Why does `required` change the value of a property?

mjs225

I'm learning this example https://doc.qt.io/qtforpython-6/examples/example_quick_models_objectlistmodel.html#example-quick-models-objectlistmodel.

I made a small change to the example: the delegate was implemented in a separate QML. Then the code stopped working. After some trials, I found it was because the property was declared as required. Why does required change the value of a property? Thanks very much.

// view.qml
import QtQuick

ListView {
    width: 100; height: 100

    delegate: MyItem {
        mdl: model
    }
}
// MyItem.qml
import QtQuick
Rectangle {
    property var mdl // adding `required` will make mdl undefined
    color: mdl.modelData.color
    height: 25
    width: 100
    Text { text: mdl.modelData.name }
}

The python file is the same as the example.

GrecKo

and do fix it you should do:

    delegate: MyItem {
        required property var model
        mdl: model
    }

(or rename mdl to model in MyItem.qml)

Kai Nickel

Strange but true: by using "required" in a delegate, you basically opt in a different mode of operation.

See this section of the documentation for details: https://doc.qt.io/qt-6/qtquick-modelviewsdata-modelview.html#models

Especially note this paragraph from the documentation: "The model, index, and modelData roles are not accessible if the delegate contains required properties, unless it has also required properties with matching names."

This is what happens in your example: "model" is undefined and so "mdl" becomes undefined.

GrecKo

and do fix it you should do:

    delegate: MyItem {
        required property var model
        mdl: model
    }

(or rename mdl to model in MyItem.qml)