# Flutter Testing Part 1: Unit Tests Any piece of software should function reliably. Reliability comes from having a deterministic outcome. One way to bring in reliability to software development is writing tests. Tests not only help you catch bugs early in development cycle, but makes sure that new code does not break old functionality. It forces you to write modular code. In this tutorial you will learn to write unit tests for a [flutter](http://flutter.dev) application. # Starter Project Clone the starter project from [here](https://github.com/shounakmulay/flutter-testing). The repo is based on the **[Wednesday Flutter Template](https://github.com/wednesday-solutions/flutter_template)** but is trimmed down for the purpose of this tutorial. Once you clone the project, open it in VS Code or Android studio and checkout the `unit-test-start` branch. You will see the following directory structure. ![Screenshot_2022-03-29_at_2.51.38_PM.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649227216745/kiWw71G6v.png) Build and run the application. You should see an app as shown below run on your simulator/emulator. ![Screenshot_1649153331.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649227243795/rM3ZLRiAw.png) Play with the application to understand what it does.. Search for any city in the search box and mark a few as favourites. # What are unit tests? A unit test is a test that verifies the behaviour of a small and isolated piece of code. **Small** and **Isolated** are 2 important parameters to consider while writing a unit test. A unit test will usually target a single function. Any inputs to the function will be provided by you while writing the test. Any dependencies, such as other functions that are called internally, should be mocked. Unit tests are quick to run and don’t require a lot of setup. > In a production application, the quantity of unit tests will be much higher than any other types of tests. # Writing testable code Before you proceed with writing any test cases, you need to make sure that the code you are writing is testable in the first place. Consider the following example. We have a **Calculator** class with a function called **add**. The calculator class internally creates an object of the **CalculatorAPIService** to perform the calculations. ![no-di.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649227655991/Kfdi5M-YL.png) This is bad for testing the `add` function because we do not have control over the `CalculatorAPIService`. - Any failure in the **CalculatorAPIService** can fail our test. - The **CalculatorAPIService** may have more dependencies of its own which will need to be created as well. - Testing failure conditions may depend on **CalculatorAPIService.** Testing for error states will become difficult. The way to solve this problem is using the principle of **Dependency Injection** or **Inversion of Control**. Instead of letting the **Calculator** class create its own dependencies, we provide it to the class via its constructor. ![calc-di.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649227681837/mxc1PCOP_.png) > Going over the concepts of Dependency Injection is out of scope of this tutorial. In this repo we are using get_it as the Dependency Injection solution and you can read more about it [here](https://pub.dev/packages/get_it). # Why mock? Mocking means replacing the dependencies of the piece of code under test, with a fake or duplicate version that we have control over. Mocking the dependencies of the subject under test will let you achieve predictable behaviour on every run of a test. With a mock, you can have a function return a particular value or throw an error whenever you want. Mocks also allow us to verify if a particular function was called a certain number of times. You will see all of this in practice when you write actual tests in the following sections. With the above setup in place, you can now pass the mock version of the dependencies to the class under tests. You will use **mocktail** to mock the required dependencies. You can read more about **mocktail** [here](https://pub.dev/packages/mocktail). # Anatomy of a unit test Every unit test file has some common structure. It can be broadly divided into 3 stages, the **pre test stage**, the **testing stage**, and the **post test stage**. - Pre test stage is for test setup, like initialising mocks, initialising the class under test, etc. - Then in the testing stage all the test cases are executed. - Post test stage is for cleanup, like resetting all mock values. ![test--anatomy.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649227782060/yQkrX-s6U.png) # Creating a test file In flutter, all test files must end with `_test.dart` and must be put under the `test` directory. You will see an empty `test` directory in the starter project. This is where we will write all the testing code. You will write tests for `weather_repository_impl.dart`. It is located at `repository/weather`. ![Screenshot_2022-03-29_at_4.42.07_PM.