How to use DropdownButton.selectedItemBuilder

dropdown_selected_item_builder.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.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  List<String> options = <String>['One', 'Two', 'Free', 'Four'];
  String dropdownValue = 'One';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(widget.title)),
      body: Container(
        color: Colors.blue,
        child: Center(
          child: DropdownButton<String>(
            value: dropdownValue,
            onChanged: (String newValue) {
              setState(() {
                dropdownValue = newValue;
              });
            },
            selectedItemBuilder: (BuildContext context) {
              return options.map((String value) {
                return Text(
                  dropdownValue,
                  style: TextStyle(color: Colors.white),
                );
              }).toList();
            },
            items: options.map<DropdownMenuItem<String>>((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
          ),
        ),
      ),
    );
  }
}

'preciate it!