Drag and Drop

Drop Actions

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 has an additional complication for the programmer: The program doesn't know whether the user wants to cut or copy the information until the operation is complete. This often makes no difference when dragging information between applications, but within an application it is important to check which drop action was used.

We can reimplement the mouseMoveEvent() for a widget, and start a drag and drop operation with a combination of possible drop actions. For example, we may want to ensure that dragging always moves objects in the widget:

    void DragWidget::mouseMoveEvent(QMouseEvent *event)
    {
        if (!(event->buttons() & Qt::LeftButton))
            return;
        if ((event->pos() - dragStartPosition).manhattanLength()
             < QApplication::startDragDistance())
            return;

        QDrag *drag = new QDrag(this);
        QMimeData *mimeData = new QMimeData;

        mimeData->setData(mimeType, data);
        drag->setMimeData(mimeData);

        Qt::DropAction dropAction = drag->start(Qt::CopyAction | Qt::MoveAction);
        ...
    }

The action returned by the start() function may default to a CopyAction if the information is dropped into another application but, if it is dropped in another widget in the same application, we may obtain a different drop action.

The proposed drop actions can be filtered in a widget's dragMoveEvent() function. However, it is possible to accept all proposed actions and let the user decide which they want to accept later:

    void DragWidget::dragMoveEvent(QDragMoveEvent *event)
    {
        event->acceptProposedAction();
    }

When a drop occurs in the widget, the dropEvent() handler function is called, and we can deal with each possible action in turn. First, we deal with drag and drop operations within the same widget:

    void DragWidget::dropEvent(QDropEvent *event)
    {
        if (event->source() == this && event->possibleActions() & Qt::MoveAction)
            return;

In this case, we refuse to deal with move operations. Each type of drop action that we accept is checked and dealt with accordingly:

        if (event->possibleActions() == Qt::MoveAction) {
            event->acceptProposedAction();
            // Process the data from the event.
        } else if (event->possibleActions() == Qt::CopyAction) {
            event->acceptProposedAction();
            // Process the data from the event.
        } else {
            // Ignore the drop.
            return;
        }
        ...
    }

Note that we checked for individual drop actions in the above code. It is often more helpful to check for the presence of each action in the value supplied by possibleActions() and set the accepted action with the event's setDropAction() function.

    void Window::dragMoveEvent(QDragMoveEvent *event)
    {
        if (event->mimeData()->hasFormat("text/plain")
            && event->answerRect().intersects(dropFrame->geometry()))

            event->acceptProposedAction();
    }

        clipboard = QApplication::clipboard();

        clipboard->setText(lineEdit->text(), QClipboard::Clipboard);

        connect(clipboard, SIGNAL(dataChanged()), this, SLOT(updateClipboard()));

    void ClipWindow::updateClipboard()
    {
        QStringList formats = clipboard->mimeData()->formats();
        QByteArray data = clipboard->mimeData()->data(format);
        ...
    }