将未来<var>分配给变量

Question

我用dart编写了一些代码，其中我使用提供商包来更新地图上一个大头针的位置。我想让它做的是让初始位置等于用户的当前位置，然后如果他们拖动大头针，它会更新到大头针掉落的位置。

我的问题是初始位置变量需要Future<LatLng>，然而，当我更新位置时，它最终只是LatLng，我不能将其分配给_location变量。

class LocationProvider with ChangeNotifier {
Future<LatLng> _location = LocationService().getLocation();
// Error here, wants it to be Future<LatLng>
LatLng get location => _location; 
void calculateNewLocation(oldLocation, zoom, offset) {
var newPoint = const Epsg3857().latLngToPoint(oldLocation, zoom) +
CustomPoint(offset.dx, offset.dy);
LatLng? newLocation = const Epsg3857().pointToLatLng(newPoint, zoom);
// Error here again for the same reason
_location = newLocation ?? _location;
notifyListeners();
}
}

我怎样才能使这两个值都分配给_location?

Answer 1

你可以在provider文件

中提供一个方法

class LocationProvider with ChangeNotifier {
LatLng? _location;
LatLng? get location => _location; 
Future<void> initializeLocation() async {
_location = await LocationService().getLocation();
notifyListeners();
}
void calculateNewLocation(oldLocation, zoom, offset) {
var newPoint = const Epsg3857().latLngToPoint(oldLocation, zoom) +
CustomPoint(offset.dx, offset.dy);
LatLng? newLocation = const Epsg3857().pointToLatLng(newPoint, zoom);
_location = newLocation ?? _location;
notifyListeners();
}
}

然后，当你想初始化它时，你必须调用initializeLocation，如:

Future<void>? _myFuture;
final _provider = Provider.of<LocationProvider>(listen: false);
_myFuture = _provider.initializeLocation();

，然后在FutureBuilder中，在future

中提供_myFuture

PS:?可以被排除，如果你不在null safe模式下使用dart

Answer 2

根据您的代码，

LocationService().getLocation() returns a future, so you have to either await/async or use then().

试试这些

Future<LatLng> _location = LocationService().getLocation();
LatLng get location = await _location;  // put this in a separate method with async keyword

或

LocationService().getLocation().then((value) { location = value } );