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:
- Add the package dependencies.
- Create a function to test.
- Create a test file with a mock
http.Client
. - Write a test for each condition.
- 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:
- Provide an
http.Client
to the function. This allows providing the correcthttp.Client
depending on the situation. For Flutter and server-side projects, provide anhttp.IOClient
. For Browser apps, provide anhttp.BrowserClient
. For tests, provide a mockhttp.Client
. - Use the provided
client
to fetch data from the internet, rather than the statichttp.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.parse('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
.
http.Client
3. Create a test file with a mock 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:http/http.dart' as http;
import 'package:mocking/main.dart';
import 'package:mockito/annotations.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:
- Returns an
Album
if the http call succeeds - 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:mocking/main.dart';
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.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.parse('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.parse('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.parse('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;
const 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(const MyApp());
class MyApp extends StatefulWidget {
const MyApp({super.key});
@override
State<MyApp> 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: const 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 const CircularProgressIndicator();
},
),
),
),
);
}
}
test/fetch_album_test.dart
import 'package:flutter_test/flutter_test.dart';
import 'package:http/http.dart' as http;
import 'package:mocking/main.dart';
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.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.parse('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.parse('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.