Flutter Unit Testing: From Necessities to Complicated Eventualities

Curiosity in Flutter is at an all-time excessive—and it’s lengthy overdue. Google’s open-source SDK is suitable with Android, iOS, macOS, internet, Home windows, and Linux. A single Flutter codebase helps all of them. And unit testing is instrumental in delivering a constant and dependable Flutter app, guaranteeing towards errors, flaws, and defects by preemptively enhancing the high quality of code earlier than it’s assembled.

On this tutorial, we share workflow optimizations for Flutter unit testing, exhibit a primary Flutter unit take a look at, then transfer on to extra complicated Flutter take a look at instances and libraries.

The Circulate of Unit Testing in Flutter

We implement unit testing in Flutter in a lot the identical method that we do in different expertise stacks:

1. Consider the code.
2. Arrange information mocking.
3. Outline the take a look at group(s).
4. Outline take a look at perform signature(s) for every take a look at group.
5. Write the checks.

To exhibit unit testing, I’ve ready a pattern Flutter undertaking and encourage you to make use of and take a look at the code at your leisure. The undertaking makes use of an exterior API to fetch and show a listing of universities that we are able to filter by nation.

Just a few notes about how Flutter works: The framework facilitates testing by autoloading the flutter_test library when a undertaking is created. The library allows Flutter to learn, run, and analyze unit checks. Flutter additionally autocreates the take a look at folder through which to retailer checks. It’s vital to keep away from renaming and/or shifting the take a look at folder, as this breaks its performance and, therefore, our skill to run checks. It’s also important to incorporate _test.dart in our take a look at file names, as this suffix is how Flutter acknowledges take a look at recordsdata.

Check Listing Construction

To advertise unit testing in our undertaking, we carried out MVVM with clear structure and dependency injection (DI), as evidenced within the names chosen for supply code subfolders. The mixture of MVVM and DI ideas ensures a separation of considerations:

1. Every undertaking class helps a single goal.
2. Every perform inside a category fulfills solely its personal scope.

We’ll create an organized cupboard space for the take a look at recordsdata we’ll write, a system the place teams of checks can have simply identifiable “properties.” In mild of Flutter’s requirement to find checks throughout the take a look at folder, let’s mirror our supply code’s folder construction below take a look at. Then, after we write a take a look at, we’ll retailer it within the applicable subfolder: Simply as clear socks go within the sock drawer of your dresser and folded shirts go within the shirt drawer, unit checks of Mannequin lessons go in a folder named mannequin, for instance.

Adopting this file system builds transparency into the undertaking and affords the staff a simple technique to view which parts of our code have related checks.

We at the moment are able to put unit testing into motion.

A Easy Flutter Unit Check

We’ll start with the mannequin lessons (within the information layer of the supply code) and can restrict our instance to incorporate only one mannequin class, ApiUniversityModel. This class boasts two features:

• Initialize our mannequin by mocking the JSON object with a Map.
• Construct the College information mannequin.

To check every of the mannequin’s features, we’ll customise the common steps described beforehand:

1. Consider the code.
2. Arrange information mocking: We’ll outline the server response to our API name.
3. Outline the take a look at teams: We’ll have two take a look at teams, one for every perform.
4. Outline take a look at perform signatures for every take a look at group.
5. Write the checks.

After evaluating our code, we’re prepared to perform our second goal: to arrange information mocking particular to the 2 features throughout the ApiUniversityModel class.

To mock the primary perform (initializing our mannequin by mocking the JSON with a Map), fromJson, we’ll create two Map objects to simulate the enter information for the perform. We’ll additionally create two equal ApiUniversityModel objects to signify the anticipated results of the perform with the supplied enter.

