Problem with access to ListElement's proporty after delegate's function calling

[Artiom]

Hello everybody!

A have a little problem with ListModels.

Below I have attached a simple example of my problem. There I create 3 ListElements of my delegate object. My delegate object has onClecked handler, that changes text property. Also I wont to chenge ListElements's text property from my main.qml file. I do it in separate Button object, in his onClicked handler.

When I just change text property by Button's handler all ok, when I just change text property by delegates's handler all ok too. But when I try to chenge text by Button's handler after changing by delegate's handler, Button's handler has no effect.

main.qml

Window {
    title: qsTr("Hello World")
    visible: true
    width: 300
    height: 300
    Item {
        width: 300
        height: 30
        ListModel {
            id: myModel
            ListElement { name: "ONE" }
            ListElement { name: "TWO" }
            ListElement { name: "THREE" }
        }
        Component {
            id: myDelegate
            MyDelegate {
                name: model.name
            }
        }

        ListView {
            id: myList
            delegate: myDelegate
            model: myModel
            anchors.fill: parent
            orientation: Qt.Horizontal
            layoutDirection: Qt.RightToLeft
        }
    }
    Button {
        x: 100
        y: 100
        width: 100
        height: 30
        text: "thisOnClicked"
        onClicked: {
            for(var j=0; j<myModel.count; j++) {
                 myModel.get(j).name = "byBottomButton"
            }
        }
    }

}

MyDelegate.qml

Button {
    id: itemDelegate
    property string name: "NONE"
    property bool check: false
    text: name
    onClicked: {
        if(check) {
            check = false
            text = "unchecked"
        } else {
            check = true
            text = "checked"
        }
    }
}

I try to understand, that happens here, but I can't... Thank you for reading this.

[IntruderExcluder]

This is because your text property of MyDelegate is binded to name property, but not backwards (since there is no two-way bindings in QML), so setting text manually will not change name property. I've wrote a working example down:

    import QtQuick 2.12
    import QtQuick.Controls 2.12
    import QtQuick.Window 2.12
    
    Window {
        visible: true
        width: 640
        height: 480
        title: qsTr("Hello World")
    
        ListView {
            id: view
            anchors.fill: parent
            spacing: 10.0
    
            model: ListModel {
                ListElement { name: "One" }
                ListElement { name: "Two" }
                ListElement { name: "Three" }
            }
    
            delegate: ItemDelegate {
                width: ListView.view.width
                text: model.name
                checkable: true
                onClicked: {
                    view.model.setProperty(model.index, "name", checked ? "Checked" : "Unchecked")
                }
                background: Rectangle {
                    color: "lightsteelblue"
                }
            }
        }
    }

[IntruderExcluder]

Probably some bindings brokes when you are writing values directly into properties. The right way to set any data in your case is to use setProperty method of ListModel.

[Artiom]

Thanks for answer, IntruderExcluder.

I replaced myModel.get(j).name = "byBottomButton" to
myModel.setProperty(j, "name", "byBottomButton"), but it still don't work right..

[IntruderExcluder]

This is because your text property of MyDelegate is binded to name property, but not backwards (since there is no two-way bindings in QML), so setting text manually will not change name property. I've wrote a working example down:

    import QtQuick 2.12
    import QtQuick.Controls 2.12
    import QtQuick.Window 2.12
    
    Window {
        visible: true
        width: 640
        height: 480
        title: qsTr("Hello World")
    
        ListView {
            id: view
            anchors.fill: parent
            spacing: 10.0
    
            model: ListModel {
                ListElement { name: "One" }
                ListElement { name: "Two" }
                ListElement { name: "Three" }
            }
    
            delegate: ItemDelegate {
                width: ListView.view.width
                text: model.name
                checkable: true
                onClicked: {
                    view.model.setProperty(model.index, "name", checked ? "Checked" : "Unchecked")
                }
                background: Rectangle {
                    color: "lightsteelblue"
                }
            }
        }
    }

[GrecKo]

@IntruderExcluder said in Problem with access to ListElement's proporty after delegate's function calling:

onClicked: {
    view.model.setProperty(model.index, "name", checked ? "Checked" : "Unchecked")
}

Note that this is equivalent to:

onClicked: model.name = checked ? "Checked" : "Unchecked"

The latter has the advantage to work with all models and not only ListModel, it calls QAbstractItemModel::setData.

[Artiom]

It really works!

Thank you, IntruderExcluder, GrecKo.