Qt 可拖拽 大小可调 方框

您可以使用Qt中的QGraphicsView和QGraphicsItem来实现可拖拽大小可调的方框。以下是一个简单的示例：

#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsItem>
#include <QGraphicsScene>
#include <QGraphicsRectItem>
#include <QGraphicsSceneMouseEvent>

class ResizableRectItem : public QGraphicsRectItem
{
public:
    ResizableRectItem(QGraphicsItem *parent = nullptr) : QGraphicsRectItem(parent)
    {
        setFlags(QGraphicsItem::ItemIsMovable | QGraphicsItem::ItemIsSelectable | QGraphicsItem::ItemSendsGeometryChanges);
    }

protected:
    void mousePressEvent(QGraphicsSceneMouseEvent *event) override
    {
        m_startPos = pos();
        m_mousePos = event->scenePos();
        m_rect = rect();
        QGraphicsRectItem::mousePressEvent(event);
    }

    void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override
    {
        QPointF delta = event->scenePos() - m_mousePos;
        if (isSelected())
        {
            if (event->modifiers() & Qt::ShiftModifier)
            {
                if (delta.x() > delta.y())
                {
                    delta.setY(delta.x());
                }
                else
                {
                    delta.setX(delta.y());
                }
            }
            setPos(m_startPos + delta);
        }
        QGraphicsRectItem::mouseMoveEvent(event);
    }

    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override
    {
        QGraphicsRectItem::mouseReleaseEvent(event);
    }

    QVariant itemChange(GraphicsItemChange change, const QVariant &value) override
    {
        if (change == ItemPositionChange)
        {
            QPointF newPos = value.toPointF();
            QRectF rect = m_rect.translated(newPos - pos());
            setRect(rect);
        }
        return QGraphicsRectItem::itemChange(change, value);
    }

private:
    QPointF m_startPos;
    QPointF m_mousePos;
    QRectF m_rect;
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QGraphicsScene scene;
    QGraphicsView view(&scene);

    ResizableRectItem *rectItem = new ResizableRectItem();
    rectItem->setRect(0, 0, 100, 100);
    scene.addItem(rectItem);

    view.show();
    return a.exec();
}

在这个示例中，我们继承了QGraphicsRectItem并重写了它的mousePressEvent、mouseMoveEvent、mouseReleaseEvent和itemChange函数。在mousePressEvent中，我们记录下方框的位置、鼠标位置和大小。在mouseMoveEvent中，我们根据鼠标移动的距离来更新方框的位置，并根据按下的Shift键来等比例调整方框的大小。在itemChange函数中，我们根据方框的位置变化来调整方框的大小。