Why we had a code to implement outside main (above) in the example and how to alternate this approach?

[A7mdFathi]

Hi, I see the code in the example and found out that we implement initialization for local notification and firebase background method outside main (above main) and I don't know why you did this approach.
and how to change this approach to be suitable for clean architecture.

@pragma('vm:entry-point')
Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
  await Firebase.initializeApp(options: DefaultFirebaseOptions.currentPlatform);
  await setupFlutterNotifications();
  showFlutterNotification(message);
  // If you're going to use other Firebase services in the background, such as Firestore,
  // make sure you call `initializeApp` before using other Firebase services.
  print('Handling a background message ${message.messageId}');
}

/// Create a [AndroidNotificationChannel] for heads up notifications
late AndroidNotificationChannel channel;

bool isFlutterLocalNotificationsInitialized = false;

Future<void> setupFlutterNotifications() async {
  if (isFlutterLocalNotificationsInitialized) {
    return;
  }
  channel = const AndroidNotificationChannel(
    'high_importance_channel', // id
    'High Importance Notifications', // title
    description:
        'This channel is used for important notifications.', // description
    importance: Importance.high,
  );

  flutterLocalNotificationsPlugin = FlutterLocalNotificationsPlugin();

  /// Create an Android Notification Channel.
  ///
  /// We use this channel in the `AndroidManifest.xml` file to override the
  /// default FCM channel to enable heads up notifications.
  await flutterLocalNotificationsPlugin
      .resolvePlatformSpecificImplementation<
          AndroidFlutterLocalNotificationsPlugin>()
      ?.createNotificationChannel(channel);

  /// Update the iOS foreground notification presentation options to allow
  /// heads up notifications.
  await FirebaseMessaging.instance.setForegroundNotificationPresentationOptions(
    alert: true,
    badge: true,
    sound: true,
  );
  isFlutterLocalNotificationsInitialized = true;
}

void showFlutterNotification(RemoteMessage message) {
  RemoteNotification? notification = message.notification;
  AndroidNotification? android = message.notification?.android;
  if (notification != null && android != null && !kIsWeb) {
    flutterLocalNotificationsPlugin.show(
      notification.hashCode,
      notification.title,
      notification.body,
      NotificationDetails(
        android: AndroidNotificationDetails(
          channel.id,
          channel.name,
          channelDescription: channel.description,
          // TODO add a proper drawable resource to android, for now using
          //      one that already exists in example app.
          icon: 'launch_background',
        ),
      ),
    );
  }
}

/// Initialize the [FlutterLocalNotificationsPlugin] package.
late FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin;

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp(options: DefaultFirebaseOptions.currentPlatform);
  // Set the background messaging handler early on, as a named top-level function
  FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);

  if (!kIsWeb) {
    await setupFlutterNotifications();
  }

  runApp(MessagingExampleApp());
}

/// Entry point for the example application.
class MessagingExampleApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Messaging Example App',
      theme: ThemeData.dark(),
      routes: {
        '/': (context) => Application(),
        '/message': (context) => MessageView(),
      },
    );
  }
}

// Crude counter to make messages unique
int _messageCount = 0;

/// The API endpoint here accepts a raw FCM payload for demonstration purposes.
String constructFCMPayload(String? token) {
  _messageCount++;
  return jsonEncode({
    'token': token,
    'data': {
      'via': 'FlutterFire Cloud Messaging!!!',
      'count': _messageCount.toString(),
    },
    'notification': {
      'title': 'Hello FlutterFire!',
      'body': 'This notification (#$_messageCount) was created via FCM!',
    },
  });
}

/// Renders the example application.
class Application extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _Application();
}

class _Application extends State<Application> {
  String? _token;
  String? initialMessage;
  bool _resolved = false;

  @override
  void initState() {
    super.initState();

    FirebaseMessaging.instance.getInitialMessage().then(
          (value) => setState(
            () {
              _resolved = true;
              initialMessage = value?.data.toString();
            },
          ),
        );

    FirebaseMessaging.onMessage.listen(showFlutterNotification);

    FirebaseMessaging.onMessageOpenedApp.listen((RemoteMessage message) {
      print('A new onMessageOpenedApp event was published!');
      Navigator.pushNamed(
        context,
        '/message',
        arguments: MessageArguments(message, true),
      );
    });
  }

