qt5base-lts/examples/widgets/tools/plugandpaint/app/paintarea.h

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#ifndef PAINTAREA_H
#define PAINTAREA_H

#include <QColor>
#include <QImage>
#include <QPainterPath>
#include <QWidget>

class BrushInterface;

class PaintArea : public QWidget
{
    Q_OBJECT

public:
    PaintArea(QWidget *parent = nullptr);

    bool openImage(const QString &fileName);
    bool saveImage(const QString &fileName, const char *fileFormat);
    void setImage(const QImage &image);
    void insertShape(const QPainterPath &path);
    void setBrushColor(const QColor &color);
    void setBrushWidth(int width);
    void setBrush(BrushInterface *brushInterface, const QString &brush);

    QImage image() const { return theImage; }
    QColor brushColor() const { return color; }
    int brushWidth() const { return thickness; }
    QSize sizeHint() const override;

protected:
    void paintEvent(QPaintEvent *event) override;
    void mousePressEvent(QMouseEvent *event) override;
    void mouseMoveEvent(QMouseEvent *event) override;
    void mouseReleaseEvent(QMouseEvent *event) override;

private:
    void setupPainter(QPainter &painter);

    QImage theImage = {500, 400, QImage::Format_RGB32};
    QColor color = Qt::blue;
    int thickness = 3;

    BrushInterface *brushInterface = nullptr;
    QString brush;
    QPoint lastPos = {-1, -1};

    QPainterPath pendingPath;
};

#endif