Android & Annotations

This tutorial lets you write an Android 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

Gradle Setup

Let's configure the KSP Plugin like this, and the following dependencies:

plugins {
    alias(libs.plugins.ksp)
}

dependencies {
    // ...

    implementation(libs.koin.annotations)
    ksp(libs.koin.ksp)
}

// Compile time check
ksp {
    arg("KOIN_CONFIG_CHECK","true")
}

note

See libs.versions.toml for current versions

Application Overview

The idea of the application is to manage a list of users, and display it in our MainActivity class with a Presenter or a ViewModel:

Users -> UserRepository -> UserService -> (Presenter or ViewModel) -> MainActivity

The "User" Data

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

data class User(val name: String, val email: 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 findUserOrNull(name: String): User?
    fun addUsers(users: List<User>)
}

@Single
class UserRepositoryImpl : UserRepository {

    private val _users = arrayListOf<User>()

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

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

The UserService Component

Let's write a service component to manage user operations:

interface UserService {
    fun getUserOrNull(name: String): User?
    fun loadUsers()
    fun prepareHelloMessage(user: User?): String
}

@Single
class UserServiceImpl(
    private val userRepository: UserRepository
) : UserService {

    init {
        loadUsers()
    }

    override fun getUserOrNull(name: String): User? = userRepository.findUserOrNull(name)

    override fun loadUsers() {
        userRepository.addUsers(listOf(
            User("Alice", "alice@example.com"),
            User("Bob", "bob@example.com"),
            User("Charlie", "charlie@example.com")
        ))
    }

    override fun prepareHelloMessage(user: User?): String {
        return user?.let { "Hello '${user.name}' (${user.email})! 👋" } ?: "❌ User not found"
    }
}

The Koin module

Let's declare a AppModule module class like below:

@Module
@ComponentScan("org.koin.sample")
@Configuration
class AppModule

• @Module - Declares this class as a Koin module
• @ComponentScan("org.koin.sample") - Automatically scans and registers all Koin definitions in the "org.koin.sample" package
• @Configuration - Enables automatic module discovery when used with @KoinApplication

With component scanning enabled, we simply add annotations to our classes:

@Single
class UserRepositoryImpl : UserRepository {
    // ...
}

@Single
class UserServiceImpl(private val userRepository: UserRepository) : UserService {
    // ...
}

The @Single annotation declares these classes as singletons in Koin.

Displaying User with Presenter

Let's write a presenter component to display a user:

@Factory
class UserPresenter(private val userService: UserService) {

    fun sayHello(name: String): String {
        val user = userService.getUserOrNull(name)
        val message = userService.prepareHelloMessage(user)
        return "[UserPresenter] $message"
    }
}

UserService is referenced in UserPresenter's constructor

We declare UserPresenter with the @Factory annotation, to create a new instance each time it's requested (avoids memory leaks with Android lifecycle):

@Factory
class UserPresenter(private val userService: UserService) {
    // ...
}

Injecting Dependencies in Android

The UserPresenter component will be created, resolving the UserService instance with it. To get it into our Activity, let's inject it with the by inject() delegate function:

class MainActivity : AppCompatActivity() {

    private val presenter: UserPresenter by inject()

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

That's it, your app is ready.

info

The by inject() function allows us to retrieve Koin instances, in Android components runtime (Activity, fragment, Service...)

Start Koin

We need to start Koin with our Android application. With the @KoinApplication annotation, Koin automatically discovers and loads all modules marked with @Configuration:

import org.koin.android.ext.koin.androidContext
import org.koin.core.annotation.KoinApplication
import org.koin.ksp.generated.*

@KoinApplication
class MainApplication : Application() {

    override fun onCreate() {
        super.onCreate()

        startKoin {
            androidContext(this@MainApplication)
        }
    }
}

Key Points:

• @KoinApplication - Automatically discovers all modules annotated with @Module and @Configuration
• No need to manually call modules(AppModule().module) - modules are loaded automatically!
• The import org.koin.ksp.generated.* import is required for generated Koin content
• You only need to configure Android-specific settings like androidContext

info

The @KoinApplication annotation works with @Configuration on your module to automatically discover and load all dependencies at compile time via KSP.

Displaying User with ViewModel

Let's write a ViewModel component to display a user:

@KoinViewModel
class UserViewModel(private val userService: UserService) : ViewModel() {

    fun sayHello(name: String): String {
        val user = userService.getUserOrNull(name)
        val message = userService.prepareHelloMessage(user)
        return "[UserViewModel] $message"
    }
}

UserService is referenced in UserViewModel's constructor

The UserViewModel is tagged with @KoinViewModel annotation to declare the Koin ViewModel definition. This ensures proper lifecycle management and avoids memory leaks.

Injecting ViewModel in Android

The UserViewModel component will be created, resolving the UserService instance with it. To get it into our Activity, let's inject it with the by viewModel() delegate function:

class MainActivity : AppCompatActivity() {

    private val viewModel: UserViewModel by viewModel()

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

Compile Time Checks

Koin Annotations allows to check your Koin configuration at compile time. This is available by jusing the following Gradle option:

ksp {
    arg("KOIN_CONFIG_CHECK","true")
}