如何使用flutter google_maps插件获取可见地图的边界？

import 'package:flutter/material.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;
  LatLngBounds _visibleBounds;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Map'),
      ),
      body: GoogleMap(
        initialCameraPosition: CameraPosition(
          target: LatLng(37.4219999, -122.0840575),
          zoom: 12,
        ),
        onMapCreated: (GoogleMapController controller) {
          _mapController = controller;
        },
        onCameraMove: (CameraPosition position) {
          _visibleBounds = _mapController.getVisibleRegion().latLngBounds;
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          if (_visibleBounds != null) {
            // 打印可见地图的边界
            print('Visible Bounds: $_visibleBounds');
          }
        },
        child: Icon(Icons.location_searching),
      ),
    );
  }
}