track_position_service.dart

  Future<void> _requestTrackPositions(
      String correlationId, String deviceId, String station, String trackNumber) async {
    _messagingService.request(
        topicRetrieveTrackPositionsQuery,
        jsonEncode(RetrieveTrackPositionsQuery(
            mobileDeviceId: deviceId,
            correlationId: correlationId,
            station: station,
            trackNumber: trackNumber)));
  }

  Stream<TrackPositionsRetrievedResponse> _trackPositionRetrieved$(
      String correlationId, String mobileDeviceId) {
    return _messagingService
        .updatesFor('$topicTrackPositionsRetrievedResponse/$mobileDeviceId')
        .map((mqttMessage) => TrackPositionsRetrievedResponse.fromJson(mqttMessage.json));
  }