/dart-add-unit-test

name: dart-add-unit-test
description: Write and organize unit tests for functions, methods, and classes using `package:test`. Use when creating new logic or fixing bugs to ensure code remains correct and regression-free.
metadata:
  model: models/gemini-3.1-pro-preview
  last_modified: Mon, 03 Aug 2026 21:51:24 GMT

Testing Dart and Flutter Applications

Contents

• [Structuring Test Files](#structuring-test-files)
• [Writing Tests](#writing-tests)
• [Executing Tests](#executing-tests)
• [Test Implementation Workflow](#test-implementation-workflow)
• [Examples](#examples)

Structuring Test Files

Organize test files to mirror the `lib` directory structure to maintain predictability.

• Place all test code within the `test` directory at the root of the package.
• Append `_test.dart` to the end of all test file names (e.g., `lib/src/utils.dart` should be tested in `test/src/utils_test.dart`).
• If writing integration tests, place them in an `integration_test` directory at the root of the package.

Writing Tests

Utilize `package:test` as the standard testing library for Dart applications.

• Import `package:test/test.dart` (or `package:flutter_test/flutter_test.dart` for Flutter).
• Group related tests using the `group()` function to provide shared context.
• Define individual test cases using the `test()` function.
• Validate outcomes using the `expect()` function alongside matchers (e.g., `equals()`, `isTrue`, `throwsA()`).
• Write asynchronous tests using standard `async`/`await` syntax. The test runner automatically waits for the `Future` to complete.
• Manage test setup and teardown using `setUp()` and `tearDown()` callbacks.
• If testing code that relies on dependency injection, use `package:mockito` alongside `package:test` to generate mock objects, configure fixed scenarios, and verify interactions.

Executing Tests

Select the appropriate test runner based on the project type and test location.

• If working on a pure Dart project, execute tests using the `dart test` command.
• If working on a Flutter project, execute tests using the `flutter test` command.
• If running integration tests, explicitly specify the directory path, as the default runner ignores it: `dart test integration_test` or `flutter test integration_test`.

Test Implementation Workflow

Follow this sequential workflow when implementing new test suites. Copy the checklist to track your progress.

Task Progress

• [ ] 1. Create the test file in the `test/` directory, ensuring the `_test.dart` suffix.
• [ ] 2. Import `package:test/test.dart` and the target library.
• [ ] 3. Define a `main()` function.
• [ ] 4. Initialize shared resources or mocks using `setUp()`.
• [ ] 5. Write `test()` cases grouped by functionality using `group()`.
• [ ] 6. Execute the test suite using the appropriate CLI command.
• [ ] 7. **Feedback Loop**: Run test -> Review stack trace for failures -> Fix implementation or assertions -> Re-run until passing.

Examples

Standard Unit Test Suite

Demonstrates grouping, setup, synchronous, and asynchronous testing.

import 'package:test/test.dart';
import 'package:my_package/calculator.dart';

void main() {
  group('Calculator', () {
    late Calculator calc;

    setUp(() {
      calc = Calculator();
    });

    test('adds two numbers correctly', () {
      expect(calc.add(2, 3), equals(5));
    });

    test('handles asynchronous operations', () async {
      final result = await calc.fetchRemoteValue();
      expect(result, isNotNull);
      expect(result, greaterThan(0));
    });
  });
}

Mocking with Mockito

Demonstrates configuring a mock object for dependency injection testing.

import 'package:test/test.dart';
import 'package:mockito/mockito.dart';
import 'package:mockito/annotations.dart';
import 'package:my_package/api_client.dart';
import 'package:my_package/data_service.dart';

// Generate the mock using build_runner: dart run build_runner build
@GenerateNiceMocks([MockSpec<ApiClient>()])
import 'data_service_test.mocks.dart';

void main() {
  group('DataService', () {
    late MockApiClient mockApiClient;
    late DataService dataService;

    setUp(() {
      mockApiClient = MockApiClient();
      dataService = DataService(apiClient: mockApiClient);
    });

    test('returns parsed data on successful API call', () async {
      // Configure the mock
      when(mockApiClient.get('/data')).thenAnswer((_) async => '{"id": 1}');

      // Execute the system under test
      final result = await dataService.fetchData();

      // Verify outcomes and interactions
      expect(result.id, equals(1));
      verify(mockApiClient.get('/data')).called(1);
    });
  });
}