Mock http.Client in Dart gives exception

Question

I have a problem when testing my classes that make http requests. I want to Mock the client so that every time the client makes a request, I can answer with a Mocked response. At the moment my code looks like this:

  final fn = MockClientHandler;
  final client = MockClient(fn as Future<Response> Function(Request));

  when(client.get(url)).thenAnswer((realInvocation) async =>
    http.Response('{"userId": 1, "id": 2, "title": "mock"}', 200));

However, when I run the test I get the following exception: type '_FunctionType' is not a subtype of type '(Request) => Future<Response>' in type cast test/data_retrieval/sources/fitbit_test.dart 26:32 main

According to Flutter/Dart Mockito should be used like this:

 final client = MockClient();

  // Use Mockito to return a successful response when it calls the
  // provided http.Client.
  when(client.get(Uri.parse('https://jsonplaceholder.typicode.com/albums/1')))
      .thenAnswer((_) async => http.Response('{"userId": 1, "id": 2, "title":"mock"}', 200));

In the example, the client is mocked without parameters, but I guess this has been changed since the documentation of MockClient now also accepts a parameter. I have no idea why this exception occurs and nothing can be found on the internet, so I was wondering if someone here knows why this exception is happening.

Answer 1

package:http's MockClient is a bit of a misnomer. It's not really a mock (and it's certainly not related to package:mockito's Mock); it's more like a stub (or arguably a fake): it has an implementation that is meant to simulate a normal http.Client object.

Note that Flutter's Mockito examples create a MockClient object using package:mockito, but this is unrelated to the MockClient class provided by package:http itself.

final fn = MockClientHandler;
final client = MockClient(fn as Future<Response> Function(Request));

package:http's MockClient expects an instance of a MockClientHandler as an argument, but you are attempting to pass the MockClientHandler type itself. You are expected to provide an actual function (again, because MockClient has an actual implementation).

In other words, if you want to use package:http's MockClient, then you should do:

Future<Response> requestHandler(Request request) async {
   return http.Response('{"userId": 1, "id": 2, "title": "mock"}', 200));
}

final client = MockClient(requestHandler);

Or if you want to use package:mockito, you need to follow https://flutter.dev/docs/cookbook/testing/unit/mocking exactly and generate a Mockito-based MockClient class.