mock-dependencies-mockito.md

Intro

Mocks allow emulating a live web service or database and return specific results depending on the situation. To achieve this, you can:

• write these alternative implementations by hand.
• make use of the Mockito package as a shortcut.

Setup

To add the needed dependencies for mocking, run flutter pub add:

$ flutter pub add http dev:mockito dev:build_runner

Create a function to test:

import 'dart:async';
import 'dart:convert';

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

// The API function to tests
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());

// Main widget UI
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();
            },
          ),
        ),
      ),
    );
  }
}

Create a test file and add the annotation @GenerateMocks([http.Client]) to the main function to generate a MockClient class with mockito:

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() {
}

Generate the mocks running the following command:

$ flutter pub run build_runner build

→ The generated mocks will be in {file-name}_test.mocks.dart. Import this file to use them.

Write tests with mocks:

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);
    });
  });
}

Run tests:

$ flutter test test/fetch_album_test.dart