How to test code that uses DateTime.now in Flutter?

Question

I have this class:

import 'package:flutter/material.dart';

class AgeText extends StatelessWidget {
  final String dateOfBirth;

  const AgeText({Key key, @required this.dateOfBirth}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final age = _calculateAge();
    return Text(age.toString());
  }

  int _calculateAge() {
    final dateOfBirthDate = DateTime.parse(dateOfBirth);
    final difference = DateTime.now().difference(dateOfBirthDate);
    final age = difference.inDays / 365;

    return age.floor();
  }
}

I'd like to test that it produces the correct age when a date of birth is passed into it. What is the best way to do this in Flutter?

---

SOLUTION: For those interested, here's the solution using @Günter Zöchbauer's suggestion of the clock package.

My widget class:

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

class AgeText extends StatelessWidget {
  final String dateOfBirth;
  final Clock clock;

  const AgeText({Key key, @required this.dateOfBirth, this.clock = const Clock()}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final age = _calculateAge();
    return Text(age.toString());
  }

  int _calculateAge() {
    final dateOfBirthDate = DateTime.parse(dateOfBirth);
    final difference = clock.now().difference(dateOfBirthDate);
    final age = difference.inDays / 365;

    return age.floor();
  }
}

and my test class:

import 'package:clock/clock.dart';
import 'package:flutter/material.dart';
import 'package:flutter_app/age.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets("shows age 30 when date of birth is 30 years ago", (WidgetTester tester) async {
    final mockClock = Clock.fixed(DateTime(2000, 01, 01));
    final testableWidget = MaterialApp(
      home: AgeText(
        dateOfBirth: "1970-01-01T00:00:00",
        clock: mockClock,
      ),
    );

    await tester.pumpWidget(testableWidget);

    expect(find.text("30"), findsOneWidget);
  });
}

Answer 1

If you use the clock package for code depending on DateTime.now() you can easily mock it.

Other than creating a custom wrapper around DateTime.now(), I don't think there is a better way than what the clock package provides.