Dart stopwatch

main.dart

import 'dart:async';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Stopwatch app',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Stopwatch app'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});
  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Duration _duration = const Duration();
  Timer? _timer;

  void addTimeToDuration() {
    int totalSec = _duration.inSeconds + 1;
    if (totalSec < 0) {
      _timer?.cancel();
    } else {
      _duration = Duration(seconds: totalSec);
    }

    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Text(
                getRunningTime(),
                style: Theme.of(context)
                    .textTheme
                    .headlineMedium
                    ?.copyWith(color: Colors.black, fontSize: 20),
              ),
              TextButton(
                onPressed: () {
                  if (_timer == null) {
                    _timer =
                        Timer.periodic(const Duration(seconds: 1), (timer) {
                      return addTimeToDuration();
                    });
                  } else {
                    _timer?.cancel();
                    _timer = null;
                  }
                  setState(() {});
                },
                child: Text(_timer != null
                    ? 'Pause timer'
                    : (getRunningTime() == '0'
                        ? 'Start timer'
                        : 'Resume timer')),
              ),
              TextButton(
                onPressed: () {
                  _timer?.cancel();
                  _timer = null;
                  _duration = const Duration();
                  setState(() {});
                },
                child: const Text('Reset timer'),
              ),
            ],
          ),
        ));
  }

  String getRunningTime() {
    List<String> time = [];
    print('duration $_duration');
    if (_duration.inHours > 0) {
      time.add(_duration.inHours.toString());
    }

    if (_duration.inHours > 0 || _duration.inMinutes > 0) {
      time.add(_duration.inMinutes.toString());
    }
    if (_duration.inSeconds > 0) {
      time.add(_duration.inSeconds.toString());
    }

    if (time.isEmpty) {
      return '0';
    }
    return time.join(':');
  }
}