Upload to Firestore in background with Flutter

I am using christocracy's flutter_background_geolocation package to build a crowdsensing app. This app relies on the geofencing function of the aforementioned package quite heavily. In the main function, I have implemented a callback function that is as follows (partial code):

void _onGeofence(bg.GeofenceEvent event) async {
    await showGeofenceNotification(flutterLocalNotificationsPlugin,
        title: "Geofence", body: "$event", id: notification_id);

    if (action == "ENTER") {
      // update certain variables
      BarometerService.startBarometerService();
      BarometerService.floorChange.listen((floorChanges) {
        // update floor
        updateDatabase();
      });
    }
    else if (action == "EXIT") {
      // update certain variables
      BarometerService.stopBarometerService();
    }
    updateDatabase();
    setState(() {
      // update UI
    });
  }

The code works perfectly when the app is open and in focus. However, when in background, the barometer service stops. The updateDatabase() function is also not carried out as my Firestore console doesn't get updated.

Here is the code for updating the database:

Future updateUserState(String matric, bool inLWN, bool inVaughan, String activity, int confidence, int floor) async {
    return await userCollection.document(uid).setData({
      'matric': matric,
      'inLWN': inLWN,
      'inVaughan': inVaughan,
      'activity': activity,
      'confidence': confidence,
      'floor': floor,
    });
  }

And here is the code for BarometerService (which uses Flutter sensors plugin):

import 'package:sensors/sensors.dart';
static startBarometerService() {
    Stream<BarometerEvent> barometer10Events = barometerEvents.throttle(Duration(seconds:PERIOD_SECONDS));
    subscription = barometer10Events.listen(onBarometer);
    streamController = new StreamController();
  }

How do I make my services run even when app is closed or terminated? I have implemented the same code in my headless callback functions (except updating UI), but nothing besides updating my (local) variables and showing local flutter notifications is working.

Headless task for reference:

void headlessTask(bg.HeadlessEvent headlessEvent) async {
  print('[BackgroundGeolocation HeadlessTask]: $headlessEvent');
  switch(headlessEvent.name) {
    case bg.Event.GEOFENCE:
      bg.GeofenceEvent geofenceEvent = headlessEvent.event;
      onHeadlessGeofence(geofenceEvent);
      print('- [Headless] GeofenceEvent: $geofenceEvent');
      break;
    case bg.Event.ACTIVITYCHANGE:
      bg.ActivityChangeEvent event = headlessEvent.event;
      onHeadlessActivityChange(event);
      print('- [Headless] ActivityChangeEvent: $event');
      break;
  }
}

onHeadlessGeofence is almost identical to the callback _onGeofence, besides the setState().

The full code can be found here