How to drag and drop a specific QPixmap into a QGraphicsView?

Question

On a subclassed QListWidget I have several items. Every QListWidget item (e.g. "ROS Init", "Images" etc) that is shown below is associated with a specific icon.

The problem I have is that I am trying to drag and drop the specific icon corresponding to that QListWidget item, but nothing happens.

Below the function responsible for the dragging:

void ListView::startDrag(Qt::DropActions supportedActions)
{
    QMap<int, QString> icons;
    icons.insert(IT_RosInit, "ROS Init");
    icons.insert(IT_Images, "Images");
    icons.insert(IT_Path, "Path");
    icons.insert(IT_RosShutDown, "ROS Shutdown");

    if (supportedActions & Qt::CopyAction)
    {
        const QList<QListWidgetItem *> &m_items(selectedItems());
        if (m_items.isEmpty())
            return;

        QPixmap pixmapLaser("/home/images/laserscan.png");
        QPixmap pixmapPCloud2("/home/images/pcloud2.png");
        // etc ...

        QStringList iconImages;

        for(int i = 0; i < icons.count(); ++i)
        {
            for (const QString &tableType : iconImages) {
                if (tableType == "ROS Init")
                {
                    auto *data = mimeData(m_items);
                    auto *drag = new QDrag(this);
                    drag->setPixmap(pixmapLaser);
                    drag->setMimeData(data);
                    drag->setHotSpot(pixmapLaser.rect().center());
                    drag->exec(Qt::CopyAction);
                 }
                else if(tableType == "Images")
                {
                    auto *data2 = mimeData(m_items);
                    auto *drag2 = new QDrag(this);
                    drag2->setPixmap(pixmapPCloud2);
                    drag2->setMimeData(data2);
                    drag2->setHotSpot(pixmapPCloud2.rect().center());
                    drag2->exec(Qt::CopyAction);
                }
            }
        }
    }
    else
    {
        QListWidget::startDrag(supportedActions);
    }
}

After subclassing the QListWidget I just reimplemented the usual drag and drop function. All other function are working properly but the startDrag and in fact as I try to drag the proper QPixmap, I actually see nothing being dragged.

I consulted this source, useful, and also this other source which was useful but it didn't reimplement the startDrag but instead dropEvent which for me is not a problem because that is working well.

I also consulted this source and this other source but that also didn't help fixing the problem.

Thanks for shedding light on this matter for solving the problem

Answer 1

Solution

I would approach this problem in the following way:

1. Set the ItemIsDragEnabled flag of QListWidgetItem to enable the item for dragging:

    item->setFlags(Qt::ItemIsEnabled | Qt::ItemIsSelectable | Qt::ItemIsDragEnabled);
   

2. Set the desired data for each item:

    item->setData(Qt::UserRole, type);
   

where type is one of the enumerated values IT_RosInit, IT_Images, etc.

3. Enable the drag functionality of the ListWidget:

    setDragEnabled(true);
    setDragDropMode(QAbstractItemView::DragOnly);
   

4. Use the item settings to setup the QDrag object, created in startDrag.

Example

Here is an example I have prepared for you to demonstrate how the proposed solution could be implemented:

MainWindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QWidget(parent)
{
    auto *l = new QVBoxLayout(this);
    auto *list = new ListView(this);

    list->addItem(createItem(":/pix/images/laserscan.png", tr("RosInit"), IT_RosInit));
    list->addItem(createItem(":/pix/images/icons/pcloud2.png", tr("Images"), IT_Images));
    list->addItem(createItem(":/pix/images/icons/some_icon.png", tr("Path"), IT_Path));
    list->addItem(createItem(":/pix/images/icons/another_icon.png", tr("RosShutDown"), IT_RosShutDown));

    l->addWidget(list);

    resize(300, 400);
    setWindowTitle("IconDrag");
}

QListWidgetItem *MainWindow::createItem(const QString &pm, const QString &text, int type)
{
    auto *item = new QListWidgetItem(QIcon(QPixmap(pm)), text);

    item->setFlags(Qt::ItemIsEnabled | Qt::ItemIsSelectable | Qt::ItemIsDragEnabled);
    item->setData(Qt::UserRole, type);

    return item;
}

ListView.cpp

ListView::ListView(QWidget *parent) :
    QListWidget(parent)
{
    setDragEnabled(true);
    setDragDropMode(QAbstractItemView::DragOnly);
    setSelectionBehavior(QAbstractItemView::SelectRows);
    setSelectionMode(QAbstractItemView::SingleSelection);
}

void ListView::startDrag(Qt::DropActions supportedActions)
{
    if (supportedActions & Qt::CopyAction) {
        const QList<QListWidgetItem *> &items(selectedItems());

        if (items.isEmpty())
            return;

        const QPixmap &pm(items.first()->icon().pixmap(64));
        auto *item = items.first();
        auto *mimeData = new QMimeData();
        auto *drag = new QDrag(this);

        mimeData->setData("text/plain", item->data(Qt::UserRole).toByteArray());

        drag->setPixmap(pm);
        drag->setMimeData(mimeData);
        drag->setHotSpot(pm.rect().center());
        drag->exec(Qt::CopyAction);
    }
}