Custom QStandardItem drag and drop not working properly

[Yohdu]

Hello,

I come to you cause I do not manage to make my custom QStandardItem/QStandardItemModel working as I want it to.

I basically have these 4 classes :

class Map
{
public:
    Map();
    Map(const sf::String& name);
    Map(const Map& map);

    //accessor

private:
    sf::String m_name;
};
/*****************************************/

class MapItem : public QStandardItem
{
public:
    MapItem(Map* map);

    Map* map() const;

    QVariant data(int role = Qt::UserRole + 1) const;
    void setData(const QVariant &value, int role = Qt::UserRole + 1);

    QStandardItem* clone() const;

private:
    Map* m_map;
};

/*****************************************/

class MapItemModel : public QStandardItemModel
{
public:
    MapItemModel();
};

/*****************************************/

class MapItemView : public QTreeView
{
public:
    MapItemView(QWidget* parent = Q_NULLPTR);

    QSize sizeHint() const;
};

So there is a custom QStandardItem (MapItem) which contains a pointer on a Map object. My first question is : is it the correct way to have a QStandardItem containing custom data ? Or should I use the setData method with a custom userRole (which I tried but did not seem to be really pertinent in my opinion) ?

Then, I have a custom model which I aliment with MapItems. My model is set in an MapItemView, which is simply a QTreeView in which I intend to implement custom behaviors. My second and main problem is that I cannot manage to make internal move operations work properly. Here is the relevant source code:

//***********MapItem

MapItem::MapItem(Map* map)
{
    m_map = map;
}

Map* MapItem::map() const
{
    return m_map;
}

QVariant MapItem::data(int role) const
{
    if(role == Qt::DisplayRole || role == Qt::EditRole)
    {
        return QVariant(map()->name().toAnsiString().c_str());
    }

    return QStandardItem::data(role);
}

void MapItem::setData(const QVariant &value, int role)
{
    if(role == Qt::DisplayRole || role == Qt::EditRole)
    {
        QString mapName = value.toString();
        map()->setName(mapName.toStdString().c_str());
        emitDataChanged();
    }
    else
    {
        QStandardItem::setData(value, role);
    }
}

QStandardItem* MapItem::clone() const
{
    Map* map = new Map(*m_map);
    MapItem* mapItem = new MapItem(map);
    return mapItem;
}

//***********MapItemModel

MapItemModel::MapItemModel() :
    QStandardItemModel()
{
    setItemPrototype(new MapItem(new Map()));

    QStandardItem* parentItem = invisibleRootItem();
    MapItem* mapItem = new MapItem(new Map("test1"));
    parentItem->appendRow(mapItem);
    mapItem->appendRow(new MapItem(new Map("test11")));
    mapItem->appendRow(new MapItem(new Map("test12")));
    parentItem->appendRow(new MapItem(new Map("test2")));
    parentItem->appendRow(new MapItem(new Map("test3")));
}

//***********MapItemView

MapItemView::MapItemView(QWidget* parent) :
    QTreeView(parent)
{
    setModel(new MapItemModel);
    setHeaderHidden(true);
    expandAll();

    //drag and drop
    setSelectionMode(QAbstractItemView::SingleSelection);
    setDragEnabled(true);
    viewport()->setAcceptDrops(true);
    setDropIndicatorShown(true);
    setDragDropMode(QAbstractItemView::InternalMove);
}

When I perform a move operation of a MapItem, the MapItem i moved "dissapears" and a new default empty MapItem is provided. According to the documentation, it must be caused by the

setItemPrototype(new MapItem(new Map()));

call. But if I do not call this method, the moved MapItem "becomes" a QStandardItem (thus with no Map object).

My second and main question is then : how to implement correct drag and drop operations with custom QStandardItem which needs to own custom data ?

With thanks

[raven-worx]

@Yohdu said:

My second and main question is then : how to implement correct drag and drop operations with custom QStandardItem which needs to own custom data ?

With the standard item widgets/models you can't drag-n-drop custom item roles.
This makes sense, since the model doesn't know which UserRole+ was set.
It would need to iterate all possible item-role values when creating the drop-data.

The only solution i see is to create your own custom model and reimplement the needed drag-n-drop methods. This also give you full control over your data structure storing the data inside the model.
This is more work, but you will learn a lot and gain some performance (by avoiding the standard-item classes)