How to mock a static method in Flutter with Mockito?

Question

I have a file a function fetchPosts() which is in charge of getting new Posts from a server and store them in a local sqlite database.

As recommended on the sqflite doc, I store a single ref to my database.

Here is the content of my database.dart file:

import 'dart:async';
import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';

class DBProvider {
  DBProvider._();
  static final DBProvider db = DBProvider._();

  static Database _database;

  static Future<Database> get database async {
    if (_database != null) return _database;
    // if _database is null, we instantiate it
    _database = await _initDB();
    return _database;
  }

  static Future<Database> _initDB() async {
    final dbPath = await getDatabasesPath();
    String path = join(dbPath, 'demo.db');

    return await openDatabase(path, version: 1, onCreate: _onCreate);
  }

  static Future<String> insert(String table, Map<String, dynamic> values) async { /* insert the record*/ }

  // Other functions like update, delete etc.
}

Then I use it as such in my fetchPosts.dart file

import 'dart:convert';
import 'package:http/http.dart' as http;
import '../services/database.dart';

const url = 'https://myapp.herokuapp.com';

Future<void> fetchPosts() {
  final client = http.Client();
  return fetchPostsUsingClient(client);
}

Future<void> fetchPostsUsingClient(http.Client client) async {
  final res = await client.get(url);
  final posts await Post.fromJson(json.decode(response.body));

  for (var i = 0; i < posts.length; i++) {
    await DBProvider.insert('posts', posts[i]);
  }
}

In my test, how can I verify that DBProvider.insert() has been called?

fetchPosts_test.dart

import 'package:test/test.dart';
import 'package:http/http.dart' as http;
import 'package:mockito/mockito.dart';
import 'package:../services/fetchPosts.dart';

// 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 {}

void main() {
  group('fetchPosts', () {
    test('update local db', () async {
      final client = MockClient();

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

      await fetchPostsWithClient(client);

      verify(/* DBProvider.insert has been called ?*/);
    });
  });
}

Answer 1

Eventually, I had to rewrite my database.dart to make it testable / mockable.
Here's the new file:

import 'dart:async';
import 'package:path/path.dart';
import 'package:sqflite/sqflite.dart';

class DBProvider {
  static final DBProvider _singleton = DBProvider._internal();

  factory DBProvider() {
    return _singleton;
  }

  DBProvider._internal();

  static Database _db;

  static Future<Database> _getDatabase() async {
    if (_db != null) return _db;
    // if _database is null, we instantiate it
    _db = await _initDB();
    return _db;
  }

  static Future<Database> _initDB() async {
    final dbPath = await getDatabasesPath();
    String path = join(dbPath, 'demo.db');

    return openDatabase(path, version: 1, onCreate: _onCreate);
  }

  Future<String> insert(String table, Map<String, dynamic> values) async {
    final db = await _getDatabase();
    return db.insert(table, values);
  }

  // ...
}

Now I can use the same trick as with the http.Client. Thank you @RémiRousselet

Answer 2

Let's say we want to test [TargetClass.someMethodCallOtherStaticMethod]

Class StaticMethodClass {
  static int someStaticMethod() {};
}

Class TargetClass {
  int someMethodCallOtherStaticMethod() {
    return StaticMethodClass.someStaticMethod();
  }
}

We should refactor [[TargetClass.someMethodCallOtherStaticMethod]] for testing, like this:

Class TargetClass {
  int someMethodCallOtherStaticMethod({@visibleForTesting dynamic staticMethodClassForTesting}) {
    if (staticMethodClassForTesting != null) {
      return staticMethodClassForTesting.someStaticMethod();
    } else {
      return StaticMethodClass.someStaticMethod();
    }        
  }
}

Now you can write your test case like this:

// MockClass need to implement nothing, just extends Mock
MockClass extends Mock {}
test('someMethodCallOtherStaticMethod', () {
  // We MUST define `mocked` as a dynamic type, so that no errors will be reported during compilation
  dynamic mocked = MockClass();
  TargetClass target = TargetClass();
  when(mocked.someStaticMethod()).thenAnswer((realInvocation) => 42);
  expect(target.someMethodCallOtherStaticMethod(staticMethodClassForTesting: mocked), 42); 
})