# Models and views

Use a model with QML change notifications when the view must react to mutation.
A plain JavaScript array is suitable for static data, while `ListModel` or a `QAbstractItemModel` supports observable changes.

```qml
import QtQuick
import QtQuick.Controls

ListModel {
    id: tasks
    ListElement { title: "Review"; done: false }
}

ListView {
    model: tasks
    clip: true

    delegate: CheckDelegate {
        required property int index
        required property string title
        required property bool done

        width: ListView.view.width
        text: title
        checked: done
        onToggled: tasks.setProperty(index, "done", checked)
    }
}
```

Use required delegate properties for roles and `index` because this is explicit and visible to tooling.
Do not retain model objects or delegate instances after a delegate leaves the view.

`ListView` creates delegates around its viewport rather than all rows at once.
Delegate instance reuse is optional and controlled by `reuseItems`; never rely on delegate-local state surviving.
Put durable row state in the model.

Use `Repeater` when every delegate should be instantiated as a child of its parent.
Use a view when content can exceed the viewport or delegate population must stay bounded.
Measure before making fixed item-count rules.