Random ball numbers that start and stop interactively.

timer_random_numbers.dart

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(
          title: Text('Random Roulette'),
        ),
        body: MyWidget(),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  int number;
  Timer timer;

  Timer createTimer() {
    Random random = Random();
    return Timer.periodic(
      Duration(milliseconds: 20),
      (t) => setState(
        () => number = random.nextInt(32),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        children: [
          Wrap(
            direction: Axis.horizontal,
            runAlignment: WrapAlignment.center,
            spacing: 10,
            children: <Widget>[
              BallNumber(number: number == null ? 0 : Random(number + 12).nextInt(10)),
              BallNumber(number: number == null ? 0 : Random(number + 78).nextInt(10)),
              BallNumber(number: number == null ? 0 : Random(number + 25).nextInt(10)),
              BallNumber(number: number == null ? 0 : Random(number + 43).nextInt(10)),
            ],
          ),
          RaisedButton(
            child: Text(timer?.isActive ?? false ? 'Stop' : 'Start'),
            onPressed: () => setState(
              () => timer?.isActive ?? false
                  ? timer.cancel()
                  : timer = createTimer(),
            ),
          ),
        ],
      ),
    );
  }
}

class BallNumber extends StatelessWidget {
  final int number;
  const BallNumber({Key key, this.number}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(20),
      decoration: BoxDecoration(
        shape: BoxShape.circle,
        color: Colors.green,
      ),
      child: Text(
        '$number',
        style: TextStyle(
          fontSize: 40,
          fontWeight: FontWeight.bold,
          color: Colors.white,
        ),
      ),
    );
  }
}