How to update ListElement (QML) value field with dynamically updating property

Question

In the below code, the property __operatingModus_season updating every 30 seconds. But in frontend, I dont see any value or changes. Please give some heads up if i am wrong.

ListModel{
    id: tileListModelBetriebsmodus
    Component.onCompleted: {
        if(__is_automatik_mode_ON){
            tileListModelBetriebsmodus.append({"tileListSource": "../../images/HausScreen/operatingModeAuto.png",
                                                  "tileListText": getFrontendText("AUTO"),
                                                  "tileListValue": __operatingModus_season
                                              })
        }else{
            tileListModelBetriebsmodus.append({"tileListSource": "../../images/HausScreen/hamburgerfinger.png",
                                                  "tileListText": getFrontendText("MANUAL"),
                                                  "tileListValue": __operatingModus_season
                                              })
        }
    }
}

Answer 1

This way of adding items to the ListModel doesn't make them dynamic, the values are stored on the spot.

You might be able to Create Property Bindings like this:

ListModel{
    id: tileListModelBetriebsmodus
    Component.onCompleted: {
            if(__is_automatik_mode_ON){
                tileListModelBetriebsmodus.append({"tileListSource": "../../images/HausScreen/operatingModeAuto.png",
                                                      "tileListText": getFrontendText("AUTO"),
                                                      "tileListValue": Qt.binding(function() { return __operatingModus_season })
                                                  })
            }else{
                tileListModelBetriebsmodus.append({"tileListSource": "../../images/HausScreen/hamburgerfinger.png",
                                                      "tileListText": getFrontendText("MANUAL"),
                                                      "tileListValue": Qt.binding(function() { return __operatingModus_season }
                                                  })
            }
        }
    }

Note I did not test this, if you would add a minimal working example I could have done so.

Answer 2

Amfasis' answer suggesting the use of Qt.binding() is on the right track, except Qt won't allow assigning a binding object directly to a ListElement. However, it allows assigning plain JS function objects. So you have to do it in two steps instead:

Step 1: Define your dynamically evaluated ListElement role as a JS function (tip: you can also use the arrow syntax to make this more readable), for example:

ListElement {
   dynamicLabel: () => config.useLabelA ? labelA : labelB
}

Step 2: Set up a binding inside your delegate between the target item and the model role:

ListView {
    delegate: Text {
        Component.onCompleted: {
            text: Qt.binding(model.dynamicLabel)
        }
    }
}

Note the need to go through Component.onCompleted as you can't do text: Qt.binding(model.dynamicLabel) directly (QML spits out an error "Invalid use of Qt.binding() in a binding declaration.").

You can't just do text: model.dynamicLabel either as that would only evaluate the JS function once at initial assignment, but not update if eg config.useLabelA changed. That's what you need the Qt.binding() wrapper for.

Answer 3

Building on Romain Pokrzywka's answer:

text: valuedUpdated() ? model.dynamicLabel : "" text: valuedUpdated() ? model.dynamicLabel : "" will be evaluated whenever value is updated. valuedUpdated() is a Q_PROPERTY, which returns true. Notify signal can be emitted whenever value is updated.