QT应用编程: 解决QGraphicsScene重写鼠标事件无法直接得到鼠标坐标问题

class my_graphicsScene : public QGraphicsScene
{
protected:
    void mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *mouseEvent);
    void mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent);
}
......
void my_graphicsScene::mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
    //鼠标左键按下
    if(mouseEvent->button()==Qt::LeftButton)
    {
        //记录鼠标按下的点
        start_point=mouseEvent->pos(); //相对窗口坐标---->这里得到的坐标全是0
    }
}

/*
工程: ECRS
日期: 2021-01-02
作者: DS小龙哥
环境: win10 QT5.12.6 MinGW32
功能: 鼠标移动事件
*/
void my_graphicsScene::mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
    QGraphicsScene::mousePressEvent(mouseEvent);
    qDebug()<<"坐标:"<<mouseEvent->scenePos();
}