dropdown_hint.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Sample',
      home: Scaffold(
        appBar: AppBar(title: const Text('Sample')),
        body: MyStatefulWidget(),
      ),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key}) : super(key: key);

  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  final List<String> items = <String>['1', '2', '3'];
  String selectedItem = '1';

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          Text('Enabled DropdownButton:'),
          DropdownButton<String>(
            value: null,
            hint: Text('Hint text here'),
            items: items.map((item) {
              return DropdownMenuItem(
                value: item,
                child: Text(item),
              );
            }).toList(),
            onChanged: (String value) {
              setState(() { selectedItem = value; });
            },
          ),
          Divider(),
          Text('Disabled DropdownButton:'),
          DropdownButton<String>(
            value: selectedItem,
            hint: Text('Hint text here'),
            items: items.map((item) {
              return DropdownMenuItem(
                value: item,
                child: Text(item),
              );
            }).toList(),
            onChanged: null,
          ),
        ],
      ),
    );
  }
}