valuegetter_example_page.dart

import 'package:flutter/material.dart';

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

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

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

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  void initState() {
    super.initState();

    listenFuture((value) {
      print('value $value');
    });
  }

  void listenFuture(ValueSetter<String> callback) {
    Future.delayed(Duration(seconds: 1), () {
      callback('Brian');
    });
    
    Future.delayed(Duration(seconds: 2), () {
      callback('Frank');
    });
    
    Future.delayed(Duration(seconds: 3), () {
      callback('CervecereoDev');
    });
    
    Future.delayed(Duration(seconds: 4), () {
      callback('Grinch Code');
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Hello Grinch'),
      ),
      body: Center(
        child: Text(
          'Holi',
        ),
      ),
    );
  }
}