Disable Android system back button using backButtonDispatcher (in Flutter)

Question

We all know about WillPopScope.

Example:

    return WillPopScope(
        onWillPop: () async => return false,
        child: Scaffold(...));

That works fine in most cases but not in mine. I'm using Beamer in a complicated way and MaterialApp.router so even if I use WillPopScope the back button still has an effect (exiting the app no matter where I'm at).

So I'd like to disable to back button at a deeper level. And the next level down looks like using the backButtonDispatcher in the MaterialApp.router like so:

class DoNothingBackButtonDispatcher extends BackButtonDispatcher
    with WidgetsBindingObserver {
  DoNothingBackButtonDispatcher();

  @override
  void addCallback(ValueGetter<Future<bool>> callback) {}

  @override
  void removeCallback(ValueGetter<Future<bool>> callback) {}

  @override
  Future<bool> didPopRoute() => invokeCallback(Future<bool>.value(false));

  @override
  ChildBackButtonDispatcher createChildBackButtonDispatcher() => ChildBackButtonDispatcher(this);


  @override
  void deferTo(ChildBackButtonDispatcher child) {}

  @override
  void forget(ChildBackButtonDispatcher child) {}

  @override
  Future<bool> invokeCallback(Future<bool> defaultValue) =>
      SynchronousFuture<bool>(false);

  bool onPop(BuildContext context, Route<dynamic> route) => false;

  @override
  void takePriority() {}
}

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  MyApp({Key? key}) : super(key: key);

  final rootBeamerDelegate = BeamerDelegate(
    initialPath: Sailor.initialPath,
    locationBuilder: RoutesLocationBuilder(
      routes: {
        '*': (context, state, data) => const HomePage(),
      },
    ),
  );

  @override
  Widget build(BuildContext context) {
    return MaterialApp.router(
      debugShowCheckedModeBanner: false,
      routerDelegate: rootBeamerDelegate,
      routeInformationParser: BeamerParser(),
      backButtonDispatcher: DoNothingBackButtonDispatcher(), // <----- HERE
    );
  }
}

Unfortunately, even this closes the app when the user hits the system back button.

So, the question is: how do I implement the abstract BackButtonDispatcher class in order to essentially ignore the button press?