typedef EventHandler<T> = void Function(T args);

class _MapEntry {
  late dynamic owner;

  late dynamic handler;
}

class MessageService {
  static final MessageService _singleton = MessageService._internal();

  final Map<String, Set<_MapEntry>> _callbackMaps = <String, Set<_MapEntry>>{};

  factory MessageService() {
    return _singleton;
  }

  MessageService._internal();

  subscribe<T>(String topic, dynamic owner, EventHandler<T>? handler) {
    var entry = _MapEntry();
    entry.owner = owner;
    entry.handler = handler;

    if (_callbackMaps.containsKey(topic)) {
      _callbackMaps[topic]!.remove(entry);
      _callbackMaps[topic]!.add(entry);
    } else {
      _callbackMaps[topic] = <_MapEntry>{};
      _callbackMaps[topic]!.add(entry);
    }
  }

  unsubscribe<T>(String topic, dynamic owner) {
    if (_callbackMaps.containsKey(topic)) {
      for (var entry in _callbackMaps[topic]!) {
        if (entry.owner == owner) {
          _callbackMaps[topic]!.remove(entry);

          break;
        }
      }
    }
  }

  publish<T>(String topic, {T? arguments}) {
    var entries = _callbackMaps[topic];

    if (entries != null) {
      for (var entry in entries) {
        entry.handler(arguments);
      }
    }
  }
}