flutter-examples/dropdown_button/lib/main.dart

import 'package:flutter/material.dart';

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

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

class MyAppState extends State<MyApp> {
  List _fruits = ["Apple", "Banana", "Pineapple", "Mango", "Grapes"];

  List<DropdownMenuItem<String>> _dropDownMenuItems;
  String _selectedFruit;

  @override
  void initState() {
    _dropDownMenuItems = buildAndGetDropDownMenuItems(_fruits);
    _selectedFruit = _dropDownMenuItems[0].value;
    super.initState();
  }

  List<DropdownMenuItem<String>> buildAndGetDropDownMenuItems(List fruits) {
    List<DropdownMenuItem<String>> items = new List();
    for (String fruit in fruits) {
      items.add(new DropdownMenuItem(value: fruit, child: new Text(fruit)));
    }
    return items;
  }

  void changedDropDownItem(String selectedFruit) {
    setState(() {
      _selectedFruit = selectedFruit;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      debugShowCheckedModeBanner: false,
      home: new Scaffold(
        appBar: new AppBar(
          title: new Text("DropDown Button Example"),
        ),
        body: new Container(
          child: new Center(
              child: new Column(
            crossAxisAlignment: CrossAxisAlignment.center,
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              new Text("Please choose a fruit: "),
              new DropdownButton(
                value: _selectedFruit,
                items: _dropDownMenuItems,
                onChanged: changedDropDownItem,
              )
            ],
          )),
        ),
      ),
    );
  }
}