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Dependency Injection with KODEIN Ubiratan Soares January / 2017

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DI 101

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Dependency Injection is all about a separation of concerns between some object creation and usage

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class Controller(private val source: DataSource) { fun doSomeWork() { val service = Service(ds) val formatter = OutputFormatter() render(formatter.format(service.getResults())) } }

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class Controller(private val source: DataSource) { fun doSomeWork() { val service = Service(ds) val formatter = OutputFormatter() render(formatter.format(service.getResults())) } }

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class Controller( private val service: Service, private val formatter: OutputFormatter) { fun doSomeWork() { val results = service.getResults() render(formatter.format(results)) } } class Service(private val source: DataSource) { } class DataSource( private val logger: Logger, private val tracker: ErrorTracker, // other dependencies ) class OutputFormatter { }

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class Controller( private val service: Service, private val formatter: OutputFormatter) { fun doSomeWork() { val results = service.getResults() render(formatter.format(results)) } } class Service(private val source: DataSource) { } class DataSource( private val logger: Logger, private val tracker: ErrorTracker, // other dependencies ) class OutputFormatter { }

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Enforcing DI means that any class with an available public constructor gets all its collaborators at instantiation time

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"NO MERCY !!! NO PRISIONERS!!! ALL SINGLETONS MUST DIE !!!"

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"Dont call us, we call you" The Hollywood Principle

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Data Source Controller Service External RestAPI Error Tracker Logger Data Parser Cache Colaborates with

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Logger Controller Service DataSource DataParser ErrorTracker Cache External API Direct Acyclic Graph

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Dependencies Chain Controller A Dependencies Chain Controller B Dependencies Chain Controller N Application …

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WHO BUILDS THE OBJECT GRAPH ???

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Usually the application itself bootstraps the dependencies graph at launch time

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• Manual code setup of all objects creation and DAG relationships • Extra code needed for scoped- based dependencies • Reflection-free • No DI errors at runtime • Boilterplate • Semi automated code setup, driven by JSR330 annotations standard • Reflection-based (Guice, etc) or reflection-free (Dagger2) • Reflection-based solutions may crash at runtime due to dependency resolution errors • Less code than static factories Static Factories JSR330

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Dependency Injection and Dependency Inversion are two distinct concepts, but related in practice

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The Dependency Inversion Principle (from SOLID) "High level policies should not depend directly on low-level details; instead, low-level details should depend on high level policies. To accomplish this, runtime dependencies can be decoupled from source code dependencies via a shared polymorphic abstraction"

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interface Logger { fun log(severity: Level, toLog : String) } class SL4JLogger : Logger { override fun log(severity: Level, toLog: String) { // Print to console } } class LogEntriesLogger : Logger { override fun log(severity: Level, toLog: String) { // Send to external logging service } }

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Dependencies Chain App Controller Logger Instances Factory

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Dependencies Chain App Controller Instances Factory SL4JLogger Factory.createLogger( )

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OVERVIEW • 100% Kotlin dependency injection framework • Heavily based on Kotlin features : type system and lazy properties • Reflection-based • Mirrors some API style from Guice, but has its own style as well • Supports Kotlin-JVM, Android, JS and Kotlin-native

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object Injector { val graph = Kodein.lazy { bind() with singleton { Gson() } bind() with singleton { OkHttpClient.Builder().build() } bind() with provider { RestCaller(httpClient = instance()) } // More bindings ...

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object Injector { val graph = Kodein.lazy { bind() with singleton { Gson() } bind() with singleton { OkHttpClient.Builder().build() } bind() with provider { RestCaller(httpClient = instance()) } // More bindings ... The inferred LazyKodein subtype represents the object graph

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object Injector { val graph = Kodein.lazy { bind() with singleton { Gson() } bind() with singleton { OkHttpClient.Builder().build() } bind() with provider { RestCaller(httpClient = instance()) } // More bindings ... Instances will be created in a lazy way

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object Injector { val graph = Kodein.lazy { bind() with singleton { Gson() } bind() with singleton { OkHttpClient.Builder().build() } bind() with provider { RestCaller(httpClient = instance()) } // More bindings ... Defines a binding, eg, a way to provide an object instance

