dart-generate-test-mocks

Annotate your test file with @GenerateNiceMocks([MockSpec<YourType>()]), import the generated .mocks.dart file, then run dart run build_runner build. The mock classes are generated automatically and can be instantiated in your tests.

Inject http.Client through your class constructor, then generate a MockClient with @GenerateNiceMocks([MockSpec<http.Client>()]). Stub responses with when(mockClient.get(any)).thenAnswer((_) async => http.Response(...)) and verify calls with verify().

@GenerateNiceMocks is preferred because it avoids missing stub exceptions that @GenerateMocks throws when an unstubbed method is called. Nice mocks return benign default values instead of failing the test unexpectedly.

The error occurs when thenReturn is used for methods returning a Future or Stream. Replace it with thenAnswer((_) async => value) so the stub returns the correct asynchronous type.

Build failures usually come from a mismatched import path for the .mocks.dart file or syntax errors in the test file. Ensure the import name matches the test file name exactly and rerun dart run build_runner build.