An example how to use mockito and Dio to tests your calls to API

dio_test.dart

class DioAdapterMock extends Mock implements HttpClientAdapter {}

const dioHttpHeadersForResponseBody = {
  Headers.contentTypeHeader: [Headers.jsonContentType],
};

void main() {
 group('UserRemoteDataSource', () {
   final Dio dio = Dio();
   UserRemoteDataSource dataSource;
   DioAdapterMock dioAdapterMock;

   setUp(() {
     dioAdapterMock = DioAdapterMock();
     dio.httpClientAdapter = dioAdapterMock;
     dataSource = UserRemoteDataSource(dio);
   });

   group('login', () {
     test('when credentials are valid should return UserLoginResponse with token', () async {
       final userCredentials = UserLoginRequestBody(password: 'password', email: 'email');
       final responsePayload = json.encode({ 'token': 'token' });
       final httpResponse = ResponseBody.fromString(responsePayload, 200, headers: dioHttpHeadersForResponseBody,);

       when(dioAdapterMock.fetch(any, any, any)).thenAnswer((_) async => httpResponse);

       final subject = await dataSource.login(userCredentials);
       final expected = UserLoginResponse(token: 'token');

       expect(subject, expected);
     });
   });
 });
}