To mock the second perform (constructing the College information mannequin), toDomain, we’ll create two College objects, that are the anticipated consequence after having run this perform within the beforehand instantiated ApiUniversityModel objects:

void predominant() {
    Map<String, dynamic> apiUniversityOneAsJson = {
        "alpha_two_code": "US",
        "domains": ["marywood.edu"],
        "nation": "United States",
        "state-province": null,
        "web_pages": ["http://www.marywood.edu"],
        "title": "Marywood College"
    };
    ApiUniversityModel expectedApiUniversityOne = ApiUniversityModel(
        alphaCode: "US",
        nation: "United States",
        state: null,
        title: "Marywood College",
        web sites: ["http://www.marywood.edu"],
        domains: ["marywood.edu"],
    );
    College expectedUniversityOne = College(
        alphaCode: "US",
        nation: "United States",
        state: "",
        title: "Marywood College",
        web sites: ["http://www.marywood.edu"],
        domains: ["marywood.edu"],
    );
 
    Map<String, dynamic> apiUniversityTwoAsJson = {
        "alpha_two_code": "US",
        "domains": ["lindenwood.edu"],
        "nation": "United States",
        "state-province":"MJ",
        "web_pages": null,
        "title": "Lindenwood College"
    };
    ApiUniversityModel expectedApiUniversityTwo = ApiUniversityModel(
        alphaCode: "US",
        nation: "United States",
        state:"MJ",
        title: "Lindenwood College",
        web sites: null,
        domains: ["lindenwood.edu"],
    );
    College expectedUniversityTwo = College(
        alphaCode: "US",
        nation: "United States",
        state: "MJ",
        title: "Lindenwood College",
        web sites: [],
        domains: ["lindenwood.edu"],
    );
}

Subsequent, for our third and fourth goals, we’ll add descriptive language to outline our take a look at teams and take a look at perform signatures:

    void predominant() {
    // Earlier declarations
        group("Check ApiUniversityModel initialization from JSON", () {
            take a look at('Check utilizing json one', () {});
            take a look at('Check utilizing json two', () {});
        });
        group("Check ApiUniversityModel toDomain", () {
            take a look at('Check toDomain utilizing json one', () {});
            take a look at('Check toDomain utilizing json two', () {});
        });
}

We’ve outlined the signatures of two checks to verify the fromJson perform, and two to verify the toDomain perform.

To satisfy our fifth goal and write the checks, let’s use the flutter_test library’s count on methodology to check the features’ outcomes towards our expectations:

void predominant() {
    // Earlier declarations
        group("Check ApiUniversityModel initialization from json", () {
            take a look at('Check utilizing json one', () {
                count on(ApiUniversityModel.fromJson(apiUniversityOneAsJson),
                    expectedApiUniversityOne);
            });
            take a look at('Check utilizing json two', () {
                count on(ApiUniversityModel.fromJson(apiUniversityTwoAsJson),
                    expectedApiUniversityTwo);
            });
        });

        group("Check ApiUniversityModel toDomain", () {
            take a look at('Check toDomain utilizing json one', () {
                count on(ApiUniversityModel.fromJson(apiUniversityOneAsJson).toDomain(),
                    expectedUniversityOne);
            });
            take a look at('Check toDomain utilizing json two', () {
                count on(ApiUniversityModel.fromJson(apiUniversityTwoAsJson).toDomain(),
                    expectedUniversityTwo);
            });
        });
}

Having achieved our 5 goals, we are able to now run the checks, both from the IDE or from the command line.

At a terminal, we are able to run all checks contained throughout the take a look at folder by getting into the flutter take a look at command, and see that our checks cross.

Alternatively, we might run a single take a look at or take a look at group by getting into the flutter take a look at --plain-name "ReplaceWithName" command, substituting the title of our take a look at or take a look at group for ReplaceWithName.

Unit Testing an Endpoint in Flutter

Having accomplished a easy take a look at with no dependencies, let’s discover a extra attention-grabbing instance: We’ll take a look at the endpoint class, whose scope encompasses:

• Executing an API name to the server.
• Remodeling the API JSON response into a distinct format.

