Flutter - Drag marker and get new position

If you allow me, I would suggest a different approach to this problem. I think in the current version of google maps for Flutter (0.4.0) you don't have a method to get the new position after dragging the marker, so my solution was to use the camera position to move the marker around and then use the current camera position to get the new coordinates. So in your code, you could do something like this:

    child: GoogleMap(
        mapType: MapType.hybrid,
        initialCameraPosition: CameraPosition(
          target: LatLng(lat, lng),
          zoom: 5,
        ),
        markers: Set<Marker>.of(
          <Marker>[
            Marker(
              draggable: true,
              markerId: MarkerId("1"),
              position: LatLng(lat, lng),
              icon: BitmapDescriptor.defaultMarker,
              infoWindow: const InfoWindow(
                title: 'Usted está aquí',
              ),
            )
          ],
        ),
        onCameraMove: ((_position) => _updatePosition(_position)),            
        onMapCreated: (GoogleMapController controller) {
          mapController = controller;
        },
      ))

Then you can set your marker's new state each time the user moves the camera around and use this coordinates for any other purpose you need:

void _updatePosition(CameraPosition _position) {
    Position newMarkerPosition = Position(
        latitude: _position.target.latitude,
        longitude: _position.target.longitude);
    Marker marker = markers["1"];

    setState(() {
      markers["1"] = marker.copyWith(
          positionParam: LatLng(newMarkerPosition.latitude, newMarkerPosition.longitude));
    });
  }

I hope it helps!

Marker have onDragEnd property. using onDragEnd property give new latitude and longitude.

Marker(
          onTap: () {
            print('Tapped');
          },
          draggable: true,
          markerId: MarkerId('Marker'),
          position: LatLng(value.latitude, value.longitude),
          onDragEnd: ((newPosition) {
            print(newPosition.latitude);
            print(newPosition.longitude);
          }))