去Flutter校园看看有你想要的东西

发生问题的原因是贴图在屏幕上渲染，但仍未生成多段线。
你需要使用腕尺，它管理所有这些异步间隙，以获取当前位置并查找polyLines。
你可以参考下面的代码：
MapsCubit.dart:

import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:flutter_polyline_points/flutter_polyline_points.dart';
import 'package:geolocator/geolocator.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';

part 'maps_state.dart';

class MapsCubit extends Cubit<MapsState> {
  MapsCubit() : super(MapsInitial());

  Future<void> getCurrentLocation() async {
    emit(MapsLoadingState());

    try {
      Position position = await Geolocator.getCurrentPosition(
          desiredAccuracy: LocationAccuracy.high);
      double lat = position.latitude;
      double long = position.longitude;

      LatLng location = LatLng(lat, long);

      emit(ObtainedCurrentLocation(location: location));
    } catch (e) {
      emit(MapsErrorState(
          errorMessage: 'Error while fetching the current location.'));
    }
  }

  Future<void> getPolyLines(LatLng currentLocation, LatLng destination) async {
    emit(MapsLoadingState());
    try {
      List<LatLng> latLngList = [];

      PolylinePoints points = PolylinePoints();

      PolylineResult polylineResult = await points.getRouteBetweenCoordinates(
        'API KEY',
        PointLatLng(currentLocation.latitude, currentLocation.latitude ?? 0),
        PointLatLng(destination.latitude, destination.longitude),
        travelMode: TravelMode.transit,
      );

      polylineResult.points.forEach((ele) {
        latLngList.add(LatLng(ele.latitude, ele.longitude));
      });

      List<Polyline> polyLines = [];
      polyLines.add(Polyline(
          polylineId: const PolylineId('PolylineId'), points: latLngList));

      emit(ObtainedPolyLines(polyLines: polyLines));
    } catch (e) {
      emit(MapsErrorState(errorMessage: 'Error while finding route'));
    }
  }
}

MapsSate.dart:

part of 'maps_issue_cubit.dart';

abstract class MapsState {}

class MapsInitial extends MapsState {}

class MapsLoadingState extends MapsState {}

class ObtainedCurrentLocation extends MapsState{
  LatLng location;

  ObtainedCurrentLocation({required this.location});
}

class ObtainedPolyLines extends MapsState {
  List<Polyline> polyLines;

  ObtainedPolyLines({required this.polyLines});
}

class MapsErrorState extends MapsState {
  String errorMessage;

  MapsErrorState({required this.errorMessage});
}

和Maps.dart：

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';
import 'package:mobile_delivery_v2/maps_issue/maps_issue_cubit.dart';

class Maps extends StatefulWidget {
  const Maps({super.key});

  @override
  State<Maps> createState() => _MapsState();
}

class _MapsState extends State<Maps> {
  late MapsCubit cubit;

  late GoogleMapController mapController;
  late LatLng destination;
  late LatLng currentLocation;
  Set<Marker> markers = {};

  @override
  void initState() {
    super.initState();
    destination = const LatLng(4.66968, 101.07338);
    currentLocation = const LatLng(0, 0);
    cubit = MapsCubit();
    cubit.getCurrentLocation();
  }

  void _onMapCreated(GoogleMapController controller) {
    mapController = controller;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: const Text('Map'),
        ),
        body: BlocConsumer(
          bloc: cubit,
          listener: (context, state) {
            if (state is ObtainedCurrentLocation) {
              currentLocation = state.location;
              cubit.getPolyLines(
                  currentLocation, destination);
              markers.addAll([
                Marker(
                  markerId: const MarkerId('1'),
                  position: currentLocation,
                ),
                Marker(markerId: const MarkerId('2'), position: destination)
              ]);
            }
          },
          builder: (context, state) {
            if (state is ObtainedPolyLines) {
              return GoogleMap(
                myLocationEnabled: true,
                onMapCreated: _onMapCreated,
                markers: markers,
                polylines: Set<Polyline>.of(state.polyLines),
                initialCameraPosition: CameraPosition(
                  target: currentLocation,
                  zoom: 16.0,
                ),
              );
            } else if (state is MapsLoadingState) {
              return const Center(
                child: CircularProgressIndicator(),
              );
            } else if (state is MapsErrorState) {
              return Center(
                child: Text(state.errorMessage),
              );
            }
            return const SizedBox.shrink();
          },
        ));
  }
}

现在，cubit方法getCurrentLocation和getPolyLines将分别管理获取当前位置和生成polyLines的异步操作，并将发出一个状态，而block consumer将捕获它并显示谷歌Map。