enum Topic { none, computing, general }

extension TopicString on String {
  Topic get topic {
    switch (this) {
      case 'computing':
        return Topic.computing;
      case 'general':
        return Topic.general;
      case 'none':
        return Topic.none;
    }
  }
}

extension TopicExtension on Topic {
  String get string {
    switch (this) {
      case Topic.computing:
        return 'computing';
      case Topic.general:
        return 'general';
      case Topic.none:
        return 'none';
    }
  }
}

Fruit f = Fruit.values.firstWhere((e) => e.toString() == 'Fruit.' + str);