flutter - The argument type 'double?' can't be assigned to the parameter type 'double'. dart(argument_type_not_assignable)

Question

I'm trying to get the user current location, but I got this error on l.latitude and l.longitude

The argument type 'double?' can't be assigned to the parameter type 'double'.

void _onMapCreated(GoogleMapController _cntlr) {
    _controller = _cntlr;
    _location.onLocationChanged.listen((l) {
      _controller.animateCamera(
        CameraUpdate.newCameraPosition(
          CameraPosition(
            target: LatLng(l.latitude, l.longitude),
            zoom: 15,
          ),
        ),
      );
    });
  }

Answer 1

The error you get is from null-safety, the type double? means that it could be either a double, or null, but your parameter only accepts a double, and no null.

For this, you can "force" the use of 'non-null' variable by adding a ! at the end of your variable, but be careful when doing this.

CameraPosition(
    target: LatLng(l.latitude!, l.longitude!),
    zoom: 15,
)

You can learn more about null-safety syntax and principles on the official documentation: https://flutter.dev/docs/null-safety

Answer 2

You could also null check the local variables, thereby making your code null safe:

    when location changes
      if (lat/lon are not null) {
        animate camera
      }

So something like this might work:

  void _onMapCreated(GoogleMapController _cntlr) {
    _controller = _cntlr;
    _location.onLocationChanged.listen((l) {
      if (l.latitude != null && l.longitude != null) {
        _controller.animateCamera(
          CameraUpdate.newCameraPosition(
            CameraPosition(
              target: LatLng(l.latitude, l.longitude),
              zoom: 15,
            ),
          ),
        );
      }
    });
  }

Logically, it wouldn't make sense to animate to a null latitude/longitude, so you can skip that listener call altogether if that's the case.

Filip talks about this situation & handling here.