QML ListView is not updated on model reset - qt

Question

Qt 5.8, Windows 10.
Quick Controls 2 application. In QML I have a ListView with a model derived from QAbstractListModel.
In the model I have the following code:
void MediaPlaylistModel::update()
{
beginResetModel();
{
std::unique_lock<std::mutex> lock(m_mutex);
m_ids = m_playlist->itemsIds();
}
endResetModel();
}
Nothing happens in QML view after calling this method: list is not updated.
If I go back and then forward (to the page with the ListView) - it'll contain updated data. Model object instance is the same always.
Am I doing something wrong?
Update #1:
The only methods I override are:
QHash<int, QByteArray> roleNames() const override;
int rowCount(const QModelIndex &parent) const override;
QVariant data(const QModelIndex &index, int role) const override;
Update #2:
C++ model code:
MediaPlaylistModel::MediaPlaylistModel(
QSharedPointer<AbstractMediaPlaylist> playlist,
QObject *parent) :
base_t(parent),
m_playlist(playlist)
{
Q_ASSERT(m_playlist);
connect(playlist.data(), &AbstractMediaPlaylist::changed,
this, &MediaPlaylistModel::update);
update();
}
QHash<int, QByteArray> MediaPlaylistModel::roleNames() const
{
QHash<int, QByteArray> result;
result[IdRole] = "id";
result[TitleRole] = "title";
result[DurationRole] = "duration";
return result;
}
void MediaPlaylistModel::update()
{
Q_ASSERT_SAME_THREAD;
beginResetModel();
{
std::unique_lock<std::mutex> lock(m_mutex);
m_ids = m_playlist->itemsIds();
}
endResetModel();
}
int MediaPlaylistModel::rowCount(
const QModelIndex &parent) const
{
Q_UNUSED(parent);
std::unique_lock<std::mutex> lock(m_mutex);
return static_cast<int>(m_ids.size());
}
QVariant MediaPlaylistModel::data(
const QModelIndex &index,
int role) const
{
auto row = static_cast<size_t>(index.row());
int id = 0;
{
std::unique_lock<std::mutex> lock(m_mutex);
if (row >= m_ids.size())
return QVariant();
id = m_ids[row];
}
if (role == IdRole)
return id;
QVariant result;
auto item = m_playlist->item(id);
switch(role)
{
case Qt::DisplayRole:
case TitleRole:
result = item.title;
break;
case DurationRole:
result = item.duration;
break;
}
return result;
}
QML code:
import QtQuick 2.7
import QtQuick.Controls 2.0
import com.company.application 1.0
Page
{
id : root
property int playlistId
property var playlistApi: App.playlists.playlist(playlistId)
ListView
{
id : playlist
anchors.fill: parent
model: playlistApi.model
delegate: ItemDelegate
{
text: model.title
width: parent.width
onClicked: App.player.play(playlistId, model.id)
}
ScrollIndicator.vertical: ScrollIndicator {}
}
}
Update #3:
When the model is updated (one item added), something strange happens with QML ListView: in addition to fact that it's not updated (and it does not call
MediaPlaylistModel::data to retrieve new items), existing items got damaged. When I click on existed item, it's model.id property is always 0. E.g. at app start its model.id was 24, after one item added its model.id became 0.
Update #4:
App.playlists.playlist(playlistId) returns pointer to this class instance:
class CppQmlPlaylistApi :
public QObject
{
Q_OBJECT
Q_PROPERTY(QObject* model READ model NOTIFY modelChanged)
Q_PROPERTY(QString title READ title WRITE setTitle NOTIFY titleChanged)
public:
explicit CppQmlPlaylistApi(
int playlistId,
QWeakPointer<CorePlaylistsManager> playlistsMgr,
QObject *parent = 0);
QObject* model() const;
QString title() const;
void setTitle(const QString &title);
signals:
void modelChanged();
void titleChanged();
void loadPlaylistRequested(int id);
protected slots:
void onPlaylistLoaded(int id);
void onPlaylistRemoved(int id);
protected:
int m_playlistId = 0;
QWeakPointer<CorePlaylistsManager> m_playlistsMgr;
QSharedPointer<QAbstractItemModel> m_model;
};

Answer 1

The model was in non-GUI thread.
I was getting these debug messages (thanks to AlexanderVX for pointing me out):
QObject::connect: Cannot queue arguments of type 'QQmlChangeSet'
(Make sure 'QQmlChangeSet' is registered using qRegisterMetaType().)
Moving the model object to GUI thread fixed the problem.

Answer 2

Your code, as provided, is good. On the QML side, as long as your model is bound, and not dynamically re-created in JS, you should be good too.
ListView {
model: mediaPlaylistModel
}
Problems can arise if you overloaded beginResetModel or endResetModel by accident. For tests purposes, you can try to emit the QAbstractItemModel::modelReset() signal, and see if it changes anything.
It's quite easy to miss something with QAbstractItemModel, resulting in nothing working anymore !