Wrestling with Qt’s Model/View API – Filtering in Tree Models

As I described in one of my last posts, Qt4’s model/view API can be kind of a challenge sometimes. Well, prepare for a even harder fight when sorting and filtering come into play.

Let’s say you finally managed to get the data into your model and to provide correct implementations of the required methods in order for the attached views to display it properly. One of your next assignments after that is very likely something like implementing some kind of sorting and filtering of the model data. Qt provides a simple-at-first-sight proxy architecture for this with API class QSortFilterProxyModel as main ingredient.

Small preliminary side note: Last time I checked it was good OO practice to have only one responsibility for a given class. And wasn’t that even more important for good API design? Well, let’s not distract us with such minor details.

With my model implementation, none of the standard filtering mechanisms, like setting a filter regexp, were applicable, so I had to override method

QVariant filterAcceptsRow ( int source_row, const QModelIndex& sourceParent ) const

in order to make it work. Well, the rows disappeared as they should, but unfortunately so did all the columns except the first one. So what to do now? One small part of the documentation of QSortFilterProxyModel made me a little uneasy:

“… This simple proxy mechanism may need to be overridden for source models with more complex behavior; for example, if the source model provides a custom hasChildren() implementation you should also provide one in the proxy model.”

What on earth should I do with that? “… may need to be overridden“? “… for example.. hasChildren()…” Why can’t they just say clearly what methods must be overridden in which cases???

After a lot more trial and error I found that for whatever reason,

int columnCount ( const QModelIndex& parent ) const

had to be overridden in order for the columns to reappear. The implementation looks like what I had thought the proxy model would do already:

int MyFilter::columnCount ( const QModelIndex& parent ) const
{
   return sourceModel()->columnCount(parent);
}

So beware of QSortFilterProxyModel! It’s not as easy to use as it looks, and with that kind of fuzzy documentation it is even harder.