尝试使用 google 地图时，查找已停用的小部件的祖先是不安全的

Question

第一次运行时，当我返回时出现此错误，请帮助。我无法解决这个问题。我尝试在我的应用程序中使用地图。谁能帮我？状态管理呢？尝试将地图集成到应用程序

我想知道为什么会出现这种情况并且应用程序出现错误？使用最新版本的 flutter.

import 'dart:async';

import 'package:flutter/foundation.dart';
import 'package:geolocator/geolocator.dart';

import 'package:google_maps_flutter/google_maps_flutter.dart';
import 'package:medicine/models/geometry.dart';
import 'package:medicine/models/location.dart';
import 'package:medicine/models/place.dart';
import 'package:medicine/models/place_search.dart';
import 'package:medicine/services/geolocator_service.dart';
import 'package:medicine/services/marker_service.dart';
import 'package:medicine/services/places_service.dart';

class ApplicationBloc with ChangeNotifier {
  final geoLocatorService = GeolocatorService();
  final placesService = PlacesService();
  final markerService = MarkerService();

  //Variables
  Position? currentLocation;
  List<PlaceSearch>? searchResults;
  StreamController<Place?> selectedLocation = StreamController<Place?>();
  StreamController<LatLngBounds> bounds = StreamController<LatLngBounds>();
  Place? selectedLocationStatic;
  String? placeType;
  List<Place>? placeResults;
  List<Marker> markers = [];

  ApplicationBloc() {
    setCurrentLocation();
  }

  setCurrentLocation() async {
    currentLocation = await geoLocatorService.getCurrentLocation();
    selectedLocationStatic = Place(
      name: null,
      geometry: Geometry(
        location: Location(
            lat: currentLocation!.latitude, lng: currentLocation!.longitude),
      ),
    );
    notifyListeners();
  }

  searchPlaces(String searchTerm) async {
    searchResults = await placesService.getAutocomplete(searchTerm);
    notifyListeners();
  }

  setSelectedLocation(String placeId) async {
    var sLocation = await placesService.getPlace(placeId);
    selectedLocation.add(sLocation);
    selectedLocationStatic = sLocation;
    searchResults = null;
    notifyListeners();
  }

  clearSelectedLocation() {
    selectedLocation.add(null);
    selectedLocationStatic = null;
    searchResults = null;
    placeType = null;
    notifyListeners();
  }

  togglePlaceType(String value, bool selected) async {
    if (selected) {
      placeType = value;
    } else {
      placeType = null;
    }

    if (placeType != null) {
      var places = await placesService.getPlaces(
          selectedLocationStatic!.geometry.location.lat,
          selectedLocationStatic!.geometry.location.lng,
          placeType!);
      markers = [];
      if (places.length > 0) {
        var newMarker = markerService.createMarkerFromPlace(places[0], false);
        markers.add(newMarker);
      }

      var locationMarker =
          markerService.createMarkerFromPlace(selectedLocationStatic!, true);
      markers.add(locationMarker);

      var _bounds = markerService.bounds(Set<Marker>.of(markers));
      bounds.add(_bounds!);

      notifyListeners();
    }
  }

  @override
  void dispose() {
    selectedLocation.close();
    bounds.close();
    super.dispose();
  }
}

这是map_screen.dart是

import 'dart:async';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

import 'package:google_maps_flutter/google_maps_flutter.dart';
import 'package:medicine/blocs/application_bloc.dart';
import 'package:medicine/models/place.dart';

import 'package:provider/provider.dart';

class MapScreen extends StatefulWidget {
  MapScreen({
    Key? key,
  }) : super(key: key);

  @override
  _MapScreenState createState() => _MapScreenState();
}

class _MapScreenState extends State<MapScreen> {
  Completer<GoogleMapController> _mapController = Completer();
  late StreamSubscription locationSubscription;
  late StreamSubscription boundsSubscription;
  final _locationController = TextEditingController();

  @override
  void initState() {
    final applicationBloc =
        Provider.of<ApplicationBloc>(context, listen: false);

    //Listen for selected Location
    locationSubscription =
        applicationBloc.selectedLocation.stream.listen((place) {
      if (place != null) {
        _locationController.text = place.name ?? "";
        _goToPlace(place);
      } else
        _locationController.text = "";
    });

    applicationBloc.bounds.stream.listen((bounds) async {
      final GoogleMapController controller = await _mapController.future;
      controller.animateCamera(CameraUpdate.newLatLngBounds(bounds, 50));
    });

    super.initState();
  }

  @override
  void dispose() {
    final applicationBloc =
        Provider.of<ApplicationBloc>(context, listen: false);
    applicationBloc.dispose();
    _locationController.dispose();
    locationSubscription.cancel();
    boundsSubscription.cancel();

