[Flutter] Flutter: Android local notifications to launch app using Work Manager / Question...

I am working on a code that will check a list of items that are loaded from a json file, managed with Riverpod state management, and send local notifications. I read the official documentation on local notifications plugin. I wish to use notifications to inform about the items that are within due date in 3 days. App is working fine but the notification feature. I understand that I need to initialize on main and use the work manager to manage notifications while the app is in deep sleep/not running, but with below approach I am not quite seeing how that would work because the items list is not initialized with riverpod state management yet, but I don't know where to approach to this. Besides running the app gives below error:

main.dart:

@pragma('vm:entry-point')
void callbackDispatcher() {
Workmanager().executeTask((task, inputData) async {
await NotificationService().checkDueDate();
return Future.value(true);
});
}

Future<void> main() async {
WidgetsFlutterBinding.ensureInitialized();
NotificationService().initialize();
Workmanager().initialize(callbackDispatcher);
Workmanager().registerPeriodicTask(
"dueDateChecker",
"dueDateCheckerTask",
frequency: const Duration(days: 1),
);
storage = Storage();
runApp(ProviderScope(child: MyApp()));
}


NotificationService.dart:

class NotificationService {
static final NotificationService _instance = NotificationService._internal();
factory NotificationService() {
return _instance;
}
FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin =
FlutterLocalNotificationsPlugin();

NotificationService._internal();
Future<void> initialize() async {
const AndroidInitializationSettings initializationSettingsAndroid =
AndroidInitializationSettings('app_icon');

const InitializationSettings initializationSettings =
InitializationSettings(
android: initializationSettingsAndroid,
);
//iOS: initializationSettingsDarwin);

await flutterLocalNotificationsPlugin.initialize(
initializationSettings,
onDidReceiveNotificationResponse: notificationTapForeground,
onDidReceiveBackgroundNotificationResponse: notificationTapBackground,
);

flutterLocalNotificationsPlugin
.getNotificationAppLaunchDetails()
.then((NotificationAppLaunchDetails? details) {
if (details != null && details.didNotificationLaunchApp) {
print("tapped when app is not running");
navigatorKey.currentState?.context.push("/inventory");
// TODO: Handle notification that caused the app to launch
// You can access details.notificationResponse.payload here
}
});
}
Future<void> _showNotification(List<Item> items) async {
const AndroidNotificationDetails androidNotificationDetails =
AndroidNotificationDetails('0', 'Impending Due Date',
channelDescription: 'Impending Due Date Notification',
actions: <AndroidNotificationAction>[
AndroidNotificationAction('0', 'Go to Inventory'),
AndroidNotificationAction('1', 'Close'),
],
importance: Importance.max,
priority: Priority.high,
ticker: 'ticker');
const NotificationDetails notificationDetails =
NotificationDetails(android: androidNotificationDetails);
String dueItems = items.map((item) => item.name).join(', ');
await flutterLocalNotificationsPlugin.show(
0, 'Impending Due Date', 'Your $dueItems is due soon!',
notificationDetails,
payload: 'item x');
}

Future<void> notificationTapBackground(
NotificationResponse notificationResponse) async {
switch (notificationResponse.actionId) {
case '0':
navigatorKey.currentState?.context.push("/inventory");
print('Action 1 was pressed');
break;
case '1':
print('Action 2 was pressed');
break;
default:
// Handle the main notification tap (if the notification itself was tapped, not an action button)
//print('Notification tapped, payload: ${notificationResponse.payload}');
}
}

Future<void> notificationTapForeground(
NotificationResponse notificationResponse) async {
switch (notificationResponse.actionId) {
case '0':
navigatorKey.currentState?.context.push("/inventory");
print('Action 1 was pressed');
break;
case '1':
print('Action 2 was pressed');
break;
default:
// Handle the main notification tap (if the notification itself was tapped, not an action button)
//print('Notification tapped, payload: ${notificationResponse.payload}');
}
}

Future<void> checkDueDate() async {
final currentItemList = ProviderContainer().read(jsonInventoryServiceNotifierProvider);
late var dueDateList;
currentItemList.when(
data: (item) {
dueDateList = item
.where((element) =>
element.dueDate != null &&
element.calculateDueDate() <=
3)
.toList();
},
loading: () => print("loading.."),
error: (e, st) => print("error.."),
);
_showNotification(dueDateList);
}
}


Error on running the app:

The backgroundHandler needs to be either a static function or a top level function to be accessible as a Flutter entry point.

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