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object Injector { val graph = Kodein.lazy { bind() with singleton { Gson() } bind() with singleton { OkHttpClient.Builder().build() } bind() with provider { RestCaller(httpClient = instance()) } // More bindings ... Defines the lifecycle time - or scope - of this object

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object Injector { val graph = Kodein.lazy { bind() with singleton { Gson() } bind() with singleton { OkHttpClient.Builder().build() } bind() with provider { RestCaller(httpClient = instance()) } // More bindings ... Pulls a dependency transitively from the graph, with a proper scope

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bind() with provider { APIGateway( homeService = instance(), brokerService = instance(), walletService = instance() ) } You should use named parameters to clarify the dependencies that are being pulled transitively

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bind(UITHREAD) with singleton { AndroidSchedulers.mainThread() } bind(WORKER) with singleton { Schedulers.io() } bind() with provider { SomeInfrastructure( storage = instance(), webService = instance(), worker = instance(WORKER) ) } companion object { val WORKER = "worker" val UITHREAD = "main" }

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You can bind multiple instances of same type using tags bind(UITHREAD) with singleton { AndroidSchedulers.mainThread() } bind(WORKER) with singleton { Schedulers.io() } bind() with provider { SomeInfrastructure( storage = instance(), webService = instance(), worker = instance(WORKER) ) } companion object { val WORKER = "worker" val UITHREAD = "main" }

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bind() with singleton { OkHttpClient.Builder().build() } bind() with provider { RestCaller(httpClient = instance()) }

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bind() with singleton { OkHttpClient.Builder().build() } bind() with provider { RestCaller(httpClient = instance()) } This dependency will live as long the Kodein instance lives

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bind() with singleton { OkHttpClient.Builder().build() } bind() with provider { RestCaller(httpClient = instance()) } Each time this dependency will be required, a new instance will be created

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You may have a dependency bounded with a custom scope, eg, this dependency will live as long the provided object lives bind() with scopedSingleton(androidActivityScope) { Presenter( scheduler = instance(UITHREAD), view = it ) } The object itself is accessible at binding time

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fun main(args: Array) { val gateway: APIGateway by Injector.graph.instance() gateway.start() }

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fun main(args: Array) { val gateway: APIGateway by Injector.graph.instance() gateway.start() } Our class holds the LazyKodein property No class reference needed to retrieve an instance from the graph

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class App : Application(), KodeinAware { override val kodein by Injection(context = this).graph } class SomeActivity : AppCompatActivity(), SomeView { private val kodein by lazy { LazyKodein(appKodein) } private val presenter by kodein.with(this).instance() // Do your work … }

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class App : Application(), KodeinAware { override val kodein by Injection(context = this).graph } class SomeActivity : AppCompatActivity(), SomeView { private val kodein by lazy { LazyKodein(appKodein) } private val presenter by kodein.with(this).instance() // Do your work … } This interface defines an instance that can hold a dependency graph

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class App : Application(), KodeinAware { override val kodein by Injection(context = this).graph } class SomeActivity : AppCompatActivity(), SomeView { private val kodein by lazy { LazyKodein(appKodein) } private val presenter by kodein.with(this).instance() // Do your work … } Provided by the Android support

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class App : Application(), KodeinAware { override val kodein by Injection(context = this).graph } class SomeActivity : AppCompatActivity(), SomeView { private val kodein by lazy { LazyKodein(appKodein) } private val presenter by kodein.with(this).instance() // Do your work … } Provides the custom dependency scope

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MUCH MORE • More ways to provide and retrieve dependencies • More ways to define dependencies scopes • Modules support, including dependency overriding • Multibindings support • JSR330 inter-operability for injections • Great documentation

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FINAL
 REMARKS

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CONCLUSIONS • Kodein is a great Kotlin first-class DI framework • You dont have to follow any JSR330 conventions to define the object graph • Kodein has same pain-points that reflection-based DI framework have : not so fast when compared with reflection-free solutions, errors may occur at runtime due to dependencies resolution failures • Great alternative for DI on Android applications

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https://speakerdeck.com/ubiratansoares

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UBIRATAN SOARES Computer Scientist by ICMC/USP Software Engineer, curious guy Google Developer Expert for Android Teacher, speaker, etc, etc

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THANK YOU @ubiratanfsoares ubiratansoares.github.io https://br.linkedin.com/in/ubiratanfsoares