What is dependency injection, and why would you use it?

A class depends on other objects to do its job: a repository, an API client, a clock. With dependency injection, it doesn’t create them; it receives them, usually through the constructor.

// Without DI: the ViewModel builds its own repository and can't be tested with a fake
class ProfileViewModel : ViewModel() {
    private val repository = ProfileRepository(RetrofitClient.api)
}

// With DI: the repository is passed in
class ProfileViewModel(private val repository: ProfileRepository) : ViewModel()

Why it helps:

• Testing. In a unit test you pass a fake repository that returns fixed data, with no network.
• Flexibility. You can switch from Retrofit to Ktor, or from a real clock to a fixed one, without touching the class.
• Clear dependencies. The constructor shows exactly what the class needs.

Frameworks like Hilt, Koin or Spring create and wire the objects for you, which helps in big apps. But passing a parameter to a constructor is already dependency injection.