如何从Firestore中检索位置并在Google Map flutter上显示

import 'package:flutter/material.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

class MapScreen extends StatefulWidget {
  @override
  _MapScreenState createState() => _MapScreenState();
}

class _MapScreenState extends State<MapScreen> {
  GoogleMapController mapController;
  List<Marker> markers = [];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Map'),
      ),
      body: StreamBuilder<QuerySnapshot>(
        stream: FirebaseFirestore.instance.collection('locations').snapshots(),
        builder: (context, snapshot) {
          if (!snapshot.hasData) {
            return Center(
              child: CircularProgressIndicator(),
            );
          }
          markers.clear();
          snapshot.data.docs.forEach((doc) {
            GeoPoint location = doc.data()['location'];
            markers.add(
              Marker(
                markerId: MarkerId(doc.id),
                position: LatLng(location.latitude, location.longitude),
              ),
            );
          });
          return GoogleMap(
            initialCameraPosition: CameraPosition(
              target: LatLng(0, 0),
              zoom: 10,
            ),
            markers: Set<Marker>.from(markers),
            onMapCreated: (controller) {
              setState(() {
                mapController = controller;
              });
            },
          );
        },
      ),
    );
  }
}