Kotlin

This tutorial lets you write a Kotlin application and use Koin dependency injection to retrieve your components. You need around 10 min to do the tutorial.

note

update - 2024-10-21

Get the code

info

The source code is available at on Github

Setup

First, check that the koin-core dependency is added like below:

dependencies {
    
    // Koin for Kotlin apps
    implementation "io.insert-koin:koin-core:$koin_version"
}

Application Overview

The idea of the application is to manage a list of users, and display it in our UserApplication class:

Users -> UserRepository -> UserService -> UserApplication

The "User" Data

We will manage a collection of Users. Here is the data class:

data class User(val name : String)

We create a "Repository" component to manage the list of users (add users or find one by name). Here below, the UserRepository interface and its implementation:

interface UserRepository {
    fun findUser(name : String): User?
    fun addUsers(users : List<User>)
}

class UserRepositoryImpl : UserRepository {

    private val _users = arrayListOf<User>()

    override fun findUser(name: String): User? {
        return _users.firstOrNull { it.name == name }
    }

    override fun addUsers(users : List<User>) {
        _users.addAll(users)
    }
}

The Koin module

Use the module function to declare a Koin module. A Koin module is the place where we define all our components to be injected.

val appModule = module {
    
}

Let's declare our first component. We want a singleton of UserRepository, by creating an instance of UserRepositoryImpl

val appModule = module {
    single<UserRepository> { UserRepositoryImpl() }
}

The UserService Component

Let's write the UserService component to request the default user:

class UserService(private val userRepository: UserRepository) {

    fun getDefaultUser() : User = userRepository.findUser(DefaultData.DEFAULT_USER.name) ?: error("Can't find default user")
}

UserRepository is referenced in UserPresenter`s constructor

We declare UserService in our Koin module. We declare it as a single definition:

val appModule = module {
     single<UserRepository> { UserRepositoryImpl() }
     single { UserService(get()) }
}

The get() function allow to ask Koin to resolve the needed dependency.

Injecting Dependencies in UserApplication

The UserApplication class will help bootstrap instances out of Koin. It will resolve the UserService, thanks to KoinComponent interface. This allows to inject it with the by inject() delegate function:

class UserApplication : KoinComponent {

    private val userService : UserService by inject()

    // display our data
    fun sayHello(){
        val user = userService.getDefaultUser()
        val message = "Hello '$user'!"
        println(message)
    }
}

That's it, your app is ready.

info

The by inject() function allows us to retrieve Koin instances, in any class that extends KoinComponent

Start Koin

We need to start Koin with our application. Just call the startKoin() function in the application's main entry point, our main function:

fun main() {
    startKoin {
        modules(appModule)
    }

    UserApplication().sayHello()
}

info

The modules() function in startKoin load the given list of modules

Koin module: classic or constructor DSL?

Here is the Koin module declaration for our app:

val appModule = module {
    single<UserRepository> { UserRepositoryImpl() }
    single { UserService(get()) }
}

We can write it in a more compact way, by using constructors:

val appModule = module {
    singleOf(::UserRepositoryImpl) { bind<UserRepository>() }
    singleOf(::UserService)
}