Unable to update Listview using QAbstractItemModel class

James A

I am trying to update the Listview from c++ data model which is the subclass of QAbstractItemModel. When the run the program the listview is empty
During debugging I observed that the following functions are not called which I think it is required for displaying the model data to the view

QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;
QHash<int, QByteArray> roleNames() const;

I have added the required snippets of the code below

================testmodel.h=================================

#include <QAbstractItemModel>

class Data {
QString name ;
int strength;

public:

Data(QString name ,int strength);
int getStrength() const;
void setStrength(int value);
QString getName() const;
void setName(const QString &value);

};

class TestModel : public QAbstractItemModel
{
Q_OBJECT

public:

enum Roles {
    nameRole = Qt::UserRole + 1,
    strengthRole
};

explicit TestModel(QObject *parent = nullptr);

int rowCount(const QModelIndex &parent = QModelIndex()) const override;

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

void addRows(Data data);

protected:
QHash<int, QByteArray> roleNames() const;

private:
QList<Data> list;

};

================testmodel.cpp=================================

QVariant TestModel::data(const QModelIndex &index, int role) const
{
if (index.row() < 0 || index.row() >= list.count())
return QVariant();

const Data &animal = list[index.row()];
if (role == nameRole)
    return animal.getName();
else if (role == strengthRole)
    return animal.getStrength();

return QVariant();

}

void TestModel::addRows(Data data)
{
beginInsertRows(QModelIndex(), rowCount(), rowCount());
list << data;
endInsertRows();
}

QHash<int, QByteArray> TestModel::roleNames() const
{
QHash<int, QByteArray> roles;
roles[nameRole] = "name";
roles[strengthRole] = "strength";
return roles;

}

==============main.cpp========================================

int main(int argc, char ** argv)
{
QGuiApplication app(argc, argv);

TestModel testmodel;

testmodel.addRows(Data("name1",5));
testmodel.addRows(Data("name2",10));

QQuickView view;
view.setResizeMode(QQuickView::SizeRootObjectToView);

QQmlContext *ctxt = view.rootContext();
ctxt->setContextProperty("myModel", &testmodel);

view.setSource(QUrl("qrc:view.qml"));
view.show();

return app.exec();

}

==================view.qml====================================

import QtQuick 2.0

Item {
width: 200; height: 250

ListView {
    width: 200; height: 250

    anchors.fill: parent
    model: myModel
    delegate: Text {  text: "mydata" + name + ", " + strength }
}

}

raven-worx

@James-A
since you inherit from QAbstractItemModel you should also implement columnCount() - and return at least 1 column.
Maybe inheriting from QAbstractListModel is more suited for you? It implements a few methods for lists already.

James A

@raven-worx
I tried inheriting from the QAbstractListModel class and implemented columnCount() function and returned columnCount as 2 , sines there are 2 roles , but the listview is not updating after this change

raven-worx

@James-A

either implement columnCount() OR inherit from QAbstractListModel
no need to return a column count of 2, since a list always has 1 column
each model index can return multiple roles

Does the count property in the listview change?

ListView {
    onCountChanged: console.log("count changed: " + count)
}

James A

@raven-worx

Now I tried inheriting from QAbstractListModel class and the count property shows :
qml: count changed: 0

raven-worx

@James-A
i dont see any issue in your code. The last thing that could be wrong is the rowCount() implementation?

James A

@raven-worx

Thanks for pointing it out. It was the issue with rowCount() implementation.
I commented the old code and updated as shown below , Now the Listview is updating

int TestModel::rowCount(const QModelIndex &parent) const
{
// if (!parent.isValid())
// return 0;
// return list.count();

Q_UNUSED(parent);
return list.count();

}

And while going through the documentation , I found the below virtual function for inserting rows into the model. But how to pass the "Data" Object to this function ?

bool insertRows(int row, int count, const QModelIndex &parent = QModelIndex()) override;

I want to replace this user defined function testmodel.addRows(Data ) with the insertRows virtual function

raven-worx

@James-A
actually the following would be more accurate:

int TestModel::rowCount(const QModelIndex &parent) const
{
    if (parent.isValid())
        return 0;
    return list.count();
}

The insertRow() method is used by other Qt classes (drag-n-drop, etc.) for example.
Going this route means calling insertRow() to add a (default) item at the given position and call setData() on that newly inserted item.

So the way you are currently using is way more efficient and ok.

James A

@raven-worx

Thanks for the Update