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car.cpp Example File
dbus/remotecontrolledcar/car/car.cpp

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 #include "car.h"
 #include <QtGui/QtGui>
 #include <math.h>

 static const double Pi = 3.14159265358979323846264338327950288419717;

 QRectF Car::boundingRect() const
 {
     return QRectF(-35, -81, 70, 115);
 }

 Car::Car() : color(Qt::green), wheelsAngle(0), speed(0)
 {
     startTimer(1000 / 33);
     setFlag(QGraphicsItem::ItemIsMovable, true);
     setFlag(QGraphicsItem::ItemIsFocusable, true);
 }

 void Car::accelerate()
 {
     if (speed < 10)
         ++speed;
 }

 void Car::decelerate()
 {
     if (speed > -10)
         --speed;
 }

 void Car::turnLeft()
 {
     if (wheelsAngle > -30)
         wheelsAngle -= 5;
 }

 void Car::turnRight()
 {
     if (wheelsAngle < 30)
        wheelsAngle += 5;
 }

 void Car::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
 {
     Q_UNUSED(option);
     Q_UNUSED(widget);

     painter->setBrush(Qt::gray);
     painter->drawRect(-20, -58, 40, 2); // front axel
     painter->drawRect(-20, 7, 40, 2); // rear axel

     painter->setBrush(color);
     painter->drawRect(-25, -79, 50, 10); // front wing

     painter->drawEllipse(-25, -48, 50, 20); // side pods
     painter->drawRect(-25, -38, 50, 35); // side pods
     painter->drawRect(-5, 9, 10, 10); // back pod

     painter->drawEllipse(-10, -81, 20, 100); // main body

     painter->drawRect(-17, 19, 34, 15); // rear wing

     painter->setBrush(Qt::black);
     painter->drawPie(-5, -51, 10, 15, 0, 180 * 16);
     painter->drawRect(-5, -44, 10, 10); // cocpit

     painter->save();
     painter->translate(-20, -58);
     painter->rotate(wheelsAngle);
     painter->drawRect(-10, -7, 10, 15); // front left
     painter->restore();

     painter->save();
     painter->translate(20, -58);
     painter->rotate(wheelsAngle);
     painter->drawRect(0, -7, 10, 15); // front left
     painter->restore();

     painter->drawRect(-30, 0, 12, 17); // rear left
     painter->drawRect(19, 0, 12, 17);  // rear right
 }

 void Car::timerEvent(QTimerEvent *event)
 {
     Q_UNUSED(event);

     const qreal axelDistance = 54;
     qreal wheelsAngleRads = (wheelsAngle * Pi) / 180;
     qreal turnDistance = ::cos(wheelsAngleRads) * axelDistance * 2;
     qreal turnRateRads = wheelsAngleRads / turnDistance;  // rough estimate
     qreal turnRate = (turnRateRads * 180) / Pi;
     qreal rotation = speed * turnRate;

     rotate(rotation);
     translate(0, -speed);
     update();
 }