Future<void> initPlatformState() async {
    if (!mounted) return;

    OneSignal.Debug.setLogLevel(OSLogLevel.verbose);
    OneSignal.Debug.setAlertLevel(OSLogLevel.none);
    OneSignal.consentRequired(requireConsent);
    // NOTE: Replace with your own app ID from https://www.onesignal.com
    OneSignal.initialize(oneSignalAppID);
    // The promptForPushNotificationsWithUserResponse function will show the iOS or Android push notification prompt. We recommend removing the following code and instead using an In-App Message to prompt for notification permission
    OneSignal.Notifications.requestPermission(true);
    
    OneSignal.LiveActivities.setupDefault();
    OneSignal.Notifications.clearAll();
    OneSignal.User.pushSubscription.addObserver((state) {
      if (kDebugMode) {
        debugPrint('OneSignal User 1---${state.current.jsonRepresentation()}');
      }
    });

    OneSignal.User.addObserver((state) {
      final userState = state.jsonRepresentation();
      if (kDebugMode) {
        print('OneSignal user changed: $userState');
      }
    });

    OneSignal.Notifications.addPermissionObserver((state) {
      if (kDebugMode) {
        print("Has permission $state");
      }
    });




    OneSignal.Notifications.addClickListener((event) {
      if (kDebugMode) {
        print(
            'NOTIFICATION CLICK additionalData: ${event.notification.additionalData}');
       }
      event.preventDefault();

      /// Do async work

      /// notification.display() to display after preventing default
      event.notification.display();
      if (event.notification.additionalData != null) {
        final Redirection target = Redirection.fromString(
            event.notification.additionalData!['targetPage']);

        if (routes.containsKey("/${target.page ?? ""}")) {
          final int subRoute = routes["/${target.page ?? ""}"]!.indexWhere(
            (element) =>
                element.toLowerCase() == (target.subRoute ?? "").toLowerCase(),
          );
          try {
            AppRouter.router.push(
              "/${target.page ?? ""}",
              extra: subRoute == -1 ? 0 : subRoute,
            );
          } catch (e) {
            throw Exception(e);
          }
        } else {
          AppRouter.router.push(Routes.HOME);
        }
      }else {
        AppRouter.router.push(Routes.HOME);
      }
    });

    OneSignal.Notifications.addForegroundWillDisplayListener((event) {
      print(
          'NOTIFICATION WILL DISPLAY LISTENER CALLED WITH: ${event.notification.jsonRepresentation()}');

      /// Display Notification, preventDefault to not display
      event.preventDefault();

      /// Do async work

      /// notification.display() to display after preventing default
      event.notification.display();
      if (event.notification.additionalData != null) {
        final Redirection target = Redirection.fromString(
            event.notification.additionalData!['targetPage']);

        if (routes.containsKey("/${target.page ?? ""}")) {
          final int subRoute = routes["/${target.page ?? ""}"]!.indexWhere(
                (element) =>
            element.toLowerCase() == (target.subRoute ?? "").toLowerCase(),
          );
          try {
            AppRouter.router.push(
              "/${target.page ?? ""}",
              extra: subRoute == -1 ? 0 : subRoute,
            );
          } catch (e) {
            throw Exception(e);
          }
        } else {
          AppRouter.router.push(Routes.HOME);
        }
      }else {
        AppRouter.router.push(Routes.HOME);
      }
    });


    if (kDebugMode) {
      print('_debugLabelString: $_debugLabelString');
    }

    OneSignal.InAppMessages.paused(true);
  }

It is working when app is in background but not working when app is completely closed or cleared from background. And also added NotificationServiceExtension and register it in Mainfeast.