How to implement nested editable lists in C++ as models in Qt (QML oriented)

I'm developing an application in QML + C++. I have list of models (say, ParentList) with each item having list of items (say, ChildList). User should be able to add/remove items from ParentList as well as add/remove items in ChildList of each ParentList item.

Which architecture is best suited for it? Can you provide an example?

Well, depends on your data on each level.

One option could be to have a list of object for the first level, each having the item's data as properties, plus an additional list of bjects property for the children, each again, carrying the data in properties.

Or having the second level as a list model

Or having both levels as list models.

Cheers,
_

Let's suppose I have List of objects and each of them has a StringList (and we can add/remove items in StringList as well as parent Lists). What I'm asking is how to implement it in C++ correctly to ensure all bindings will work in QML?

Best of you implement it as a full fledged QAbstractItemModel subclass, expose it to QML and equip with a nice API. Then you can use VisualDataModel to traverse this model easily in the ListView.

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Originally Posted by wysota

Best of you implement it as a full fledged QAbstractItemModel subclass, expose it to QML and equip with a nice API. Then you can use VisualDataModel to traverse this model easily in the ListView.

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Ok, I tried to implement it as QAbstractListModel, but I failed to return inner list in override of data() method as QVariant. What is the correct way to expose inner property (which is a model but not an ordinary property) to QML?

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Originally Posted by ribtoks

Let's suppose I have List of objects and each of them has a StringList (and we can add/remove items in StringList as well as parent Lists).

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If your objects are QObject derived then this is trivial: have the list of them as a property on some object or as a context property and their QStringList as a property of each object.

If you objects are not QObject derived, create a simple list model (derive from QAbstractListModel), as wysota suggested.

Cheers,
_

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Originally Posted by anda_skoa

If your objects are QObject derived then this is trivial: have the list of them as a property on some object or as a context property and their QStringList as a property of each object.

If you objects are not QObject derived, create a simple list model (derive from QAbstractListModel), as wysota suggested.

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I've read in docs, that I should return that property with list in method data(role) which would return child item's property value with role as a key (property name). data() returns QVariant according to docs. Would it be correct to return QVariant::fromValue(myInnerListHere)? My control needs to provide edit functionality both to parent and child items. If I'll return child's item through QVariant::fromValue() in method data() would all my bindings (which allow to add/remove/edit) work correct?

QVariant::fromValue() on the stringlist sounds correct, the QML engine should be able to handle that as an array of strings on its side.

For the editing just have some slots or Q_INVOKABLEs in your model class. Don't forget to make the model emit the right signals (e.g. dataChanged() for changes to the string lists)

Cheers,
_

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Originally Posted by anda_skoa

For the editing just have some slots or Q_INVOKABLEs in your model class.
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Could you please be more specific on that? Which slots? Link to the docs will be ok.

Your own slots. Methods to manipulate your data.

Cheers,
_

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Originally Posted by anda_skoa

Your own slots. Methods to manipulate your data.

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Ok, I'll read more on that. Sorry for stupid questions.

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Originally Posted by ribtoks

Ok, I tried to implement it as QAbstractListModel,

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Why not QAbstractItemModel?

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Originally Posted by wysota

Why not QAbstractItemModel?

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Because it looked easier (I hadn't idea why I need QAbstractItemModel). I had class for List of Lists which was QAbstractListModel and child List which also was QAbstractListModel (which I planned to return as a property through parent's data() method).
Why this is bad? (or why this was bad? I'm going to rewrite it based on responses in this thread)

You want a hierarchical model thus it seems to me implementing a proper model will do the job. I don't think returning a list in each model which will in turn return a list of its own is a better approach.

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Originally Posted by wysota

Why not QAbstractItemModel?

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Easier to implement a list with QAbstractListModel at its base.
QAbstractItemModel requires implementations for columnCount(), index() and parent(), the latter two usually not trivial, and neither is needed for a list.

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Originally Posted by wysota

You want a hierarchical model thus it seems to me implementing a proper model will do the job.

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Aside from ribtoks data not being hierachical as it turns out, I don't think QtQuick can deal with tree models at this point.

Cheers,
_

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Originally Posted by anda_skoa

I don't think QtQuick can deal with tree models at this point.

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It can deal with tree models with the use of VisualDataModel element which allows to set the root index to traverse different layers of the model within the view.