Intro

Today, I would like to share how to test Firestore in a Flutter environment. If you tried already, then you probably know that the Firestore connection does not work in test mode. So, you need to mock the Firestore instance. When it comes to Flutter, you can use fake_cloud_firestore which is mentioned in FlutterFire.

In this post, I will explain how to write repository unit tests that interact with Firestore.

If you just want to learn how to test Firestore, you wouldn’t need an actual Firebase Project.

Setup

First, create a test project.

flutter create test_fake_cloud_firestore

Second, you need to install the packages below.

flutter pub add cloud_firestore
flutter pub add fake_cloud_firestore

• cloud_firestore → need for using type
• fake_cloud_firestore → need for mocking Firestore

Write TestRepository using Firestore

Now, under the lib directory, create test_repository.dart

cd test_fake_cloud_firestore
touch ./lib/test_repository.dart

Then, fill out the above file with TestRepository class that has a method for interacting with Firestore. Since this is only for testing, you don’t need to have an actual Firebase project. The path of collection or doc will be used in the test file, so make up any path you want and use that path later on.

Below is the class that I’ve tested with.

import 'package:cloud_firestore/cloud_firestore.dart';

class TestRepository {
  final FirebaseFirestore firestore;
  TestRepository({required this.firestore});

  late final countDoc = firestore.doc('test/doc_12345');

  Future<int?> fetch() async {
    final snapshot = await countDoc.get();
    return snapshot.data()?['count'];
  }
}

Note that I handed over the Firestore instance through the constructor. By injecting dependency(in this case, Firestore instance), you will have a repository that is easy to test with the mock instance.

Create test_repository_test.dart

After that, you need to create a test file whose name ends with _test under the test directory.

touch ./test/test_repository_test.dart

In a test file, declare void main() function which will be run when you run flutter test command.

void main() {}

Write test content using FakeFirestore

Now, fill the main function with the test code.

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:fake_cloud_firestore/fake_cloud_firestore.dart';
import 'package:test_fake_cloud_firestore/test_repository.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  group('Test fake_cloud_firestore', () {
    // 1
    final firestore = FakeFirebaseFirestore();
    late TestRepository testRepository;
    setUp(
      () {
        // 2
        testRepository = TestRepository(firestore: firestore);
        // 3
        setDummyFirestore(firestore);
      },
    );

    test('Test fetch data', () async {
      // 4
      final result = await testRepository.fetch();
      expect(result, 2);
    });
  });
}

setDummyFirestore(FirebaseFirestore firestore) {
  firestore.collection('test').doc('doc_12345').set({
    'count': 4,
  });
}

• First, use flutter_test package to run the test easily. (group, setUp, test, expect)
• I will explain the steps by number.
  1. Initialize FakeFirebaseFirestore at first.
  2. In setUp, initialize testRepository with a fake Firestore instance.
  3. Then, set up data in fake Firestore for the test. In this case, need to set ‘count’ field in ‘test/doc_12345’ with a number.
  4. Run the test method and check if result variable has the right value.

Finally, run the test with the below command in a project root directory.

flutter test

This test should fail followed by logging different values (2 and 4).

This means that now you can test your Firebase Firestore connection logic safely with the mock instance.

Conclusion

Firebase and Flutter are often used together. I hope this article helped your testing practice with them!

Cheers!