Flutter / Firebase: How to use StreamBuilder to read Googlemap markers

Question

This code works but nothing happens if I add a new marker in the "POSITIONS" collection of Firebase

   void initState() {
    super.initState();
    getFirebaseMarkers();
    ....
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children:[
          GoogleMap(
            markers: Set<Marker>.of(_markers),
       .....

  void getFirebaseMarkers() {
    firestore.collection('POSITIONS').get().then(
          (QuerySnapshot querySnapshot) => {
        for (var doc in querySnapshot.docs){
          _addMarker(doc), // add a new marker into _markers
        },
      },
    );
  }

I know I need to use 'streamBuilder'. I haven't been able to get the few examples I found on the internet to work.

Can someone help me please?