Now uses `tester.binding.clock`

stopwatch_widget_test_update.dart

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:quiver/async.dart';
import 'package:quiver/testing/time.dart';

void main() {
  group('CounterTimer', () {
    testWidgets('counts down from 10 to 0', (WidgetTester tester) async {
      // Create a Timer
      final timer = CountdownTimer(
        Duration(seconds: 10),
        Duration(milliseconds: 100),
        stopwatch: FakeStopwatch(
          // The function will simply return our fake "Now" time.
          () => tester.binding.clock.now().microsecondsSinceEpoch,
          Duration(seconds: 1).inMicroseconds,
        ),
      );

      // Create the Widget we'll rebuild over time
      final widget = Directionality(
        textDirection: TextDirection.ltr,
        child: CountdownTimerRunner(countdownTimer: timer),
      );

      // Build the Widget the first time and make sure it shows the right thing
      await tester.pumpWidget(widget);
      expect(find.text('10'), findsOneWidget);

      // Wait 10 seconds to make sure the Widget is at 0
      await tester.pumpWidget(widget, Duration(seconds: 10));
      expect(find.text('0'), findsOneWidget);

      // Important: Cancel the Timer, or this test will throw an error that
      // there are active async calls in progress!
      timer.cancel();
    });
  });
}

class CountdownTimerWidget extends StatefulWidget {
  @override
  _CountdownTimerWidgetState createState() => _CountdownTimerWidgetState();
}

class _CountdownTimerWidgetState extends State<CountdownTimerWidget> {
  CountdownTimer _countdownTimer;

  @override
  void dispose() {
    _countdownTimer?.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    if (_countdownTimer == null) {
      return CountdownTimerInitializer(
        Duration.zero,
        onDurationSelected: (Duration duration) {
          setState(() {
            _countdownTimer =
                CountdownTimer(duration, Duration(milliseconds: 250));
          });
        },
      );
    }
    return CountdownTimerRunner(
      countdownTimer: _countdownTimer,
    );
  }
}

typedef void OnDurationSelected(Duration duration);

class CountdownTimerInitializer extends StatefulWidget {
  final Duration initialDuration;
  final OnDurationSelected onDurationSelected;

  CountdownTimerInitializer(this.initialDuration,
      {@required this.onDurationSelected, Key key})
      : super(key: key);

  @override
  CountdownTimerInitializerState createState() {
    return new CountdownTimerInitializerState(initialDuration);
  }
}

class CountdownTimerInitializerState extends State<CountdownTimerInitializer> {
  Duration _duration;

  CountdownTimerInitializerState([Duration initialDuration = Duration.zero])
      : _duration = initialDuration;

  void _increment() {
    setState(() {
      _duration = _duration + Duration(seconds: 1);
    });
  }

  void _decrement() {
    setState(() {
      var updatedDuration = _duration - Duration(seconds: 1);
      if (updatedDuration <= Duration.zero) {
        updatedDuration = Duration.zero;
      }
      _duration = updatedDuration;
    });
  }

  bool get _isValid {
    return _duration > Duration.zero;
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        Padding(
          padding: const EdgeInsets.all(8.0),
          child: Text(
            _duration.inSeconds.toString(),
            style: Theme.of(context).textTheme.display1,
            key: ValueKey("configure-duration-text"),
          ),
        ),
        Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            RaisedButton(
              key: ValueKey("configure-decrement"),
              onPressed: _isValid ? _decrement : null,
              child: Text("-"),
            ),
            RaisedButton(
              key: ValueKey("configure-increment"),
              onPressed: _increment,
              child: Text("+"),
            ),
          ],
        ),
        RaisedButton(
          key: ValueKey("configure-start"),
          onPressed:
              _isValid ? () => widget.onDurationSelected(_duration) : null,
          child: Text("Start"),
        )
      ],
    );
  }
}

class CountdownTimerRunner extends StatelessWidget {
  final CountdownTimer countdownTimer;

  const CountdownTimerRunner({Key key, @required this.countdownTimer})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        StreamBuilder<CountdownTimer>(
          initialData: countdownTimer,
          stream: countdownTimer,
          builder: (context, snapshot) {
            if (snapshot.hasData) {
              return Text(
                countdownTimer.remaining.inSeconds.toString(),
                key: ValueKey("runner-duration-text"),
              );
            } else {
              return Container();
            }
          },
        ),
      ],
    );
  }
}