Unit testing DateTime.now() with the help of Dart extensions

The Problem…

Often times within our applications, both on the UI and Data sides we will need to set “defaults” for dates, the simplest way to do that in most cases would be to use DateTime.now(), after all, we are most likely going to use the current time as the default value.

While this is very handy, it can create complications when it comes time to test. Within Dart it is not possible to simply override methods or functions globally, you can only mock arguments that are passed into classes, and adding an argument of DateTime to a class while easy enough, but not seldom wanted and can more then likely just lead to unnecessary bloat.

With widgets, however, that is a completely different story, you would either need to pass DateTime into the widget or create a custom class and add that to some dependency injection in order to be able to mock it in the test.

A Solution…

One solution I discovered, which is made possible with extensions which was introduced in dart 2.7

Extension methods, introduced in Dart 2.7, are a way to add functionality to existing libraries. (Link)

Using this, we are able to create an extension on DateTime with which we can both get the “current” time as well as implement a setter to allow us to override that “current” time.

extension ExtendedDateTime on DateTime {
  static DateTime? _customTime;

  static DateTime get current {
    return _customTime ?? DateTime.now();
  }

  static set customTime(DateTime customTime) {
    _customTime = customTime;
  }
}

We have a nullable DateTime which when we use ExtendedDateTime.current within our code will return the value of DateTime.now().

That in itself is nothing special, the set method is what we really care about.

Take the following class as an example.

class User {
  final String userName;
  final DateTime createdDate;

  User(this.userName, this.createdDate);

  static User createUser(String userName) => User(userName, DateTime.now());
}

From within the logic of our code, we wish to create a new user, so I wish to verify that when I call User.createUser that it returns a new user with the name passed in as well as the current DateTime. However, during the execution of the test, a few milliseconds would pass between calling the function and verifying the expectation.

final result = User.createUser('Reme');
expect(result, equals(User('Reme', DateTime.now());

The above example is almost certain to fail as a result of those few milliseconds.

Now that we have added our extension to our project we can refactor our code to rather look like:

class User {
  final String userName;
  final DateTime createdDate;

  User(this.userName, this.createdDate);

  static User createUser(String userName) => User(userName, ExtendedDateTime.current);
}

After which our test would look something like:

ExtendedDateTime.current = DateTime.parse(
  "2020-05-15 13:07:53.531Z",
);

final result = User.createUser('Reme');

expect(result, equals(User('Reme', "2020-05-15 13:07:53.531Z");