It is an example of how to use flutterLocalnotifications to schedule a lot of notifications with isolateHandler

isolate_handler_example.dart

final IsolateHandler isolateHandler = IsolateHandler();

void scheduleNotificationsIsolate(String _reminders) async {
  await new Future.delayed(new Duration(milliseconds: 500));
  
  // ... (describe settings)
  flutterLocalNotificationsPlugin.initialize(
      settings,
      onSelectNotification: onSelectNotification,
  );
  await flutterLocalNotificationsPlugin.cancelAll();
  List<dynamic> _remindersParsed = jsonDecode(_reminders);
 
  for (// iterate over your entities to show the message) {
    int generatedId = id ?? random.nextInt(1000000000);

    await flutterLocalNotificationsPlugin.schedule(
      generatedId,
      title,
      message,
      scheduledNotificationDateTime,
      platformSpecifics,
      payload: payload,
    );
  }
  killCurrentScheduleNotifications();
}

startScheduleNotifications(String _remindersAsString) {
  killCurrentScheduleNotifications();
  isolateHandler.spawn<String>(
    entryPoint,
    name: "scheduleNotifications",
    onReceive: scheduleNotificationsIsolate,
    onInitialized: () => isolateHandler.send(
      _remindersAsString,
      to: "scheduleNotifications",
    ),
  );
}

void killCurrentScheduleNotifications() {
  if (isolateHandler.isolates.containsKey('scheduleNotifications'))
    isolateHandler.kill('scheduleNotifications');
}

void entryPoint(Map<String, dynamic> context) {
  final messenger = HandledIsolate.initialize(context);
  messenger.listen((message) {
    messenger.send(message);
  });
}
    
/// At my production code i create some abstractions above flutterLocalNotifications:
/// 1 - a class called NotificationManager is the object that i use in the application it is a singleton 
/// and it creates a specific provider that i initialized just once int the application. (Because Isolate its a separeted 
/// context i have to initialize it them to every time i call)
/// 2- an interface called NotificationProvider and your implementation where i put the common methods
/// 3 - The specific implementations NotificationProviderIOS and NotificationProviderAndroid that extends 
/// NotificationProviderImpl where i put the configs necessary to create an specific platform notification.
    
/// An Isolate should be a simple function created directly inside a file, declared outside of a class.
/// In other plugins that i tested i remember to declare the methodChannel used by flutterLocalNotifications. 
/// This is a thing that i didn't have to do with flutter isolate.
    
/// the parameter passed to an isolate should be compound only by primitives. I serialize it as string to decode as JSON.
    
// I realized tests with a few thousands of notifications and performance was good, the phone slows down a bit. 
/// A good strategie is schedule the minimun that is needed(a week, a month) an create routines in your application to schedule
/// the next ones when user hits some action.

// I hope that this content should help. If you get in trouble with this feel free to contact me by e-mail.

Hey I just posted a question on Stack Overflow (https://stackoverflow.com/questions/69955481/creating-multiple-flutter-nofitications-with-flutter-local-notification) to which I think you could answer, would you mind sharing more code on how you managed to create scheduled notifications on a second thread with flutter_local_notification?