如何在flutter中显示Firebase中的google地图标记？

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

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

class _MapScreenState extends State<MapScreen> {
  GoogleMapController mapController;
  Set<Marker> markers = {};

  @override
  void initState() {
    super.initState();
    // 连接到Firebase项目
    FirebaseDatabase.instance.reference().child('markers').once().then((DataSnapshot snapshot) {
      // 从Firebase获取标记数据
      Map<dynamic, dynamic> data = snapshot.value;
      data.forEach((key, value) {
        // 创建标记
        Marker marker = Marker(
          markerId: MarkerId(key),
          position: LatLng(value['latitude'], value['longitude']),
          infoWindow: InfoWindow(
            title: value['title'],
            snippet: value['description'],
          ),
        );
        // 添加标记到集合中
        markers.add(marker);
      });
      // 刷新界面
      setState(() {});
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Google Map'),
      ),
      body: GoogleMap(
        initialCameraPosition: CameraPosition(
          target: LatLng(37.42796133580664, -122.085749655962),
          zoom: 10,
        ),
        onMapCreated: (GoogleMapController controller) {
          mapController = controller;
        },
        markers: markers,
      ),
    );
  }
}