Drag and Drop

In the clipboard model, the user can cut or copy the source information, then later paste it. Similarly in the drag-and-drop model, the user can drag a copy of the information or they can drag the information itself to a new place (moving it). The drag-and-drop model however has an additional complication for the programmer: the program doesn't know whether the user wants to cut or copy until the drop (paste) is done! For dragging between applications, it makes no difference, but for dragging within an application, the application must take a little extra care not to tread on its own feet. For example, to drag text around in a document, the drag start point and the drop event might look like this:

void MyEditor::startDrag()
{
    QDragObject *d = new QTextDrag(myHighlightedText(), this);
    if ( d->drag() && d->target() != this )
        cutMyHighlightedText();
}

void MyEditor::dropEvent(QDropEvent* event)
{
    QString text;

    if ( QTextDrag::decode(event, text) ) {
        if ( event->source() == this && event->action() == QDropEvent::Move ) {
            // Careful not to tread on my own feet
            event->acceptAction();
            moveMyHighlightedTextTo(event->pos());
        } else {
            pasteTextAt(text, event->pos());
        }
    }
}

Some widgets are more specific than just a "yes" or "no" response when data is dragged onto them. For example, a CAD program might only accept drops of text onto text objects in the view. In these cases, the dragMoveEvent() is used and an area is given for which the drag is accepted or ignored:

void MyWidget::dragMoveEvent(QDragMoveEvent* event)
{
    if ( QTextDrag::canDecode(event) ) {
        MyCadItem* item = findMyItemAt(event->pos());
        if ( item )
            event->accept();
    }
}

void MyWidget::dragMoveEvent(QDragMoveEvent* event)
{
    if ( QTextDrag::canDecode(event) ) {
        MyCadItem* item = findMyItemAt(event->pos());
        if ( item ) {
            QRect r = item->areaRelativeToMeClippedByAnythingInTheWay();
            if ( item->type() == MyTextType )
                event->accept( r );
            else
                event->ignore( r );
        }
    }
}

The dragMoveEvent() can also be used if you need to give visual feedback as the drag progresses, to start timers, to scroll the window, or whatever is appropriate (don't forget to stop the scrolling and timers in a dragLeaveEvent() though).

The QApplication object (available as the qApp global) also provides some drag and drop related functions: QApplication::setStartDragTime(), QApplication::setStartDragDistance(), and their corresponding getters, QApplication::startDragTime() and QApplication::startDragDistance().

Inter-operating with Other Applications