QML ListView on prepend items to QAbstractListModel, ListView jumps to first element

[blacky_i]

Hello everyone!

I'm writing messaging app and want to implement fetching older messages. On atYBeginning I call prependFetchMore() and it adds to QAbstractListModel new items. But it immediately jumps to element with index 0, but I'm looking to stay on previous element

Here is listView.qml:

import QtQuick 2.6
import QtQuick.Window 2.2
import org.test1.0
Window {
   id:window
   visible: true
   width: 640
   height: 480
   title: qsTr("Hello World")
   
   Column {
       width:parent.width
       Rectangle {
           id:busyInd
           property bool running: false
           color:"black"
           width: parent.width
           height:running ? 40 : 0
           focus: running
       }
       ListView{
           id:listView
           
           
           
           onContentYChanged: if ( atYBeginning ) {
                                  modelRef.prependFetchMore()
                              }
           onCurrentIndexChanged: console.log("current index - " + currentIndex)
           width: parent.width
           height: window.height - busyInd.height
           
           model:     MyModel {
               id:modelRef
               initialIndex: 100
           }
           delegate: Rectangle {
               width:parent.width
               height: 100
               border.color: 'black'
               border.width: 2
               color: element_ref > 0 ? "cyan" : "transparent"
               Row {
                   anchors.horizontalCenter: parent.horizontalCenter
                   Text {
                       text: "index = " + index
                   }
                   Item {
                       width: 50
                       height: 1
                   }
                   Text {
                       id: elementId
                       text: "id = " + element_id
                   }
                   Item {
                       width: 50
                       height: 1
                   }
                   Text {
                       id: elementRef
                       text: "ref_id = " + element_ref
                   }
               }
           }
           header: Rectangle {
               z:1000
               width: parent.width
               height: 20
               color: "red"
               MouseArea {
                   anchors.fill: parent
                   onClicked: listView.positionViewAtBeginning()
               }
           }
           headerPositioning:ListView.OverlayHeader
           
           footerPositioning:ListView.OverlayFooter
           footer: Rectangle {
               z:1000
               width: parent.width
               height: 20
               color: "blue"
               MouseArea {
                   anchors.fill: parent
                   onClicked: listView.positionViewAtEnd()
               }
           }
       }
   }
}

Here is my model:

#include "mymodel.h"
#include <QDebug>

MyModel::MyModel(QObject *parent) : QAbstractListModel(parent),
    m_initialIndex(0),
    m_currentStack(0)
{
    Top.append(-1);
    //int[] ids = {8,260,370};
}

int MyModel::rowCount(const QModelIndex &parent) const
{
    return m_elements.size();
}

QVariant MyModel::data(const QModelIndex &index, int role) const
{
    if( index.row() < 0)
        return QVariant();
    int rowIndex = index.row();
    switch (role) {
    case ID:
        return m_elements[rowIndex].id;
        break;
    case REF:
        return m_elements[rowIndex].ref;
        break;
    }
    return QVariant();
}

void MyModel::fetchMore(const QModelIndex &parent)
{
    if(!fetching) {
        fetching = true;

        if (m_elements.size() == 0) {
            auto elements = getNewElements(m_initialIndex,MAX_PER_PAGE);
            appendElements(elements,m_currentStack);
        } else {
            auto elements = getNewElements(m_elements[Top[m_currentStack]].id,MAX_PER_PAGE);
            appendElements(elements,m_currentStack);
        }
        fetching = false;
    }
}

bool MyModel::canFetchMore(const QModelIndex &/* parent*/) const
{
    return !fetching && m_elements.size() < 600;
}

QHash<int, QByteArray> MyModel::roleNames() const
{
    QHash<int, QByteArray> roles;
    roles[ID] = "element_id";
    roles[REF] = "element_ref";
    return roles;
}

int MyModel::initialIndex() const
{
    return m_initialIndex;
}

void MyModel::setInitialIndex(int initialIndex)
{
    if (m_initialIndex == initialIndex)
        return;

    m_initialIndex = initialIndex;
    emit initialIndexChanged(m_initialIndex);
}

void MyModel::prependFetchMore()
{
    if(!fetching) {

        if (m_elements.size() != 0)  {
            fetching = true;
            auto elements = getNewElements(m_elements.first().id - MAX_PER_PAGE - 1,MAX_PER_PAGE);
            if(elements.size()>0)
                prependElements(elements,m_currentStack);
        }
        fetching = false;
    }
}

QList<Element> MyModel::getNewElements(const int lastId, const int size)
{
    QList<Element> result;
    if(lastId >=600)
        return result;
    int off_size = size;
    if(lastId+size >=600)
        off_size = 600 - lastId;
    if(lastId >= 0) {
        for (int i = lastId + 1; i < lastId + 1 + off_size;i++) {
            if(i == 0)
                result.append(Element(i,250));
            else if(i == 250)
                result.append(Element(i,350));
            else if(i == 350)
                result.append(Element(i,550));
            else
                result.append(Element(i,0));
        }
    }
    return result;
}

void MyModel::appendElements(const QList<Element> &elements, const int &stackNumber)
{
    int lastIndex = Top[stackNumber];
    if(elements.size() > 0) {
        beginInsertRows(QModelIndex(),lastIndex + 1,lastIndex + elements.size());
        foreach (auto el, elements) {
            lastIndex++;

            m_elements.insert(lastIndex,el);
        }
        for (int i = stackNumber; i < Top.size(); i++)
            Top[i] = Top[i] + elements.size();

        endInsertRows();
    }
}

void MyModel::prependElements(const QList<Element> &elements, const int &stackNumber)
{
    int lastIndex = 0;
    if(stackNumber != 0)
        lastIndex = Top[stackNumber - 1] + 1;

    beginInsertRows(QModelIndex(),lastIndex,lastIndex + elements.size() - 1);

    for(int i = elements.size() - 1; i >= 0; i--) {
        m_elements.insert(lastIndex, elements[i]);
    }
    for (int i = stackNumber; i < Top.size(); i++)
        Top[i] = Top[i] + elements.size();

    endInsertRows();
}

So how to preverve element position?