Dependency Injection for Kotlin Apps with Koin

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Dependency injection for Kotlin apps with Koin Arnaud Giuliani @arnogiu

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! Freelance Software Eng " Mobile First Lead of Koin project Arnaud GIULIANI

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Why a new dependency injection framework?

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The CoffeeMaker App ☕ * aka the thermosiphon dagger’s nightmare sample app

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Pump +pump() Thermosiphon Heater +off() +on() +isHot() ElectricHeater CoffeeMaker +brew()

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The Heater

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interface Heater { fun on() fun off() fun isHot() : Boolean }

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class ElectricHeater : Heater { var heating: Boolean = false override fun on() { println("~ ~ ~ heating ~ ~ ~") heating = true } override fun off() { heating = false } override fun isHot(): Boolean = heating }

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The Thermosiphon

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interface Pump { fun pump() }

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class Thermosiphon(val heater: Heater) : Pump{ override fun pump() { if (heater.isHot()){ println("=> => pumping => =>") } } }

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The CoffeeMaker

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class CoffeeMaker(val pump: Pump, val heater: Heater) { fun brew() { heater.on() pump.pump() println(" [_]P coffee! [_]P ") heater.off() } }

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The CoffeeApp

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class CoffeeApp { val coffeeMaker: CoffeeMaker fun run() { coffeeMaker.brew() } }

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Pump +pump() Thermosiphon Heater +off() +on() +isHot() ElectricHeater CoffeeMaker +brew() how to assemble it?

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class ElectricHeater : Heater class Thermosiphon(val heater: Heater) : Pump class CoffeeMaker(val pump: Pump, val heater: Heater) // bootstrap from an injector // ---- // heater : Heater = ElectricHeater() // pump : Pump = Thermosiphon(heater) // coffeeMaker = CoffeeMaker(heater,pump) val coffeeMaker = Injector.get()

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{ Thermosiphon(get()) } { ElectricHeater() } Provide Provide

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Koin

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- Intuitive DSL to describe it - Lightweight container to run it - Simple API to use it - Pure Kotlin!

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repositories { jcenter() } dependencies { // Koin for Kotlin implementation ‘org.koin:koin-core:$version’ } Gradle Setup

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// declare definitions here module { }

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module { // definition single | factory { instance creation expression } }

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class ElectricHeater : Heater class Thermosiphon(val heater: Heater) : Pump class CoffeeMaker(val pump: Pump, val heater: Heater)

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class ElectricHeater : Heater class Thermosiphon(val heater: Heater) : Pump class CoffeeMaker(val pump: Pump, val heater: Heater) val coffeeMakerModule = module { single { CoffeeMaker(get(),get()) } single{ Thermosiphon(get()) } single { ElectricHeater() } } * also multi modules

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class ElectricHeater : Heater class Thermosiphon(val heater: Heater) : Pump class CoffeeMaker(val pump: Pump, val heater: Heater) val coffeeMaker = module { single { CoffeeMaker(get(),get()) } } val coffeeParts = module { single{ Thermosiphon(get()) } single { ElectricHeater() } } * also multi modules

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Koin Component a class that can use the Koin container

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✅ open access to Kotlin extensions: inject(), get() identify a class linked to the Koin API KoinComponent is an interface marker bootstrap & help integrate with runtime (Android…)

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class CoffeeApp : KoinComponent { val coffeeMaker: CoffeeMaker by inject() fun run() { coffeeMaker.brew() } }

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The CoffeeApp

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startKoin to start the Koin container

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class CoffeeApp : KoinComponent { val coffeeMaker: CoffeeMaker by inject() fun run() { coffeeMaker.brew() } } fun main(vararg args: String) { }

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fun run() { coffeeMaker.brew() } } fun main(vararg args: String) { startKoin { } }

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fun run() { coffeeMaker.brew() } } fun main(vararg args: String) { startKoin { modules(coffeeMakerModule) } }

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fun run() { coffeeMaker.brew() } } fun main(vararg args: String) { startKoin { modules(coffeeMakerModule) } CoffeeApp().run() }

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~ ~ ~ heating ~ ~ ~ => => pumping => => [_]P coffee! [_]P

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Can’t create Android components directly

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Activity, Fragment, Service … are extended as KoinComponent! out of the box! ✅ Koin extensions for Android

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repositories { jcenter() } dependencies { // Koin for Android implementation ‘org.koin:koin-android:$version’ } Gradle Setup

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class MyApplication : Application() { override fun onCreate() { super.onCreate() // Add Android Koin Logger startKoin { androidLogger() modules(coffeeMakerModule) } } }

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class MyApplication : Application() { override fun onCreate() { super.onCreate() // Reference Android context in Koin startKoin { androidLogger() androidContext(this@MyApplication) modules(coffeeMakerModule) } } }

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class MyActivity : AppCompatActivity() { val coffeeMaker: CoffeeMaker by inject() }

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Get the Android context?