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649228238663/r0fxzefo1.png) It is a good idea to mimic the directory structure of **lib** in **test** as it makes it easier to locate relevant test files. Since `weather_repository_impl.dart` in located in `lib/repository/weather`, create a new file `weather_repository_impl_test.dart` in `test/repository/weather`. ![Screenshot_2022-03-29_at_4.50.02_PM.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649227982722/OUz771x9Z.png) Add a **main** method. All test code should be inside this main method. ```dart void main() { // Test code here! } ``` # Initial test setup To test a function in the `weather_repository_impl.dart` we first need to create an instance of the repository. The `setUp` function is the perfect place to do it. Add the `setUp` function to the test file you created and create an instance of **WeatherRepositoryImpl** here. ```dart import 'package:flutter_test/flutter_test.dart'; import 'package:flutter_testing/repository/weather/weather_repository_impl.dart'; void main() { late WeatherRepositoryImpl weatherRepository; setUp(() { weatherRepository = WeatherRepositoryImpl(); }); } ``` You will notice that the `WeatherRepositoryImpl` requires some other classes to be passed in its constructor. Since you are testing only the `WeatherRepositoryImpl` class here, you do not want any other classes to influence the result of a test case. This is where mocking plays an important role. We will provide all the dependencies as mocks so that we can control their behaviour as required. You will use `mocktail` as the mocking library. Open `pubspec.yaml` and add the `mocktail` dependency to the `dev_dependencies` section. Don’t forget to run `flutter pub get` after adding the dependency. ```yaml dev_dependencies: mocktail: ^0.3.0 # other dependencies ``` - Mocks can be reused in multiple test files, so it’s better to extract them to a separate file. Create a new director under `test` called `mocks` and add a file called `mocks.dart` to that directory. ![Screenshot_2022-03-30_at_11.16.24_AM.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649228038875/YR9CfjFAO.png) - The `WeatherRepositoryImpl` a couple of services, a few mappers and a repository as its dependencies. Services are classes that give access to data sources. Mappers are classes that convert service data classes to domain data classes. Create mocks for all the dependencies. To create a mock just extends the `Mock` class. Here we will also implement the interface that is used by the original class so that it is identified as a valid object. ```dart import 'package:flutter_testing/repository/date/date_repository.dart'; import 'package:flutter_testing/repository/weather/domain_city_mapper.dart'; import 'package:flutter_testing/repository/weather/local_city_mapper.dart'; import 'package:flutter_testing/repository/weather/local_day_weather_mapper.dart'; import 'package:flutter_testing/repository/weather/local_weather_mapper.dart'; import 'package:flutter_testing/services/weather/local/weather_local_service.dart'; import 'package:flutter_testing/services/weather/remote/weather_remote_service.dart'; import 'package:mocktail/mocktail.dart'; // Service class MockWeatherLocalService extends Mock implements WeatherLocalService {} class MockWeatherRemoteService extends Mock implements WeatherRemoteService {} // Mappers class MockDomainCityMapper extends Mock implements DomainCityMapper {} class MockLocalCityMapper extends Mock implements LocalCityMapper {} class MockLocalWeatherMapper extends Mock implements LocalWeatherMapper {} class MockLocalDayWeatherMapper extends Mock implements LocalDayWeatherMapper {} // Repositories class MockDateRepository extends Mock implements DateRepository {} ``` - With the mocks declared, you can now go back to `weather_repository_impl_test.dart` and use the mocks as dependencies to the `WeatherRepositoryImpl` class. ```dart void main() { late WeatherLocalService weatherLocalService; late WeatherRemoteService weatherRemoteService; late DomainCityMapper domainCityMapper; late LocalCityMapper localCityMapper; late LocalWeatherMapper localWeatherMapper; late LocalDayWeatherMapper localDayWeatherMapper; late DateRepository dateRepository; late WeatherRepository weatherRepository; setUp(() { weatherLocalService = MockWeatherLocalService(); weatherRemoteService = MockWeatherRemoteService(); domainCityMapper = MockDomainCityMapper(); localCityMapper = MockLocalCityMapper(); localWeatherMapper = MockLocalWeatherMapper(); localDayWeatherMapper = MockLocalDayWeatherMapper(); dateRepository = MockDateRepository(); weatherRepository = WeatherRepositoryImpl( weatherLocalService: weatherLocalService, weatherRemoteService: weatherRemoteService, domainCityMapper: domainCityMapper, localCityMapper: localCityMapper, localWeatherMapper: localWeatherMapper, localDayWeatherMapper: