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 -> (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)

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

Let's declare a AppModule module class like below.

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

• We use the @Module to declare our class as Koin module
• The @ComponentScan("org.koin.sample") allow to scan any Koin definition in "org.koin.sample"package

Let's simply add @Single on UserRepositoryImpl class to declare it as singleton:

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

Displaying User with Presenter

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

class UserPresenter(private val repository: UserRepository) {

    fun sayHello(name : String) : String{
        val foundUser = repository.findUser(name)
        return foundUser?.let { "Hello '$it' from $this" } ?: "User '$name' not found!"
    }
}

UserRepository is referenced in UserPresenter`s constructor

We declare UserPresenter in our Koin module. We declare it as a factory definition with the @Factory annotation, to not keep any instance in memory (avoid any leak with Android lifecycle):

@Factory
class UserPresenter(private val repository: UserRepository) {
    // ...
}

Injecting Dependencies in Android

The UserPresenter component will be created, resolving the UserRepository 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. Just call the startKoin() function in the application's main entry point, our MainApplication class:

// generated
import org.koin.ksp.generated.*

class MainApplication : Application(){
    override fun onCreate() {
        super.onCreate()
        
        startKoin{
            androidLogger()
            androidContext(this@MainApplication)
            modules(AppModule().module)
        }
    }
}

The Koin module is generated from AppModule with the .module extension: Just use the AppModule().module expression to get the Koin module from the annotations.

info

The import org.koin.ksp.generated.* import is required to allow to use generated Koin module content

Displaying User with ViewModel

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

@KoinViewModel
class UserViewModel(private val repository: UserRepository) : ViewModel() {

    fun sayHello(name : String) : String{
        val foundUser = repository.findUser(name)
        return foundUser?.let { "Hello '$it' from $this" } ?: "User '$name' not found!"
    }
}

UserRepository is referenced in UserViewModel`s constructor

The UserViewModel is tagged with @KoinViewModel annotation to declare the Koin ViewModel definition, to not keep any instance in memory (avoid any leak with Android lifecycle).

Injecting ViewModel in Android

The UserViewModel component will be created, resolving the UserRepository 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")
}

Verifying your App!

We can ensure that our Koin configuration is good before launching our app, by verifying our Koin configuration with a simple JUnit Test.

Gradle Setup

Add the Koin Android dependency like below:

// Add Maven Central to your repositories if needed
repositories {
    mavenCentral()    
}

dependencies {
    
    // Koin for Tests
    testImplementation "io.insert-koin:koin-test-junit4:$koin_version"
}

Checking your modules

The androidVerify() function allow to verify the given Koin modules:

class CheckModulesTest : KoinTest {

    @Test
    fun checkAllModules() {

        AppModule().module.androidVerify()
    }
}

With just a JUnit test, you can ensure your definitions configuration are not missing anything!