shouldly is an assertion concept that focuses on

• writing assertions as plain English sentences
• and giving better error messages when the assertion fails while being simple and terse.

Inspired from Fluent Assertion, Shouldly, should.js

Features

• More readable test code
• Better test failure messages
• Conjunction support (and)
• Custom assertions

Better readability

More readable test code as plain English sentence.

void main() {
  const playerCharacter = PlayerCharacter(
    'Arthur',
    100,
    ['Axe', 'Sword', 'Staff of Wonder'],
  );

  test('equality', () {
    // non shouldly
    expect(playerCharacter.health, 100);
    // 😍 shouldly
    playerCharacter.health.should.be(100);
  });

  test('array contains', () {
    // non shouldly
    expect(playerCharacter.weapons, contains('Staff of Wonder'));
    // 😍 shouldly
    playerCharacter.weapons.should.contain('Staff of Wonder');
  });

  test('nullability of string', () {
    // non shouldly
    expect(playerCharacter.nickname, isNotNull);
    // 😍 shouldly
    playerCharacter.nickname.should.not.beNullOrEmpty();
  });
}

Better test failure messages

To get more readable and contextual information.

Expected: <10>
  Actual: <100>

Expected playerCharacter.health
    should be
10
    but was
100

Expected: null
  Actual: 'Arthur'

Expected nickname
    should be null or empty
but was
    `Arthur`

Expected: contains 'Staff of Wonderr'
  Actual: ['Axe', 'Sword', 'Staff of Wonder']

Expected playerCharacter.weapons
    should contain
Staff of Wonder
    but was actually
[Axe, Sword]

No more: Mix Up with parameters

You can mix up with Expected or Actual 🤔. But with shouldly there is no way to mix up.

// without shouldly
expect(playerCharacter.health, 0);
expect(0, playerCharacter.health);

// shouldly
playerCharacter.health.should.be(0);

No more single heap of assertion methods

Before

You could write such crazy assertions.

expect(2, isTrue);
expect(2, contains(2));
expect(2, startsWith('1'));
expect(2, isEmpty);
expect(2, throwsException);

After

Every single type of class has his own assertions. Easy to find required assertion method.

Conjunctions

This is a real English sentence, is it not?

13.should.beOdd().and.beGreaterOrEqualThan(13);
participants.should.contain('Andrew').and.not.contain('Bobby');

SatisfyAllConditions

satisfyAllConditions will show all errors at once, not only first fail assertion.

test('should satisfy all conditions', () {
    Should.satisfyAllConditions([
      () => playerCharacter.nickname.should.as('nickname').not.beNullOrEmpty(),
      () => playerCharacter.weapons.should.as('weapons').contain('Staff of Wonder'),
      () => playerCharacter.health.should.as('health').be(100),
    ]);
  });

Example report can be:

Expected satisfy all conditions specified, but does not.
The following errors were found ...

------------- Error 1 -------------
Expected nickname
    should not be null or empty
but was
    ''

------------- Error 2 -------------
Expected weapons
    should contain
Staff of Wonder
    but was actually
[Axe, Sword]

------------- Error 3 -------------
Expected health
    should be
100
    but was
99

------------------------------------

Custom matchers

For existing one - just create an extension for it

extension CustomNumAssertions on NumericAssertions {
  NumericAssertions get beNegative {
    if (subject >= 0) {
      throw ShouldlyTestFailureError('Number\n  should be negative');
    }
    return NumericAssertions(subject);
  }
}

Or more exotic matchers - fot custom types

1. First, create a Assertions class for your custom type which extends BaseAssertions

class CustomerAssertions extends BaseAssertions<Customer, CustomerAssertions> {
  // ...

  CustomerAssertions get beMarried {
    if (isReversed) {
      if (subject!.isMarried) {
        throw ShouldlyTestFailureError('Customer should not be married');
      }
    } else {
      if (!subject!.isMarried) {
        throw ShouldlyTestFailureError('Customer should be married');
      }
    }
    return CustomerAssertions(subject);
  }

  CustomerAssertions get beMale {
    if (isReversed) {
      Execute.assertion
          .forCondition(subject!.gender == Gender.male)
          .failWith('$subjectLabel should not be male');
    } else {
      Execute.assertion
          .forCondition(subject!.gender != Gender.male)
          .failWith('$subjectLabel should be male');
    }

    Execute.assertion
        .forCondition(subject!.gender != Gender.male)
        .failWith('$subjectLabel should be male');

    return CustomerAssertions(subject);
  }

  // ...
}

2. Second, create an extension on your class which has should property with type of your custom assertions class

extension CustomerExtension on Customer {
  CustomerAssertions get should => CustomerAssertions(this);
}

3. Third, you are ready to go use your own shouldly assertions

test('Custom matchers', () {
  bobby.should.beMale.and.beMarried;
  kate.should.beMarried.and.not.beMale;
});

Complete in

To verify how long your function is executed

test('should complete in a duration', () async {
  await Should.completeIn(
    Duration(seconds: 1),
    func: () => slowFunction(
      Duration(milliseconds: 900),
    ),
  );
});

test('should complete after a period of time', () async {
  await Should.completeAfter(
    const Duration(seconds: 3),
    func: () => verySlowFunction(
      const Duration(seconds: 4),
    ),
  );
});

