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  1. Qt
  2. QTBUG-123842

Excessive delegate creation+destruction when calling list<T>.shift() on ListView.model

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    Description

      Code

      import QtQuick
import QtQuick.Controls.Basic
import QtQuick.Layouts

Window {
    id: root
    width: 640
    height: 480
    visible: true

    property list<int> numbers: [11, 22, 33]

    ColumnLayout {
        anchors.fill: parent

        Button {
            text: "Remove First Number"
            onClicked: {
                let num = root.numbers.shift()
                console.log("Removed", num)
            }
        }
        ListView {
            Layout.preferredWidth: 100
            Layout.fillHeight: true
            model: root.numbers
            delegate: Label {
                required property int modelData
                text: modelData
                Component.onCompleted: console.log("Delegate created for value", modelData)
                Component.onDestruction: console.log("Delegate destroyed for value", modelData)
            }
        }
    }
}

       

      Outcomes
      Program startup:

      qml: Delegate created for value 11
qml: Delegate created for value 22
qml: Delegate created for value 33

      1st click:

      qml: Delegate destroyed for value 22
qml: Delegate destroyed for value 33
qml: Delegate destroyed for value 11
qml: Delegate created for value 22
qml: Delegate created for value 22
qml: Delegate created for value 33
qml: Delegate destroyed for value 22
qml: Delegate destroyed for value 33
qml: Delegate destroyed for value 22
qml: Delegate created for value 22
qml: Delegate created for value 33
qml: Delegate created for value 33
qml: Delegate destroyed for value 33
qml: Delegate destroyed for value 33
qml: Delegate destroyed for value 22
qml: Delegate created for value 22
qml: Delegate created for value 33
qml: Delegate created for value 0
qml: Delegate destroyed for value 33
qml: Delegate destroyed for value 0
qml: Delegate destroyed for value 22
qml: Delegate created for value 22
qml: Delegate created for value 33
qml: Removed 11

      2nd click:

      qml: Delegate destroyed for value 33
qml: Delegate destroyed for value 22
qml: Delegate created for value 33
qml: Delegate created for value 33
qml: Delegate destroyed for value 33
qml: Delegate destroyed for value 33
qml: Delegate created for value 33
qml: Delegate created for value 0
qml: Delegate destroyed for value 0
qml: Delegate destroyed for value 33
qml: Delegate created for value 33
qml: Removed 22

      3rd click:

      qml: Delegate destroyed for value 33
qml: Delegate created for value 0
qml: Delegate destroyed for value 0
qml: Removed 33

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        Issue Links

          resulted from

          Bug - A problem which impairs or prevents the functions of the product. QTBUG-123882 QML list of length N emits (N+1) value change signals when shift() is called

          • P2: Important - Urgent, should be fixed, but will not stop the release.
          • Open
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              qt.team.quick.subscriptions Qt Quick and Widgets Team
              skoh-qt Sze Howe Koh
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              Dates

                Created:
                Updated:

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