Advanced mocking
- Dart course
- Expert
- Lesson 39/47
- 5 min
Learn advanced mock and stub.
Advanced mocking
Advanced mocking mimics a dependency's behavior. when(mock.fetch()).thenReturn(5) sets a fixed return value; verify(mock.fetch()) checks call expectations.
Stub: returns a canned value when called. Spy: both returns and records calls. Mock: verifies call expectations.
These tools let you test domain logic without a real database or API. Tests are fast, repeatable and isolated.
Stub vs spy
A stub only returns a fixed value; it does not record calls. It is fast; used for simple return values.
A spy both returns and records calls. spy.callCount reads how many times it was called. A spy is used when behavior verification is needed.
A mock adds call expectation checks on top of a spy: verify(mock.fetch()).called(2). So mock = stub + spy + expectation checks.
Practice Define a MockUserService
Define a User class with final String name; User(this.name);. Define a class MockUserService with a User _stubbed; field, void stubFetch(User u) { _stubbed = u; } and User fetchUser() => _stubbed; methods. In main create final mock = MockUserService();; call mock.stubFetch(User('Ayşe'));; add print(mock.fetchUser().name);.
Starter code
void main() {
// define a User class (name field)
// define a MockUserService class (stubFetch, fetchUser methods)
// create a mock; stub a value; print via fetchUser
}Sample solution
class User {
final String name;
User(this.name);
}
class MockUserService {
User _stubbed;
void stubFetch(User u) {
_stubbed = u;
}
User fetchUser() => _stubbed;
}
void main() {
final mock = MockUserService();
mock.stubFetch(User('Ayşe'));
print(mock.fetchUser().name);
}Expected output
Ayşe
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the mock parts
| Term | Meaning |
|---|---|
Mock |
Fake class |
when |
Stub start |
thenReturn |
Return value |
verify |
Call verification |
2 What does when(mock.fetch()).thenReturn(5) do?
- Runs fetch for real
- Slows fetch down
- Deletes fetch
- Returns 5 when fetch is called Correct answer
Explanation: thenReturn returns a fixed value when called.
3 The thenReturn call
when(mock.fetch()).____(5);when(mock.fetch()).thenReturn(5);Explanation: thenReturn sets a fixed return value.