Draggable Icons Example

DragWidget Class Implementation

The DragWidget constructor sets an attribute on the widget that ensures that it will be deleted when it is closed:

 DragWidget::DragWidget(QWidget *parent)
     : QFrame(parent)
 {
     setMinimumSize(200, 200);
     setFrameStyle(QFrame::Sunken | QFrame::StyledPanel);
     setAcceptDrops(true);

     QLabel *boatIcon = new QLabel(this);
     boatIcon->setPixmap(QPixmap(":/images/boat.png"));
     boatIcon->move(20, 20);
     boatIcon->show();
     boatIcon->setAttribute(Qt::WA_DeleteOnClose);

     QLabel *carIcon = new QLabel(this);
     carIcon->setPixmap(QPixmap(":/images/car.png"));
     carIcon->move(120, 20);
     carIcon->show();
     carIcon->setAttribute(Qt::WA_DeleteOnClose);

     QLabel *houseIcon = new QLabel(this);
     houseIcon->setPixmap(QPixmap(":/images/house.png"));
     houseIcon->move(20, 120);
     houseIcon->show();
     houseIcon->setAttribute(Qt::WA_DeleteOnClose);
 }

To enable dragging from the icon, we need to act on a mouse press event. We do this by reimplementing QWidget::mousePressEvent() and setting up a QDrag object.

 void DragWidget::mousePressEvent(QMouseEvent *event)
 {
     QLabel *child = static_cast<QLabel*>(childAt(event->pos()));
     if (!child)
         return;

     QPixmap pixmap = *child->pixmap();

     QByteArray itemData;
     QDataStream dataStream(&itemData, QIODevice::WriteOnly);
     dataStream << pixmap << QPoint(event->pos() - child->pos());

Since we will be sending pixmap data for the icon and information about the user's click in the icon widget, we construct a QByteArray and package up the details using a QDataStream.

For interoperability, drag and drop operations describe the data they contain using MIME types. In Qt, we describe this data using a QMimeData object:

     QMimeData *mimeData = new QMimeData;
     mimeData->setData("application/x-dnditemdata", itemData);

We choose an unofficial MIME type for this purpose, and supply the QByteArray to the MIME data object.

The drag and drop operation itself is handled by a QDrag object:

     QDrag *drag = new QDrag(this);
     drag->setMimeData(mimeData);
     drag->setPixmap(pixmap);
     drag->setHotSpot(event->pos() - child->pos());

Here, we pass the data to the drag object, set a pixmap that will be shown alongside the cursor during the operation, and define the position of a hot spot that places the position of this pixmap under the cursor.