Is there a way to fake DateTime.now() in a Flutter test?

Question

I can pass in a time to my widget, but my widget is supposed to be instantiating the time in itself, not receiving a DateTime from a parent widget.

Answer 1

The clock package: https://pub.dartlang.org/documentation/clock/latest/clock/clock-library.html lets you inject a fake clock into the test.

Answer 2

extension CustomizableDateTime on DateTime {
  static DateTime customTime;
  static DateTime get current {
    if (customTime != null)
      return customTime;
    else
      return DateTime.now();
  }
}

Just use CustomizableDateTime.current in the production code. You can modify the returned value in tests like that: CustomizableDateTime.customTime = DateTime.parse("1969-07-20 20:18:04");. There is no need to use third party libraries.

Answer 3

As Mary said, a very neat way is to use the clock package, which is maintained by the Dart team.

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.

Just note that for widget tests, you need to wrap pumpWidget, pump and expect in the withClock callback - only wrapping pumpWidget does not work.