我从方法中得到 null 返回值。扑

Question

我是 Flutter/Dart 的新手，我正在使用 flutter_bloc bloc/repository 模式。

我正在努力从存储库方法中获取值，但我在 bloc 方法中得到了 null。

bloc 部分工作正常，但我从存储库方法中获得的预期 LatLng 值为 null。

bloc 的值和 Event to State 方法：

LatLng locationStream;    

Stream<MapState> _mapGetLocationStreamToState(
          GetLocationStream event) async* {
        print('_mapGetLocationStreamToState event received : $event');

        locationStream = await _mapRepository.getLocationStream();
        print('_mapGetLocationStreamToState() locationStream is: $locationStream ');
        yield LocationStream(locationStream);
      } 

打印：

颤振：_mapGetLocationStreamToState() locationStream 为：null

flutter: 事务是转换 { currentState: LocationStream {location: null}，事件：“GetLocationStream”实例，nextState： LocationStream {location: null} }

我尝试了 T 和 Future<T> 存储库方法的返回类型，从不同的范围返回值（请参阅注释掉的返回），但无论我从哪里返回值，我总是得到 null。 你能发现问题出在哪里吗？ 它应该在getLocation() 方法中，对吗？

非常感谢您的宝贵时间和帮助。

T：

    LatLng getLocationStream() {
    print('getLocationStream() called');
    LatLng location;
    LocationOptions locationOptions = LocationOptions(
        accuracy: LocationAccuracy.bestForNavigation, distanceFilter: 0);
    try {
      if (isTracking == true) {
        _positionStreamSubsciption.cancel();
        isTracking = !isTracking;
      } else {
        _positionStreamSubsciption = locationManager
            .getPositionStream(locationOptions)
            .listen((Position position) {
          if (position != null) {
            location = LatLng(position.latitude, position.longitude);
//            return location; // stops the stream
          }
          isTracking = !isTracking;
          print('getLocationStream() location is : $location');
//          return location; // returns null
        });
//        return location; // returns null
      }
//      return location; // returns null
    } catch (error) {
      print('startTracking error: $error');
    }
    return location; // returns null
  }

打印：

flutter: getLocationStream() 位置是：LatLng(latitude:37.33022, 经度：-122.023788）颤振：getLocationStream（）位置是： LatLng（纬度：37.330219，经度：-122.023748）颤动： getLocationStream() 位置是：LatLng(latitude:37.330219, 经度：-122.023708）颤振：getLocationStream（）位置是： LatLng（纬度：37.330221，经度：-122.023668）

未来：

Future<LatLng> getLocationStream() {
    print('getLocationStream() called');
    print('isTracking was : $isTracking');
    Future<LatLng> location;
    LocationOptions locationOptions = LocationOptions(
        accuracy: LocationAccuracy.bestForNavigation, distanceFilter: 0);
    try {
      if (isTracking == true) {
        _positionStreamSubsciption.cancel();
//        isTracking = !isTracking;
//        print('isTracking was ${!isTracking} and now is : $isTracking');
      } else {
        _positionStreamSubsciption = locationManager
            .getPositionStream(locationOptions)
            .listen((Position position) {
          if (position != null) {
            location =
                LatLng(position.latitude, position.longitude) as Future<LatLng>;
//            return location;

          }
          print('getLocationStream() location is : $location');
//          return location;
        });
//        return location;
      }

      isTracking = !isTracking;
      print('isTracking is : $isTracking');
//      return location;
    } catch (error) {
      print('startTracking error: $error');
    }
  }

打印：

颤振：getLocationStream() 位置是：LatLng(latitude:37.324746, 经度：-122.021542）颤振：getLocationStream（）位置是： LatLng（纬度：37.324746，经度：-122.021573）颤动： getLocationStream() 位置是：LatLng(latitude:37.324746, 经度：-122.021605)

Answer 1

在修改了一下代码后，我决定改变方法，似乎我之前选择了错误的方法。 我决定转换Stream<Position> coming fromGeolocatorAPI directly into aStream，而不是从存储库方法返回 LatLng，并从 bloc 收听它。

现在状态正在按预期流动，并带有新的位置值。

剩下要解决的唯一问题（我实际上认为我没有）是 MapScren BlocBuilder 没有获取来自新状态的值，并且在使用它时我得到 null _mapController.move(userLocation, 16);.

LatLng userLocation = (state as LocationStream).location;不是获取它的正确方法吗？

更新的代码是：

BlocBuilder：

bloc: MapBloc(mapRepository: _mapRepository),
        builder: (BuildContext context, MapState state) {
          LatLng userLocation = (state as LocationStream).location;
          return Scaffold(
..

存储库：

Stream<LatLng> getLocationStream() {
    print('getLocationStream() called');
    print('isTracking was : $isTracking');
    Stream<LatLng> locationStream;
    Stream<Position> _positionStream;
    LocationOptions locationOptions = LocationOptions(
        accuracy: LocationAccuracy.bestForNavigation, distanceFilter: 0);
    try {
      if (isTracking == true) {
        _positionStreamSubsciption.cancel();
      } else {
        _positionStream = locationManager.getPositionStream(locationOptions);

        handleData(Position position, EventSink<LatLng> sink) =>
            sink.add(LatLng(position.latitude, position.longitude));

        final transformer = StreamTransformer<Position, LatLng>.fromHandlers(
            handleData: handleData);
        locationStream = _positionStream.transform(transformer);
        return locationStream;
      }

      isTracking = !isTracking;
      print('isTracking is : $isTracking');
    } catch (error) {
      print('startTracking error: $error');
    }
  }

集团：

    MapState get initialState => LocationStream(locationStream);

  @override
  Stream<MapState> mapEventToState(MapEvent event) async* {
    // user location
    if (event is GetLocationStream) {
      print('MapBloc event received : $event');
      yield* _mapGetLocationStreamToState(event);
    }
    if (event is LocationUpdated) {
      yield* _mapLocationUpdatedToState(event);
    }
  }


  Stream<MapState> _mapGetLocationStreamToState(
      GetLocationStream event) async* {
    print('_mapGetLocationStreamToState event received : $event');

    _locationStreamSubscription =
        _mapRepository.getLocationStream().listen((LatLng location) {
          locationStream = location;
          add(LocationUpdated(locationStream));
          print(
              '_mapGetLocationStreamToState() locationStream is: $locationStream ');
        });

//    yield LocationStream(locationStream);
  }

控制台打印：

flutter：事件是 LocationUpdated { 更新的位置： LatLng(latitude:37.3246, longitude:-122.02457)} 颤动： _mapGetLocationStreamToState() locationStream is: LatLng(latitude:37.3246, longitude:-122.02457) flutter: Transaction is 过渡{ currentState：LocationStream {location： LatLng（纬度：37.324603，经度：-122.024523）}，事件： LocationUpdated { 更新位置：LatLng(latitude:37.3246, 经度：-122.02457)}，下一个状态：LocationStream {位置： LatLng（纬度：37.3246，经度：-122.02457）} }