What is the equivalent of onExpansionChanged for Flutter ExpandablePanel

Use an ExpandableControllerand an ExpandableNotifier:

class MyWidget extends StatefulWidget {
  const MyWidget({Key? key}) : super(key: key);

  @override
  State<StatefulWidget> createState() => _MyWidget();
}

class _MyWidget extends State<MyWidget> {
  final ExpandableController expandableController = ExpandableController();

  void onExpandableControllerStateChanged() {
    if (expandableController.expanded) {
      // Do your stuff when panel got expanded here
    } else {
      // Do your stuff when panel got collapsed here
    }
  }

  @override
  void initState() {
    super.initState();
    expandableController.addListener(onExpandableControllerStateChanged);
  }

  @override
  void dispose() {
    expandableController.removeListener(onExpandableControllerStateChanged);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return ExpandableNotifier(
      controller: expandableController,
      child: ExpandablePanel(
        header: HeaderWidget(),
        collapsed: CollapsedWidget(),
        expanded: ExpandedWidget(),
      ),
    );
  }
}