我从一个方法中得到null返回值.颤振

Question

我是Flutter/Dat的新手，我使用的是Flutter_bloc block/repository模式。

我很难从存储库方法中获得值，而在bloc方法中我得到了null。

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

集团价值和事件到状态方法：

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：

flutter:_mapGetLocationStreamToState((locationStream为：null

flutter:事务正在转换｛currentState:LocationStream｛location:null｝，事件："GetLocationStream"的实例，nextState:LocationStream｛location:null｝｝

我尝试了存储库方法的T和Future<T>返回类型，从不同的范围返回值(请参阅注释掉的返回(，但无论从哪里返回值，我都会得到null。你能发现问题出在哪里吗？应该是getLocation()方法，对吧？

非常感谢您的时间和帮助。

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((位置为：LatLng(纬度：37.33022，经度：-122.023788(flutter:getLocationStream((位置为：LatLng(纬度：37.330219，经度：-122.023748(颤振：getLocationStream((位置为：LatLng(纬度：37.330219，经度：-122.023708(flutter:getLocationStream((位置为：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：

flutter:getLocationStream((位置为：LatLng(纬度：37.324746，经度：-122.021542(flutter:getLocationStream((位置为：LatLng(纬度：37.324746，经度：-122.021573(颤振：getLocationStream((位置为：LatLng(纬度：37.324746，经度：-122.021605(

Answer 1

在对代码进行了一些修改后，我决定改变方法，而且似乎以前选择了错误的方法。我没有从存储库方法返回LatLng，而是决定转换Stream<Position> coming fromGeolocatorAPI directly into aStream，并从block中侦听它。

现在，各州正像预期的那样流动，携带着新的位置价值。

剩下的唯一需要解决的问题(我实际上认为我没有(是MapScrenBlocBuilder无法获得来自新状态的值，并且当我像在_mapController.move(userLocation, 16);中那样使用它时，会得到null。

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

更新代码为：

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:事件为LocationUpdated｛更新位置：LatLng(纬度：37.3246，经度：-122.02457(}颤振：_mapGetLocationStreamToState((locationStream为：LatLng(纬度：37.3246，经度：-122.02457(flutter:事务为转换｛currentState:LocationStream｛location:LatLng(纬度：37.324603，经度：-122.024523(}，事件：LocationUpdated｛upstaded location:LatLng(纬度：37.3246，经度：-122.02457(｝，nextState:LocationStream｛location:LatLng(纬度：37.3246，经度：-122.02457(}}