How to get selected index of DropDownButton in Flutter

In this Google flutter code example we are going to learn how to to get selected index of DropDownButton in Flutter.

The code is based on flutter version 1.0.0

Main.dart (App entry point)

import 'package:flutter/material.dart';
import 'week2/dropdown_selected_index.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      //home: MyHomePage(title: 'Flutter Demo Home Page'),
      home: SelectedIndexDropdown(),
    );
  }
}

dropdown_selected_index.dart

import 'package:flutter/material.dart';

class SelectedIndexDropdown extends StatefulWidget {
  @override
  SelectedIndexDropdownState createState() {
    return new SelectedIndexDropdownState();
  }
}

class SelectedIndexDropdownState extends State<SelectedIndexDropdown> {
  final List<String> _dropdownValues = [
    "One",
    "Two",
    "Three",
    "Four",
    "Five"
  ]; //The list of values we want on the dropdown
  String _currentlySelected = ""; //var to hold currently selected value

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Selected Index - DropdownButton'),
      ),
      //display currently selected item on dropdown
      body: Center(
          child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        mainAxisSize: MainAxisSize.max,
        children: <Widget>[
          Text(_currentlySelected),
          SizedBox(
            height: 20.0,
          ),
          DropdownButton(
            //map each value from the lIst to our dropdownMenuItem widget
            items: _dropdownValues
                .map((value) => DropdownMenuItem(
                      child: Text(value),
                      value: value,
                    ))
                .toList(),
            onChanged: (String value) {
              //once dropdown changes, update the state of out currentValue
              setState(() {
                _currentlySelected = value;
              });
            },
            //this wont make dropdown expanded and fill the horizontal space
            isExpanded: false,
            //make default value of dropdown the first value of our list
            value: _dropdownValues.first,
          ),
        ],
      )),
    );
  }
}

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