[Interest] Proxied view not updating live when source model does

Björn Schäpers qt-maillist at hazardy.de
Wed Sep 22 20:45:44 CEST 2021

You emit dataChanged with the given role (which is Qt::EditRole), but not for 
Qt::DisplayRole. Is it that?


Am 20.09.2021 um 19:15 schrieb Murphy, Sean:
> I'm having an issue where my proxy model and its associated view aren't updating live when changes
> are made in the source model.
> My proxy model simply is attempting to invert the rows of the source model. So it inherits from a
> QSortFilterProxyModel and implements mapFromSource() and mapToSource() as follows:
>      QModelIndex invertRowsModel::mapFromSource(const QModelIndex &sourceIndex) const
>      {
>          if(sourceIndex.isValid() && rowCount() > 0)
>          {
>              return this->index(rowCount() - sourceIndex.row() - 1, sourceIndex.column());
>          }
>          return QModelIndex();
>      }
>      QModelIndex invertRowsModel::mapToSource(const QModelIndex &proxyIndex) const
>      {
>          if(proxyIndex.isValid() && sourceModel() && rowCount() > 0)
>          {
>              return sourceModel()->index(rowCount() - proxyIndex.row() - 1, proxyIndex.column());
>          }
>          return QModelIndex();
>      }
> I load up my source model with some data, connect the models up to their respective views,
> connect the source model to the proxy and launch the application. The original views are
> correct - the proxied rows are flipped when compared to the source model. But then if the
> user makes a change to some of the data in the source model's view, the corresponding index
> in proxy model's view is NOT updated automatically. HOWEVER, if the user then clicks within
> the proxy view, or clicks outside of my application, the proxy model will then repaint and the
> updated information is correctly shown.
> I've attached a minimal example that shows the behavior. Things to note:
> 1. When the source model emits its dataChanged(), this seems to trigger a dataChanged()
>    emission in the proxy model as well. The weird (at least to me) thing is that both of those
>    signals have the SAME model index row & column numbers. I would have expected that
>    when the source model emits data changed, that the proxy model would translate the
>    source model's indices into the proper proxy model's indices via the mapFromSource()
>    function and then emit dataChanged() with those indices. I've got debug statements in
>    my example to illustrate that no dataChanged() translation is happening - if the user
>    updates data in row 0 in the source model's view, the proxy model also emits that data
>    changed in its row 0, but that isn't the correct row for the proxy.
> 2. We discovered that if we connect the proxy model's dataChanged() signal to a custom
>    slot within the proxy model, and have that slot emit layoutChanged() then we can get
>    the live update behavior when the source model updates. This is at line 25 of
>    invertrowsmodel.cpp. It's commented out initially, but if you uncomment it and re-run
>    the app then proxy view does update instantly. Other similar signals would work as well -
>    we just need something triggers the proxy view to refresh the data in the proxied view,
>    but we don't think we should really have to do this...
> 3. I have a second view attached to the source model and both source model views update
>    each other live - changes in one source model view are instantly updated in the other
>    source model view, but not in the proxy model view without further user interaction
>    (or by emitting that layoutChanged() signal)
> Any thoughts as to why the proxy model isn't updating correctly?
> Sean
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