实现在一个QWidget上显示另一个QWidget，并通过鼠标拖拽子窗口的四条边和四个角来改变子窗口的大小_qtwidget拖动边缘改变大小

#include <QtWidgets>
 
class ChildWidget : public QWidget {
public:
    ChildWidget(QWidget *parent = nullptr) : QWidget(parent) {
        setAutoFillBackground(true);
        setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    }
 
protected:
    void paintEvent(QPaintEvent *event) override {
        Q_UNUSED(event);
 
        // 绘制子窗口的颜色
        QPainter painter(this);
        painter.fillRect(rect(), Qt::blue);
    }
};
 
class ParentWidget : public QWidget {
public:
    ParentWidget(QWidget *parent = nullptr) : QWidget(parent) {
        setWindowTitle("Parent Widget");
 
        // 创建子窗口对象
        childWidget = new ChildWidget(this);
 
        // 设置子窗口对象为透明，以显示父窗口的背景颜色
        childWidget->setAttribute(Qt::WA_TransparentForMouseEvents);
 
        // 创建布局管理器
        layout = new QVBoxLayout(this);
 
        // 将子窗口添加到布局中
        layout->addWidget(childWidget);
 
        // 设置布局管理器为父窗口的布局
        setLayout(layout);
    }
 
    void mousePressEvent(QMouseEvent *event) override {
        if (event->button() == Qt::LeftButton) {
            // 记录鼠标点击时父窗口的位置和子窗口的大小
            startPos = event->pos();
            startSize = childWidget->size();
        }
    }
 
    void mouseMoveEvent(QMouseEvent *event) override {
        if (event->buttons() & Qt::LeftButton) {
            // 计算鼠标拖拽的偏移量
            
            int dx = event->pos().x() - startPos.x();
            int dy = event->pos().y() - startPos.y();
 
            // 改变子窗口的大小
            childWidget->resize(startSize.width() + dx, startSize.height() + dy);
        }
    }
 
private:
    QVBoxLayout *layout;
    ChildWidget *childWidget;
    QPoint startPos;
    QSize startSize;
};
 
int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
 
    ParentWidget parentWidget;
    parentWidget.show();
 
    return app.exec();
}

#include <QtWidgets>
 
class ResizableWidget : public QWidget
{
public:
    ResizableWidget(QWidget *parent = nullptr) : QWidget(parent)
    {
        setWindowFlags(Qt::FramelessWindowHint);  // 设置无边框窗口
        setMouseTracking(true);  // 开启鼠标跟踪
    }
 
protected:
    void mousePressEvent(QMouseEvent *event) override
    {
        if (event->buttons() & Qt::LeftButton) {
            // 记录当前鼠标位置和窗口位置
            m_startPos = event->globalPos();
            m_startGeometry = geometry();
            event->accept();
        }
    }
 
    void mouseMoveEvent(QMouseEvent *event) override
    {
        if (event->buttons() & Qt::LeftButton) {
            // 计算鼠标移动的偏移量
            QPoint offset = event->globalPos() - m_startPos;
 
            // 判断鼠标位置，调整窗口大小
            if (m_resizeLeft) {
                QRect newGeometry = m_startGeometry.adjusted(offset.x(), 0, 0, 0);
                setGeometry(newGeometry);
            } else if (m_resizeRight) {
                QRect newGeometry = m_startGeometry.adjusted(0, 0, offset.x(), 0);
                setGeometry(newGeometry);
            } else if (m_resizeTop) {
                QRect newGeometry = m_startGeometry.adjusted(0, offset.y(), 0, 0);
                setGeometry(newGeometry);
            } else if (m_resizeBottom) {
                QRect newGeometry = m_startGeometry.adjusted(0, 0, 0, offset.y());
                setGeometry(newGeometry);
            } else if (m_resizeTopLeft) {
                QRect newGeometry = m_startGeometry.adjusted(offset.x(), offset.y(), 0, 0);
                setGeometry(newGeometry);
            } else if (m_resizeTopRight) {
                QRect newGeometry = m_startGeometry.adjusted(0, offset.y(), offset.x(), 0);
                setGeometry(newGeometry);
            } else if (m_resizeBottomLeft) {
                QRect newGeometry = m_startGeometry.adjusted(offset.x(), 0, 0, offset.y());
                setGeometry(newGeometry);
            } else if (m_resizeBottomRight) {
                QRect newGeometry = m_startGeometry.adjusted(0, 0, offset.x(), offset.y());
                setGeometry(newGeometry);
            }
 
