future_demo_start.dart

import 'package:flutter/material.dart';
import 'dart:math';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Future Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: FuturePage(),
    );
  }
}

class FuturePage extends StatefulWidget {
  @override
  _FuturePageState createState() => _FuturePageState();
}

class _FuturePageState extends State<FuturePage> {
  int result;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Back to the Future'),
      ),
      body: Center(
        child: Column(
          children: [
            Spacer(),
            RaisedButton(
              child: Text('GO!'),
              onPressed: () {
               setState(() {
                 result = find42s();
               });
              },),
            Spacer(),
            Text(result.toString()),
            Spacer(),
            CircularProgressIndicator(),
            Spacer(),
          ]
        ),
      ),
    );
  }

  int find42s() {
    int counter = 0;
    var rnd = Random();
    for (int i = 0; i<10000000; i++) {
      int myNumber = rnd.nextInt(100);
      if (myNumber == 42) {
        counter++;
      } 
    }
    return counter;
  }



}