Signal when changes occur to *data* property

[A Former User]

Is it possible to get notified when an items data property changes?

There's a onChildrenChanged signal which works for visual items but I'm looking for the non-visuals

The following doesn't work:

import QtQuick 2.0

Item {
    id: modes

    Connections {
        target: modes
        onDataChanged: {
            for(var m in modes.data) {
                var d = modes.data[m]
            }
        }
    }

    onDataChanged: {
          for(var m in modes.data) {
              var d = modes.data[m]
          }
    }
}

Both gives me the error:

Cannot assign to non-existent property "onDataChanged"