KDFunctionalSortFilterProxyModel A Functional Sort/Filter Proxy Model

15.12.2021 Giuseppe D'Angelo No comments

Another day, another small addition to KDToolBox, KDAB’s collection of miscellaneous useful C++ classes and stuff.

In this post, we’re going to talk about KDFunctionalSortFilterProxyModel, which is a convenience subclass of QSortFilterProxyModel.

What’s a proxy model to begin with, and what’s QSortFilterProxyModel? Well, that explanation requires a bigger space than what I have in this blog post. I’ll leave that one to Jesper:

Most of the time, QSortFilterProxyModel works just fine “out-of-the-box,” at least when it comes to sorting. But when it comes to filtering, we typically have to subclass it in order to override its row-accept and/or column-accept functions. This is easy, but in a project that makes extensive use of proxy filters, it’s also quite annoying that every time we need a different filter we also have to create a different datatype (that is, a different subclass), give it a proper and unique name, document it, etc.

KDFunctionalSortFilterProxyModel tries to improve on this situation by offering a functional interface to QSortFilterProxyModel. If you need to customize what a given proxy instance should do, all you need to do is to set a function-like object (a free function, a lambda, etc.) that provides custom filtering and/or custom sorting. For instance:

// create the proxy
KDFunctionalSortFilterProxyModel *proxy = new KDFunctionalSortFilterProxyModel(parent);

// use this lambda as the filtering function
auto acceptFunction = [](const QAbstractItemModel *model, int source_row, const QModelIndex &parent)
{
    // decide whether to accept `source_row` from `model`, under `parent`;
    // return true to accept, false to filter the row out
};

proxy->setFilterAcceptsRowFunction(acceptFunction);

// use it!
proxy->setSourceModel(sourceModel);
view->setModel(proxy);

KDToolBox is freely available here. Thanks for reading, and have a great day!