Implementing Dagger2 with Kotlin

Most of you have heard about Kotlin which is now officially supported from Google itself. Thus I wont deep-dive into Kotlin, but you can follow this and this link as a starting point.

I am going to explain on how to implement Dagger 2 in your Android projects for using with Kotlin.

Firstly you have to configure Gradle build tool for supporting Kotlin

• Add Kotlin Gradle Plugin in your top build.gradle file:

classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"

• Add Kotlin plugins in your inner build.gradle file:

//Kotlin android extensions
apply plugin: 'kotlin-android' 
apply plugin: 'kotlin-android-extensions'
//Kotlin Annotation Processing (KAPT) plugin
apply plugin: 'kotlin-kapt'

• Add dependency to your dependencies in inner build.gradle file:

implementation "com.google.dagger:dagger:$daggerVersion"
kapt "com.google.dagger:dagger-compiler:$daggerVersion"

Creating Module

Suppose we want to create module named AppModule which takes our App class as a constructor parameter:

@Module
class AppModule(val app: App) {

    @Provides
    fun provideApp(): App = app
}

this will be equivalent to:

@Module
public class AppModule {
  private final App app;
  public AppModule(App app) {
     this.app = app;
  }

  @Provides
  Application provideApp() {
     return app;
  }
}

Creating component

We create AppComponent by injecting App class as below:

@Singleton
@Component(modules = arrayOf(AppModule::class))
interface AppComponent {

    fun inject(app: App)

}

which is equivalent to:

@Singleton
@Component(modules = AppModule.class)
public interface AppComponent {

  void inject(App app);
}

if you want to include a dependency on your @Component you can do it same as module: dependencies = arrayOf(AnotherComponent::class)

and finally we implement it on our App:

class App : Application() {

    val appComponent: AppComponent by lazy {
        DaggerAppComponent
                .builder()
                .appModule(AppModule(this))
                .build()
    }

    override fun onCreate() {
        super.onCreate()

        appComponent.inject(this)
    }

    fun appComponent() : AppComponent = appComponent
}

Creating custom scope

If you want to create a custom scope you can do it as following:

@Qualifier
@Retention(AnnotationRetention.RUNTIME)
annotation class AppContextScope

notice that AnnotationRetention is a kotlin annotation(kotlin.annotation)

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