qt - Sort and filter a C++ model via QML functors?

Question

I have a polymorphic (as in arbitrary roles) QObject model that is mostly instantiated declaratively from QML, as in this answer, and I would like to be able to have custom data "views" that sort and filter the model via arbitrary, and potentially - runtime generated from code strings JS functors, something like that:

  DataView {
    sourceModel: model
    filter: function(o) { return o.size > 3 }
    sort: function(a, b) { return a.size > b.size }
  }

The QSortFilterProxyModel interface doesn't seem to be particularly well suited to the task, instead being fixated on static roles and pre-compiled rules.

I tried using QJSValue properties on the C++ side, but it seems like it is not possible, the C++ code just doesn't compile with that property type. And if I set the property type to QVariant I get error messages from QML that functions can only be bound to var properties. Evidently, var to QVariant conversion doesn't kick in here as it does for return values.

Answer 1

As you mentionned, you could use QJSValue. But that's pretty static. What if you want to use a filter like filter: function(o) { return o.size > slider.value; } with a dynamic slider ? You'll have to manually call invalidateFilter().

As a more practical alternative, you could instead use QQmlScriptString as a property & QQmlExpression to execute it. Using QQmlExpression allows you to be notified of context changes with setNotifyOnValueChanged.

Your syntax would change to be like so : filter: o.size > slider.value.

If you are looking for an out of the box solution, I've implemented this in a library of mine : SortFilterProxyModel on GitHub

You can take a look at ExpressionFilter & ExpressionSorter, those do the same as what you initially wanted. You can check the complete source code in the repo.

How to use it :

import SortFilterProxyModel 0.2

// ...

SortFilterProxyModel {
    sourceModel: model
    filters: ExpressionFilter  { expression: model.size > 3 }
    sorters: ExpressionSorter { expression: modelLeft.size < modelRight.size }
}

But as @dtech mentionned, the overhead of going back and forth between qml and c++ for each row of the model is quite noticeable. That's why I created more specific filters and sorters. In your case, we would use RangeFilter and RoleSorter :

import SortFilterProxyModel 0.2

// ...

SortFilterProxyModel {
    sourceModel: model
    filters: RangeFilter  {
        roleName: "size"
        minimumValue > 3
        minimumInclusive: true
    }
    sorters: RoleSorter { roleName: "size" }
}

Doing like this, we have a nice declarative API and the parameters are only passed once from qml to c++. All the filtering and sorting is then entirely done on the c++ side.