How to enable background processes in Flutter?

I have this code in flutter that should send a notification every 3 seconds in the background (even when the app is completely closed):

    import 'dart:async';
    import 'dart:ui';
    import 'package:flutter_background_service_android/flutter_background_service_android.dart';
    import 'package:flutter/material.dart';
    import 'package:flutter_background_service/flutter_background_service.dart';
    import 'package:p2p_app/services/notifivcation_setrvice.dart';

    @pragma('vm:entry-point')
    void onStart(ServiceInstance service) {
      DartPluginRegistrant.ensureInitialized();

      int i = 0;

      Timer.periodic(const Duration(seconds: 3), (timer) async {
        i++;

        if (service is AndroidServiceInstance) {
          if (await service.isForegroundService()) {
            LocalNoticeService().addNotification(
                title: 'Test', body: i.toString(), id: i, channel: 'channel-alice');
            print('Hello $i');
          }
        }
      });
    }

    void main() async {
      WidgetsFlutterBinding.ensureInitialized();

      await LocalNoticeService().setup();
      await initializeService();

      runApp(const MyApp());
    }

    Future<void> initializeService() async {
      final service = FlutterBackgroundService();

      await service.configure(
        androidConfiguration: AndroidConfiguration(
          onStart: (service) => onStart(service),
          autoStart: true,
          isForegroundMode: true,
        ),
        iosConfiguration: IosConfiguration(),
      );
    }

    class MyApp extends StatefulWidget {
      const MyApp({super.key});

      @override
      _MyAppState createState() => _MyAppState();
    }

    class _MyAppState extends State<MyApp> {
      @override
      void initState() {
        super.initState();
      }

      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          home: Scaffold(
            body: Column(children: [
              ElevatedButton(onPressed: () async {}, child: const Text('Create')),
              ElevatedButton(onPressed: () async {}, child: const Text('Stop')),
            ]),
          ),
        );
      }
    }

But my application can’t even start because of this persistent error:

    E/flutter (12081): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: onStart method must be a top-level or static function
    E/flutter (12081): #0      FlutterBackgroundServiceAndroid.configure (package:flutter_background_service_android/flutter_background_service_android.dart:90:7)
    E/flutter (12081): #1      FlutterBackgroundService.configure (package:flutter_background_service/flutter_background_service.dart:20:17)
    E/flutter (12081): #2      initializeService (package:p2p_app/main.dart:140:17)
    E/flutter (12081): #3      main (package:p2p_app/main.dart:100:9)
    E/flutter (12081): <asynchronous suspension>

Does anyone know how to fix this?

Ignore this text: people write to me that my post consists mainly of code. People write to me that my post consists mainly of code. People write to me that my post consists mainly of code.

>Solution :

Replace

AndroidConfiguration(
  onStart: (service) => onStart(service),
  autoStart: true,
  isForegroundMode: true,
)

with

AndroidConfiguration(
  onStart: onStart,
  autoStart: true,
  isForegroundMode: true,
)

The former, the type of onStart: is a lambda. The latter, is a function.