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

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        ** This file is part of the documentation of the Qt Toolkit.
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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));
                    }
                }
            }
        }