How do you know if DraggableScrollableSheet is collapsed or expanded - flutter

Question

In my app there is a DraggableScrollableSheet and a FAB. I want the FAB be invisible if the DraggableScrollableSheet is expanded. I need to check the event of expansion
I tried to attach a listener to the scrollController and check the value of scrollController.offset. But I realized that the listener is triggered just when the DraggableScrollableSheet is completly expanded and not before. Is there another way to check if it is expanded or collapsed?
double appbarSize = 0.08;
double offsetVisibility=100.0;
bool FAB_visibility=true;
#override
Widget build(BuildContext context) {
return Scaffold(
floatingActionButton: Visibility(
visible: FAB_visibility,
child: FloatingActionButton(
child: Icon(Icons.add),
),
),
body: SizedBox.expand(
child: DraggableScrollableSheet(
maxChildSize: 0.8,
minChildSize: appbarSize,
initialChildSize: appbarSize,
builder: (BuildContext context, ScrollController scrollController) {
_scrollListener() {
if(FAB_visibility==false && scrollController.offset<=offsetVisibility){
setState(() {
FAB_visibility=true;
});
}
else if(FAB_visibility==true && scrollController.offset>offsetVisibility){
setState(() {
FAB_visibility=false;
});
}
}
scrollController.addListener(_scrollListener);
return Container(
decoration: new BoxDecoration(
color: Colors.red,
borderRadius: new BorderRadius.only(
topLeft: const Radius.circular(10.0),
topRight: const Radius.circular(10.0))),
child: ListView.builder(
controller: scrollController,
itemCount: 25,
itemBuilder: (BuildContext context, int index) {
return ListTile(title: Text('Item $index'));
},
),
);
},
)),
);
}

Answer 1

Thanks to flutter team I understood how to do it!
We should wrap DraggableScrollableSheet in a NotificationListener
This is the working code:
class _MyHomePageState extends State<MyHomePage> {
double appbarSize = 0.08;
double offsetVisibility = 100.0;
bool FAB_visibility = true;
#override
Widget build(BuildContext context) {
return Scaffold(
body: SizedBox.expand(
child: NotificationListener<DraggableScrollableNotification>(
onNotification: (DraggableScrollableNotification DSNotification){
if(FAB_visibility && DSNotification.extent>=0.2){
setState(() {
FAB_visibility=false;
});
}
else if(!FAB_visibility && DSNotification.extent<0.2){
setState(() {
FAB_visibility=true;
});
}
},
child: DraggableScrollableSheet(
maxChildSize: 0.8,
minChildSize: appbarSize,
initialChildSize: appbarSize,
builder: (BuildContext context, ScrollController scrollController) {
return Container(
child: ListView.builder(
controller: scrollController,
itemCount: 25,
itemBuilder: (BuildContext context, int index) {
return ListTile(title: Text('Item $index'));
},
),
);
},
),
)),
floatingActionButton: Visibility(
visible: FAB_visibility,
child: FloatingActionButton(
child: Icon(Icons.add),
),
),
);
}
}