QT的拖放（Drag/Drop）的实现有关问题

QT的拖放（Drag/Drop）的实现问题？
以下代码，已经验证OK了。

但有个问题，QVariant(Qt::transparent).toByteArray()的传送出去，是为了什么？（说明一下，这是项目中的，看了下，它仅仅是作为判定数据合法性）

C/C++ code

#define DATA_ID1        "Text/stringids"#define DATA_ID2        "Text/colordata"//发送方void MyWidget1::mousePressEvent(QMouseEvent *pMouseEvent){    qDebug ("%d: %s: %s enter", __LINE__, __FILE__, __FUNCTION__);    QWidget::mousePressEvent(pMouseEvent);    QDrag* pDrag = new QDrag(this); //Qt自动释放    QMimeData* pMimeData = new QMimeData(); //Qt自动释放    if ((NULL!=pDrag) && (NULL!=pMimeData))    {        //set the mime data        QString strData = "this is a test for drag and drop";        pMimeData->setData(DATA_ID1, strData.toAscii());        pMimeData->setData(DATA_ID2, QVariant(Qt::transparent).toByteArray());        pDrag->setMimeData(pMimeData);        pDrag->exec(Qt::CopyAction | Qt::MoveAction);    }}//接收方MyWidget2::MyWidget2(QWidget *parent)    : QWidget(parent){    setAcceptDrops(true);}void MyWidget2::dragEnterEvent(QDragEnterEvent *pDragEnterEvent){    qDebug ("%d: %s: %s enter", __LINE__, __FILE__, __FUNCTION__);    QString strData(pDragEnterEvent->mimeData()->data(DATA_ID1));    QVariant varColor(pDragEnterEvent->mimeData()->data(DATA_ID2));    QColor color = varColor.value<QColor> ();    if (color.isValid())    {        pDragEnterEvent->acceptProposedAction();        pDragEnterEvent->accept();        this->setMouseTracking(true);    }}void MyWidget2::dropEvent(QDropEvent *pDropEvent){    qDebug ("%d: %s: %s enter", __LINE__, __FILE__, __FUNCTION__);    //get the mime data    QString strData(pDropEvent->mimeData()->data(DATA_ID1));    QVariant varColor(pDropEvent->mimeData()->data(DATA_ID2));    QColor color = varColor.value<QColor>();    if (color.isValid())    {        //todo:    }}