How to mock the FirebaseApp in flutter

I had the same problem. Using this answer I found the solution.

1. Copy the contents of: https://github.com/FirebaseExtended/flutterfire/blob/master/packages/firebase_auth/firebase_auth/test/mock.dart into a file, that you can import into your tests, where you need to initialize a Firebase app.

2. Call setupFirebaseAuthMocks(); at the top of your main function for all your tests.

3. In your setUpAll function, call await Firebase.initializeApp(); (You can also put this in your main function under setupFirebaseAuthMocks(); will still work).

Now you should have a mocked Firebase app.

Here is a full example:

import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:flutter_test/flutter_test.dart';
import '../lib/authentication/your_auth_using_firebase.dart';
import './mock.dart'; // from: https://github.com/FirebaseExtended/flutterfire/blob/master/packages/firebase_auth/firebase_auth/test/mock.dart



void main() {
  // TestWidgetsFlutterBinding.ensureInitialized(); Gets called in setupFirebaseAuthMocks()
  setupFirebaseAuthMocks();

  setUpAll(() async {
    await Firebase.initializeApp();
  });

  testWidgets('Your Test', (WidgetTester tester) async {
    final YourFirebaseAuthClass authService = YourFirebaseAuthClass();
    // Tests to write
  });
}

TO mock Firebase app you can use the following method from firerbase_core_plarform_interface plugin.

import 'package:firebase_core_platform_interface/firebase_core_platform_interface.dart';
import 'package:flutter/services.dart';
import 'package:flutter_test/flutter_test.dart';

typedef Callback = void Function(MethodCall call);

void setupFirebaseAuthMocks([Callback? customHandlers]) {
  TestWidgetsFlutterBinding.ensureInitialized();

  setupFirebaseCoreMocks();
}

Future<T> neverEndingFuture<T>() async {
  while (true) {
    await Future.delayed(const Duration(minutes: 5));
  }
}

Maybe the fake_cloud_firestore package is useful for you:

Fakes to write unit tests for Cloud Firestore. Instantiate a MockFirestoreInstance, then pass it around your project as if it were a FirestoreInstance. This fake acts like Firestore except it will only keep the state in memory. To help debug, you can use MockFirestoreInstance.dump() to see what's in the fake database. This is useful to set up the state of your database, then check that your UI behaves the way you expect.

Example from the docs:

import 'package:fake_cloud_firestore/fake_cloud_firestore.dart';

void main() {
  final instance = FakeFirebaseFirestore();
  await instance.collection('users').add({
    'username': 'Bob',
  });
  final snapshot = await instance.collection('users').get();
  print(snapshot.docs.length); // 1
  print(snapshot.docs.first.get('username')); // 'Bob'
  print(instance.dump());
}

In a UI test you would (example from the docs):

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:fake_cloud_firestore/fake_cloud_firestore.dart';
import 'package:firestore_example/main.dart';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

const MessagesCollection = 'messages';
    
        

void main() {
  testWidgets('shows messages', (WidgetTester tester) async {
    // Populate the fake database.
    final firestore = FakeFirebaseFirestore();
    await firestore.collection(MessagesCollection).add({
      'message': 'Hello world!',
      'created_at': FieldValue.serverTimestamp(),
    });

    // Render the widget.
    await tester.pumpWidget(MaterialApp(
        title: 'Firestore Example', home: MyHomePage(firestore: firestore)));
    // Let the snapshots stream fire a snapshot.
    await tester.idle();
    // Re-render.
    await tester.pump();
    // // Verify the output.
    expect(find.text('Hello world!'), findsOneWidget);
    expect(find.text('Message 1 of 1'), findsOneWidget);
  });
}

EDIT: For those combing from cloud_firestore_mocks the author renamed this package to fake_cloud_firestore.

DI: You might also want to have a look at get_it, which allows you to combine things with dependency injection. This is helpful for component tests in which you want to instantiate a service that depends on the mock Firestore. Make everything you need to mock out a constructor parameter and inject it using get_it. In the tests you can then inject the fake_cloud_firestore mocks if you want to instantiate such a service.

think that is for use FutureBuilder as first Widget o root widget of the tree, flutter needs always a MaterialApp or others(dont remmember right now).

Recomendation, refactor FutureBuilder -> Wrap whit widget -> MaterialApp(home:FutureBuilder(...

and check MyWidget() first widget, maybe same MaterialApp get error, but not sure.

if you are using routes method to navigate use initialRoute.

Luck!