如何在flutter中显示firestore地图上的标记？

import 'package:flutter/material.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:google_maps_flutter/google_maps_flutter.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('Firestore Map'),
      ),
      body: StreamBuilder<QuerySnapshot>(
        stream: FirebaseFirestore.instance.collection('markers').snapshots(),
        builder: (context, snapshot) {
          if (!snapshot.hasData) {
            return Center(
              child: CircularProgressIndicator(),
            );
          }

          markers.clear();

          snapshot.data.docs.forEach((doc) {
            final marker = Marker(
              markerId: MarkerId(doc.id),
              position: LatLng(
                doc.data()['latitude'],
                doc.data()['longitude'],
              ),
              infoWindow: InfoWindow(
                title: doc.data()['name'],
                snippet: doc.data()['description'],
              ),
            );

            markers.add(marker);
          });

          return GoogleMap(
            initialCameraPosition: CameraPosition(
              target: LatLng(0, 0),
              zoom: 10,
            ),
            markers: Set<Marker>.from(markers),
            onMapCreated: (controller) {
              setState(() {
                mapController = controller;
              });
            },
          );
        },
      ),
    );
  }
}