Koin

Koin Dependency Injection (Android and KMP)

Pragmatic Kotlin DI. Koin vs Hilt: Koin runs in commonMain (Hilt doesn't) and verifies the graph via verify() instead of codegen; Hilt is Android-only with codegen + deep Jetpack integration (@HiltViewModel / hiltViewModel()). This reference covers the traps and boundaries, not the Classic DSL basics (single / factory / scoped / viewModel { } with get() resolution and runtime params, koinViewModel / koinInject, startKoin { androidLogger(); androidContext(this@MyApp); modules(...) }). Related: android-skills:kmp-ktor, android-skills:android-data-layer. Deps via koin-bom: koin-core, koin-android, koin-androidx-compose, koin-compose-viewmodel (KMP), koin-compose-viewmodel-navigation (Nav 3), koin-test, + optional koin-annotations / koin-ksp-compiler.

KSP Annotations — the generated-module accessor

@Module @ComponentScan discovers annotated classes and emits a module you pass to startKoin as UserModule().module — the generated .module extension on the annotated class is the part most often missed. Pick one style per module (don't mix Classic DSL + annotations inside a single module; switching between modules is fine).

@Single class UserRepositoryImpl(private val service: UserService) : UserRepository
@Factory class UserFormValidator
@KoinViewModel class UserDetailViewModel(
    @InjectedParam private val userId: String,
    private val repository: UserRepository,
) : ViewModel()
@Module @ComponentScan("com.example.feature.user") class UserModule
// startKoin { modules(UserModule().module) }

KMP source-set layout

Most bindings live in commonMain; platform-typed ones (HTTP engine, Context, Keychain) go behind expect val platformModule: Module with an actual per platform (mirrors android-skills:kmp-ktor's engine pattern). iOS startup: call InitKoinKt.doInitKoin(config: nil) from iOSApp.init() — Swift reserves init, hence the do prefix.

// commonMain
expect val platformModule: Module
// androidMain: actual = module { single<HttpClientEngine> { OkHttp.create() }; single<TokenStorage> { DataStoreTokenStorage(get()) } }
// iosMain:     actual = module { single<HttpClientEngine> { Darwin.create() }; single<TokenStorage> { KeychainTokenStorage() } }

Compose: the call-site failure

koinViewModel / koinInject require koin-androidx-compose (Android) or koin-compose-viewmodel (KMP). Without that artifact the call compiles but fails at the call site — koin-core knows nothing about ViewModel or the Compose runtime. For testability, default composable params to koinInject(): fun Screen(service: AnalyticsService = koinInject()).

val vm: UserDetailViewModel = koinViewModel { parametersOf(userId) }
// keyed per entity: koinViewModel(key = "detail_$userId", parameters = { parametersOf(userId) })

Scopes

Survives configuration changes (the common case) → activityRetainedScope — backed by the Activity's retained ViewModel, so it lives across rotation and closes on real finish. Declare with activityRetainedScope { } (or @ActivityRetainedScope) and access via AndroidScopeComponent; do not hand-roll createScope / close for this case.

val checkoutModule = module {
    activityRetainedScope {                 // retained across config changes
        scoped { CheckoutCart() }
        scoped { CheckoutPricing(get()) }
    }
}
class CheckoutActivity : AppCompatActivity(), AndroidScopeComponent {
    override val scope: Scope by activityRetainedScope()
    private val cart: CheckoutCart by inject()
}

Custom lifetime not tied to an Activity (e.g. a multi-step flow keyed by an order id) → declare a scope<T> and own its lifecycle: getKoin().createScope<CheckoutFlow>("checkout-$orderId"), resolve with scope.get(), and scope.close() when the flow ends.

Nav 3

koinEntryProvider() resolves ViewModels per destination; register destinations inside modules, not inline at the NavDisplay site.

val navigationModule = module {
    navigation<HomeRoute> { HomeScreen(viewModel = koinViewModel()) }
    navigation<DetailRoute> { route -> DetailScreen(viewModel = koinViewModel { parametersOf(route.id) }) }
}
NavDisplay(backStack = backStack, backStack.removeLastOrNull() }, entryProvider = koinEntryProvider())

Testing — verify() / checkModules()

Walks each declaration's constructor and confirms every dependency is declared — missing bindings become test failures instead of a runtime NoDefinitionFoundException (recovering one of Hilt's advantages). Run it in CI. For constructor params resolved at runtime (e.g. SavedStateHandle), pass extraTypes or verify() false-fails. Override real bindings with fakes (e.g. Ktor MockEngine — see android-skills:kmp-ktor). KoinTestRule (JUnit 4) / KoinTestExtension (JUnit 5) installs a context per test.

class ModuleVerificationTest : KoinTest {
    @Test fun `all modules resolve cleanly`() {
        koinApplication { modules(appModule, networkModule, platformModule) }.checkModules()
    }
    // Per-module with runtime-resolved types:
    // featureModule.verify(extraTypes = listOf(SavedStateHandle::class))
}