Fridge Magnets Example

Let's take a look at the https://() event handler, where drag and drop operations begin:

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

    QPoint hotSpot = event->pos() - child->pos();

    QByteArray itemData;
    QDataStream dataStream(&itemData, QIODevice::WriteOnly);
    dataStream << child->labelText() << QPoint(hotSpot);

Mouse events occur when a mouse button is pressed or released inside a widget, or when the mouse cursor is moved. By reimplementing the https://() method we ensure that we will receive mouse press events for the widget containing the fridge magnets.

Whenever we receive such an event, we first check to see if the position of the click coincides with one of the labels. If not, we simply return.

If the user clicked a label, we determine the position of the hot spot (the position of the click relative to the top-left corner of the label). We create a byte array to store the label's text and the hot spot, and we use a https:// object to stream the data into the byte array.

With all the information in place, we create a new https:// object. As mentioned above, https:// objects associate the data that they hold with the corresponding MIME types to ensure that information can be safely transferred between applications. The https://() method sets the data associated with a given MIME type. In our case, we associate our item data with the custom application/x-fridgemagnet type.

    QMimeData *mimeData = new QMimeData;
    mimeData->setData(fridgetMagnetsMimeType(), itemData);
    mimeData->setText(child->labelText());

Note that we also associate the magnet's text with the text/plain MIME type using https://'s https://() method. Below, we will see how our widget detects both these MIME types with its event handlers.

Finally, we create a https:// object. It is the https:// class that handles most of the details of a drag and drop operation, providing support for MIME-based drag and drop data transfer. The data to be transferred by the drag and drop operation is contained in a https:// object. When we call https://'s https://() method the ownership of our item data is transferred to the https:// object.

We call the https://() function to set the pixmap used to represent the data during the drag and drop operation. Typically, this pixmap shows an icon that represents the MIME type of the data being transferred, but any pixmap can be used. In this example, we simply use the pixmap used by the label itself to make it look like the fridge magnet itself is being moved.

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

    child->hide();

We also specify the cursor's hot spot, its position relative to the top-level corner of the drag pixmap, to be the point we calculated above. This makes the process of dragging the label feel more natural because the cursor always points to the same place on the label during the drag operation.

We start the drag operation using https://'s https://() function, requesting that the magnet is copied when the drag is completed.

    if (drag->exec(Qt::MoveAction | Qt::CopyAction, Qt::CopyAction) == Qt::MoveAction)
        child->close();
    else
        child->show();
}

The function returns the drop action actually performed by the user (this can be either a copy or a move action in this case); if this action is equal to https:// we will close the activated fridge magnet widget because we will create a new one to replace it (see the https:// implementation). Otherwise, if the drop is outside our main widget, we simply show the widget in its original position.

When a a drag and drop action enters our widget, we will receive a drag enter event. https:// inherits most of its functionality from https://, which in turn inherits most of its functionality from https://. Note that we must accept this event in order to receive the drag move events that are sent while the drag and drop action is in progress. The drag enter event is always immediately followed by a drag move event.

In our dragEnterEvent() implementation, we first determine whether we support the event's MIME type or not:

void DragWidget::dragEnterEvent(QDragEnterEvent *event)
{
    if (event->mimeData()->hasFormat(fridgetMagnetsMimeType())) {
        if (children().contains(event->source())) {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        } else {
            event->acceptProposedAction();
        }

If the type is application/x-fridgemagnet and the event origins from any of this application's fridge magnet widgets, we first set the event's drop action using the https://() method. An event's drop action is the action to be performed on the data by the target. https:// indicates that the data is moved from the source to the target.

Then we call the event's https://() method to indicate that we have handled the event. In general, unaccepted events might be propagated to the parent widget. If the event origins from any other widget, we simply accept the proposed action.

