Welcome to the second lesson in our exploration of Test Driven Development (TDD) with Swift and XCTest. Previously, you learned about using dummies to isolate dependencies. In this lesson, we'll focus on another type of test double — stubs.

By the end of this lesson, you'll understand what stubs are and how to implement them in your tests, specifically for isolating dependencies like external services.

Test doubles help us isolate parts of our application for testing. We've previously discussed dummies; now, let's move to a slightly more useful test double: stubs. Stubs are predefined answers to method calls made during testing. They don't track how they're used, unlike more complex test doubles that we'll learn about later.

Stubs are beneficial when you're testing a function that relies on an external service or complex dependency. They let you simulate the function's output, making your tests faster and more predictable. Keep in mind, though, that stubs primarily ensure your application logic functions as intended and don't necessarily verify the correctness of external dependencies. It’s also important to understand that overusing stubs can lead to tests that are tightly coupled to the stubbed behavior. If the real external service changes its behavior, your tests might still pass, giving a false sense of security. This is why stubs should be used to test your own logic, not to assume the correctness of services you don't control.

Stubs are used to provide consistent, predefined responses to specific method calls in tests, allowing you to control the behavior of certain dependencies without implementing their actual functionality. Unlike dummies, which are mere placeholders, stubs actively simulate responses, making them useful when you need predictable outcomes from dependencies. This predictability allows you to isolate and test your application's logic without needing to rely on the behavior of external systems, which is especially helpful when those systems may be complex or introduce variability.