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# circlewidget.cpp Example File
painting/concentriccircles/circlewidget.cpp

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     #include <QtGui>

     #include "circlewidget.h"

     #include <stdlib.h>

     CircleWidget::CircleWidget(QWidget *parent)
         : QWidget(parent)
     {
         floatBased = false;
         antialiased = false;
         frameNo = 0;

         setBackgroundRole(QPalette::Base);
         setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
     }

     void CircleWidget::setFloatBased(bool floatBased)
     {
         this->floatBased = floatBased;
         update();
     }

     void CircleWidget::setAntialiased(bool antialiased)
     {
         this->antialiased = antialiased;
         update();
     }

     QSize CircleWidget::minimumSizeHint() const
     {
         return QSize(50, 50);
     }

     QSize CircleWidget::sizeHint() const
     {
         return QSize(180, 180);
     }

     void CircleWidget::nextAnimationFrame()
     {
         ++frameNo;
         update();
     }

     void CircleWidget::paintEvent(QPaintEvent *)
     {
         QPainter painter(this);
         painter.setRenderHint(QPainter::Antialiasing, antialiased);
         painter.translate(width() / 2, height() / 2);

         for (int diameter = 0; diameter < 256; diameter += 9) {
             int delta = abs((frameNo % 128) - diameter / 2);
             int alpha = 255 - (delta * delta) / 4 - diameter;
             if (alpha > 0) {
                 painter.setPen(QPen(QColor(0, diameter / 2, 127, alpha), 3));

                 if (floatBased) {
                     painter.drawEllipse(QRectF(-diameter / 2.0, -diameter / 2.0,
                                                diameter, diameter));
                 } else {
                     painter.drawEllipse(QRect(-diameter / 2, -diameter / 2,
                                               diameter, diameter));
                 }
             }
         }
     }