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;
});

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()
              }),
         );
      });
}

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(),
);

Please refer to the below working code. I created a new Stateful widget(ModalBottomSheet) for the showModalBottomSheet. On button press, we are rebuilding the ModalBottomSheet only which is much cleaner now. We can use AnimationController if need animation for changing the height.

import 'dart:async';
import 'package:flutter/material.dart';

class ModalBottomSheet extends StatefulWidget {
  _ModalBottomSheetState createState() => _ModalBottomSheetState();
}

class _ModalBottomSheetState extends State<ModalBottomSheet>
    with SingleTickerProviderStateMixin {
  var heightOfModalBottomSheet = 100.0;

  Widget build(BuildContext context) {
    return Container(
      height: heightOfModalBottomSheet,
      child: RaisedButton(
          child: Text("Press"),
          onPressed: () {
            heightOfModalBottomSheet += 100;
            setState(() {});
          }),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return new _MyHomePageState();
  }
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    Future(() => showModalBottomSheet(
        context: context,
        builder: (context) {
          return ModalBottomSheet();
        }));
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Modal example"),
      ),
    );
  }
}

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(title: 'Flutter Demo', home: new MyHomePage());
  }
}

create a separate StatefulWidget for the showModalBottomSheet(), like

 showModalBottomSheet(
    context: context,
    builder: (ctx) {
      return MapBottomSheet();
    });

Bottom Sheet Statefulwidget

class MapBottomSheet extends StatefulWidget {
  @override
  _MapBottomSheetState createState() => _MapBottomSheetState();
}

class _MapBottomSheetState extends State<MapBottomSheet> {
  List<String> places = [];

  void _setPlaces(String place) {
    setState(() {
      places.add(place);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.black12,
      child: Column(
        children: [
          AppTextField(
            hint: "Search",
            onEditingComplete: () {},
            onChanged: (String text) {},
            onSubmitted: (String text) async {
              // Await the http get response, then decode the json-formatted response.
              var response = await http.get(Uri.parse(
                  'https://api.mapbox.com/geocoding/v5/mapbox.places/$text.json?access_token=pk.eyJ1IjoidjNyc2lvbjkiLCJhIjoiY2ttNnZldmk1MHM2ODJxanh1ZHZqa2I3ZCJ9.e8pZsg87rHx9FSM0pDDtlA&country=PK&fuzzyMatch=false&place=park'));
              if (response.statusCode == 200) {
                Map<String, dynamic> data = jsonDecode(response.body);
                print(data.toString());

                List<dynamic> features = data['features'];

                features.forEach((dynamic feature) {
                  setState(() {
                    _setPlaces(feature['place_name']);
                  });
                });
              } else {
                print('Request failed with status: ${response.statusCode}.');
              }
            },
          ),
          Expanded(
            child: Container(
              height: 250.0,
              width: double.infinity,
              child: ListView.builder(
                  itemCount: places.length,
                  itemBuilder: (ctx, idx) {
                    return Container(
                      child: Text(places[idx]),
                    );
                  }),
            ),
          ),
        ],
      ),
    );
    
  }
}