QAbstractListModel as member of QmlSingleton in Python not working

[LS-KS]

I ran into an issue and it seems like I understood something wrong.

I want to create a project similar to QML Oscilloscope example:
I have a generator class that has a list of signal-elements, each represents a sine-signal. Controlled by a timer the generator class loops over all signals and evaluates the equations, create a QPointF which is then added to a QLineSeries and rendered live in a ChartView. Although there are some issues with the performance it works. I'll have some ideas to address this later.

My problem is: I want to render all SignalElements in a ListView so the user may add, remove and edit entries. Because the generator has to interfere with the model I thought it would be good to keep the instance of the QAbstractListModel as a member of my controller instance. like:

QML_IMPORT_NAME = "io.qt.signalcontroller"
QML_IMPORT_MAJOR_VERSION = 1
QML_IMPORT_MINOR_VERSION = 0


@QmlElement
@QmlSingleton
class SignalController(QObject):
    newValue = Signal(float, float)  # Time, Value

    def __init__(self):
        super().__init__(None)
        self.generator = SignalGenerator(self)
        self.signalModel = SignalModel()
        self.series: list[QPointF] = []

and us it inside my QML-file:

ListView{
                    id: listView
                    anchors.fill: parent
                    anchors.margins: 5
                    implicitWidth: 200
                    clip:true
                    model: SignalController.signalModel
                    delegate:
                    Row {
                        spacing: 5
                        Layout.maximumWidth: listRect.width
                        Label {
                            text: "Amplitude"
                            Layout.maximumWidth: 100
                        }
                       TextField {
                            text: model.amp
                            Layout.maximumWidth: 60
                        }
                      .
                      .
                      .

This way I could later replace the generator by a class that reads data from COM-ports.
But the app starts with an empty list view. And no error is thrown.
a print-statement in the model's data method shows it is never called.
I would like to understand how this should be done correctly.

I code for 3 years now but since I'm self taught it is fairly possible I missed something.

Another idea to approach this would be to make the SignalModel also as a Singleton and overloading the __ new __ method so it always returns the existing instance. But unfortunately it always returned a NoneType -object.

Sure... I could make the model a QmlElement and pass everything from controller to model by linking signals inside a .qml file. But this feels odd.

Any hints?

Here is my implementation of my QAbstractListModel:

class SignalModel(QAbstractListModel):

    def __init__(self, parent=None):
        super().__init__(parent)
        self._data: list[SignalElement] = []

    def rowCount(self, parent=QModelIndex()):
        return len(self._data)

    def roleNames(self):
        return {
            Qt.UserRole + 0 : b"amp",
            Qt.UserRole + 1 : b"off",
            Qt.UserRole + 2 : b"freq",
            Qt.UserRole + 3 : b"phase",
        }

    def data(self, index, role=Qt.DisplayRole):
        print(f"Data called: {index.row()} {role}")
        if not index.isValid() or not 0 <= index.row() < len(self._data):
            return None
        if role == Qt.UserRole + 0:
            return self._data[index.row()].amplitude
        elif role == Qt.UserRole + 1:
            return self._data[index.row()].offset
        elif role == Qt.UserRole + 2:
            return self._data[index.row()].frequency
        elif role == Qt.UserRole + 3:
            return self._data[index.row()].phase
        return None

[jeremy_k]

@LS-KS said in QAbstractListModel as member of QmlSingleton in Python not working:

But the app starts with an empty list view. And no error is thrown.

You might need to turn on the appropriate logging categories.

a print-statement in the model's data method shows it is never called.

Are you certain that SignalController.signalModel in the QML is referencing self.signalModel from the Python-defined singleton? In the code posted, I don't see a definition of a Qt property. With PyQt, exposing a Python object member as a property to QML requires a little code:

class Object(QObect):
    @pyqtProperty(str)
    def myProperty():
        return "some text"

Text {
    text: Object.myProperty
}

[LS-KS]

@jeremy_k Thank you for your answer!

I tried the following after reading this link:

from PySide6.QtCore import QObject, Property

class SignalController(QObject):
  def __init__(self):
    super()__init__(None)
    self.generator = SignalGenerator(self)
    self.signalModel = SignalModel()


  @Property(SignalModel)
  def signalModel(self):
    return self._signalModel

  @signalModel.setter
  def signalModel(self, model: SignalModel):
    self._signalModel = model

This throws an exception:

QQmlExpression: Expression file://...../main.qml:46:21 depends on non-NOTIFYable properties:
    SignalController::signalModel

Don't know what to do with this exception. Couldn't find anything that made sense to me.

Edit:

I changed the code to:

from PySide6.QtCore import QObject, Property

class SignalController(QObject):

 signalModelChanged = Signal()
 
 def __init__(self):
    super()__init__(None)
    self.generator = SignalGenerator(self)
    self.signalModel = SignalModel()


  def get_signalModel(self):
    return self._signalModel

  def set_signalModel(self, model: SignalModel):
    self._signalModel = model

  signalModel = Property(fget=get_signalModel, fset=set_signalModel, notify=signalModelChanged())

No QML-Exception. But still no list entry.

[LS-KS]

The error was somewhere else. Both solutions of my last post are working. Closed

[jeremy_k]

Congratulations. Can you share the root problem and solution, to help future readers?