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# cannonfield.cpp Example File
tutorials/tutorial/t11/cannonfield.cpp

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     #include <QPaintEvent>
     #include <QPainter>
     #include <QTimer>

     #include <math.h>

     #include "cannonfield.h"

     CannonField::CannonField(QWidget *parent)
         : QWidget(parent)
     {
         currentAngle = 45;
         currentForce = 0;
         timerCount = 0;
         autoShootTimer = new QTimer(this);
         connect(autoShootTimer, SIGNAL(timeout()), this, SLOT(moveShot()));
         shootAngle = 0;
         shootForce = 0;
         setPalette(QPalette(QColor(250, 250, 200)));
         setAutoFillBackground(true);
     }

     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);
     }

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

     void CannonField::shoot()
     {
         if (autoShootTimer->isActive())
             return;
         timerCount = 0;
         shootAngle = currentAngle;
         shootForce = currentForce;
         autoShootTimer->start(5);
     }

     void CannonField::moveShot()
     {
         QRegion region = shotRect();
         ++timerCount;

         QRect shotR = shotRect();

         if (shotR.x() > width() || shotR.y() > height()) {
             autoShootTimer->stop();
         } else {
             region = region.unite(shotR);
         }
         update(region);
     }

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

         paintCannon(painter);
         if (autoShootTimer->isActive())
             paintShot(painter);
     }

     void CannonField::paintShot(QPainter &painter)
     {
         painter.setPen(Qt::NoPen);
         painter.setBrush(Qt::black);
         painter.drawRect(shotRect());
     }

     const QRect barrelRect(30, -5, 20, 10);

     void CannonField::paintCannon(QPainter &painter)
     {
         painter.setPen(Qt::NoPen);
         painter.setBrush(Qt::blue);

         painter.save();
         painter.translate(0, height());
         painter.drawPie(QRect(-35, -35, 70, 70), 0, 90 * 16);
         painter.rotate(-currentAngle);
         painter.drawRect(barrelRect);
         painter.restore();
     }

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

     QRect CannonField::shotRect() const
     {
         const double gravity = 4;

         double time = timerCount / 20.0;
         double velocity = shootForce;
         double radians = shootAngle * 3.14159265 / 180;

         double velx = velocity * cos(radians);
         double vely = velocity * sin(radians);
         double x0 = (barrelRect.right() + 5) * cos(radians);
         double y0 = (barrelRect.right() + 5) * sin(radians);
         double x = x0 + velx * time;
         double y = y0 + vely * time - 0.5 * gravity * time * time;

         QRect result(0, 0, 6, 6);
         result.moveCenter(QPoint(qRound(x), height() - 1 - qRound(y)));
         return result;
     }