QTableView: endMoveRows in model resets horizontal header sizes - c++

Question

I'm trying to implement drag'n'drop reordering of rows in a QTableView.
The operation needs to actually reorder the underlying data, therefore reordering the vertical header rows doesn't seem like a good solution.
I instead opted for the underlying model to handle this by providing the row indices in its mimeData function and accepting this list in dropMimeData. In the dropMimeData function, I use the beginMoveRows and endMoveRows to move individual rows visually, in addition to moving the underlying data. This approach seems to work, with only one shortcoming: as soon as I call endMoveRows, the QTableView's horizontal header resets all its section sizes to defaults.
Am I missing something I need to do, or is this a bug in Qt (or even intended behavior)? Is there any other way to handle drag'n'drop reordering?
Here is a simplified piece of code that shows the same behavior:
class Model: public QAbstractTableModel
{
public:
Model():
m_Items({"A", "B", "C", "D", "E"})
{
}
virtual QVariant data(const QModelIndex & a_Parent, int a_Role) const override
{
return QString("%1 - %2").arg(m_Items[a_Parent.row()]).arg(a_Parent.column());
}
virtual int rowCount(const QModelIndex & a_Parent) const override
{
return m_Items.count();
}
virtual int columnCount(const QModelIndex & a_Parent) const override
{
return 4;
}
void moveNow()
{
// Moves the last row as the 3rd row:
auto count = m_Items.count();
beginMoveRows(QModelIndex(), count, count, QModelIndex(), 2);
m_Items.insert(2, m_Items.last());
m_Items.pop_back();
endMoveRows();
}
protected:
QStringList m_Items;
};
static Model g_Model;
MainWindow::MainWindow(QWidget * parent):
QMainWindow(parent),
ui(new Ui::MainWindow)
{
// "ui" created by QtCreator, has a "tableView" and a "pushButton"
ui->setupUi(this);
ui->tableView->setModel(&g_Model);
ui->tableView->horizontalHeader()->resizeSection(0, 200);
ui->tableView->horizontalHeader()->resizeSection(1, 100);
ui->tableView->horizontalHeader()->resizeSection(2, 50);
ui->tableView->horizontalHeader()->resizeSection(3, 80);
connect(ui->pushButton, &QPushButton::clicked,
[this]()
{
// Outputs 200 on first click
qDebug() << ui->tableView->horizontalHeader()->sectionSize(0);
g_Model.moveNow();
// Outputs 100
qDebug() << ui->tableView->horizontalHeader()->sectionSize(0);
}
);
}

Answer 1

I found out that calling emit layoutAboutToBeChanged() just before the move operation fixes the issue:
void moveNow()
{
emit layoutAboutToBeChanged();
auto count = m_Items.count();
beginMoveRows(QModelIndex(), count, count, QModelIndex(), 2);
m_Items.insert(2, m_Items.last());
m_Items.pop_back();
endMoveRows();
}
This seems to contradict the documentation for beginMoveRows, which indicates that it is a replacement for calling this function. Also there must not be a matching emit layoutChanged() after endMoveRows, otherwise the columns are, again, reset.
This all together leads me to believe it's an actual bug in Qt.