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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));
                }
            }
        }
    }