Question

In certain cases, unit tests may depend on classes that fetch data from live web services or databases. This is inconvenient for a few reasons:

• Calling live services or databases slows down test execution.
• A passing test may start failing if a web service or database returns unexpected results. This is known as a “flaky test.”
• It is difficult to test all possible success & failure scenarios using a live web service or database.

Therefore, rather than relying on a live web service or database, you can “mock” these dependencies. Mocks allow us to emulate a live web service or database and return specific results depending on the situation.

Generally speaking, you can mock dependencies by creating an alternative implementation of a class. You can write these alternative implementations by hand or make use of the Mockito package as a shortcut.

This recipe demonstrates the basics of mocking using the Mockito package. For more information, please see the Mockito package documentation.

Directions

1. Add the mockito & test dependencies
2. Create a function to test
3. Create a test file with a mock http.Client
4. Write a test for each condition
5. Run the tests

1. Add the mockito dependency

In order to use the mockito package, you first need to add it to the pubspec.yaml file along with the flutter_test dependency in the dev_dependencies section.

You’ll also be using the http package in this example, and will define that dependency in the dependencies section.

dependencies:
  http: <newest_version>
dev_dependencies:
  test: <newest_version>
  mockito: <newest_version>

2. Create a function to test

In this example, you’ll want to unit test the fetchPost function from the Fetch data from the internet recipe. In order to test this function, you need to make two changes:

1. Provide an http.Client to the function. This allows you to provide the correct http.Client depending on the situation. For Flutter and server-side projects, you can provide an http.IOClient. For Browser apps, you can provide an http.BrowserClient. For tests, you provide a mock http.Client.
2. Use the provided client to fetch data from the internet, rather than the static http.get method, which is difficult to mock.

The function should now look like this:

Future<Post> fetchPost(http.Client client) async {
  final response =
      await client.get('https://jsonplaceholder.typicode.com/posts/1');

  if (response.statusCode == 200) {
    // If the call to the server was successful, parse the JSON
    return Post.fromJson(json.decode(response.body));
  } else {
    // If that call was not successful, throw an error.
    throw Exception('Failed to load post');
  }
}

3. Create a test file with a mock http.Client

Next, create a test file along with a MockClient class. Following the advice in the Introduction to unit testing recipe, create a file called fetch_post_test.dart in the root test folder.

The MockClient class implements the http.Client class. This allows you to pass the MockClient to the fetchPost function, and allow you to return different http responses in each test.

// Create a MockClient using the Mock class provided by the Mockito package.
// Create new instances of this class in each test.
class MockClient extends Mock implements http.Client {}

main() {
  // Tests go here
}

4. Write a test for each condition

If you think about the fetchPost function, it will do one of two things:

1. Return a Post if the http call succeeds
2. Throw an Exception if the http call fails

Therefore, you’ll want to test these two conditions. You can use the MockClient class to return an “Ok” response for the success test, and an error response for the unsuccessful test.

To achieve this, use the when function provided by Mockito.

// Create a MockClient using the Mock class provided by the Mockito package.
// Create new instances of this class in each test.
class MockClient extends Mock implements http.Client {}

main() {
  group('fetchPost', () {
    test('returns a Post if the http call completes successfully', () async {
      final client = MockClient();

      // Use Mockito to return a successful response when it calls the
      // provided http.Client.
      when(client.get('https://jsonplaceholder.typicode.com/posts/1'))
          .thenAnswer((_) async => http.Response('{"title": "Test"}', 200));

      expect(await fetchPost(client), isInstanceOf<Post>());
    });

    test('throws an exception if the http call completes with an error', () {
      final client = MockClient();

      // Use Mockito to return an unsuccessful response when it calls the
      // provided http.Client.
      when(client.get('https://jsonplaceholder.typicode.com/posts/1'))
          .thenAnswer((_) async => http.Response('Not Found', 404));

      expect(fetchPost(client), throwsException);
    });
  });
}

5. Run the tests

Now that you have a fetchPost function with tests in place, run the tests.

$ dart test/fetch_post_test.dart

You can also run tests inside your favorite editor by following the instructions in the Introduction to unit testing recipe.

Summary

In this example, you’ve learned how to use Mockito to test functions or classes that depend on web services or databases. This is only a short introduction to the Mockito library and the concept of mocking. For more information, see the documentation provided by the Mockito package.