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class MyComponent(val context : Context) val myModule = module { single { MyComponent(androidContext()) } }

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class MyComponent(val context : Context) val myModule = module { single { MyComponent(get()) } }

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Dealing with transient components? ie. presenters …

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class MyPresenter(val coffeeMaker : CoffeeMaker) val myModule = module { // will create MyPresenter on each call factory { MyPresenter(get()) } }

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class MyPresenter(val coffeeMaker : CoffeeMaker) val myModule = module { // will create MyPresenter on each call factory { MyPresenter(get()) } } class MyActivity : AppCompatActivity() { // new instance on each call val presenter: MyPresenter by inject() }

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Ready for Android Architecture Components

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repositories { jcenter() } dependencies { // Koin for Android + ViewModel features implementation ‘org.koin:koin-android-viewmodel:$version’ implementation ‘org.koin:koin-androidx-viewmodel:$version’ } Gradle Setup

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✅ Koin extensions for Android ViewModel

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class MyViewModel(val coffeeMaker : CoffeeMaker) : ViewModel() val coffeeMakerModule = module { viewModel { MyViewModel(get()) } }

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//... viewModel { MyViewModel(get()) } } class MyActivity : AppCompatActivity() { // inject ViewModel val myViewModel: MyViewModel by viewModel()

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//... viewModel { MyViewModel(get()) } } class MyFragment : Fragment() { // inject ViewModel val myViewModel: MyViewModel by viewModel()

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//... viewModel { MyViewModel(get()) } } class MyFragment : Fragment() { // inject ViewModel from parent activity val myViewModel: MyViewModel by sharedViewModel()

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Injecting Bundle State in ViewModel

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repositories { jcenter() } dependencies { // Koin for Android Fragments feature implementation ‘org.koin:koin-androidx-viewmodel:$version’ } Gradle Setup 2.1.0-alpha-4

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// Inject your state class StateViewModel(val state: SavedStateHandle) : ViewModel() // use injection parameter to get state viewModel { (handle: SavedStateHandle, id: String) -> StateViewModel(handle, id, get()) } // Provide initial state as parameter class MVVMActivity : AppCompatActivity() { val mySavedVM: StateViewModel by viewModel { parametersOf(Bundle(), "vm1") } }

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// Inject your state class StateViewModel( val handle: SavedStateHandle, val id: String, val service: SimpleService) : ViewModel() // use injection parameter to get state viewModel { (handle: SavedStateHandle) -> StateViewModel(handle, id, get()) } // Provide initial state as parameter class MVVMActivity : AppCompatActivity() { val mySavedVM: StateViewModel by viewModel { parametersOf(Bundle(), "vm1") } }

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// Inject your state class StateViewModel( val handle: SavedStateHandle, val id: String, val service: SimpleService) : ViewModel() // use injection parameter to get state viewModel { (handle: SavedStateHandle, id: String) -> StateViewModel(handle, id, get()) } // Provide initial state as parameter class MVVMActivity : AppCompatActivity() { val mySavedVM: StateViewModel by viewModel { parametersOf(Bundle()) } }

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Inject your Fragments

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repositories { jcenter() } dependencies { // Koin for Android Fragments feature implementation ‘org.koin:koin-androidx-fragment:$version’ } Gradle Setup 2.1.0-alpha-4

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// Use constructor injection :) class MyFragment(val session: Session) : Fragment() // Declare your fragment val module = module { single { Session() } fragment { MyFragment(get()) } } // Setup in Activity class MyMActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { // setup factory setupKoinFragmentFactory() super.onCreate(savedInstanceState)

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// Setup in Activity class MyMActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { // setup factory setupKoinFragmentFactory() super.onCreate(savedInstanceState) // Call your Fragment supportFragmentManager.beginTransaction() .replace(R.id.fragment, MyFragment::class.java, null, null) .commit() } }

