Dagger: Component, Module & @Inject

Dagger 2 is a fully static, compile-time dependency injection framework for Java/Kotlin and Android. Rather than resolving dependencies via reflection at runtime, it generates plain, readable dependency-wiring code at compile time -- both faster and able to catch configuration errors as build errors, not runtime crashes.

@Inject: Classes Dagger Can Construct Directly

// A class you've written yourself -- Dagger can inspect its
// constructor directly, no separate @Provides method needed
class UserRepository @Inject constructor(
    private val apiClient: ApiClient,
)

Modules: Constructing What Dagger Can't Figure Out Alone

// Third-party classes (Retrofit) -- Dagger has no visibility into
// their constructor, so a Module explicitly shows how to build one
@Module
class NetworkModule {
    @Provides
    @Singleton  // ONE shared instance for the Component's lifetime,
                // not a fresh one every time it's requested
    fun provideRetrofit(): Retrofit =
        Retrofit.Builder().baseUrl(BASE_URL).build()
}

// @Binds: tells Dagger which implementation to use for an interface --
// more efficient than an equivalent @Provides that manually constructs it
@Module
abstract class RepositoryModule {
    @Binds
    abstract fun bindUserRepository(impl: UserRepositoryImpl): UserRepository
}

Components: The Generated Bridge

@Component(modules = [NetworkModule::class, RepositoryModule::class])
interface AppComponent {
    fun inject(activity: MainActivity)
}

// Dagger generates DaggerAppComponent.kt at BUILD time -- open it in
// your IDE to see the actual, readable constructor-calling logic when
// debugging a confusing DI error