Mocktail Skill

This skill defines how to correctly use the mocktail package for mocking in Dart and Flutter tests.

1. Mock vs. Fake vs. Real Object

| Use | When | |---|---| | Real object | Prefer over mocks when practical. | | Fake (extends Fake) | Lightweight custom implementation; override only the methods you need. Prefer over mocks when you don't need interaction verification. | | Mock (extends Mock) | Only when you need to verify interactions (call counts, arguments) or stub dynamic responses. |

• Never add @override methods or implementations to a class extending Mock.
• Only use mocks if your test has verify assertions; otherwise prefer real or fake objects.

2. Creating Mocks

dart
class MockMyService extends Mock implements MyService {}
class FakeMyEvent extends Fake implements MyEvent {}

No code generation required — unlike Mockito, Mocktail uses noSuchMethod at runtime.

3. Registering Fallback Values

Register fallback values before using custom types with argument matchers. Do this in setUpAll or at the top of your test:

dart
setUpAll(() {
  registerFallbackValue(FakeMyEvent());
});

• Required for non-nullable custom types used with any(), captureAny(), or captureThat().
• Register fallback values for any custom type used with argument matchers.

4. Stubbing

dart
final mock = MockMyService();

// Return a value
when(() => mock.fetchData()).thenReturn('result');

// Throw an error
when(() => mock.fetchData()).thenThrow(Exception('error'));

// Dynamic/async response
when(() => mock.fetchData()).thenAnswer((_) async => 'result');

// Future<void>
when(() => mock.doWork()).thenAnswer((_) async {});

• Always stub async methods (returning Future or Future<void>) with thenAnswer.
• Stub every method you expect to be called, even if it's not the focus of your test.

5. Named Parameters

Always include all named parameters in both when and verify calls. Use any(named: 'paramName') for those you don't care about:

dart
when(() => mock.fetch(
  id: any(named: 'id'),
  headers: any(named: 'headers'),
)).thenReturn(response);

• If a method has default values for named parameters, Mocktail still expects them all to be matched.

6. Verification

dart
verify(() => mock.fetchData());             // called at least once
verifyNever(() => mock.fetchData());        // never called
verify(() => mock.fetchData()).called(2);   // called exactly twice

7. Argument Matchers

dart
// Any positional argument
when(() => mock.process(any())).thenReturn(true);

// Capture arguments for later assertions
final captured = verify(() => mock.process(captureAny())).captured;
print(captured.last);

• Use any() for positional parameters when you don't care about the exact value.
• Use captureThat() for conditional capturing.
• When matching string output, be aware of what .toString() returns for the type.

References

• Mocktail GitHub Repository