Injecting in Android - Koin

Once you have declared some modules and started Koin, how can you retrieve your instances in your Android Activity, Fragments, or Services?

Ready for Android Classes

Activity, Fragment & Service are extended with Koin extensions. Any ComponentCallbacks class has access to:

by inject() - lazy evaluated instance from Koin container
get() - eager fetch instance from Koin container
by viewModel() - lazy ViewModel instance
getViewModel() - eager ViewModel instance

Defining Dependencies

Compiler Plugin DSL

kotlin

val appModule = module {
    factory<Presenter>()
    viewModel<UserViewModel>()
}

Annotations

kotlin

@Factory
class Presenter(private val repository: UserRepository)

@KoinViewModel
class UserViewModel(private val repository: UserRepository) : ViewModel()

Classic DSL

kotlin

val appModule = module {
    factory { Presenter(get()) }
    viewModel { UserViewModel(get()) }
}

Injecting in Activity

kotlin

class DetailActivity : AppCompatActivity() {

    // Lazy inject Presenter
    private val presenter: Presenter by inject()

    // Lazy inject ViewModel
    private val viewModel: UserViewModel by viewModel()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // Use presenter and viewModel
    }
}

Injecting in Fragment

kotlin

class UserFragment : Fragment() {

    // Fragment's own ViewModel
    private val viewModel: UserViewModel by viewModel()

    // Shared ViewModel with Activity
    private val sharedViewModel: SharedViewModel by activityViewModel()

    // Regular dependency
    private val presenter: Presenter by inject()
}

Injecting in Service

kotlin

class MyService : Service() {

    private val repository: UserRepository by inject()

    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
        repository.doSomething()
        return START_STICKY
    }
}

Eager vs Lazy Injection

kotlin

class DetailActivity : AppCompatActivity() {

    // Lazy - created on first access
    private val presenter: Presenter by inject()

    // Eager - created immediately
    private val service: MyService = get()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        // Or get eagerly in a function
        val anotherPresenter: Presenter = get()
    }
}

Method	When Created	Use Case
`by inject()`	On first access	Most cases, avoids unnecessary creation
`get()`	Immediately	When you need the instance right away

:::info If your class doesn't have Koin extensions, implement the KoinComponent interface to access inject() or get(). :::

Injection with Parameters

Pass parameters at injection time:

kotlin

@Factory
class UserPresenter(
    @InjectedParam val userId: String,
    val repository: UserRepository
)

kotlin

class UserActivity : AppCompatActivity() {

    private val presenter: UserPresenter by inject { parametersOf("user_123") }
}

Injection with Qualifiers

When you have multiple definitions of the same type:

kotlin

val appModule = module {
    single<Database>(named("local")) { LocalDatabase() }
    single<Database>(named("remote")) { RemoteDatabase() }
}

kotlin

class MyActivity : AppCompatActivity() {

    private val localDb: Database by inject(named("local"))
    private val remoteDb: Database by inject(named("remote"))
}

Using Android Context in Definitions

Once your Application class configures Koin with androidContext, you can resolve it in your definitions.

Annotations

With annotations, simply declare a Context or Application parameter - it will be automatically injected:

kotlin

@Factory
class MyPresenter(private val context: Context)

@Singleton
class MyRepository(private val application: Application)

DSL

Use androidContext() or androidApplication() functions in your module:

kotlin

val appModule = module {
    factory {
        MyPresenter(androidContext())
    }
    single {
        MyRepository(androidApplication())
    }
}

Android Scope & Context Resolution

When you have a scope binding the Context type, you may need to resolve Context from different levels:

kotlin

class MyPresenter(val context: Context)

val appModule = module {
    scope<MyActivity> {
        scoped { MyPresenter(get()) }
    }
}

Context resolution:

get() - resolves closest Context, here MyActivity
androidContext() - resolves closest Context, here MyActivity
androidApplication() - resolves Application from Koin setup