Solving the Mockito BocTest Error: “type ‘Null’ is not a subtype of type ‘Future‘”

Are you tired of encountering the frustrating Mockito BocTest error “type ‘Null’ is not a subtype of type ‘Future‘”? This error can be a real showstopper, especially when you’re trying to write unit tests for your Flutter application. Fear not, dear developer, for we’re about to dive into the world of Mockito and BlocTest to solve this pesky issue once and for all!

What’s the Problem?

Before we dive into the solution, let’s take a step back and understand what’s causing this error. When using Mockito with BlocTest, you’re essentially creating a mock object that mimics the behavior of a real object. In this case, the error occurs when the mock object returns a null value, which is not compatible with the expected Future return type.

This error typically arises when you’re trying to test a Bloc that relies on a Future-based API call. The issue lies in the fact that Mockito doesn’t know how to handle the Future return type, resulting in a null value being returned instead.

The Solution

So, how do we solve this problem? Fear not, for the solution is quite straightforward. We’ll use a combination of Mockito’s when method and thenAnswer to return a Future that complies with the expected return type. But before we dive into the code, let’s set up a simple example.

A Simple Example

Let’s say we have a simple UserRepository that retrieves user data from an API:

abstract class UserRepository {
  Future<UserModel> getUserData();
}

We can create a mock implementation of this repository using Mockito:

@Mock()
UserRepository userRepository;

Now, let’s write a test that uses BlocTest to verify that our Bloc correctly handles the user data:

void main() {
  group('UserBlocTest', () {
    late UserBloc userBloc;

    setUp(() {
      userBloc = UserBloc(userRepository: userRepository);
    });

    test('loads user data', () async {
      // Arrange
      when(userRepository.getUserData()).thenAnswer((_) async => UserModel());

      // Act
      userBloc.loadUserData();

      // Assert
      await untilCalled(userRepository.getUserData());
      verify(userBloc.state).userData(ActivityResult.success(UserModel()));
    });
  });
}

In this example, we’re using Mockito’s when method to specify that when getUserData() is called, it should return a Future that resolves to a UserModel instance. However, this is where the error occurs, as Mockito doesn’t know how to handle the Future return type.

Fixing the Error

So, how do we fix this error? The solution lies in using Mockito’s thenAnswer method to return a Future that complies with the expected return type. Here’s the modified code:

void main() {
  group('UserBlocTest', () {
    late UserBloc userBloc;

    setUp(() {
      userBloc = UserBloc(userRepository: userRepository);
    });

    test('loads user data', () async {
      // Arrange
      when(userRepository.getUserData()).thenAnswer((_) => Future.value(UserModel()));

      // Act
      userBloc.loadUserData();

      // Assert
      await untilCalled(userRepository.getUserData());
      verify(userBloc.state).userData(ActivityResult.success(UserModel()));
    });
  });
}

By using thenAnswer with a callback function that returns a Future using Future.value, we’re telling Mockito to return a Future that resolves to a UserModel instance. This fixes the error and allows our test to run successfully!

Best Practices

When using Mockito with BlocTest, it’s essential to follow some best practices to avoid common pitfalls:

• Use when and thenAnswer carefully

  Make sure to specify the correct return type when using when and thenAnswer. In this case, we used Future.value to return a Future that resolves to a UserModel instance.

• Avoid using any as a return type

  Avoid using any as a return type, as it can lead to unpredictable behavior. Instead, specify the exact return type expected by your Bloc.

• Verify mock interactions

  Always verify that the expected interactions occurred between your Bloc and the mock repository. This ensures that your test is covering the correct scenarios.

Conclusion

In conclusion, the “type ‘Null’ is not a subtype of type ‘Future‘” error can be frustrating, but it’s easily solvable with the right approach. By using Mockito’s when and thenAnswer methods correctly, you can write robust unit tests for your Flutter application. Remember to follow best practices and verify mock interactions to ensure your tests are accurate and reliable.

Frequently Asked Questions

Got more questions about Mockito and BlocTest? Here are some frequently asked questions:

Q	A
What is the purpose of Mockito?	Mockito is a popular mocking framework for unit testing in Java and other languages. It allows you to create mock objects that mimic the behavior of real objects, making it easier to write unit tests.
What is BlocTest?	BlocTest is a testing library for Flutter that provides a simple way to write unit tests for your Bloc-based application. It allows you to verify the behavior of your Bloc and its interactions with other components.
How do I fix the “type ‘Null’ is not a subtype of type ‘Future‘” error?	To fix this error, use Mockito’s when and thenAnswer methods to return a Future that complies with the expected return type. In this case, we used Future.value to return a Future that resolves to a UserModel instance.

I hope this article has helped you solve the “type ‘Null’ is not a subtype of type ‘Future‘” error and improve your unit testing skills with Mockito and BlocTest. Happy coding!

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