Getting started

Simple add shouldly dependency into your project.

dev_dependencies:
  shouldly: <latest>

Usage

Objects

Every single object has following assertion methods:

test('should be not null', () {
  final obj = Object();
  obj.should.not.beNull();
});

test('should be null', () {
  const Object? obj = null;
  obj.should.beNull();
});

test('should be type of `int`', () {
  const obj = 1;
  obj.should.beOfType<int>();
});

test('should be assignable to `num`', () {
  const obj = 1;
  obj.should.beAssignableTo<num>();
});

Booleans

test('false should be `false`', () {
  false.should.beFalse();
});

test('false should not be `true`', () {
  false.should.not.beTrue();
});

Numbers

test('Int should be type of `int`', () {
  2.should.beEven();
  10.should.beGreaterThan(9);
  9.99.should.not.beCloseTo(10.0, delta: 0.01);
});

Strings

test('should not start with substring', () {
  'Flutter'.should.not.startWith('A');
});

DateTimes

// before
DateTime(2021, 9, 9).should.beBefore(DateTime(2021, 9, 10));
DateTime(2021, 9, 9).should.not.beBefore(DateTime(2021, 9, 9));

// close to
DateTime(2021, 9, 9, 1, 1, 1, 2).should.beCloseTo(
      DateTime(2021, 9, 9, 1, 1, 1, 3),
      delta: Duration(milliseconds: 1),
    );

Iterables

test('should contain', () {
  [1, 200, 3].should.contain(200);
});

test('should not contain', () {
  [1, 2, 4].should.not.contain(3);
});

test('with every element in collection is true for predicate', () {
  [3, 5, 7, 9].should.every((item) => item < 10);
});

test('with some elements in collection is true for predicate', () {
  [3, 5, 7, 9].should.any((item) => item > 8);
});

Maps

final subject = {
  'name': 'John',
  'age': 18,
};

test('should contain key', () {
  subject.should.containKey('name');
});

test('should contain key with exact value', () {
  subject.should.containKeyWithValue('age', 18);
});

Functions

Should.throwException(() => someMethodWitchThrowException(params:));
Should.throwError<ExactError>(() => someMethodWitchThrowExactError(params:));

test('async function should throw exception', () async {
  await Should.throwAsync(() {
    Future.delayed(Duration(milliseconds: 100));
    throw Exception('test');
  });
});

test('async function should throw exact exception', () async {
  await Should.throwAsync<CustomException>(() {
    Future.delayed(Duration(milliseconds: 100));
    throw CustomException('custom exception test');
  });
});

test('should complete in a duration', () async {
  await Should.completeIn(
    Duration(seconds: 1),
    func: () => slowFunction(
      Duration(milliseconds: 900),
    ),
  );
});

Enums

test('should not be equal', () {
  seasons.spring.should.not.be(seasons.winter);
});

test('should not be type of', () {
  seasons.spring.should.not.beOfType<level>();
});

test('should be assignable to `Enum`', () {
  seasons.spring.should.beAssignableTo<Enum>();
});

More examples here

Writing Custom Matchers

extension CustomerExtension on Customer {
  CustomerAssertions get should => CustomerAssertions(this);
}

class CustomerAssertions extends BaseAssertions<Customer, CustomerAssertions> {
  CustomerAssertions(
    Customer? subject, {
    bool isReversed = false,
    String? subjectLabel,
  }) : super(subject, isReversed: isReversed, subjectLabel: subjectLabel);

  CustomerAssertions get beMarried {
    if (isReversed) {
      if (subject!.isMarried) {
        throw ShouldlyTestFailure('Customer should not be married');
      }
    } else {
      if (!subject!.isMarried) {
        throw ShouldlyTestFailure('Customer should be married');
      }
    }
    return CustomerAssertions(subject);
  }

  CustomerAssertions get beMale {
    if (isReversed) {
      if (subject!.gender == Gender.male) {
        throw ShouldlyTestFailure('Customer should be female');
      }
    } else {
      if (subject!.gender != Gender.male) {
        throw ShouldlyTestFailure('Customer should be male');
      }
    }

    return CustomerAssertions(subject);
  }

  @override
  CustomerAssertions copy(
    Customer? subject, {
    bool isReversed = false,
    String? subjectLabel,
  }) =>
      CustomerAssertions(
        subject,
        isReversed: isReversed,
        subjectLabel: subjectLabel,
      );
}

Recommendations

You can improve the readability of the rest of your test code with given_when_then_unit_test, which enhances the test report readability as well.

Changelog

Please see the Changelog page to know what's recently changed.

Contributing

Feel free to contribute to this project.

If you find a bug or want a feature, but don't know how to fix/implement it, please fill an issue.
If you fixed a bug or implemented a new feature, please send a pull request.

We accept the following contributions:

• Ideas how to improve
• Reporting issues
• Fixing bugs
• More tests
• More class integrations (Functions, Futures, Streams, Durations)
• Improving documentation and comments

Maintainers

• Andrew Piterov