Use when generating mocks, stubbing methods, verifying interactions, capturing arguments, or choosing between mocks, fakes, and real objects (Mockito).
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---
name: mockito
description: "Use when generating mocks, stubbing methods, verifying interactions, capturing arguments, or choosing between mocks, fakes, and real objects (Mockito)."
license: MIT
---
# Mockito Skill
This skill defines how to correctly use the `mockito` 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. |
- **Data models** should not be mocked if they can be constructed with stubbed data.
- Only use mocks if your test has `verify` assertions; otherwise prefer real or fake objects.
---
## 2. Generating Mocks
```dart
@GenerateMocks([MyClass])
// or for nice mocks (return simple legal values for missing stubs):
@GenerateNiceMocks([MockSpec<MyClass>()])
void main() { ... }
```
```bash
dart run build_runner build
```
- Only annotate files under `test/` for mock generation by default.
- Use a `build.yaml` if you need to generate mocks outside of `test/`.
- Never add `@override` methods or implementations to a class extending `Mock`.
- Never stub responses in a mock's constructor or inside the mock class — always stub in your tests.
---
## 3. Stubbing
```dart
final mock = MockCat();
// Return a value
when(mock.sound()).thenReturn('Meow');
// Throw an error
when(mock.sound()).thenThrow(Exception('No sound'));
// Calculate response at call time
when(mock.sound()).thenAnswer((_) => computedValue);
// Return values in sequence
when(mock.sound()).thenReturnInOrder(['Meow', 'Purr']);
```
- Always stub methods/getters **before** using them if you need specific return values.
- Missing stub behavior: `@GenerateMocks` → throws; `@GenerateNiceMocks` → returns a simple legal value.
- Use `throwOnMissingStub(mock)` to throw on any unstubbed call.
---
## 4. Verification
```dart
verify(mock.sound()); // called at least once
verifyNever(mock.eat(any)); // never called
verify(mock.sound()).called(2); // called exactly twice
```
**Async:**
```dart
await untilCalled(mock.sound()); // wait for the interaction
```
---
## 5. Argument Matchers
```dart
// Flexible stubbing
when(mock.eat(any)).thenReturn(true);
when(mock.eat(argThat(isNotNull))).thenReturn(true);
// Named arguments
when(mock.fetch(any, headers: any)).thenReturn(response);
```
- Do **not** use `null` as an argument adjacent to an argument matcher.
- For named arguments, use `any` or `argThat` as values, not as argument names.
---
## 6. Capturing Arguments
```dart
final captured = verify(mock.eat(captureAny)).captured;
print(captured.last); // last captured argument
```
Use `captureThat` for conditional capturing.
---
## 7. Resetting Mocks
```dart
reset(mock); // clear all stubs AND interactions
clearInteractions(mock); // clear only recorded interactions
```
---
## 8. Mocking Function Types
To mock a function type (e.g., a callback), define an abstract class with the required signature and generate mocks for it:
```dart
abstract class Callback {
void call(String value);
}
@GenerateMocks([Callback])
```
---
## 9. Debugging
```dart
logInvocations([mock1, mock2]); // print all collected invocations
```
---
## References
- [Mockito GitHub Repository](https://github.com/dart-lang/mockito)
