Mock dependencies using Mockito

Sometimes, unit tests might 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 might 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 and failure scenarios by 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 emulating 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. Write these alternative implementations by hand or make use of the Mockito package as a shortcut.

This recipe demonstrates the basics of mocking with the Mockito package using the following steps:

  1. Add the package 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.

For more information, see the Mockito package documentation.

1. Add the package dependencies

To use the mockito package, add it to the pubspec.yaml file along with the flutter_test dependency in the dev_dependencies section.

This example also uses the http package, so define that dependency in the dependencies section.

mockito: 5.0.0 supports Dart’s null safety thanks to code generation. To run the required code generation, add the build_runner dependency in the dev_dependencies section.

dependencies:
  http: <newest_version>
dev_dependencies:
  flutter_test:
    sdk: flutter
  mockito: <newest_version>
  build_runner: <newest_version>

2. Create a function to test

In this example, unit test the fetchAlbum function from the Fetch data from the internet recipe. To test this function, make two changes:

  1. Provide an http.Client to the function. This allows providing the correct http.Client depending on the situation. For Flutter and server-side projects, provide an http.IOClient. For Browser apps, provide an http.BrowserClient. For tests, 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<Album> fetchAlbum(http.Client client) async {
  final response =
      await client.get(Uri.https('jsonplaceholder.typicode.com', 'albums/1'));

  if (response.statusCode == 200) {
    // If the server did return a 200 OK response,
    // then parse the JSON.
    return Album.fromJson(jsonDecode(response.body));
  } else {
    // If the server did not return a 200 OK response,
    // then throw an exception.
    throw Exception('Failed to load album');
  }
}

In your app code, you can provide an http.Client to the fetchAlbum method directly with fetchAlbum(http.Client()). http.Client() creates a default http.Client.

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

Next, create a test file.

Following the advice in the Introduction to unit testing recipe, create a file called fetch_album_test.dart in the root test folder.

Add the annotation @GenerateMocks([http.Client]) to the main function to generate a MockClient class with mockito.

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

The generated mocks will be located in fetch_album_test.mocks.dart. Import this file to use them.

import 'package:flutter_test/flutter_test.dart';
import 'package:http/http.dart' as http;
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';

import '../lib/main.dart';
import 'fetch_album_test.mocks.dart';

// Generate a MockClient using the Mockito package.
// Create new instances of this class in each test.
@GenerateMocks([http.Client])
void main() {

Next, generate the mocks running the following command:

$ flutter pub run build_runner build

4. Write a test for each condition

The fetchAlbum() function does one of two things:

  1. Returns an Album if the http call succeeds
  2. Throws an Exception if the http call fails

Therefore, you want to test these two conditions. Use the MockClient class to return an “Ok” response for the success test, and an error response for the unsuccessful test. Test these conditions using the when() function provided by Mockito:

import 'package:flutter_test/flutter_test.dart';
import 'package:http/http.dart' as http;
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';

import '../lib/main.dart';
import 'fetch_album_test.mocks.dart';

// Generate a MockClient using the Mockito package.
// Create new instances of this class in each test.
@GenerateMocks([http.Client])
void main() {
  group('fetchAlbum', () {
    test('returns an Album 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(Uri.https('jsonplaceholder.typicode.com', 'albums/1')))
          .thenAnswer((_) async => http.Response('{"userId": 1, "id": 2, "title": "mock"}', 200));

      expect(await fetchAlbum(client), isA<Album>());
    });

    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(Uri.https('jsonplaceholder.typicode.com', 'albums/1')))
          .thenAnswer((_) async => http.Response('Not Found', 404));

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

5. Run the tests

Now that you have a fetchAlbum() function with tests in place, run the tests.

$ flutter test test/fetch_album_test.dart

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

Complete example

lib/main.dart
import 'dart:async';
import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

Future<Album> fetchAlbum(http.Client client) async {
  final response =
      await client.get(Uri.https('jsonplaceholder.typicode.com', 'albums/1'));

  if (response.statusCode == 200) {
    // If the server did return a 200 OK response,
    // then parse the JSON.
    return Album.fromJson(jsonDecode(response.body));
  } else {
    // If the server did not return a 200 OK response,
    // then throw an exception.
    throw Exception('Failed to load album');
  }
}

class Album {
  final int userId;
  final int id;
  final String title;

  Album({required this.userId, required this.id, required this.title});

  factory Album.fromJson(Map<String, dynamic> json) {
    return Album(
      userId: json['userId'],
      id: json['id'],
      title: json['title'],
    );
  }
}

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  MyApp({Key? key}) : super(key: key);

  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  late final Future<Album> futureAlbum;

  @override
  void initState() {
    super.initState();
    futureAlbum = fetchAlbum(http.Client());
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Fetch Data Example',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text('Fetch Data Example'),
        ),
        body: Center(
          child: FutureBuilder<Album>(
            future: futureAlbum,
            builder: (context, snapshot) {
              if (snapshot.hasData) {
                return Text(snapshot.data!.title);
              } else if (snapshot.hasError) {
                return Text("${snapshot.error}");
              }

              // By default, show a loading spinner.
              return CircularProgressIndicator();
            },
          ),
        ),
      ),
    );
  }
}
test/fetch_album_test.dart
import 'package:flutter_test/flutter_test.dart';
import 'package:http/http.dart' as http;
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';

import '../lib/main.dart';
import 'fetch_album_test.mocks.dart';

// Generate a MockClient using the Mockito package.
// Create new instances of this class in each test.
@GenerateMocks([http.Client])
void main() {
  group('fetchAlbum', () {
    test('returns an Album 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(Uri.https('jsonplaceholder.typicode.com', 'albums/1')))
          .thenAnswer((_) async => http.Response('{"userId": 1, "id": 2, "title": "mock"}', 200));

      expect(await fetchAlbum(client), isA<Album>());
    });

    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(Uri.https('jsonplaceholder.typicode.com', 'albums/1')))
          .thenAnswer((_) async => http.Response('Not Found', 404));

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

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.