QPaintEvent * /* event */)
 {
     QPainter painter(this);

     painter.setPen(Qt::NoPen);
     painter.setBrush(Qt::blue);

     painter.translate(0, height());
     painter.drawPie(QRect(-35, -35, 70, 70), 0, 90 * 16);
     painter.rotate(-currentAngle);
     painter.drawRect(QRect(30, -5, 20, 10));
 }

We paint as in Chapter 9.

 QRect CannonField::cannonRect() const
 {
     QRect result(0, 0, 50, 50);
     result.moveBottomLeft(rect().bottomLeft());
     return result;
 }

The cannonRect() function returns the rectangle enclosing the cannon in widget coordinates. First we create a rectangle with the size 50 x 50 and then move it so its bottom-left corner is equal to the widget's own bottom-left corner.

The QWidget::rect() function returns the widget's enclosing rectangle in the widget's own coordinates. The top-left corner of the rectangle is always (0, 0).

 MyWidget::MyWidget(QWidget *parent)
     : QWidget(parent)
 {

     LCDRange *force = new LCDRange;
     force->setRange(10, 50);

     connect(force, SIGNAL(valueChanged(int)),
             cannonField, SLOT(setForce(int)));
     connect(cannonField, SIGNAL(forceChanged(int)),
             force, SLOT(setValue(int)));

     QVBoxLayout *leftLayout = new QVBoxLayout;
     leftLayout->addWidget(angle);
     leftLayout->addWidget(force);

     QGridLayout *gridLayout = new QGridLayout;
     gridLayout->addWidget(quit, 0, 0);
     gridLayout->addLayout(leftLayout, 1, 0);
     gridLayout->addWidget(cannonField, 1, 1, 2, 1);
     gridLayout->setColumnStretch(1, 10);

     force->setValue(25);