Flutter Unit Test for a Firebase Function - flutter

Question

I'm brand new to flutter and I want to try to make a Unit Test for one functions that I've created. The function simply test if the email & password that the user provides to login are correct or not. So, the function provides a connection to the database and verify if the email & password are valid.
I tried to make a Unit Test with mockito to emulate the Firebase but it don't work very well. My function return an "UserCredential" type and I don't know how to verify it with Mocks. I tried to create a Mock for this type, but it says that "type 'Null' is not a subtype of type 'Future'"...
Anyone can make a clear explanation ? :)
Firebase Function to test
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flexmes/data/models/user_model.dart';
class UserAPI{
final CustomUser _customUser = CustomUser();
final FirebaseAuth auth;
UserAPI({required this.auth});
Future<UserCredential?> signInWithEmailAndPassword(String email, String password) async {
try{
UserCredential result = await auth.signInWithEmailAndPassword(email: email, password: password);
print (result);
return result;
} catch (error){
print (error);
return null;
}
}
}
Unit Test for the signInWithEmailAndPassword function
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flexmes/data/data_providers/user_provider.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';
class MockFirebaseAuth extends Mock implements FirebaseAuth{}
class MockUserCredential extends Mock implements UserCredential{}
void main(){
final MockFirebaseAuth mockAuth = MockFirebaseAuth();
final MockUserCredential userCredential = MockUserCredential();
group('description', () {
late UserAPI userAPI;
setUp(() {
userAPI = UserAPI(auth: mockAuth);
});
tearDown(() {
});
test('Sign in with Email & Password', () async {
when(mockAuth.signInWithEmailAndPassword(email: "admin#admin.com", password: "password")).
thenAnswer((_) => Future<MockUserCredential>.value(userCredential));
expect(await userAPI.signInWithEmailAndPassword("admin#admin.com", "password"), userCredential);
});
});
}

Answer 1

There is mock authentication package available firebase_auth_mocks and google_sign_in_mocks
You can replace your code like this
group('description', () {
final _mockAuth = MockFirebaseAuth(mockUser: _mockUser);
late UserAPI userAPI;
setUp(() {
userAPI = UserAPI(auth: _mockAuth);
});
tearDown(() {});
const _email = 'ilyas#yopmail.com';
const _uid = 'sampleUid';
const _displayName = 'ilyas';
const _password = 'Test#123';
final _mockUser = MockUser(
uid: _uid,
email: _email,
displayName: _displayName,
);
test('signIn function test', () async {
final user = await userAPI.signInWithEmailAndPassword(_email, _password);
expect(user?.uid, _mockUser.uid);
});
});
google_sign_in_mocks this you can use for GoogleSignIn test