Koin – Dependency Injection for Kotlin

Intro

Koin is a lightweight dependency injection library for Kotlin and written in pure Kotlin. using functional resolution only: no proxy, no code generation, no reflection. Koin is a DSL, using Kotlin’s DSLs, that describes your dependencies into modules and sub-modules. You describe your definitions with certain key functions that mean something in the Koin context.

In this article, we are going to pass through the basic steps to setup and take advantage of dependency injection with Koin for a simple project.

1. Setup

// Add Jcenter to your repositories if needed
repositories {
    jcenter()
}
dependencies {
    // Koin for Android
    implementation 'org.koin:koin-android:{revnumber}'
}

2. Declare a module

Defines those entities which will be injected in the app.

val appModule = module {
	single { DataRepository }
}

3. Use Application layer to initialize Koin injection

Trigger the DI process and indicate which modules will be available when needed

class BaseApplication : Application() {

  override fun onCreate() {
    super.onCreate()
    startKoin(this, appModule)
  }

}

4. Use the actual injection

class MainActivity : AppCompatActivity() {
  private val repository: DataRepository by inject()
  ...
}

This was the basic usage of koin which has much more power/features available in its framework. Another great Koin feature is the integration with the Architecture Component View Model. Koin has a specif project called koin-android-viewmodel project is dedicated to inject Android Architecture ViewModel.

Architecture Components with Koin: ViewModel

1. Setup

// Add Jcenter to your repositories if needed
repositories {
    jcenter()
}
dependencies {
    // ViewModel for Android
    implementation 'org.koin:koin-android-viewmodel:{revnumber}'
    // or ViewModel for AndroidX
    implementation 'org.koin:koin-androidx-viewmodel:{revnumber}'
}

2. Declare a module for your viewModule

val appModule = module {
    // ViewModel for News View with a dependency in its constructor
    viewModel { NewsViewModel(get()) }
}

3. Use the actual injection

class MainActivity() : AppCompatActivity() {

  // lazy inject MyViewModel
  val viewModel : NewsViewModel by viewModel()
}

Koin Key Functions

• module { } - create a Koin Module or a submodule (inside a module)
• factory { } - provide a factory bean definition
• single { } - provide a bean definition
• get() - resolve a component dependency
• bind - additional Kotlin type binding for given bean definition
• getProperty() - resolve a property

Futher Details

I highly recommend the official docs and the following articles: Koin - Simple Android DI and Ready for Koin 2.0 by Arnaud Giuliani