main.dart

import 'dart:convert';

void main() {
  final event = EposEvent(EposEventType.connect, {'key': 'value'});
  final jsonObject = event.toJson();
  print(event);
  print(jsonObject); 
}


class EposEvent {
  final EposEventType type;
  final Map<String, Object> data;

  const EposEvent(this.type, this.data);

  Map<String, Object> toJson() {
    return {
      'type': type.keyFromEventTypeMap,
      'data': data,
    };
  }

  @override
  String toString() => jsonEncode(toJson());

  static EposEvent fromJson(Map<String, Object> json) {
    return EposEvent(
      eventTypesMap[json['type']],
      json['data'],
    );
  }
}


final Map<String, EposEventType> eventTypesMap = {
  'DISCOVERY': EposEventType.discovery,
  'CONNECT': EposEventType.connect,
  'DISCONNECT': EposEventType.disconnect,
  'RECONNECTING': EposEventType.reconnecting,
  'RECONNECT': EposEventType.reconnect,
  'SCAN': EposEventType.scan
};

enum EposEventType {
  discovery,
  connect,
  disconnect,
  reconnecting,
  reconnect,
  scan
}

extension EposEventTypeExtension on EposEventType {
  String get keyFromEventTypeMap {
    return eventTypesMap.keys.firstWhere((k) => eventTypesMap[k] == this);
  }
}

https://dartpad.dev/091dc8c6b4753779f5dd066ba6a90e0a