c++ 实现qt 实现橡皮擦功能擦掉之前QPaint画笔绘画代码-QT开发中文网

下面是一个简单的示例代码，展示如何在Qt中实现橡皮擦功能，并且能够擦掉之前QPaint画笔绘画的内容：

#include <QtWidgets>

class Eraser : public QObject {
    Q_OBJECT

public:
    Eraser(QObject *parent = nullptr) : QObject(parent), m_rect(0, 0, 0, 0) {}

    void setRect(const QRectF &rect) { m_rect = rect; }
    QRectF rect() const { return m_rect; }

private:
    QRectF m_rect;
};

class Canvas : public QWidget {
public:
    Canvas(QWidget *parent = nullptr) : QWidget(parent), m_image(size(), QImage::Format_ARGB32_Premultiplied), m_pressed(false)
    {
        QPainter painter(&m_image);
        painter.fillRect(rect(), Qt::white);
        update();
    }

protected:
    void mousePressEvent(QMouseEvent *event) override
    {
        if (event->button() == Qt::LeftButton) {
            m_eraser = new Eraser(this);
            m_eraser->setRect(QRectF(event->pos(), QSize(50, 50)));
            m_erasers.append(m_eraser);
            m_pressed = true;
            update();
        }
    }

    void mouseMoveEvent(QMouseEvent *event) override
    {
        if (m_pressed) {
            QRectF oldRect = m_eraser->rect();
            QPointF delta = event->pos() - event->lastPos();
            QPointF center = oldRect.center() + delta;
            QRectF newRect(center - QSizeF(oldRect.size() / 2), oldRect.size());
            newRect &= rect(); // Ensure the rectangle is inside the widget.
            m_eraser->setRect(newRect);
            update();
        }
    }

    void mouseReleaseEvent(QMouseEvent *event) override
    {
        if (event->button() == Qt::LeftButton) {
            m_pressed = false;
            m_erasers.removeOne(m_eraser);
            delete m_eraser;
            QPainter painter(&m_image);
            painter.setCompositionMode(QPainter::CompositionMode_DestinationOut);
            for (const Eraser *eraser : qAsConst(m_erasers)) {
                painter.eraseRect(eraser->rect());
            }
            update();
        }
    }

    void paintEvent(QPaintEvent *event) override
    {
        QPainter painter(this);

        // Draw the background image.
        painter.drawImage(event->rect(), m_image, event->rect());

        // Draw the erasers.
        for (const Eraser *eraser : qAsConst(m_erasers)) {
            QRectF rect = eraser->rect();
            rect.translate(-offset());
            painter.fillRect(rect, Qt::transparent);
        }
    }

private:
    QImage m_image;
    QList<Eraser *> m_erasers;
    Eraser *m_eraser = nullptr;
    bool m_pressed;
};

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    Canvas canvas;
    canvas.resize(600, 400);
    canvas.show();

    return app.exec();
}

#include "main.moc"

这个示例代码实现了一个基本的绘图区域（Canvas）和橡皮擦类（Eraser），并在mousePressEvent()、mouseMoveEvent()和mouseReleaseEvent()三个事件函数中对应鼠标按下、移动和松开时的操作。其中，QImage对象m_image用于保存当前绘图区域的内容，QList m_erasers则保存了所有已经添加到绘图区域中的Eraser对象。

在paintEvent()事件函数中，先将m_image作为背景画出，然后遍历所有已经添加到绘图区域中的Eraser对象，使用QPainter::eraseRect()函数来删除对应的矩形区域。需要注意的是，在擦掉之前QPaint画笔绘画的内容时，需要将painter设置为CompositionMode_DestinationOut模式。

希望这个示例代码可以帮助你实现橡皮擦功能并能够擦掉之前QPaint画笔绘画的内容。