main.dart

import 'package:flutter/material.dart';
void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.red,
      ),
      home: MyHomePage(title: 'Epic Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

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

String dropdownStr = 'Batman Begins';

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
          child: DropdownButton <String>(
              value: dropdownStr,
              onChanged: (String newValue) {
                setState(() {
                  dropdownStr = newValue;
                });
              },
              items: <String>[
                'Batman Begins',
                'The Dark Knight',
                'The Dark Knight Rises'
              ]
                  .map<DropdownMenuItem<String>>((String value) {
                return DropdownMenuItem<String>(
                    value: value,
                    child: Text(value)
                );
              }).toList()
          )
      ),
    );
  }
}