Changing UI

main.dart

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

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomeScreen(),
    );
  }
}

class HomeScreen extends StatefulWidget {
  const HomeScreen({Key? key}) : super(key: key);

  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  var counter = 0;
  Timer? timer;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              '${counter / 10}'.padLeft(2, '0'),
              style: TextStyle(fontSize: 30),
            ),
            SizedBox(height: 20),
            Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                ElevatedButton(
                  onPressed: () {
                    timer ??= Timer.periodic(
                      Duration(milliseconds: 100),
                      (timer) {
                        counter++;
                        setState(() {});
                      },
                    );
                  },
                  child: Text('Start'),
                ),
                SizedBox(width: 10),
                ElevatedButton(
                  onPressed: () {
                    setState(() {
                      timer?.cancel();
                    });
                  },
                  child: Text('Stop'),
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}

final colorList = [
  Colors.red,
  Colors.blue,
  Colors.amber,
  Colors.purple,
  Colors.purple.shade100,
  Colors.purple.shade900,
  Color.fromARGB(255, 15, 172, 20),
  Colors.orange,
  Colors.black,
  Colors.white,
];