http://bbs.youkuaiyun.com/topics/380245181
QGraphicsItem *selectedItem = itemAt(contextMenuEvent->scenePos());
if (!selectedItem)
{
QMenu mymenu;
mymenu.addAction("newItem");
mymenu.exec(QCursor::pos());
}
else
{
QGraphicsScene::contextMenuEvent(contextMenuEvent);
}