How to update state of a ModalBottomSheet in Flutter?

You can use Flutter's StatefulBuilder to wrap your ModalBottomSheet as follows:

showModalBottomSheet(
    context: context,
    builder: (context) {
      return StatefulBuilder(
          builder: (BuildContext context, StateSetter setState /*You can rename this!*/) {
        return Container(
          height: heightOfModalBottomSheet,
          child: RaisedButton(onPressed: () {
            setState(() {
              heightOfModalBottomSheet += 10;
            });
          }),
        );
      });
});

Please note that the new setState will override your main widget setState but sure you can just rename it so you would be able to set state of your parent widget and the modal's

//This sets modal state
setModalState(() {
    heightOfModalBottomSheet += 10;
});
//This sets parent widget state
setState(() {
     heightOfModalBottomSheet += 10;
});

Screenshot:

---

Create a class:

class MyBottomSheet extends StatefulWidget {
  @override
  _MyBottomSheetState createState() => _MyBottomSheetState();
}

class _MyBottomSheetState extends State<MyBottomSheet> {
  bool _flag = false;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        FlutterLogo(
          size: 300,
          style: FlutterLogoStyle.stacked,
          textColor: _flag ? Colors.black : Colors.red,
        ),
        RaisedButton(
          onPressed: () => setState(() => _flag = !_flag),
          child: Text('Change Color'),
        )
      ],
    );
  }
}

Usage:

showModalBottomSheet(
  context: context,
  builder: (_) => MyBottomSheet(),
);

You can maybe use the showBottomSheet from the ScaffoldState. read more here about this showBottomSheet.

This will show the bottomSheet and return a controller PersistentBottomSheetController. with this controller you can call controller.SetState((){}) which will re-render the bottomSheet.

Here is an example

PersistentBottomSheetController _controller; // <------ Instance variable
final _scaffoldKey = GlobalKey<ScaffoldState>(); // <---- Another instance variable
.
.
.
void _incrementBottomSheet(){
    _controller.setState(
        (){
            heightOfModalBottomSheet += 10;
        }
    )
}
.
void _createBottomSheet() async{
  _controller = await _scaffoldKey.currentState.showBottomSheet(
        context: context,
        builder: (context) {
           return Container(
               height: heightOfModalBottomSheet,
               child: RaisedButton(
               onPressed: () {
                  _incrementBottomSheet()
              }),
         );
      });
}