Are QML var properties passed by reference or by copy? - qt

Question

I have two QML files as below:
//Page.qml
SelectionPage {
model: localizationPageProxy.vehicleTypes //QObject* class exposed by property
currentKey: localizationPageProxy.vehicleTypes.currentDataKey //QVariant property
}
//SelectionPage.qml
Item {
property var model
property var currentKey
id: page
ColumnLayout {
height: parent.height
width: parent.width * 0.9
anchors.horizontalCenter: parent.horizontalCenter
ListView {
id: listView
anchors.fill: parent
ScrollBar.vertical: ScrollBar {}
clip: true
model: page.model.data
spacing: Number.EPSILON // I don't know why the data loading is faster with that
delegate: Item {
height: listView.height * 0.12
width: listView.width
RadioButtonItem {
height: parent.height * 0.85
width: parent.width
anchors.centerIn: parent
text: modelData.value
checked: modelData.key === page.currentKey
onClicked: page.currentKey = modelData.key //here the c++ property is changed
}
}
}
}
}
So, is currentKey property of SelectionPage.qml passed by reference?
If that was a copy I should not see the c++ model change.
Thank you for your help

Answer 1

Rather than discussing copy vs reference, we should really be talking about bindings. When you do this:
currentKey: localizationPageProxy.vehicleTypes.currentDataKey
You're creating a binding. Whenever currentDataKey's value changes, currentKey will be updated too. But it is not a two-way binding. So changing currentKey does not update currentDataKey. Two-way bindings are pretty difficult to achieve, but there are posts on SO about them.
To actually solve what you're trying to achieve, I recommend adding a Q_INVOKABLE function to your QObject called updateCurrentKey or something. Then in your onClicked handler, do something like this:
onClicked: page.model.updateCurrentKey(modelData.key)