koin-patterns

Koin Dependency Injection

Pragmatic DI for Kotlin with Koin. Current stable: BOM io.insert-koin:koin-bom:4.2.2 — pull koin-core, koin-android, koin-androidx-compose, koin-compose + koin-compose-viewmodel (KMP), koin-test / koin-test-junit4 without versions from it.

Module Setup

// AppModule.kt
val appModule = module {
    // Singletons
    single<AppDatabase> { Room.databaseBuilder(...).build() }
    single { get<AppDatabase>().userDao() }
    
    // Factories (new instance each time)
    factory { DateFormatter() }
}

// NetworkModule.kt
val networkModule = module {
    single<HttpClient> {
        HttpClient(OkHttp) {
            install(ContentNegotiation) {
                json(Json { ignoreUnknownKeys = true })
            }
        }
    }
    
    single<AuthApi> { AuthApiImpl(get()) }
    single<UserApi> { UserApiImpl(get()) }
}

// FeatureModule.kt
val homeModule = module {
    // Repository
    single<HomeRepository> { HomeRepositoryImpl(get(), get()) }
    
    // Use Cases
    factory { GetItemsUseCase(get()) }
    factory { GetItemUseCase(get()) }
    
    // ViewModel
    viewModel { HomeViewModel(get()) }
}

Constructor DSL (*Of)

Constructor references instead of lambdas — Koin resolves each constructor parameter with get():

val homeModule = module {
    singleOf(::HomeRepositoryImpl) bind HomeRepository::class
    factoryOf(::GetItemsUseCase)
    viewModelOf(::HomeViewModel)

    // Options lambda: qualifier, extra bindings, eager creation
    singleOf(::RemoteTracker) {
        named("remote")
        bind<Tracker>()
        createdAtStart()
    }
}

bind<T>() / bind T::class only add a secondary type — Koin does not check the hierarchy, so the class must actually implement T or get<T>() throws ClassCastException at resolution. Never bind to a final third-party class.

Since Koin 4.0 the ViewModel DSL is multiplatform: import viewModel/viewModelOf from org.koin.core.module.dsl (koin-core-viewmodel). The old org.koin.androidx.viewmodel.dsl imports are deprecated.