            event->accept();
        }
    }
 
    void mouseReleaseEvent(QMouseEvent *event) override
    {
        Q_UNUSED(event);
        // 重置所有的resize标志
        m_resizeLeft = false;
        m_resizeRight = false;
        m_resizeTop = false;
        m_resizeBottom = false;
        m_resizeTopLeft = false;
        m_resizeTopRight = false;
        m_resizeBottomLeft = false;
        m_resizeBottomRight = false;
    }
 
    void resizeEvent(QResizeEvent *event) override
    {
        Q_UNUSED(event);
        // 设置子窗口的背景颜色，用以区分
        setStyleSheet("background-color: yellow;");
    }
 
    void enterEvent(QEvent *event) override
    {
        Q_UNUSED(event);
        // 根据鼠标位置设置对应的resize标志
        QPoint pos = mapFromGlobal(QCursor::pos());
        m_resizeLeft = (pos.x() <= 5);
        m_resizeRight = (pos.x() >= width() - 5);
        m_resizeTop = (pos.y() <= 5);
        m_resizeBottom = (pos.y() >= height() - 5);
        m_resizeTopLeft = (m_resizeLeft && m_resizeTop);
        m_resizeTopRight = (m_resizeRight && m_resizeTop);
        m_resizeBottomLeft = (m_resizeLeft && m_resizeBottom);
        m_resizeBottomRight = (m_resizeRight && m_resizeBottom);
    }
 
private:
    QPoint m_startPos;  // 鼠标按下时的起始位置
    QRect m_startGeometry;  // 窗口的起始位置和大小
 
    bool m_resizeLeft = false;  // 是否可以改变窗口的左边界
    bool m_resizeRight = false;  // 是否可以改变窗口的右边界
    bool m_resizeTop = false;  // 是否可以改变窗口的上边界
    bool m_resizeBottom = false;  // 是否可以改变窗口的下边界
    bool m_resizeTopLeft = false;  // 是否可以改变窗口的左上角
    bool m_resizeTopRight = false;  // 是否可以改变窗口的右上角
    bool m_resizeBottomLeft = false;  // 是否可以改变窗口的左下角
    bool m_resizeBottomRight = false;  // 是否可以改变窗口的右下角
};
 
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
 
    QWidget parentWidget;
    parentWidget.setGeometry(100, 100, 400, 300);
    parentWidget.setStyleSheet("background-color: gray;");
 
    ResizableWidget childWidget(&parentWidget);
    childWidget.setGeometry(50, 50, 200, 150);
 
    parentWidget.show();
 
    return app.exec();
}

#include <QtWidgets>
 
// 自定义的子窗口类
class ChildWidget : public QWidget
{
public:
    ChildWidget(QWidget *parent = nullptr) : QWidget(parent)
    {
        setAutoFillBackground(true);
        QPalette palette;
        palette.setColor(QPalette::Background, Qt::green);
        setPalette(palette);
    }
 
    // 鼠标按下事件
    void mousePressEvent(QMouseEvent *event) override
    {
        if (event->button() == Qt::LeftButton)
        {
            // 记录鼠标按下时的位置
            m_lastPos = event->pos();
        }
    }
 
    // 鼠标移动事件
    void mouseMoveEvent(QMouseEvent *event) override
    {
        // 如果鼠标左键被按下，则进行窗口大小的改变
        if (event->buttons() & Qt::LeftButton)
        {
            // 计算鼠标移动的偏移量
            int dx = event->x() - m_lastPos.x();
            int dy = event->y() - m_lastPos.y();
 
            // 改变窗口大小
            // 这里只实现了拖拽右下角改变大小的功能，你可以根据需求调整和扩展
            QSize newSize = size() + QSize(dx, dy);
            resize(newSize);
 
            event->accept();
        }
    }
 
private:
    QPoint m_lastPos;
};
 
// 自定义的主窗口类
class MainWindow : public QMainWindow
{
public:
    MainWindow(QWidget *parent = nullptr) : QMainWindow(parent)
    {
        // 创建两个QWidget，一个作为主窗口，一个作为子窗口
        QWidget *mainWidget = new QWidget(this);
        ChildWidget *childWidget = new ChildWidget(mainWidget);
 
        // 使用垂直布局将子窗口添加到主窗口上
        QVBoxLayout *layout = new QVBoxLayout(mainWidget);
        layout->addWidget(childWidget);
 
        // 将主窗口设置为应用程序的主窗口
        setCentralWidget(mainWidget);
    }
};
 
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
 
    MainWindow mainWindow;
    mainWindow.show();
 
    return app.exec();
}