localDayWeatherMapper, dateRepository: dateRepository, ); }); } ``` - Also add the `tearDown` function to reset any state that might have been persisted in the mocks. ```dart void main() { setUp(() { // setUp code }); tearDown(() { resetMocktailState(); }); } ``` That was a lot of setup steps. Fortunately you won’t have to do this every time as some of this setup (like the mocks) can be shared between multiple test files. Now you are ready to write your first test! # Writing a unit test - The first step to writing a unit test is to pick the unit of code that you want to test. For your first test, let’s pick the `setCityAsFavorite` function from the very bottom of `WeatherRepositoryImpl`. ```dart @override Future setCityAsFavorite(City city) async { await weatherLocalService.markCityAsFavorite( city: localCityMapper.map(city), ); } ``` - Once you choose the unit of code, identity the steps involved in it. Doing so helps in determining what parts to mock and what parts to verify during a test. - `setCityAsFavorite` accepts a `City` as input. - It then converts the `City` to `LocalCity` by calling `localCityMapper.map` function. - Then it calls `markCityAsFavorite` on the `weatherLocalService` with the result of the map function. - You will use the `test` function which will run every time we call `flutter test`. It accepts 2 parameters, the name of the test and the actual test code itself. You will use the `Given, When, Then` pattern to describe a test. It breaks down a test into 3 parts - **`Given`** some context - **`When`** some action is carried out - **`Then`** a particular set of action should occur. - Write a new test by calling the `test` function. You will name it in the `Given, When, Then` pattern. ```dart test("Given a valid City object, When setCityAsFavorite is called and no error occurs, Then markCityAsFavorite is called with LocalCity object", () { // Given // When // Then }); ``` - Since the `weatherLocalService` is a mock object, calling any functions in it will not return any values. You need to tell the mocking library what value should be returned when a particular function is called with a certain set of arguments. For doing this you will also create some data objects with dummy data. For the current test function we need the the `City` and the `LocalCityCompanion` data objects. ```dart test("Given a valid City object, When setCityAsFavorite is called and no error occurs, Then markCityAsFavorite is called with LocalCity object", () { // Given final testCity = City( id: 1, title: "title", locationType: "locationType", location: "location", ); const testLocalCity = LocalCityCompanion( woeid: Value(1), title: Value("title"), locationType: Value("locationType"), location: Value("location"), ); // When // Then }); ``` - To tell the mocking library that when `localCityMapper.map` is called with `testCity` , it should return the `testLocalCity`, we will use the `when` function from `mocktail` library. You can use `thenReturn` to return normal values. To return future or stream values, you need to use the `thenAnswer` function. Since `markCityAsFavorite` returns a `Future` we just use an empty async function in `thenAnswer`. You can read more about `mocktail` syntax [here](https://pub.dev/packages/mocktail). ```dart test( "Given a valid City object, When setCityAsFavorite is called and no error occurs, Then markCityAsFavorite is called with LocalCity object", () { // Given final testCity = City(...); const testLocalCity = LocalCityCompanion(...); when(() => localCityMapper.map(testCity)).thenReturn(testLocalCity); when(() => weatherLocalService.markCityAsFavorite(city: testLocalCity)).thenAnswer((_) async {}); // When // Then }); ``` - With the mock setup done, you need to call the actual function under test. Call the `setCityAsFavorite` function, with `testCity` as the argument. Also as `setCityAsFavorite` is async, mark the test function as async as well. ```dart test( "Given a valid City object, When setCityAsFavorite is called and no error occurs, Then markCityAsFavorite is called with LocalCity object", () async { // Given ... ... // When await weatherRepository.setCityAsFavorite(testCity); // Then }); ``` - Final step in writing a test is to verify that the expected function calls were made and the expected data was returned. Since the function you are testing here does not return anything, we will look at how to check for that in a later test case. For now, let’s verify that both the `markCityAsFavorite` and the `map` function was called once with the `verify` function. ```dart test( "Given a valid City object, When setCityAsFavorite is called and no error occurs, Then markCityAsFavorite is called with LocalCity object", () async { // Given ... ... // When ... ... // Then verify(() => localCityMapper.map(testCity)).called(1); verify(() => weatherLocalService.markCityAsFavorite(city: testLocalCity)).called(1); verifyNoMoreInteractions(localCityMapper); verifyNoMoreInteractions(weatherLocalService); verifyZeroInteractions(weatherRemoteService); }); ``` - The `verifyNoMoreInteractions` function checks that no more function calls happen on the given mock and the `verifyZeroInteractions` checks the no function was ever called on the given mock for the entire duration of the test. - You can now run the test by pressing the green button next to the test function or by running `flutter test`. ![Screenshot_2022-03-30_at_12.40.14_PM.