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

[kwisatz]

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?

[topocc]

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

example doc.

[p3c0]

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();
@

[kwisatz]

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

[ondrejandrej]

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

[kwisatz]

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.

[ondrejandrej]

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);@

[kwisatz]

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.

[ondrejandrej]

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.

[kwisatz]

:D Thanks for pointing that out!

[p3c0]

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 ?

[ondrejandrej]

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

[kwisatz]

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]

[p3c0]

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 ?

[kwisatz]

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.

[kwisatz]

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!