A StatefulBuilder with a persistent value in State + some basic dropdown demo.

common_dropdown.dart

// roipeker 2020
// sample using ValueBuilder. Generic Dropdown wrapper with basic styling.

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

class CommonDropDown<T> extends StatelessWidget {
  
  final double height;
  final List<T> listData;
  final T initialValue;
  final String hintText;
  final String Function(T) itemLabelBuilder;
  final Widget Function(T) itemWidgetBuilder;
  final void Function(T) onChanged;

  const CommonDropDown({
    Key key,
    @required this.listData,
    this.height,
    this.onChanged,
    this.hintText,
    this.initialValue,
    this.itemWidgetBuilder,
    this.itemLabelBuilder,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final list =
        listData.map<DropdownMenuItem>(_buildItem).toList(growable: false);
    Widget child = ValueBuilder<T>(
      initialValue: initialValue,
      onValue: onChanged,
      builder: (ctx, value, updater) {
        // style as you want.
        return Container(
          decoration: BoxDecoration(
            border: Border(bottom: BorderSide(color: Colors.grey, width: 1)),
          ),
          alignment: Alignment.center,
          child: DropdownButton(
            isExpanded: true,
            hint: hintText != null ? Text(hintText) : null,
            isDense: true,
            underline: Container(),
            onChanged: updater,
            items: list,
            value: value,
          ),
        );
      },
    );
    if (height != null) {
      child = SizedBox(width: double.infinity, height: height, child: child);
    }
    return child;
  }

  DropdownMenuItem _buildItem(T data) {
    Widget child;
    if (itemWidgetBuilder != null) {
      child = itemWidgetBuilder(data);
    } else {
      final text = (itemLabelBuilder == null)
          ? data?.toString()
          : itemLabelBuilder(data);
      child = Text(text);
    }
    return DropdownMenuItem<T>(
      child: child,
      value: data,
    );
  }
}

dropdown_models.dart

// roipeker 2020
// demo models

import 'package:flutter/material.dart';

// option model List<String>.
final hotelTypesStrings = <String>[
  'Hotel',
  'Hostel',
  'Motel',
  'Resort',
  'Apartment',
  'Studios',
  'BnB',
  'Villa',
  'Camping',
  'Pet Hotel',
];

// option model List<HotelType>.
final hotelTypesModels = <HotelType>[
  HotelType('Hotel', Icons.business),
  HotelType('Hostel', Icons.directions_car),
  HotelType('Motel', Icons.golf_course),
  HotelType('Resort', Icons.more_vert),
  HotelType('Apartment', Icons.portrait),
  HotelType('Studios', Icons.shopping_basket),
  HotelType('BnB', Icons.replay_30),
  HotelType('Villa', Icons.fiber_dvr),
  HotelType('Camping', Icons.local_grocery_store),
  HotelType('Pet Hotel', Icons.subscriptions),
];

class HotelType {
  final IconData icon;
  final String name;
  final int index;
  HotelType(this.name, this.icon, [this.index]);
  
  // if doing this, no need to set a CommonDropDown::itemLabelBuilder...
  @override
  String toString() => name ;
}

main_demo.dart

//... some widget class.
  
  Widget buildDropdowns() {
    return Column(
      mainAxisAlignment: MainAxisAlignment.spaceEvenly,
      children: [

        CommonDropDown<HotelType>(
          listData: hotelTypesModels,
          hintText: 'Select a type',
          // 1 - only return String for the Text.
          // itemLabelBuilder: (model) => model.name,
          // 2 - generate the actual item's child widget, overrides itemLabelBuilder.
          itemWidgetBuilder: (model) =>
              Row(
                children: [
                  Icon(model.icon),
                  SizedBox(width: 10),
                  Text(model.name),
                ],
              ),
          onChanged: (value) => print("dropdown changed: $value"),
          height: 50,
        ),

        CommonDropDown<String>(
          listData: hotelTypesStrings,
          initialValue: hotelTypesStrings.first,
          // When no itemLabelBuilder, takes directly from .toString()
          //  itemLabelBuilder: (model) => model,
          onChanged: (value) => print("dropdown changed: $value"),
          height: 40,
        ),

      ],
    );
  }
//... rest of implementation 

valuebuilder.dart

// SAMPLE code:
// return ValueBuilder<bool>(
//  initialData: false,
//  builder: (_, value, updater) =>
//    Switch(
//      onChanged: updater,
//      value: value,
//    ),
// utility callbacks
//    onValue: print,
//      onDispose: () => print("Disposing widget"),
//    );

class ValueBuilder<T> extends StatefulWidget {
  final T initialData;
  final ValueUpdateBuilder builder;
  final void Function() onDispose;
  final void Function(T) onValue;

  const ValueBuilder({
    Key key,
    this.initialData,
    this.onDispose,
    this.onValue,
    @required this.builder,
  }) : super(key: key);

  @override
  _ValueBuilderState<T> createState() => _ValueBuilderState<T>();
}

class _ValueBuilderState<T> extends State<ValueBuilder<T>> {
  T value;

  @override
  void initState() {
    super.initState();
    value = widget.initialData;
  }

  @override
  Widget build(BuildContext context) => widget.builder(context, value, updater);

  void updater(T newValue) {
    if (widget.onValue != null) {
      widget.onValue(newValue);
    }
    setState(() {
      value = newValue;
    });
  }

  @override
  void dispose() {
    super.dispose();
    if (widget.onDispose != null) {
      widget.onDispose();
    }
    if (value is ChangeNotifier) {
      (value as ChangeNotifier)?.dispose();
    } else if (value is StreamController) {
      (value as StreamController)?.close();
    }
    value = null;
  }
}