After having evaluated our code, we’ll use flutter_test library’s setUp methodology to initialize the lessons inside our take a look at group:

group("Check College Endpoint API calls", () {
    setUp(() {
        baseUrl = "https://take a look at.url";
        dioClient = Dio(BaseOptions());
        endpoint = UniversityEndpoint(dioClient, baseUrl: baseUrl);
    });
}

To make community requests to APIs, I want utilizing the retrofit library, which generates a lot of the vital code. To correctly take a look at the UniversityEndpoint class, we’ll power the dio library—which Retrofit makes use of to execute API calls—to return the specified consequence by mocking the Dio class’s conduct by way of a customized response adapter.

Customized Community Interceptor Mock

Mocking is feasible because of our having constructed the UniversityEndpoint class by way of DI. (If the UniversityEndpoint class had been to initialize a Dio class by itself, there can be no method for us to mock the category’s conduct.)

So as to mock the Dio class’s conduct, we have to know the Dio strategies used throughout the Retrofit library—however we wouldn’t have direct entry to Dio. Due to this fact, we’ll mock Dio utilizing a customized community response interceptor:

class DioMockResponsesAdapter extends HttpClientAdapter {
  remaining MockAdapterInterceptor interceptor;

  DioMockResponsesAdapter(this.interceptor);

  @override
  void shut({bool power = false}) {}

  @override
  Future<ResponseBody> fetch(RequestOptions choices,
      Stream<Uint8List>? requestStream, Future? cancelFuture) {
    if (choices.methodology == interceptor.kind.title.toUpperCase() &&
        choices.baseUrl == interceptor.uri &&
        choices.queryParameters.hasSameElementsAs(interceptor.question) &&
        choices.path == interceptor.path) {
      return Future.worth(ResponseBody.fromString(
        jsonEncode(interceptor.serializableResponse),
        interceptor.responseCode,
        headers: {
          "content-type": ["application/json"]
        },
      ));
    }
    return Future.worth(ResponseBody.fromString(
        jsonEncode(
              {"error": "Request would not match the mock interceptor particulars!"}),
        -1,
        statusMessage: "Request would not match the mock interceptor particulars!"));
  }
}

enum RequestType { GET, POST, PUT, PATCH, DELETE }

class MockAdapterInterceptor {
  remaining RequestType kind;
  remaining String uri;
  remaining String path;
  remaining Map<String, dynamic> question;
  remaining Object serializableResponse;
  remaining int responseCode;

  MockAdapterInterceptor(this.kind, this.uri, this.path, this.question,
      this.serializableResponse, this.responseCode);
}

Now that we’ve created the interceptor to mock our community responses, we are able to outline our take a look at teams and take a look at perform signatures.

In our case, we now have just one perform to check (getUniversitiesByCountry), so we’ll create only one take a look at group. We’ll take a look at our perform’s response to 3 conditions:

1. Is the Dio class’s perform truly referred to as by getUniversitiesByCountry?
2. If our API request returns an error, what occurs?
3. If our API request returns the anticipated consequence, what occurs?

Right here’s our take a look at group and take a look at perform signatures:

  group("Check College Endpoint API calls", () {

    take a look at('Check endpoint calls dio', () async {});

    take a look at('Check endpoint returns error', () async {});

    take a look at('Check endpoint calls and returns 2 legitimate universities', () async {});
  });

We’re prepared to write down our checks. For every take a look at case, we’ll create an occasion of DioMockResponsesAdapter with the corresponding configuration:

group("Check College Endpoint API calls", () {
    setUp(() {
        baseUrl = "https://take a look at.url";
        dioClient = Dio(BaseOptions());
        endpoint = UniversityEndpoint(dioClient, baseUrl: baseUrl);
    });

    take a look at('Check endpoint calls dio', () async {
        dioClient.httpClientAdapter = _createMockAdapterForSearchRequest(
            200,
            [],
        );
        var consequence = await endpoint.getUniversitiesByCountry("us");
        count on(consequence, <ApiUniversityModel>[]);
    });