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Ktor Web Applications

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repositories { jcenter() } dependencies { // Koin for Ktor compile ‘org.koin:koin-ktor:$version’ // Koin SLF4J Logger compile ‘org.koin:koin-logger-slf4j:$version’ } Gradle Setup

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fun main(args: Array) { // Start Ktor embeddedServer(Netty, commandLineEnvironment(args)).start() } fun Application.main() { install(DefaultHeaders) install(CallLogging) // Routing section routing { get("/hello") { call.respondText(...) } } }

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fun Application.main() { install(DefaultHeaders) install(CallLogging) install(Koin) { slf4jLogger() modules(coffeeMakerModule) } // Routing section routing { get(“/coffee") { call.respondText(...) } } }

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fun Application.main() { install(DefaultHeaders) install(CallLogging) install(Koin) { slf4jLogger() modules(coffeeMakerModule) } val coffeeMaker by inject() // Routing section routing { get("/coffee") { call.respondText(...) } } }

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Application, Route, Routing … are extended as KoinComponent! out of the box! ✅ Koin extensions for Ktor

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class ElectricHeater : Heater class Thermosiphon(val heater: Heater) : Pump class CoffeeMaker(val pump: Pump, val heater: Heater) val coffeeMakerModule = module(createdAtStart = true) { single() singleBy() singleBy() }

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Testing easily with Koin ✅

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repositories { jcenter() } dependencies { // Koin for Tests testImplementation ‘org.koin:koin-test:$version’ } Gradle Setup In rewrite!

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Checking your modules ✔

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class ModuleTest { @Test fun `check all definitions from coffeeMakerModule`() { startKoin { modules(coffeeMakerModule) }.checkModules() } }

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✅ Koin extensions with JUnit & Mockito

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class CoffeeMakerTest : KoinTest { val coffeeMaker: CoffeeMaker by inject() val heater: Heater by inject() @Test fun testHeaterIsTurnedOnAndThenOff() { startKoin { modules(coffeeMakerModule) } } }

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class CoffeeMakerTest : KoinTest { val coffeeMaker: CoffeeMaker by inject() val heater: Heater by inject() @Test fun testHeaterIsTurnedOnAndThenOff() { startKoin { modules(coffeeMakerModule) } declareMock() given(heater.isHot()).will { true } coffeeMaker.brew() verify(heater, times(1)).on() verify(heater, times(1)).off() } }

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More about Koin✨

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- Container DSL - Qualiﬁers (string/type) - Global vs isolated context - Scope API - Android, Scopes & lifecycle - Injection Parameters - AndroidX - Koin for Java - Koin for Ktor

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Koin 2.0

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Since ﬁrst 1.0 version…

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Simple DSL module, single, factory, get

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Pragmatic API ✌ KoinComponent, get, inject

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* startup & Kotlin overhead ** split modules startup if needed

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Benchmarks https://github.com/InsertKoinIO/thermosiphon_dagger2koin https://github.com/Sloy/android-dependency-injection-performance https://github.com/arnaudgiuliani/android-dependency-injection-performance

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http://bit.ly/ready4koin

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- Container DSL - Qualiﬁers (string/type) - Global vs isolated context - Scope API - Android, Scopes & lifecycle - Injection Parameters - AndroidX - Koin for Java - Koin for Ktor

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- Container DSL - Qualiﬁers (string/type) - Global vs isolated context - Scope API - Android, Scopes & lifecycle - Injection Parameters - AndroidX - Koin for Java - Koin for Ktor

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Dependency injection & Service Locator?

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Dependency injection & Service Locator

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class Thermosiphon(val heater : Heater) : Pump class ElectricHeater() : Heater module { single{ Thermosiphon(get()) } single { ElectricHeater() } } Dependency Injection - Constructor injection conﬁgured via Koin DSL - Your classes are not dependant to Koin API - Instances are called by Koin Container - Functional DSL Conﬁguration: parameters, context, scopes…

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class CoffeeApp : KoinComponent { val coffeeMaker by inject() } Service Locator - Use the Koin API to retrieve a dependency - “Host” class not created by Koin

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Is service locator bad?