    } else if (event->mimeData()->hasText()) {
        event->acceptProposedAction();
    } else {
        event->ignore();
    }
}

We also accept the proposed action if the event's MIME type is text/plain, i.e., if https://() returns true. If the event has any other type, on the other hand, we call the event's https://() method allowing the event to be propagated further.

void DragWidget::dragMoveEvent(QDragMoveEvent *event)
{
    if (event->mimeData()->hasFormat(fridgetMagnetsMimeType())) {
        if (children().contains(event->source())) {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        } else {
            event->acceptProposedAction();
        }
    } else if (event->mimeData()->hasText()) {
        event->acceptProposedAction();
    } else {
        event->ignore();
    }
}

Drag move events occur when the cursor enters a widget, when it moves within the widget, and when a modifier key is pressed on the keyboard while the widget has focus. Our widget will receive drag move events repeatedly while a drag is within its boundaries. We reimplement the https://() method, and examine the event in the exact same way as we did with drag enter events.

Note that the https://() event handler behaves slightly differently: We first get hold of the event's MIME data.

void DragWidget::dropEvent(QDropEvent *event)
{
    if (event->mimeData()->hasFormat(fridgetMagnetsMimeType())) {
        const QMimeData *mime = event->mimeData();

The https:// class provides a container for data that records information about its MIME type. https:// objects associate the data that they hold with the corresponding MIME types to ensure that information can be safely transferred between applications, and copied around within the same application.

We retrieve the data associated with the application/x-fridgemagnet MIME type using a data stream in order to create a new DragLabel object.

        QByteArray itemData = mime->data(fridgetMagnetsMimeType());
        QDataStream dataStream(&itemData, QIODevice::ReadOnly);

        QString text;
        QPoint offset;
        dataStream >> text >> offset;

The https:// class provides serialization of binary data to a https:// (a data stream is a binary stream of encoded information which is completely independent of the host computer's operating system, CPU or byte order).

Finally, we create a label and move it to the event's position:

        DragLabel *newLabel = new DragLabel(text, this);
        newLabel->move(event->pos() - offset);
        newLabel->show();
        newLabel->setAttribute(Qt::WA_DeleteOnClose);

        if (event->source() == this) {
            event->setDropAction(Qt::MoveAction);
            event->accept();
        } else {
            event->acceptProposedAction();
        }

If the source of the event is also the widget receiving the drop event, we set the event's drop action to https:// and call the event's https://() method. Otherwise, we simply accept the proposed action. This means that labels are moved rather than copied in the same window. However, if we drag a label to a second instance of the Fridge Magnets example, the default action is to copy it, leaving the original in the first instance.

If the event's MIME type is text/plain (i.e., if https://() returns true) we retrieve its text and split it into words. For each word we create a new DragLabel action, and show it at the event's position plus an offset depending on the number of words in the text. In the end we accept the proposed action. This lets the user drop selected text from a text editor or Web browser onto the widget to add more fridge magnets.

    } else if (event->mimeData()->hasText()) {
        QStringList pieces = event->mimeData()->text().split(QRegularExpression(QStringLiteral("\\s+")),
                             QString::SkipEmptyParts);
        QPoint position = event->pos();

        for (const QString &piece : pieces) {
            DragLabel *newLabel = new DragLabel(piece, this);
            newLabel->move(position);
            newLabel->show();
            newLabel->setAttribute(Qt::WA_DeleteOnClose);

            position += QPoint(newLabel->width(), 0);
        }

        event->acceptProposedAction();
    } else {
        event->ignore();
    }
}

If the event has any other type, we call the event's https://() method allowing the event to be propagated further.

• draganddrop/fridgemagnets/draglabel.cpp

• draganddrop/fridgemagnets/draglabel.h

• draganddrop/fridgemagnets/dragwidget.cpp

• draganddrop/fridgemagnets/dragwidget.h

• draganddrop/fridgemagnets/fridgemagnets.pro

• draganddrop/fridgemagnets/fridgemagnets.qrc

• draganddrop/fridgemagnets/main.cpp