    take a look at('Check endpoint returns error', () async {
        dioClient.httpClientAdapter = _createMockAdapterForSearchRequest(
            404,
            {"error": "Not discovered!"},
        );
        Listing<ApiUniversityModel>? response;
        DioError? error;
        strive {
            response = await endpoint.getUniversitiesByCountry("us");
        } on DioError catch (dioError, _) {
            error = dioError;
        }
        count on(response, null);
        count on(error?.error, "Http standing error [404]");
    });

    take a look at('Check endpoint calls and returns 2 legitimate universities', () async {
        dioClient.httpClientAdapter = _createMockAdapterForSearchRequest(
            200,
            generateTwoValidUniversities(),
        );
        var consequence = await endpoint.getUniversitiesByCountry("us");
        count on(consequence, expectedTwoValidUniversities());
    });
});

Now that our endpoint testing is full, let’s take a look at our information supply class, UniversityRemoteDataSource. Earlier, we noticed that the UniversityEndpoint class is part of the constructor UniversityRemoteDataSource({UniversityEndpoint? universityEndpoint}), which signifies that UniversityRemoteDataSource makes use of the UniversityEndpoint class to satisfy its scope, so that is the category we’ll mock.

Mocking With Mockito

In our earlier instance, we manually mocked our Dio consumer’s request adapter utilizing a customized NetworkInterceptor. Right here we’re mocking a whole class. Doing so manually—mocking a category and its features—can be time-consuming. Luckily, mock libraries are designed to deal with such conditions and might generate mock lessons with minimal effort. Let’s use the mockito library, the trade commonplace library for mocking in Flutter.

To mock by way of Mockito, we first add the annotation “@GenerateMocks([class_1,class_2,…])” earlier than the take a look at’s code—simply above the void predominant() {} perform. Within the annotation, we’ll embody a listing of sophistication names as a parameter (rather than class_1,class_2…).

Subsequent, we run Flutter’s flutter pub run build_runner construct command that generates the code for our mock lessons in the identical listing because the take a look at. The resultant mock file’s title will probably be a mixture of the take a look at file title plus .mocks.dart, changing the take a look at’s .dart suffix. The file’s content material will embody mock lessons whose names start with the prefix Mock. For instance, UniversityEndpoint turns into MockUniversityEndpoint.

Now, we import university_remote_data_source_test.dart.mocks.dart (our mock file) into university_remote_data_source_test.dart (the take a look at file).

Then, within the setUp perform, we’ll mock UniversityEndpoint by utilizing MockUniversityEndpoint and initializing the UniversityRemoteDataSource class:

import 'university_remote_data_source_test.mocks.dart';

@GenerateMocks([UniversityEndpoint])
void predominant() {
    late UniversityEndpoint endpoint;
    late UniversityRemoteDataSource dataSource;

    group("Check perform calls", () {
        setUp(() {
            endpoint = MockUniversityEndpoint();
            dataSource = UniversityRemoteDataSource(universityEndpoint: endpoint);
        });
}

We efficiently mocked UniversityEndpoint after which initialized our UniversityRemoteDataSource class. Now we’re able to outline our take a look at teams and take a look at perform signatures:

group("Check perform calls", () {

  take a look at('Check dataSource calls getUniversitiesByCountry from endpoint', () {});

  take a look at('Check dataSource maps getUniversitiesByCountry response to Stream', () {});

  take a look at('Check dataSource maps getUniversitiesByCountry response to Stream with error', () {});
});

With this, our mocking, take a look at teams, and take a look at perform signatures are arrange. We’re prepared to write down the precise checks.

Our first take a look at checks whether or not the UniversityEndpoint perform is known as when the information supply initiates the fetching of nation data. We start by defining how every class will react when its features are referred to as. Since we mocked the UniversityEndpoint class, that’s the category we’ll work with, utilizing the when( function_that_will_be_called ).then( what_will_be_returned ) code construction.

