Update List Model created via QVariant::fromValue(QList<QObject*>))

maxwell31

Hi,

I have a

QList<QObject*> _mylist;

which I display in a TableView. To get the model for the TableView I did

_engine->rootContext()->setContextProperty("myList", QVariant::fromValue(_myList));

This is working fine, in principle. However, when I add new elements via

_myList.push_back(new MyItem(...));

I need to update the model. How is this done correctly? Right now I simply called

_engine->rootContext()->setContextProperty("myList", QVariant::fromValue(_myList));

again. However, with this approach, I also get error messages of this type:

qrc:/Screen.qml:158: TypeError: Cannot read property 'firstProperty' of null

Still, the new content gets displayed.
Thanks for your help!

Diracsbracket

@maxwell31
According to the Qt doc:
http://doc.qt.io/qt-5/qtquick-modelviewsdata-cppmodels.html
your approach of re-using setContextProperty() to update the view is correct:

Note: There is no way for the view to know that the contents of a QList has changed. 
If the QList changes, it is necessary to reset the model by calling
QQmlContext::setContextProperty() again.

To avoid the typeError warning you can check whetherobject is null before you try to use its firstProperty property, e.g.:

   someBinding: object ? object.firstProperty : someDefaultValue

Or you should defer using the object until you are sure the object has been created, e.g. using Component.onCompleted etc...

To be honest, I also still struggle with QML's object creation order issues like this, especially when the layout has multiple model-based views which depend on each other.

maxwell31

Thanks for your post. The errors come from the TableViewColumns. If I modify them according to your suggestion

  TableViewColumn {
    width: 80
    title: "property"
    delegate: Text{
      text: model ? model.firstProperty.toFixed(2):""
    }
  }

The errors disappear. I guess using onComponentCompleted is not possible here, as the model already was present before calling setContextProperty again? So there is no other solution for this than checking for null?

J.Hilk

@maxwell31
well, instead of setting the List directly as a context property, you could wrap it in a Qobject based class and make the List available via Q_Property macro

Q_PROPERTY(QVariant myModel READ myModel NOTIFY myModelChanged)

Now, when you emit myModelChanged(), the QML part will update itself

maxwell31

Good idea

AlfreddGco

@J-Hilk I think this would represent a problem anyways, because if you use the list in a repeater, QML is still not able to know when it changes so the first approach would need to be implemented again. Even if I call "emit myModelChanged", it doesn't work. It seems that the dataModel only updates with the emit when the size changes... so I remove the last element, emit de change, add it back, and emit again.

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Update List Model created via QVariant::fromValue(QList<QObject*>))