Flutter: separating background service to different class, cause method error - flutter

Question

Hi so I'm trying to use background_service for my app by using flutter_background_service plugin, I've managed to follow the example from the documentation and I want to move these function to different classes for better code writing. The problem is when I move it to the other file and call the method it show an error of:
Unhandled Exception: onStart method must be a top-level or static function
What causing this to happen ? can anyone explain it and how to fix it ? And how do I make my background_service app listen to my firestore data.. I want to detect if there's any task that almost pass the due_date.
Here's my code:
Main.dart
void main() async {
WidgetsFlutterBinding.ensureInitialized();
await BackgroundService().initializeService();
}
Background_service.dart
class BackgroundService {
static const notificationChannelId = 'my_foreground';
static const notificationId = 888;
Future<void> initializeService() async {
final service = FlutterBackgroundService();
const AndroidNotificationChannel channel = AndroidNotificationChannel(
notificationChannelId, // ID
'MY FOREGROUND SERVICE', // Title
description:
'This channel is used for important notifications.', // Description
importance: Importance.low, // importance must be at low or higher level
);
final FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin = FlutterLocalNotificationsPlugin();
if (Platform.isIOS) {
await flutterLocalNotificationsPlugin.initialize(
const InitializationSettings(
iOS: IOSInitializationSettings(),
),
);
}
await flutterLocalNotificationsPlugin
.resolvePlatformSpecificImplementation<
AndroidFlutterLocalNotificationsPlugin>()
?.createNotificationChannel(channel);
await service.configure(
androidConfiguration: AndroidConfiguration(
onStart: onStart,
autoStart: true,
isForegroundMode: true,
),
iosConfiguration: IosConfiguration(
autoStart: true,
onForeground: onStart,
onBackground: _onIosBackground,
),
);
service.startService();
}
onStart(ServiceInstance service) async {
DartPluginRegistrant.ensureInitialized();
/// Code about _onstrat method
}
#pragma('vm:entry-point')
Future<bool> _onIosBackground(ServiceInstance service) async {
/// Code about Ios background
}
}

Answer 1

top-level means the function should be sits on the top of any
hierarchy of Class or Function.
you have to put the fucntion outside of classes. read documentation here: https://dart.dev/guides/language/effective-dart/design#avoid-defining-a-class-that-contains-only-static-members
Workaround:
background_service.dart
// on start is a top-level function
#pragma('vm:entry-point')
onStart(ServiceInstance service) async {
DartPluginRegistrant.ensureInitialized();
/// Code about _onstrat method
}
// top-level function
#pragma('vm:entry-point')
Future<bool> _onIosBackground(ServiceInstance service) async {
/// Code about Ios background
}
class BackgroundService {
static const notificationChannelId = 'my_foreground';
static const notificationId = 888;
......
await service.configure(
androidConfiguration: AndroidConfiguration(
onStart: onStart,
),
iosConfiguration: IosConfiguration(
onForeground: onStart,
onBackground: _onIosBackground,
),
.......