    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    final applicationBloc = Provider.of<ApplicationBloc>(context);
    return Scaffold(
        appBar: AppBar(
          title: Text('Map'),
        ),
        body: (applicationBloc.currentLocation == null)
            ? Center(
                child: CircularProgressIndicator(),
              )
            : ListView(
                children: [
                  Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: TextField(
                      controller: _locationController,
                      textCapitalization: TextCapitalization.words,
                      decoration: InputDecoration(
                        hintText: 'Search by City',
                        suffixIcon: Icon(Icons.search),
                      ),
                      onChanged: (value) => applicationBloc.searchPlaces(value),
                      onTap: () => applicationBloc.clearSelectedLocation(),
                    ),
                  ),
                  Stack(
                    children: [
                      Container(
                        height: 300.0,
                        child: GoogleMap(
                          mapType: MapType.normal,
                          myLocationEnabled: true,
                          initialCameraPosition: CameraPosition(
                            target: LatLng(
                                applicationBloc.currentLocation!.latitude,
                                applicationBloc.currentLocation!.longitude),
                            zoom: 14,
                          ),
                          onMapCreated: (GoogleMapController controller) {
                            _mapController.complete(controller);
                          },
                          markers: Set<Marker>.of(applicationBloc.markers),
                        ),
                      ),
                      if (applicationBloc.searchResults != null &&
                          applicationBloc.searchResults!.length != 0)
                        Container(
                            height: 300.0,
                            width: double.infinity,
                            decoration: BoxDecoration(
                                color: Colors.black.withOpacity(.6),
                                backgroundBlendMode: BlendMode.darken)),
                      if (applicationBloc.searchResults != null)
                        Container(
                          height: 300.0,
                          child: ListView.builder(
                              itemCount: applicationBloc.searchResults!.length,
                              itemBuilder: (context, index) {
                                return ListTile(
                                  title: Text(
                                    applicationBloc
                                        .searchResults![index].description,
                                    style: TextStyle(color: Colors.white),
                                  ),
                                  onTap: () {
                                    applicationBloc.setSelectedLocation(
                                        applicationBloc
                                            .searchResults![index].placeId);
                                  },
                                );
                              }),
                        ),
                    ],
                  ),
                  SizedBox(
                    height: 20.0,
                  ),
                  Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: Text('Find Nearest',
                        style: TextStyle(
                            fontSize: 25.0, fontWeight: FontWeight.bold)),
                  ),
                  Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: Wrap(
                      spacing: 8.0,
                      children: [
                        FilterChip(
                            label: Text('Pharmacy'),
                            onSelected: (val) => applicationBloc
                                .togglePlaceType('pharmacy', val),
                            selected: applicationBloc.placeType == 'pharmacy',
                            selectedColor: Colors.blue),
                      ],
                    ),
                  )
                ],
              ));
  }

  Future<void> _goToPlace(Place place) async {
    final GoogleMapController controller = await _mapController.future;
    controller.animateCamera(
      CameraUpdate.newCameraPosition(
        CameraPosition(
            target: LatLng(
                place.geometry.location.lat, place.geometry.location.lng),
            zoom: 14.0),
      ),
    );
  }
}

错误是

The following assertion was thrown while finalizing the widget tree:
Looking up a deactivated widget's ancestor is unsafe.

At this point, the state of the widget's element tree is no longer stable.

To safely refer to a widget's ancestor in its dispose() method, save a reference to the ancestor by calling dependOnInheritedWidgetOfExactType() in the widget's didChangeDependencies() method.

When the exception was thrown, this was the stack
#0      Element._debugCheckStateIsActiveForAncestorLookup.<anonymous closure>
package:flutter/…/widgets/framework.dart:3944
#1      Element._debugCheckStateIsActiveForAncestorLookup
package:flutter/…/widgets/framework.dart:3958
#2      Element.getElementForInheritedWidgetOfExactType
package:flutter/…/widgets/framework.dart:3984
#3      Provider._inheritedElementOf
package:provider/src/provider.dart:327
#4      Provider.of
package:provider/src/provider.dart:284
...

Answer 1

你不能在dispose()里面调用Provider.of(context)，因为Flutter不能保证调用dispose()时上下文在树中的正确位置。

相反，您可以：

1. 在 initState()
中调用 Provider.of(context)
2. 将其存储在 State class
上的一个字段中
3. 使用dispose()
中的字段

例如：

class _MapScreenState extends State<MapScreen> {

  late final ApplicationBloc applicationBloc;

  @override
  void initState() {
    super.initState();
    // ... other logic

    applicationBloc = Provider.of<ApplicationBloc>(context, listen: false);
  }

  // build, etc

  @override
  void dispose() {
    applicationBloc.dispose();
    // other logic...
    super.dispose();
  }
}