Mock

Learn mocking with mockito.

Fakes with mocks

A mock is a fake version of a real dependency. It keeps external services (HTTP, databases) out of tests. The mockito package is common in Dart.

A mock class is written as class MockHttp extends Mock implements HttpClient {}. Then you set behavior with when(mock.get()).thenReturn('data').

So the test gets predictable results without a real network call. It is fast and stable.

when and verify

In mockito when(mock.fetch()).thenReturn('data') sets a method's return value. During the test calling mock.fetch() returns 'data'; no real call is made.

verify(mock.fetch()) checks whether the method was called. This is useful to probe interactions and to verify the expected flow.

In simple cases a hand-written fake class is enough; mockito is not always required.

Exercise A fake WeatherService

Define an abstract class WeatherService with a String temperature() method. Write a FakeWeatherService implements WeatherService class whose temperature() returns '22C'. In main create a FakeWeatherService and print the result of temperature() with print.

Show example solution

abstract class WeatherService {
  String temperature();
}

class FakeWeatherService implements WeatherService {
  @override
  String temperature() => '22C';
}

void main() {
  var s = FakeWeatherService();
  print(s.temperature());
}

Test yourself

What does a mock do?
- Provides a fake dependency
- Makes a real network call
- Sets up a loop
- Deletes a file
Show answer

Provides a fake dependency — a mock provides a fake dependency.

Importing mockito

import 'package:mockito/____.dart';

Show answer

import 'package:mockito/mockito.dart';

Mock parts

class FakeWeatherService

Fake class
when(...).thenReturn

Behavior stubbing
verify(...)

Call verification
Mock

mockito base class

Next lesson: Summary - Testing

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