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Mocked backend for testing

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A testing library for Dio based projects.

When working on a feature that requires networking it's best to confirm that the feature behaves as expected before going to production. This means testing for happy paths, unhappy paths, and for the relevant endpoint(s) integrity i.e we pass the correct query parameters, json body, headers, etc..

If you use Dio directly or Rertofit for networking, Use this library to validate your backend integration by mocking Json responses and errors.

Features #

  • Easily mock a backend response
  • Confirm your endpoint's implementation integrity by using request matchers
  • Chain multiple request into a "Scenario"

Getting started #

Add the dependency #

  mocked_backend: <latest version>

The latest version can be found in the Versions tab on

  • Make sure it's added as a dev_dependencies

Setup a Dio interceptor #

Create an instance of MockedBackendInterceptor and add it to Dio's interceptors:

mockedBackendInterceptor = MockedBackendInterceptor();
final dio = Dio(BaseOptions(baseUrl: ''));
  • For unit tests, keep a reference for the above interceptor to use it in your tests later
  • For BDD tests, you can register the interceptor in your DI of choise and then simply "get_it" when needed

Mocking an endpoint #

To mock an endpoint create a MockedRequestHandler:

  requestMatcher: RequestMatcherFactory.path('Your endpoint's path'),
  isSuccess: true,

Two parameters are required to create a MockedRequestHandler:

  1. requestMatcher - Tells the interceptor how to detect the desired endpoint
  2. isSuccess - Tells the interceptor how to treat the matched endpoint. Either success or error (boolean)

There are optional parameters that can be used as well:

  1. body - Can be used as the json response that the backebd should return (both for success or error response)
  2. statusCode - Can be used if you want to map an error code to a custom exception. It can also be used for a successful response such as 201 if your business logic requires it

Requests matchers

There are multiple requestMatcher that you can use to match an endpoint:

  1. RequestMatcherFactory.path(String path) - Will match a request if its path contains the given path
  2. RequestMatcherFactory.method(String method) - Will match a request based on its methos (get, post, put, etc..). It's best to use it in combination with at least the path matcher. Otherwise the first endpoint that has the same method will be matched and probably will fail the test
  3. RequestMatcherFactory.body(String body) - Will match a request that has the given body
  4. RequestMatcherFactory.query(Map<String, String> queryParameters) - Will match a request that contains the given queryParameters
  5. RequestMatcherFactory.multiple(List
/// Get a list of tracks:
/// vs update a list of tracks:

The best way to confirm your implementation is to combine all of the above matchers. This way you confirm that you use the correct endpoint's method, path, query parameters, json body, and soon there wiill be another matcher to match the request's headers.

  • It might look like a bit of work but it will prevent bugs if for example, the API contract has been changed and a query parameter's key is now different

The next step is to wrap the MockedRequestHandler in a Scenario. A scenario is a class that holds a list of MockedRequestHandlers.

If your business logic requires calling multiple endpoints, for example, a login form might need to call one endpoint to validates a given email and another endpoint to sends an email:

  final sendEmailSucceedsScenario = Scenario([
        requestMatcher: RequestMatcherFactory.path('/email/evaluate'),
        isSuccess: true,
      requestMatcher: RequestMatcherFactory.path('/email/send'),
      isSuccess: true,

Those two different endpoints can be easily incoporated in a single scenario.

The last step is to pass the scenario to the MockedBackendInterceptor:


Please note,

If your test calls an endpoint that isn't mocked, a RequestNotMockedException will be thrown:

Method: GET
Path: /app/tracks?limit=5
Body: {}

That's it! If you run a test and a triggered endpoint is matched then you'll get the expected mocked response.

Example app #

If you want to see more examples, checkout the example in the Github repository.

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Mocked backend for testing

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dio, flutter


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