Flutter-来自其他类的异步函数可以打印，但返回null

Question

从main.dart调用时，以下函数将打印正确的值，但返回null。 似乎该函数以某种方式不在返回之前等待该值。

希望能有所帮助。

location.dart

class Location {
  StreamSubscription<Map<String, double>> _locationSubscription;

  Future getLocation() async {
    try{
      geolocation.Location _location = new geolocation.Location();
      _locationSubscription =
          _location.onLocationChanged().listen((Map<String, double> result) {
          print(result);
          // This prints the proper value
          return(result);
          // But this returns null...

      });
    } on PlatformException {
      return null;
    } catch(e){
      print(e);
    }
  }
}

主镖

  @override
  void initState(){
    super.initState();
    getLocation();
  }

   getLocation() async {
    var location = Location(); 
     var loc = await location.getLocation();
     print(loc);
     //This prints null
  }

Answer 1

您必须定义未来的退货类型。

class Location {
StreamSubscription<Map<String, double>> _locationSubscription;
 Future<Map<String, double>> getLocation() async {
 try{
      geolocation.Location _location = new geolocation.Location();
      _locationSubscription =
      _location.onLocationChanged().listen((Map<String, double> result) {
      print(result);
      // This prints the proper value
      return(result);
      // But this returns null...

     });
    } on PlatformException {
     return null;
   } catch(e){
    print(e);
   }
  }
}