Use when creating mocks, stubbing methods, verifying interactions, registering fallback values, or choosing between mocks, fakes, and real objects (Mocktail).
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---
name: mocktail
description: "Use when creating mocks, stubbing methods, verifying interactions, registering fallback values, or choosing between mocks, fakes, and real objects (Mocktail)."
license: MIT
---
# 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](https://github.com/felangel/mocktail)
