charlatan

This package provides the ability to configure and return fake HTTP responses from your Dio HTTP Client. This makes it easy to test the behavior of code that interacts with HTTP services without having to use mocks.

It consists of two components and a few helper functions:

• Charlatan - a class for configuring and providing fake HTTP responses based on HTTP method and URI template.
• CharlatanHttpClientAdapter - an implementation of Dio's HttpClientAdapter that returns responses from a configured Charlatan instance.
• charlatanResponse and request matching helpers - utilites for concisely matching HTTP requests and generating fake responses.

Usage

Add charlatan to your pubspec.yaml's dev_dependencies:

# pubspec.yaml
dev_dependencies:
  charlatan:

Configuring fake responses

Create an instance of Charlatan and call the corresponding configuration method for the HTTP method you want to map a request to.

You can configure fakes responses using a specific path or a URI template. You can also use the request object to customize your response. The easiest way to configure a response is with the charlatanResponse helper function.

final charlatan = Charlatan();
charlatan.whenPost('/users', charlatanResponse(body: { 'id': 1, 'bilbo' }));
charlatan.whenGet('/users/{id}', charlatanResponse(body: { 'name': 'bilbo' }));
charlatan.whenPut('/users/{id}/profile', charlatanResponse(statusCode: 204));
charlatan.whenDelete('/users/{id}', (req) => CharlatanHttpResponse(statusCode: 204, body: { 'uri': req.path }));

If you need to further customize the response, you can expand your fake response handler to include whatever you need. The only requirement is that it returns a CharlatanHttpResponse. This allows you to provide dynamic values for the status code, body, and headers in the response.

charlatan.whenPost('/users', (req) {
  final data = req.body as Map<String, Object?>? ?? {};
  final name = data['name'] as String?;
  if (name == null) {
    return CharlatanHttpResponse(
      statusCode: 422,
      body: {
        'errors': {
          'name': ['cannot be blank'],
        },
      },
    );
  }

  return CharlatanHttpResponse(
    statusCode: 201,
    body: { 'id': 1, 'name': name },
  );
});

Additionally, if you need to match requests using other properties of the request or with different logic, you can use whenMatch.

charlatan.whenMatch(
  (req) => req.method == 'GET' && req.path.toLowerCase() == '/posts',
  charlatanResponse(statusCode: 200),
);

Building a fake HTTP client

Build the CharlatanHttpClientAdapter from the Charlatan instance and then assign it to your Dio instance's httpClientAdapter.

final charlatan = Charlatan();
// ... configure fake responses ...
final dio = Dio()..httpClientAdapter = charlatan.toFakeHttpClientAdapter();

Now make HTTP requests like your normally would and they will be routed through your configured fakes.

final result = await dio.get<Object?>('/users/1');
expect(result.data, {'id', 1, 'name': 'bilbo'});

FAQ

What happens if I make a request that doesn't match a configured fake response?

You get a helpful error message like this:

Unable to find matching fake http response definition for:

GET /blahhhh

Did you configure it?

The fake http response definitions configured were:
GET /users
POST /users
PUT /users
DELETE /users

How can I configure a fake response that relies upon the result of another fake request? e.g. a POST followed by a GET that can "read its own writes"

Check out the example directory.

Contributing

If you run into a bug or limitation when using Charlatan, we'd love your help in resolving it. First, it would be awesome if you could open an issue to discuss. If we feel like we should move forward with a change and you're willing to contribute, create a fork of Charlatan and open a PR against the main repo. Keep the following in mind when doing so:

• Prior to opening the PR, be sure to run dart format . in the root of Charlatan which will format the code so it passes CI checks
• When opening the PR, include one of (MINOR), (MAJOR), or (NOBUMP) at the end of your PR title. Otherwise, it will fail CI. These tokens aid in the automation of our releases. Use (MINOR) to denote that your changes warrant a minor bump (they don't break the public API). Use (MAJOR) to denote that your changes warrant a major bump (they break the public API). Lastly, use (NOBUMP) to denote that your changes don't warrant a version bump (perhaps you fixed a docs typo).