Updating sort-order indicator in QTableView/QHeaderView when model is sorted

Question

I want to know how to ensure the sort indicator in the horizontal header of a QTableView is updated when a programmatic sort is performed on the model.

Here's the problem:

QStandardItemModel model(3,1);
QTableView view;
view.setModel( &model );

// Populate the model ensuring it is not in a sorted order
for( int row = 0; row < model.rowCount(); ++row )
{
    model.setItem( row , 0 ,
                   new QStandardItem(QString::number((row+1)%model.rowCount())));
}

view.setSortingEnabled( true );
// At this point everything is consistent since enabling the sorting
// triggers a sort that matches the indicator in the horizontalHeader (see A)

model.sort( 0 , Qt::AscendingOrder );
// However at this point the sort order has been reversed but the
// header's sort indicator remains unchanged (see B)

A: B:

As you can see the sort indicator remains the same and therefore is inconsistent with the actual sort order.

In my application I have two views that interact with the same model and sorting can be triggered from either of them. I don't see anything in QAbstractItemModel that signals when a sort has been performed. It seems like QHeaderView/TableView assume that they are the only thing that can trigger a sort.

Does Qt provide facilities for coping with this that I'm missing? If not, what's the best way of keeping the sort indicator up-to-date without breaking the encapsulation of the multiple views on the model too much?

Answer 1

One of the ItemDataRole enumerators available since Qt 4.8 is InitialSortOrderRole.

http://qt-project.org/doc/qt-4.8/qt.html#ItemDataRole-enum

It should therefore be possible to transmit sort order information through the QAbstractItemModel::headerData method.

I've tried this however and found that QTableView and QHeaderView do not seem to update in response to changes in this headerData role. A customised header view would appear to be necessary...

It might be worth it because passing this information via the model allows any number of views to synchronise without any external agent having to track all the views in existence so that it can distribute notifications. It would also work seamlessly through model proxy stacks such as those built with QSortFilterModelProxy.