ListView issue when updated (current index set to 0)

[Mickael 22]

Hi all,
I meet an issue with listView when the model is updated from C++.
After each update from C++, the current index is set to 0 and the view return to beginning of it !
In the C++ side, the model exposed is declared like this :

Q_PROPERTY(QList<QObject*> debugList READ getDebugList NOTIFY debugListChanged)

in qml :

 Component {
                    id: nameDelegate
                    Item {
                        id: delegateItem
                        height: ListView.isCurrentItem ? 25 : textDebug.height
                        width:debugRx.width
                        Behavior on height {
                            NumberAnimation {
                                duration: 750
                                easing.type: "InOutQuint"
                            }
                        }
                        Text {
                            id:textDebug
                            text: model.modelData.data
                            color: model.modelData.color
                            width: debugRx.width
                            //height: 20
                            lineHeight: 15
                            lineHeightMode:  Text.FixedHeight
                            scale: parent.height/height
                            transformOrigin: Item.Left
                            wrapMode: Text.WordWrap
                            font{
                                family: "arial"
                                pixelSize:16
                            }
                        }
                    }
                }

                ListView {
                    id:listViewDebug
                    snapMode: ListView.SnapOneItem
                    property string name: "listeDebug"
                    property bool scrollDebugAuto: true
                    property int scrollDebugIndex: 0
                    bottomMargin: 5
                    anchors.fill: parent
                    anchors.topMargin: 5
                    anchors.bottomMargin: 5
                    anchors.leftMargin: 2
                    anchors.rightMargin: 2
                    clip:true
                    model:device.debugList
                    focus:true
                    delegate: nameDelegate
                    ScrollBar.vertical: ScrollBar{
                        id:scrollBarDebug
                    }

                    highlight: Rectangle {
                        width: listViewDebug.width;
                        border.color: "gray"
                        border.width: 2
                        color : "green"
                        opacity: 0.3
                        radius:5
                        Behavior on y {
                            SpringAnimation {
                                spring: 3
                                damping: 0.2
                            }
                        }
                    }
                    highlightFollowsCurrentItem: true
                    //highlightRangeMode : ListView.StrictlyEnforceRange
                    currentIndex: scrollDebugAuto ? listViewDebug.count-1 : scrollDebugIndex
                    onCurrentIndexChanged:{
                        console.log("current index update :"+currentIndex)
                    }
                    onModelChanged: {
                        console.log("current index :"+currentIndex);
                        console.log("save index : "+scrollDebugIndex)
                    }
                    Component.onCompleted: positionViewAtIndex(count - 1, ListView.Beginning)

                }
            }

When the property scrollDebugAuto is true it work fine (the view stay et end of the listView) but when it false the view goes to beginning (current index : 0)
false--> i 'm trying to add a functionality to disable the list update continuously when the user click to a button

                CheckBox {
                    id: checkBox
                    text: "\uf0c1"
                    checked: true
                    onCheckedChanged:
                    {
                        if(checked)
                            listViewDebug.scrollDebugAuto=true
                        else
                        {
                            listViewDebug.scrollDebugAuto=false
                            listViewDebug.scrollDebugIndex=listViewDebug.count-1
                            console.log("scroll debug index : "+listViewDebug.scrollDebugIndex)
                        }
                    }
                }

That's strange is when the model is updated from C++, the listView index is set to 0 !
When the scrollDebugAuto is true i have this on the log :

qml: current index update :52
qml: current index update :0
qml: current index update :52
qml: current index :53
qml: save index : 0

when is false

qml: current index update :0
qml: current index :89
qml: save index : 72

we can see that the current index is set to 0 during C++ model update .
Do you have an idea how i can resolve this ?

[KroMignon]

@Mickael-22 said in ListView issue when updated (current index set to 0):

Do you have an idea how i can resolve this ?

AFAIK, this is the normal behavior when using QList<Object *>() or QStringList() as model, because each model update will generate a full new model.
If you wan't a smooth update, you have to use QAbstractItemModel , cf. https://doc.qt.io/qt-5/qtquick-modelviewsdata-cppmodels.html

[Mickael 22]

@KroMignon Thanks for this explanations, it's more clear now.
I will have a look on QAbstractItemModel class, it just pity to add more complexity with this class to just keep list current index to it's value !

[KroMignon]

@Mickael-22 Yes QAbstractItemModel is not easy to use.

I use since many years now a template class create by Thomas Boutroue which creates an QAbstractListModel by introspection of the base QObject.
Take a look at Qt QML Models and Qt SuperMacros.

Here are some explanations at Qt World Summit 2015:
https://resources.qt.io/videos/qtws15-lightning-talk-a-new-way-for-creating-qml-models-from-c-thomas-boutroue

Perhaps this could help you.

[jeremy_k]

This is an undocumented behavior of ListView. When the model is changed, currentIndex is set to 0, or -1 if the model if the empty.

https://code.woboq.org/qt5/qtdeclarative/src/quick/items/qquickitemview.cpp.html#239:

if (d->model) {
        ...
        if (isComponentComplete()) {
            ...
            setCurrentIndex(d->model->count() > 0 ? 0 : -1);

If you have the option, updating the model rather than changing to a new one is probably a better method.

[Mickael 22]

@KroMignon Thanks a lot !

@jeremy_k
in the C++ code i just emit a signal that the list changed like this :

    m_debugList.append(new BleData(data,color,this));
    emit debugListChanged();

class BleData : public QObject
{
    Q_OBJECT

public:
    explicit BleData(QObject *parent = nullptr);
    BleData(QString data,QString color = "black",QObject *parent = nullptr);

    Q_PROPERTY(QString data READ getData NOTIFY dataChanged)
    Q_PROPERTY(QString color READ getColor NOTIFY colorChanged)

signals:

    void dataChanged(QString data);
    void colorChanged(QString color);

public slots:
    QString getData() const;
    QString getColor() const;
    void addData(QString data,QString color="black");

private :
    QString m_data;
    QString m_color;
};

I don't know how to proceed in another way

[jeremy_k]

@Mickael-22 said in ListView issue when updated (current index set to 0):

@KroMignon Thanks a lot !

@jeremy_k
in the C++ code i just emit a signal that the list changed like this :

    m_debugList.append(new BleData(data,color,this));
    emit debugListChanged();

That's the entire problem. Emitting the change signal for the list of QObjects says the entire list has changed, and should be treated as a new model.

The item models that @KroMignon refers to have signals to indicate changes within the model. If implementing a new QAbstractItemModel sounds like too much work, using a QStandardItemModel, or a QML ListView might work. QStringListView could also, with some changes to the delegate.

[Mickael 22]

Thanks for this precisions.
i will have a look on the different options and let you know the more convenient way i will use