Passing a value from child to parent and then rebuild the state - flutter

Question

I try to pass the value of my carouselwidget index to another widget . however the value returned in my second widget is null ( so for example if i drag a value in my carousel the getindex function prints the index value but the print test in the background_widget stay initialize as null
I think the pb is that i trie to passe a variable (index) from a child (carousel) to an another (Map) through the common parent (Screen stacking carousel and map together).
I'm trying to pass the index now with help of a provider but without success (and seems really overkill to build a model & a provider just to pass a f**ing variable between too child at the same level):
import 'package:provider/provider.dart';
import 'package:algua_alpha/provider/provideIndex.dart';
/*All the variables definition are not published here */
//// model class for index
class Index {
final int id;
Index(this.id);
}
class ProvideIdx with ChangeNotifier {
Index _index;
Index get index {
print(_index);
return _index;
}
void changeId() {
notifyListeners();
}
}
class RegionSelection extends StatefulWidget {
final String region;
RegionSelection(this.region);
#override
_RegionSelectionState createState() => _RegionSelectionState(region);
}
class _RegionSelectionState extends State<RegionSelection> {
// main screen where Carousel and map are stacked
Widget build(BuildContext context) {
return ChangeNotifierProvider(
create: (ctx) => ProvideIdx(),
child: Scaffold(
body: Container(
child:!isLoading?
Stack(
children: <Widget>[
Maposm(region,// this widget need the Index of the carousel slider
),
Align(
alignment: Alignment.bottomCenter,
child: CarouselSlider.builder( //========= Carousel
itemBuilder: (BuildContext c, index) {
return GestureDetector(
onTap: () {
Navigator.of(context).push(MaterialPageRoute(
builder: (context) => NewScreen(
objectlist: region,
item: index,
)));
},
);
itemCount: region.length,
options: CarouselOptions(
enableInfiniteScroll: true,
scrollDirection: Axis.horizontal,
enlargeCenterPage: true,
onPageChanged: (index, reason) { // this callback initialize the new index value
setState(() {
Index(index);
});
},
);
}
)
])
: Center(
child: CircularProgressIndicator(
backgroundColor: Colors.white,
valueColor: AlwaysStoppedAnimation<Color>(
Colors.orange[300]))
),
);
}
}
//=============================== The Map ===============================
class Maposm extends StatefulWidget {
//final carouselID;
final region;
Maposm(this.region);
#override
_MaposmState createState() =>_MaposmState(region);
}
class _MaposmState extends State<Maposm> {
#override
Widget build(BuildContext context) {
final indexes = Provider.of<Index>(context);
final carouselID = indexes.id; //get carousel id
List<Marker> markers = [];
for (var i = 0; i < region.length; i++) {
var m = Marker(
point:
LatLng(region[i].xy[0], region[i].xy[1]),
builder: (ctx) => InkWell(
child: (carouselID == i // index of marker builder == index of carousel
? Icon(
Icons.assistant,
color: Colors.pink[700],
size: 20,
)
: Icon( // else
Icons.brightness_1_sharp,
color: Colors.black87,
size: 4,
)),
onDoubleTap: null,
));
markers.add(m);
}
return FlutterMap(
options: MapOptions(
interactive: true,
slideOnBoundaries: true,
bounds: LatLngBounds(
LatLng(coordxy[0], coordxy[1]), LatLng(coordxy[2], coordxy[3])),
),
layers: [
TileLayerOptions(
urlTemplate: "https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png",
subdomains: ['a', 'b', 'c']),
MarkerLayerOptions(markers: markers)
]);
}
}