Flutter:如何从Google地图中清除折线

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

class GoogleMapPage extends StatefulWidget {
  @override
  _GoogleMapPageState createState() => _GoogleMapPageState();
}

class _GoogleMapPageState extends State<GoogleMapPage> {
  GoogleMapController _mapController;
  Set<Polyline> _polylines = {};

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Google Map'),
      ),
      body: GoogleMap(
        onMapCreated: (controller) {
          _mapController = controller;
        },
        polylines: _polylines,
        initialCameraPosition: CameraPosition(
          target: LatLng(37.7749, -122.4194), // 初始位置为旧金山
          zoom: 12.0,
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          _clearPolyline();
        },
        child: Icon(Icons.clear),
      ),
    );
  }

  void _clearPolyline() {
    setState(() {
      _polylines.clear();
    });
  }

  void _addPolyline() {
    Polyline polyline = Polyline(
      polylineId: PolylineId('polyline_1'),
      points: [
        LatLng(37.7749, -122.4194), // 起点
        LatLng(37.7749, -122.4313), // 终点
      ],
      color: Colors.blue,
      width: 5,
    );

    setState(() {
      _polylines.add(polyline);
    });
  }

  @override
  void initState() {
    super.initState();
    _addPolyline();
  }
}