A clock + its test

main.dart

import 'dart:math';

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/scheduler.dart';
import 'package:provider/provider.dart';
import 'package:clock/clock.dart';

void main() {
  runApp(
    Provider(
      create: (_) => Clock(),
      child: MyApp(),
    ),
  );
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      showPerformanceOverlay: true,
      home: MyHomePage(),
      routes: {
        '/whatever': (c) => Scaffold(
              appBar: AppBar(title: Text('Whatever')),
              body: Center(
                child: RaisedButton(
                  onPressed: () => Navigator.pop(c),
                  child: Text('Return to previous page'),
                ),
              ),
            )
      },
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Clock'),
      ),
      body: Center(
        child: AnimatedClock(),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Navigator.pushNamed(context, '/whatever');
        },
        child: Icon(Icons.edit),
      ),
    );
  }
}

class AnimatedClock extends StatefulWidget {
  @override
  AnimatedClockState createState() => AnimatedClockState();
}

@visibleForTesting
class AnimatedClockState extends State<AnimatedClock>
    with SingleTickerProviderStateMixin {
  Ticker _ticker;
  DateTime _initialTime;
  DateTime _now;

  @override
  void initState() {
    super.initState();
    _now = _initialTime = context.read<Clock>().now();

    _ticker = this.createTicker((elapsed) {
      final newTime = _initialTime.add(elapsed);
      // rebuild only if seconds changes instead of every frame
      if (_now.second != newTime.second) {
        setState(() => _now = newTime);
      }
    });
    _ticker.start();
  }

  @override
  void dispose() {
    _ticker.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    // A widget that renders an unanimated clock from a DateTime
    return _ClockRenderer(
      dateTime: _now,
    );
  }
}

class _ClockRenderer extends StatelessWidget {
  const _ClockRenderer({
    Key key,
    this.dateTime,
  }) : super(key: key);

  final DateTime dateTime;

  @override
  Widget build(BuildContext context) {
    return Container(
      width: 210,
      height: 210,
      decoration: BoxDecoration(
        border: Border.all(width: 3, color: Colors.black),
        borderRadius: BorderRadius.circular(210),
      ),
      child: Stack(
        children: <Widget>[
          Positioned(
            top: 105,
            left: 100,
            child: Transform(
              alignment: Alignment.topCenter,
              transform: Matrix4.rotationZ(pi + pi / 24 * 2 * dateTime.hour),
              child: Container(height: 90, width: 5, color: Colors.black),
            ),
          ),
          Positioned(
            top: 105,
            left: 100,
            child: Transform(
              alignment: Alignment.topCenter,
              transform: Matrix4.rotationZ(pi + pi / 60 * 2 * dateTime.minute),
              child: Container(height: 50, width: 5, color: Colors.grey),
            ),
          ),
          Positioned(
            top: 105,
            left: 101.5,
            child: Transform(
              alignment: Alignment.topCenter,
              transform: Matrix4.rotationZ(pi + pi / 60 * 2 * dateTime.second),
              child: Container(height: 90, width: 2, color: Colors.red),
            ),
          ),
        ],
      ),
    );
  }
}

widget_test.dart

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

import 'package:my_clock/main.dart';
import 'package:provider/provider.dart';

void main() {
  testWidgets('renders our clock', (tester) async {
    await tester.pumpWidget(
      Provider(
        create: (_) => tester.binding.clock,
        child: MaterialApp(
          home: Center(
            child: RepaintBoundary(
              child: AnimatedClock(),
            ),
          ),
        ),
      ),
    );

    await expectLater(
      find.byType(AnimatedClock),
      matchesGoldenFile('initial_frame.png'),
    );

    await tester.pump(Duration(hours: 2, minutes: 42, seconds: 21));

    await expectLater(
      find.byType(AnimatedClock),
      matchesGoldenFile('updated_frame.png'),
    );
  });
}

The length of the hour hand

Container(height: 90, width: 5, color: Colors.black)

is confusingly longer than the minute hand

Container(height: 50, width: 5, color: Colors.grey)

The following screenshot is a golden file rendered by _ClockRenderer, which was configured with

DateTime.parse('1989-06-04 20:18:04Z')

It took me a while to figure out that the hour hand is the black one. An hour hand and a minute hand with heights of 50 and 80 respectively would be better off: