Flutter中如何清除之前在谷歌地图中创建的折线？

Question

因此，我创建了使用 Flutter 中的 Google 地图 API 获取两个位置方向的功能。 一切正常，直到我尝试输入多个位置并且显示多条折线相互重叠。 我将如何重置之前创建的多段线，使它们完全不重叠？ 以下是我在 Flutter 应用程序中创建 Map function 时使用的代码：

import 'dart:async';
import 'dart:math';
import 'package:ehatid_driver_app/location_service.dart';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter_polyline_points/flutter_polyline_points.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Google Maps Demo',
      home: MapSample(),
    );
  }
}

class MapSample extends StatefulWidget {
  @override
  State<MapSample> createState() => MapSampleState();
}

class MapSampleState extends State<MapSample> {
  Completer<GoogleMapController> _controller = Completer();
  TextEditingController _originController = TextEditingController();
  TextEditingController _destinationController = TextEditingController();

  Set<Marker> _markers = Set<Marker>();
  Set<Polygon> _polygons = Set<Polygon>();
  Set<Polyline> _polylines = Set<Polyline>();
  List<LatLng> polygonLatLngs = <LatLng>[];

  int _polygonIdCounter = 1;
  int _polylineIdCounter = 1;

  static final CameraPosition _kGooglePlex = CameraPosition(
    target: LatLng(37.42796133580664, -122.085749655962),
    zoom: 14.4746,
  );

  /**static final Marker _kGooglePlexMarker = Marker(
      markerId: MarkerId('_kGooglePlex'),
      infoWindow: InfoWindow(title: 'Your Location'),
      icon: BitmapDescriptor.defaultMarker,
      position: LatLng(37.42796133580664, -122.085749655962),
      );
      static final CameraPosition _kLake = CameraPosition(
      bearing: 192.8334901395799,
      target: LatLng(37.43296265331129, -122.08832357078792),
      tilt: 59.440717697143555,
      zoom: 19.151926040649414);

      static final Marker _kLakeMarker = Marker(
      markerId: MarkerId('_kLakeMarker'),
      infoWindow: InfoWindow(title: 'My Destination'),
      icon: BitmapDescriptor.defaultMarkerWithHue(BitmapDescriptor.hueBlue),
      position: LatLng(37.43296265331129, -122.08832357078792),
      );

      static final Polyline _kPolyline = Polyline(
      polylineId: PolylineId('_kPolyline'),
      points: [
      LatLng(37.42796133580664, -122.085749655962),
      LatLng(37.43296265331129, -122.08832357078792),
      ],
      width: 5,
      );

      static final Polygon _kPolygon = Polygon(polygonId: PolygonId('_kPolygon'),
      points: [
      LatLng(37.43296265331129, -122.08832357078792),
      LatLng(37.42796133580664, -122.085749655962),
      LatLng(37.418, -122.092),
      LatLng(37.435, -122.092)
      ],
      strokeWidth: 5,
      fillColor: Colors.transparent,
      );**/
  @override
  void initState() {
    super.initState();

    _setMarker(LatLng(37.42796133580664, -122.085749655962));
  }

  void _setMarker(LatLng point){
    setState(() {
      _markers.add(
          Marker(markerId: MarkerId('marker'),
            position: point,
          ),
      );
    });
  }

  void _setPolygon() {
    final String polygonIdVal = 'polygon_$_polygonIdCounter';
    _polygonIdCounter++;

    _polygons.add(
      Polygon(
        polygonId: PolygonId(polygonIdVal),
        points: polygonLatLngs,
        strokeWidth: 2,
        fillColor: Colors.transparent,
    ),
    );
  }

  void _setPolyline(List<PointLatLng> points) {
    final String polylineIdVal = 'polyline_$_polylineIdCounter';
    _polylineIdCounter++;

    _polylines.add(
      Polyline(
        polylineId: PolylineId(polylineIdVal),
        width: 2,
        color: Colors.purple,
        points: points.map((point) => LatLng(point.latitude, point.longitude),
        ).toList(),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: AppBar(
        title: Text('Google Maps'),
      ),
      body: Column(
        children: [
          Row(
            children: [
              Expanded(
                child: Column(
                  children: [
                  TextFormField(
                    controller: _originController,
                    //textCapitalization: TextCapitalization.words,
                    decoration: InputDecoration(hintText: 'Your Location'),
                    onChanged: (value) {
                      print(value);
                    },
                  ),
                  TextFormField(
                    controller: _destinationController,
                    textCapitalization: TextCapitalization.words,
                    decoration: InputDecoration(hintText: 'Destination'),
                    onChanged: (value) {
                      print(value);
                    },
                  ),
                ],
                ),
              ),
              IconButton(
                onPressed: () async {
                  var directions = await LocationService().getDirections(
                      _originController.text,
                      _destinationController.text
                  );
                  _goToPlace(
                      directions['start_location']['lat'],
                      directions['start_location']['lng'],
                      directions['bounds_ne'],
                      directions['bounds_sw'],
                  );

                  _setPolyline(directions['polyline_decoded']);
                },
                icon: Icon(Icons.search),
              ),
            ],
          ),


          /**Row(
            children: [
              Expanded(child:
              ),
            ],
          ),**/
          Expanded(
            child: GoogleMap(
              mapType: MapType.normal,
              markers: _markers,
              polygons: _polygons,
              polylines: _polylines,
              initialCameraPosition: _kGooglePlex,
              onMapCreated: (GoogleMapController controller) {
                _controller.complete(controller);
              },
              onTap: (point) {
                setState(() {
                  polygonLatLngs.add(point);
                  _setPolygon();
                });
              },
            ),
          ),
        ],
      ),
    );
  }

  Future<void> _goToPlace(
      //Map<String, dynamic> place,
      double lat,
      double lng,
      Map<String, dynamic> boundsNe,
      Map<String, dynamic> boundsSw,

      ) async {
   // final double lat = place['geometry']['location']['lat'];
   // final double lng = place['geometry']['location']['lng'];
    final GoogleMapController controller = await _controller.future;
    controller.animateCamera(CameraUpdate.newCameraPosition(
      CameraPosition(target: LatLng(lat, lng), zoom: 12),
    ),
    );

    controller.animateCamera(
      CameraUpdate.newLatLngBounds(
          LatLngBounds(
              southwest: LatLng(boundsSw['lat'], boundsSw['lng']),
              northeast: LatLng(boundsNe['lat'], boundsNe['lng']),
          ),
          25),
    );

    _setMarker(LatLng(lat, lng));
  }
}