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class CoffeeApp : KoinComponent { // Lazily ask for instance val coffeeMaker : CoffeeMaker by inject() } Service Locator class CoffeeApp { lateinit var coffeeMaker : CoffeeMaker init { // ask injection of setters DI.inject(this) } } Setter Injection

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80% of your Koin usage will be conﬁguration 20% will be platform extensions Limit your dependency to Koin as much as possible

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The choice between Service Locator and Dependency Injection is less important than the principle of separating service conﬁguration from the use of services within an application. Martin Fowler

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Functional Dependency Injection

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Function and Container Context

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val coffeeMakerModule = module { single { CoffeeMaker(get(),get()) } single{ Thermosiphon(get()) } single { ElectricHeater() } } typealias Definition = Scope.(DefinitionParameters) -> T

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Deﬁnition Parameters DSL Extension Scope

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Function to inject parameters

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class MyPresenter(val id : String) val myModule = module { factory { (id : String) -> MyPresenter(id)} }

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val myModule = module { factory { (id : String) -> MyPresenter(id)} } class MyActivity : AppCompatActivity(){ val presenter : MyPresenter by inject { parametersOf("42")} }

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class MyViewModel(val id : String) val myModule = module { viewModel { (id : String) -> MyViewModel(id)} }

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val myModule = module { viewModel { (id : String) -> MyViewModel(id)} } class MyActivity : AppCompatActivity(){ val presenter : MyViewModel by viewModel { parametersOf("42")} }

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✨ Simpler declaration with Reﬂection ✨

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A.k.a “Get rid of get() get() get() get() get()”

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Functional Dependency Injection 0

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repositories { jcenter() } dependencies { // Koin for Android implementation ‘org.koin:koin-core-ext:$version’ } Gradle Setup

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class ElectricHeater : Heater class Thermosiphon(val heater: Heater) : Pump class CoffeeMaker(val pump: Pump, val heater: Heater) val coffeeMakerModule = module { single() singleBy() singleBy() }

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Available out of the box for pure JVM platforms (Ktor, TornadoFx…)

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repositories { jcenter() } dependencies { // Koin for Android implementation ‘org.koin:koin-android-ext:$version’ implementation ‘org.koin:koin-androidx-ext:$version’ } Gradle Setup

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class MyViewModel(val coffeeMaker : CoffeeMaker) : ViewModel() val coffeeMakerModule = module { viewModel() }

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Add following pro guard rules https://insert-koin.io/docs/2.0/quick-references/experimental-features/#proguard-rule

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Check performances Pure functional DSL declaration is 4x - 10x faster * deﬁnition execution time ** depending on devices (< Android 6)

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Let’s migrate to Koin, what else?

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Refactoring is not always sustainable

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« Hard to switch from our current DI » Top #1 issue

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What if you could… run it easily in a module ✨

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startKoin { modules(…) } loadKoinModules(…) Definition Definition Definition unloadKoinModules(…)

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Making dynamic modules with Koin - Load dynamic modules/deﬁnitions - Change implementation on the ﬂy - Easy to compose step by step

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Onboarding Wallet Proﬁle Transactions Proﬁle v2 Payments Card Management Onboarding Onboarding v2

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Dynamic Modules Architecture - Registry / Plugin architecture - Feature <> Libraries isolation (modules) - Navigation, Feature Flagging, A/B testing …

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What’s next?

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Tips & Best pratices

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Try the getting started projects * look at code samples

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start.insert-koin.io

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doc.insert-koin.io

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Add it step by step to your project * start with small components

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Check your modules ✅

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Keep it simple

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and …

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StackOverﬂow, Github & Slack for any question

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@insertkoin_io koin-developers

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Thank you! All the community…

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stable_version = “2.0.1" unstable_version = “2.1.0-alpha-4” repositories { jcenter() } dependencies { // Core features compile "org.koin:koin-core:$version" // Ktor compile "org.koin:koin-ktor:$version" compile "org.koin:koin-logger-slf4j:$version" // Android implementation "org.koin:koin-android:$version"

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dependencies { // Core features compile "org.koin:koin-core:$version" // Ktor compile "org.koin:koin-ktor:$version" compile "org.koin:koin-logger-slf4j:$version" // Android implementation "org.koin:koin-android:$version" // Android + ViewModel implementation "org.koin:koin-android-viewmodel:$version" // JUnit & Mockito testImplementation "org.koin:koin-test:$version" }

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dependencies { // Core features compile "org.koin:koin-core:$version" // Ktor compile "org.koin:koin-ktor:$version" compile "org.koin:koin-logger-slf4j:$version" // Android implementation “org.koin:koin-android:$version” // Android + ViewModel implementation "org.koin:koin-android-viewmodel:$version" // JUnit & Mockito testImplementation "org.koin:koin-test:$version" }