QML ListView does not reflect updates from its C++ data model [Solved]



  • My model inherits QAbstractListModel, I have a method to clear all items in the list, which is triggered by a mouse press event from the QML file, I'm able to receive this event fine. While the C++ model does get cleared, the ListView does not seem to reflect that change on the UI.

    Relevant snippet from the header file of my model.

    my_data_model.h
    @class MyDataModel : public QAbstractListModel
    {
    public:
    MyDataModel();
    int rowCount( const QModelIndex& ) const;
    QVariant data( const QModelIndex& index, int role = Qt::DisplayRole ) const;
    void clearMyData();
    private:
    QList< MyData > my_data;
    };@

    my_data_model.cpp
    @void MyDataModel::clearMyData()
    {
    my_data.clear();
    }@

    The qml file's relevant code snippet. name and content are properties derived through the data model's template structure.

    my_data.qml
    @Rectangle {
    id:listRectangle
    width: 650; height: 250

    ListView {
        id:listView
        objectName: "listView"
        width: parent.width; height: parent.height
        anchors.centerIn: parent
        clip:true
        spacing: 5
        model: myDataModel
        focus: false
    
        delegate:  Rectangle {
        id: delegate
        objectName: name
    
            Text {
                id: titleText
                text: content
            }
    
            MouseArea {
                id:ma
                anchors.fill: parent
                onClicked: {
                    listView.currentIndex = index;
                }
            }
        }
    }       
    

    }@

    The qml is invoked like this:
    model_usage_widget.cpp
    @xView = new QDeclarativeView(this);
    context = xView->engine()->rootContext();
    MyDataModel myDataModel;
    context->setContextProperty("myDataModel", &myDataModel);
    xView->setSource(QUrl("qrc:/ui/qml/my_data_model.qml"));
    xLayout = new QHBoxLayout(this);
    xLayout->setMargin(0);
    xLayout->addWidget(xView);
    this->setLayout(xLayout);
    this->show();@

    this is an object that inherits QWidget

    I've included all relevant code above. I created my list with one item, and tried to clear it but the QML does not seem to update it's view. Do I have to add anything else?



  • Q_PROPERTY(QQuickItem *target READ target WRITE setTarget NOTIFY targetChanged)
    Q_PROPERTY(int cursorPosition READ cursorPosition WRITE setCursorPosition NOTIFY cursor

    example doc.


  • Moderators

    Hi,

    my_data.clear() will just clear the list but you will the view to be notified about the changes. This can be done by wrapping it up in "beginResetModel":http://doc.qt.io/qt-5/qabstractitemmodel.html#beginResetModel and endResetModel:
    @
    beginResetModel();
    my_data.clear();
    endResetModel();
    @



  • I just tried inserting that, and my Application Crashed as soon as that code was invoked. Narrowed it down to endResetModel() being the culprit.



  • This code works for me:
    @ emit layoutAboutToBeChanged();
    my_data.clear();
    emit layoutChanged();@



  • So, with this the Application no longer crashes but my view still seems to hold the model item's in it's view but the I'm not able to scroll between the items.

    [quote author="ondrejandrej" date="1421854871"]This code works for me:
    @ emit layoutAboutToBeChanged();
    my_data.clear();
    emit layoutChanged();@[/quote]

    As an aside, it'd be nice to find a complete example of a c++ model and a qml view where items are added and removed dynamically but I'm not able to find one online. I'd really appreciate if someone could link it for me.



  • You are creating a model, but it goes out of scope immediately after constructor terminates (I suppose your last code snippet is in a constructor).

    You should ensure that the model remains in existence while the QML view is showing, e. g. by allocating on heap:
    @MyDataModel *myDataModel = new MyDataModel(this);
    context->setContextProperty("myDataModel", myDataModel);@



  • I've tried to allocate it at the heap as well and the ui remains unchanged. Originally, myDatamodel was a private member of its parent's class. The code snippet was modified to simplify my question.



  • You are using "it's" wrongly. Have a look at http://www.elearnenglishlanguage.com/blog/english-mistakes/its/ . You have it also in the title.



  • :D Thanks for pointing that out!


  • Moderators

    bq. I just tried inserting that, and my Application Crashed as soon as that code was invoked. Narrowed it down to endResetModel() being the culprit.

    I have been using this approach in all my projects where ListView is used. It worked all the time.
    Why are you using QDeclarativeView ? Aren't you using Qt 5 ?



  • [quote author="kwisatz" date="1421877712"]:D Thanks for pointing that out!
    [/quote]
    It's good that you fixed it :D



  • Sorry, should have made this clear earlier on, I'm using Qt 4.8.4.

    [quote author="p3c0" date="1421902494"]bq. I just tried inserting that, and my Application Crashed as soon as that code was invoked. Narrowed it down to endResetModel() being the culprit.

    I have been using this approach in all my projects where ListView is used. It worked all the time.
    Why are you using QDeclarativeView ? Aren't you using Qt 5 ?[/quote]


  • Moderators

    Well then I'm not sure why it crashes. Qt 4.8.4 still is quite old in Qt4 series. Can you try it on Qt 4.8.6 ?



  • I've tried this with 4.8.6 but to no avail

    At the moment, I'm unable to move to Qt5. I'm cross-compiling for an Embedded Linux System as well, there's a fair amount of porting involved to move to Qt5.



  • Okay, I did manage to get this to work finally.

    When clearing data, I used:
    @ beginResetModel();
    m_data.clear();
    endResetModel();@

    When removing an individual item in the data, I used:

    @ beginRemoveRows(QModelIndex(), index, index);
    m_data.removeAt(index);
    endRemoveRows();@

    And I worked on this bit of code separately from my main application. Will have to see what kind of fun awaits when I merge the changes with the rest of the application ;)

    Appreciate the help folks!


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