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?

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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.

I don't think there is a good way without a wrapper around DateTime.now() licke the clock package provides.

Answer 2

As Günther said, the clock package, maintained by the Dart team, provides a very neat way to achieve this.

Normal usage:

import 'package:clock/clock.dart';  void main() {   // prints current date and time   print(clock.now()); } 

Overriding the current time:

import 'package:clock/clock.dart';  void main() {   withClock(     Clock.fixed(DateTime(2000)),     () {       // always prints 2000-01-01 00:00:00.       print(clock.now());     },   ); } 

I wrote about this in more detail on my blog.

For widget tests, you need to wrap pumpWidget, pump and expect in the withClock callback.