  Future<void> sendPushMessage() async {
    if (_token == null) {
      print('Unable to send FCM message, no token exists.');
      return;
    }

    try {
      await http.post(
        Uri.parse('https://api.rnfirebase.io/messaging/send'),
        headers: <String, String>{
          'Content-Type': 'application/json; charset=UTF-8',
        },
        body: constructFCMPayload(_token),
      );
      print('FCM request for device sent!');
    } catch (e) {
      print(e);
    }
  }

  Future<void> onActionSelected(String value) async {
    switch (value) {
      case 'subscribe':
        {
          print(
            'FlutterFire Messaging Example: Subscribing to topic "fcm_test".',
          );
          await FirebaseMessaging.instance.subscribeToTopic('fcm_test');
          print(
            'FlutterFire Messaging Example: Subscribing to topic "fcm_test" successful.',
          );
        }
        break;
      case 'unsubscribe':
        {
          print(
            'FlutterFire Messaging Example: Unsubscribing from topic "fcm_test".',
          );
          await FirebaseMessaging.instance.unsubscribeFromTopic('fcm_test');
          print(
            'FlutterFire Messaging Example: Unsubscribing from topic "fcm_test" successful.',
          );
        }
        break;
      case 'get_apns_token':
        {
          if (defaultTargetPlatform == TargetPlatform.iOS ||
              defaultTargetPlatform == TargetPlatform.macOS) {
            print('FlutterFire Messaging Example: Getting APNs token...');
            String? token = await FirebaseMessaging.instance.getAPNSToken();
            print('FlutterFire Messaging Example: Got APNs token: $token');
          } else {
            print(
              'FlutterFire Messaging Example: Getting an APNs token is only supported on iOS and macOS platforms.',
            );
          }
        }
        break;
      default:
        break;
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Cloud Messaging'),
        actions: <Widget>[
          PopupMenuButton(
            onSelected: onActionSelected,
            itemBuilder: (BuildContext context) {
              return [
                const PopupMenuItem(
                  value: 'subscribe',
                  child: Text('Subscribe to topic'),
                ),
                const PopupMenuItem(
                  value: 'unsubscribe',
                  child: Text('Unsubscribe to topic'),
                ),
                const PopupMenuItem(
                  value: 'get_apns_token',
                  child: Text('Get APNs token (Apple only)'),
                ),
              ];
            },
          ),
        ],
      ),
      floatingActionButton: Builder(
        builder: (context) => FloatingActionButton(
          onPressed: sendPushMessage,
          backgroundColor: Colors.white,
          child: const Icon(Icons.send),
        ),
      ),
      body: SingleChildScrollView(
        child: Column(
          children: [
            MetaCard('Permissions', Permissions()),
            MetaCard(
              'Initial Message',
              Column(
                children: [
                  Text(_resolved ? 'Resolved' : 'Resolving'),
                  Text(initialMessage ?? 'None'),
                ],
              ),
            ),
            MetaCard(
              'FCM Token',
              TokenMonitor((token) {
                _token = token;
                return token == null
                    ? const CircularProgressIndicator()
                    : SelectableText(
                        token,
                        style: const TextStyle(fontSize: 12),
                      );
              }),
            ),
            ElevatedButton(
              onPressed: () {
                FirebaseMessaging.instance
                    .getInitialMessage()
                    .then((RemoteMessage? message) {
                  if (message != null) {
                    Navigator.pushNamed(
                      context,
                      '/message',
                      arguments: MessageArguments(message, true),
                    );
                  }
                });
              },
              child: const Text('getInitialMessage()'),
            ),
            MetaCard('Message Stream', MessageList()),
          ],
        ),
      ),
    );
  }
}

/// UI Widget for displaying metadata.
class MetaCard extends StatelessWidget {
  final String _title;
  final Widget _children;

  // ignore: public_member_api_docs
  MetaCard(this._title, this._children);

  @override
  Widget build(BuildContext context) {
    return Container(
      width: double.infinity,
      margin: const EdgeInsets.only(left: 8, right: 8, top: 8),
      child: Card(
        child: Padding(
          padding: const EdgeInsets.all(16),
          child: Column(
            children: [
              Container(
                margin: const EdgeInsets.only(bottom: 16),
                child: Text(_title, style: const TextStyle(fontSize: 18)),
              ),
              _children,
            ],
          ),
        ),
      ),
    );
  }
}