How to test a Flutter application using Firebase?

Question

I am using Flutter and Firebase as my database. I want to do some unit test of my application, but when I start this test :

testWidgets('MyWidget has a title and message', (WidgetTester tester) async {
    // Create the widget by telling the tester to build it.
    await tester.pumpWidget(LoginPage());

    expect(true, true);
});

I have the Exception :

No Firebase App '[DEFAULT]' has been created - call Firebase.initializeApp()

But Firebase.initializeApp() is called in my main.dart here :

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

And I don't know how to initialize the Firebase app in my test.

Solution

Posting this as a community wiki as it's based on @GuilhermeGabanelli's comment.

If you check this flutterfire documentation on how to perform unit tests with Firebase Services you will see that:

The Firebase libraries need to run on an actual device or emulator. So if you want to run unit tests, you'll have to use Fakes instead. A Fake is a library that implements the API of a given Firebase library and simulates its behavior.

...

When initializing your app, instead of passing the actual instance of a Firebase library (e.g. FirebaseFirestore.instance if using Firestore), you pass an instance of a fake (e.g. FakeFirebaseFirestore()). Then the rest of your application will run as if it were talking to Firebase.

Followed by an example with sample code. I believe this is the cause of the issue you are facing and if you change your code to use Fakes this should be fixed.