Flutter Unit Class with Initializer - flutter

Question

I'm trying to write the unit test for this class, but I'm having problems about the initializer... When I instantiate the class in the test file, the caller throws an error that " Null check operator used on a null value". I know that's because the UserProvider is not initialized on the test folders. But how can I mock this??
class ContactController extends ChangeNotifier {
BuildContext context;
ContactController(this.context) {
initializeData();
}
late Contact contact;
initializeData() {
var userProvider = context.read<UserProvider>();
var currentContact = userProvider?.contact;
if (currentContact != null) {
newContact = currentContact;
}
notifyListeners();
}
}

Answer 1

The code below should do the trick. The thing is that you need to provide a mocked UserProvider into the context. To do so, just use MultiProvider in the tests to inject the mocked one.
Note the use of #GenerateMocks([UserProvider]). This comes from mockito and it annotates the source to generate the MockUserProvider class. And to generate the *.mocks.dart just run flutter pub run build_runner build.
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';
import 'package:provider/provider.dart';
import 'package:so72756235_test_provider/contact_controller.dart';
import 'contact_controller_test.mocks.dart';
#GenerateMocks([UserProvider])
void main() {
testWidgets('ContactController test', (WidgetTester tester) async {
final mockUserProvider = MockUserProvider();
when(mockUserProvider.contact).thenReturn(const Contact(name: 'Test'));
await tester.pumpWidget(
MultiProvider(
providers: [
Provider<UserProvider>(create: (_) => mockUserProvider),
ChangeNotifierProvider(
create: (context) => ContactController(context)),
],
child: Consumer<ContactController>(
builder: (context, value, child) =>
Text(value.newContact.name, textDirection: TextDirection.ltr)),
),
);
expect(find.text('Test'), findsOneWidget);
});
}