The features we’re testing are asynchronous (features that return a Future object), so we’ll use the when(perform title).thenanswer( (_) {modified perform consequence} ) code construction to change our outcomes.

To verify whether or not the getUniversitiesByCountry perform calls the getUniversitiesByCountry perform throughout the UniversityEndpoint class, we’ll use when(...).thenAnswer( (_) {...} ) to mock the getUniversitiesByCountry perform throughout the UniversityEndpoint class:

when(endpoint.getUniversitiesByCountry("take a look at"))
    .thenAnswer((realInvocation) => Future.worth(<ApiUniversityModel>[]));

Now that we’ve mocked our response, we name the information supply perform and verify—utilizing the confirm perform—whether or not the UniversityEndpoint perform was referred to as:

take a look at('Check dataSource calls getUniversitiesByCountry from endpoint', () {
    when(endpoint.getUniversitiesByCountry("take a look at"))
        .thenAnswer((realInvocation) => Future.worth(<ApiUniversityModel>[]));

    dataSource.getUniversitiesByCountry("take a look at");
    confirm(endpoint.getUniversitiesByCountry("take a look at"));
});

We will use the identical ideas to write down further checks that verify whether or not our perform appropriately transforms our endpoint outcomes into the related streams of information:

import 'university_remote_data_source_test.mocks.dart';

@GenerateMocks([UniversityEndpoint])
void predominant() {
    late UniversityEndpoint endpoint;
    late UniversityRemoteDataSource dataSource;

    group("Check perform calls", () {
        setUp(() {
            endpoint = MockUniversityEndpoint();
            dataSource = UniversityRemoteDataSource(universityEndpoint: endpoint);
        });

        take a look at('Check dataSource calls getUniversitiesByCountry from endpoint', () {
            when(endpoint.getUniversitiesByCountry("take a look at"))
                    .thenAnswer((realInvocation) => Future.worth(<ApiUniversityModel>[]));

            dataSource.getUniversitiesByCountry("take a look at");
            confirm(endpoint.getUniversitiesByCountry("take a look at"));
        });

        take a look at('Check dataSource maps getUniversitiesByCountry response to Stream',
                () {
            when(endpoint.getUniversitiesByCountry("take a look at"))
                    .thenAnswer((realInvocation) => Future.worth(<ApiUniversityModel>[]));

            count on(
                dataSource.getUniversitiesByCountry("take a look at"),
                emitsInOrder([
                    const AppResult<List<University>>.loading(),
                    const AppResult<List<University>>.data([])
                ]),
            );
        });

        take a look at(
                'Check dataSource maps getUniversitiesByCountry response to Stream with error',
                () {
            ApiError mockApiError = ApiError(
                statusCode: 400,
                message: "error",
                errors: null,
            );
            when(endpoint.getUniversitiesByCountry("take a look at"))
                    .thenAnswer((realInvocation) => Future.error(mockApiError));

            count on(
                dataSource.getUniversitiesByCountry("take a look at"),
                emitsInOrder([
                    const AppResult<List<University>>.loading(),
                    AppResult<List<University>>.apiError(mockApiError)
                ]),
            );
        });
    });
}

We’ve executed numerous Flutter unit checks and demonstrated totally different approaches to mocking. I invite you to proceed to make use of my pattern Flutter undertaking to run further testing.

Flutter Unit Assessments: Your Key to Superior UX

Should you already incorporate unit testing into your Flutter tasks, this text might have launched some new choices you might inject into your workflow. On this tutorial, we demonstrated how simple it will be to include unit testing into your subsequent Flutter undertaking and find out how to sort out the challenges of extra nuanced take a look at eventualities. Chances are you’ll by no means wish to skip over unit checks in Flutter once more.

The editorial staff of the Toptal Engineering Weblog extends its gratitude to Matija Bečirević and Paul Hoskins for reviewing the code samples and different technical content material offered on this article.