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649228093424/ep4S-v0PV.png) - The test should pass. You should see a similar output in the console. ![Screenshot_2022-03-30_at_12.42.05_PM.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649228112904/G8lIoe5To.png) # Expecting results in unit test - Create a new test for the `getFavoriteCitiesList` function. The structure will be the same as before: creating test data, mocking function return values, calling the function under test, verifying expected function calls. ```dart test( "Given local service returns list of LocalCityData, When getFavoriteCitiesList is called, Then Future> is returned", () async { // Given final localCityData = [ LocalCityData( woeid: 1, title: "title", locationType: "locationType", location: "location", ) ]; final cityData = [ City( id: 1, title: "title", locationType: "locationType", location: "location", ) ]; when(() => weatherLocalService.getFavouriteCities()) .thenAnswer((_) => Future.value(localCityData)); when(() => domainCityMapper.mapList(localCityData)).thenReturn(cityData); // When final result = await weatherRepository.getFavoriteCitiesList(); // Then verify(() => weatherLocalService.getFavouriteCities()).called(1); verify(() => domainCityMapper.mapList(localCityData)).called(1); }); ``` - The current function under test also returns a result. For such functions it is important to check that the return value is as expected. To do that we will use the `expect` function. The `expect` function can check a range of values, you can read more about it [here](https://api.flutter.dev/flutter/flutter_test/expect.html). - Add the following expect functions to check if the returned results is valid. In the first expect we are checking the length of the list returned and in the second expect we are checking the entire result itself. ```dart test( "Given local service returns list of LocalCityData, When getFavoriteCitiesList is called, Then Future> is returned", () async { // Given ... ... // When ... ... // Then verify(() => weatherLocalService.getFavouriteCities()).called(1); verify(() => domainCityMapper.mapList(localCityData)).called(1); expect(result.length, localCityData.length); expect(result, cityData); }); ``` - Running this test should give the following result ![Screenshot_2022-03-30_at_1.03.50_PM.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1649228140762/TUKs5dQcG.png) # Expecting exceptions - In some situations, you may want to check that if a particular function throws an exception, then how does the function under test handle it. - Create one more test for `setCityAsFavorite`. You will test the condition where if the `localCityMapper` throws an exception, that exception is surfaced by `setCityAsFavorite`. For this you will instruct the mock to throw an exception instead of returning a value using the `thenThrow` method. ```dart test( "Given setCityAsFavorite is called with a valid City object, When localCityMapper throws an exception, Then the same exception is surfaced to the caller", () async { // Given final testCity = City( id: 1, title: "title", locationType: "locationType", location: "location", ); final testException = Exception("Test exception"); when(() => localCityMapper.map(testCity)).thenThrow(testException); // When // Then }); ``` - To test a function throws an exception, you need to combine the `when` and `then` steps into a single expect block. ```dart test( "Given setCityAsFavorite is called with a valid City object, When localCityMapper throws an exception, Then the same exception is surfaced to the caller", () async { // Given ... ... expect( // When () async => await weatherRepository.setCityAsFavorite(testCity), // Then throwsA(same(testException)), ); }); ``` That’s it! You are now capable of writing unit tests for any situation. As you might have noticed, many test require the same dummy data. You can extract these dummy test data objects into a separate file for easy re-use. # Where to go from here? To get the complete code form this tutorial, checkout the `unit-test-end` branch of the repo or [view it on GitHub](https://github.com/shounakmulay/flutter-testing/tree/unit-test-end). You can checkout the original repo, this test repo is based on at [Wednesday Flutter Template](https://github.com/wednesday-solutions/flutter_template). In Part 2, we will look at testing flutter widgets. Stay tuned! If you found this article helpful, do drop a like and share with your fellow developers.