Question

What is the way to go to prevent duplicates in a QListView which uses QStandardItemModel as its model? Data is added with drag & drop, so I tried to override QStandardItemModel::dropMimeData, which seems kind of odd since I need to override QStandardItemModel::mimeData (and reimplement encodeData/decodeData) as well. This has to be easier!

Answer 1

Well, I managed to solve this by overriding QListView::dataChanged, which checks if there is more than one item with the same data for Qt::DisplayRole in the model after dropping and deletes one of them if there is. It basically looks like that:

void MyListView::dataChanged(QModelIndex topLeft, QModelIndex bottomRight)
{
    // there can be only one item dragged at once in my use case
    if(topLeft == bottomRight)
    {
        QStandardItemModel* m = static_cast<QStandardItemModel*>(model());
        // if theres already another item with the same DisplayRole...
        if(m->findItems(topLeft.data().toString()).count() > 1)
        {
            // ... we get rid of it.
            model()->removeRow(topLeft.row());
        }
    }
    else
    {
        // let QListView decide
        QListView::dataChanged(topLeft, bottomRight);
    }
}

It’s by far not perfect (e.g. if you can drop more than one item at once) but it works for that simple use case.