QML and C++ model not shown

Mick_1

Hello,

I'm trying to learn how to create a C++ model with QAbstractListModel to display it in a QML view but I have nothing displayed despite my attempts.

I followed the video on this Qt site: C++ Model

The only part where I don't follow the explanations is the modification of main to add my model class, because I preferred to use a BackEnd class to modify my model according to an XML file (later).

So I've got a "CustomData" class with a custom item list managed from a QAbstractListModel class whose header looks like this:

#include <QAbstractListModel>

Q_MOC_INCLUDE("CustomData.h")

class CustomData;

class DataModel : public QAbstractListModel
{
    Q_OBJECT

public:
    explicit DataModel(QObject *parent = nullptr);
    ~DataModel();


    Q_PROPERTY(CustomData *dataList READ dataList WRITE setDataList NOTIFY dataListChanged)

    enum
    {
        NameRole  = Qt::UserRole,
        ValueRole = Qt::UserRole+1,
        TypeRole  = Qt::UserRole+2
    };

    // Basic functionality:
    int rowCount(const QModelIndex &parent = QModelIndex()) const override;

    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;

    // Editable:
    bool setData(const QModelIndex &index, const QVariant &value, int role = Qt::EditRole) override;

    Qt::ItemFlags flags(const QModelIndex &index) const override;

    virtual QHash<int, QByteArray> roleNames() const override;

    CustomData *dataList() const;
    void setDataList(CustomData *dataList);


signals:
    void dataListChanged();


private:
    QHash<int, QByteArray> m_roleNames;
    CustomData *m_dataList;

};

Here is the header of my BackEnd class:

/* BackEnd Class */
class BackEnd : public QObject
{
    Q_OBJECT
    Q_PROPERTY(CustomData *dataList READ dataList WRITE setDataList NOTIFY dataListChanged)

    QML_ELEMENT


public:
    /* Constructor */
    explicit BackEnd(QObject *parent = nullptr);
    /* Destructor */
    ~BackEnd();


    /* Public Methods */
    CustomData *dataList() const;
    void setDataList(CustomData *newDataList);


signals:
    void dataListChanged();


private:
    /* Private Methods */


    /* Private Attributes */
    CustomData     *m_dataList;

};

That's all I do in my BackEnd:

BackEnd::BackEnd(QObject *parent)
    : QObject{parent}
{
    m_dataList = new CustomData();
    m_dataList->appendItem();
    m_dataList->appendItem();
}

CustomData *BackEnd::dataList() const
{
    return m_dataList;
}

void BackEnd::setDataList(CustomData *newDataModel)
{
    m_dataList = newDataModel;
    emit dataListChanged();
}

My main:

qmlRegisterType<BackEnd>("BackEnd", 1, 0, "BackEnd");

qmlRegisterType<DataModel>("DataModel", 1, 0, "DataModel");

And finally, my QML view :

ListView {
        id: listView
        ...

        model: DataModel {
             id: datamodel
             dataList: backend.dataList

        }

I know that maybe I'm not showing enough code, but since there are 3 classes, I thought that the problem would already be in what I'm showing.

I think it's more how I call my model in BackEnd and my QML view because the rest is like in the video or the explanations in the Qt 6 QML book.

Thank you if you can give me your point of view.

JonB

@Mick_1 said in QML and C++ model not shown:

But with my custom model, it return 0 as count for now :/

So show your implementation of overridden rowCount()!?

GrecKo

@Mick_1 Did you give a size to your ListView? what does its count property returns?

Mick_1

@GrecKo Yes, I tried to replace my model with a ListModel and ListElement and I can see my delegates and data.

But with my custom model, it return 0 as count for now :/

Would you like more code ? I can publish my whole code here (or somewhere else more convenient ?)

JonB

@Mick_1 said in QML and C++ model not shown:

But with my custom model, it return 0 as count for now :/

So show your implementation of overridden rowCount()!?

Mick_1

Oh damn, it was that...

Example of a copy/paste tutorial without investigating the why...

My rowCount looked like this:

int DataModel::rowCount(const QModelIndex &parent) const
{
    if (!parent.isValid() || !m_dataList)
        return 0;

    return m_dataList->items().size();
}

This parameter and associated code is present by default, so I'm not sure what the point is and whether I should keep its implementation.

Removing the parent test in the condition fixed my problem. Thanks a lot !!!

JonB

@Mick_1
This code's

 if (!parent.isValid())
        return 0;

looks like it was taken from a tree model, not a flat list like you want/have? And hence is the wrong way round for you. When you have a tree model all nodes (other than the root) have a parent which is valid, and rowCount() must return the count of child items. For a flat model it is always passed an invalid parent, your !parent.isValid() will always be true and hence you will always return 0.

For a flat model I would just have

    Q_ASSERT(!parent.isValid());    // `parent` should never be passed in valid, else there is a logic error for a flat list
    if (!m_dataList)
        return 0;

    return m_dataList->items().size();

Mick_1

Ok, thank you @JonB for this explanation, I understood :)

Thanks @GrecKo and @JonB for your help ! I'm happy to finally play with C++ models ^^

jeremy_k

@JonB said in QML and C++ model not shown:

@Mick_1
This code's
 if (!parent.isValid())
        return 0;
looks like it was taken from a tree model, not a flat list like you want/have?

This is also unworkable for a tree model, if the intention is to make the top level items of the tree viewable.