What is Koin?

Koin is a pragmatic lightweight dependency injection framework for Kotlin developers.
-> Source: https://github.com/InsertKoinIO/koin
-> Official: https://insert-koin.io/

How to Setup?

See -> Setup
.. also -> koin-android

Ready MVP Android Code Before apply DI

Code over view before applying koin.
It's popular mvp.
View -> Presenter -> UseCase -> Repositry

// add a class inherited Application for setup koin
class MyApp: Application() {
    override fun onCreate() {
        super.onCreate()
    }
}

// presentation layer

object GreetContract {
    interface View {
        fun showGreet(greet: Greet): Unit
    }
    interface Presenter {
        fun loadGreet()
    }
}

class MainActivity
    : AppCompatActivity(), GreetContract.View {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        button.setOnClickListener {
            // loadGreet
        }
    }
    override fun showGreet(greet: Greet) {
        textView.text = greet.helloWords
    }
}

class GreetPresenter(
        private val view: GreetContract.View,
        private val countryProvider: () -> Country, // add lambda
        private val getGreetUseCase: GetGreetUseCase
) : GreetContract.Presenter {
    override fun loadGreet() {
        val greet = getGreetUseCase.execute(countryProvider())
        view.showGreet(greet)
    }
}

// model layer

sealed class Greet(val helloWords: String){
    object JpKonichiha: Greet("こんにちは！")
    object USHello: Greet("Hello!")
    object Emoji: Greet("\uD83D\uDC4B")
}

enum class Country {
    JP, US, OTHER;
    companion object {
        fun countryProvider(): () -> Country {
            var count = 0
            return {
                when(count++%3) { // it's not a implementation for thread safe.
                    0 -> Country.JP
                    1 -> Country.US
                    else -> Country.OTHER
                }
            }
        }
    }
}

class GetGreetUseCase(
        private val greetRepository: GreetRepository
) {
    fun execute(country: Country): Greet = greetRepository.get(country)
}

// data layer

interface GreetRepository {
    fun get(country: Country): Greet
}

class GreetRepositoryImpl: GreetRepository {
    override fun get(country: Country): Greet {
        return when(country) {
            Country.JP -> Greet.JpKonichiha
            Country.US -> Greet.USHello
            Country.OTHER -> Greet.Emoji
        }
    }
}

Apply Koin

diff

class MyApp: Application() {
    override fun onCreate() {
        super.onCreate()
+        startKoin(this, listOf(appModules))
    }

+    private val appModules = module {
+        single<GreetRepository> { GreetRepositoryImpl() } // single intance
+        factory { GetGreetUseCase(get()) }
+        factory { Country.countryProvider() }
+        factory<GreetContract.Presenter> { (cv: GreetContract.View) ->
+            GreetPresenter(cv, get(), get())
+        } // add lambda argument to get `cv` in inject timing.
+    }
}

// presentation layer

class MainActivity
    : AppCompatActivity(), GreetContract.View {
+    val presenter: GreetContract.Presenter  by inject { parametersOf(this) } // set for cv
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        button.setOnClickListener {
-            // loadGreet
+            presenter.loadGreet()
        }
    }
    override fun showGreet(greet: Greet) {
        textView.text = greet.helloWords
    }
}

build app ...

(i made gif at https://ezgif.com/video-to-gif)

Other

single<Foo>/factory<Foo> or as Foo or bind Foo:class?
- bind is for a multiple type definition like Repository and RepositoryImpl
- <Foo> and as Foo is same definition.
How to apply in MVVM(Android Architecture ViewModel)
- use viewModel injection utils. -> koin:Android Architecture ViewModel

How is it ?

I feel very easy to use Koin to Android layerd architecture for testable. It's a so Simple .

Comparation with Dagger

I haven't used dagger/dagger2 in Production I joined.