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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));
             }
         }
     }
 }