class ListWidget extends StatefulWidget {

  @override
  _ListWidgetState createState() => _ListWidgetState();

}

class _ListWidgetState extends State with 
                  AutomaticKeepAliveClientMixin{ // ** here

  @override
  Widget build(BuildContext context) {
    return Container();
  }

  @override
  bool get wantKeepAlive => true; // ** and here
}