Dagger Android 2.26 simple example with support for Singleton, PerActivity, PerFragment, and PerChildFragment scopes in Kotlin

Dagger2SimpleExample.kt

// This is a super simplified example of how to use the dagger.android framework
// This example also shows how to scope subcomponents to higher level subcomponents: 
// Activity -> Fragment -> Fragment
// It allows you to create a single instances of classes for an Activity and its Fragments.
// This example works with Dagger 2.26. 

// App.kt
class App: Application(), HasAndroidInjector {

    @Inject
    lateinit var appDependency: AppDependency

    @Inject
    lateinit var androidInjector: DispatchingAndroidInjector<Any>

    override fun onCreate() {
        super.onCreate()
        DaggerAppComponent.create().inject(this)
    }

    override fun androidInjector(): AndroidInjector<Any> {
        return androidInjector;
    }
}

// AppModule.kt
@Module(includes = [AndroidInjectionModule::class])
abstract class AppModule {
    @PerActivity
    @ContributesAndroidInjector(modules = [MainActivityModule::class])
    abstract fun mainActivityInjector(): MainActivity
}

// AppComponent.jkt
@Singleton
@Component(modules = [AppModule::class])
interface AppComponent {
    fun inject(app: App)
}

// MainActivity.kt
// Could also extend DaggerActivity instead of implementing HasAndroidInjector
class MainActivity: Activity(), HasAndroidInjector {

    @Inject
    lateinit var appDependency: AppDependency // same object from App
  
    @Inject
    lateinit var activityDependency: ActivityDependency
  
    @Inject
    lateinit var fragmentInjector: DispatchingAndroidInjector<Any>
  
    override fun onCreate(savedInstanceState: Bundle?) {
        AndroidInjection.inject(this)
        super.onCreate(savedInstanceState)
        setContentView(R.layout.main_activity)

        if (savedInstanceState == null) {
            addFragment(R.id.fragment_container, MainFragment())
        }
    }
    
    override fun androidInjector(): AndroidInjector<Any> {
        return fragmentInjector
    }

    private fun addFragment(@IdRes containerViewId: Int, fragment: Fragment) {
        getFragmentManager.beginTransaction()
            .add(containerViewId, fragment)
            .commit()
    }
}

// MainActivityModule.kt
@Module
abstract class MainActivityModule {
    @PerFragment
    @ContributesAndroidInjector(modules = [MainFragmentModule::class])
    abstract fun mainFragmentInjector(): MainFragment
}

// MainFragment.kt
class MainFragment : Fragment(), HasAndroidInjector {
    
    @Inject
    lateinit var appDependency: AppDependency // same object from App

    @Inject
    lateinit var activityDependency: ActivityDependency // same object from MainActivity

    @Inject
    lateinit var fragmentDependency: FragmentDependency

    @Inject
    lateinit var childFragmentInjector: DispatchingAndroidInjector<Any>
    
    fun onAttach(context: Context?) {
        AndroidInjection.inject(this)
        super.onAttach(context)
    }

    fun onCreateView(
        inflater: LayoutInflater, 
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        return inflater.inflate(R.layout.main_fragment, container, false)
    }

    fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        if (savedInstanceState == null) {
            addChildFragment(R.id.child_fragment_container, MainChildFragment())
        }
    }

    override fun androidInjector(): AndroidInjector<Any> {
        return childFragmentInjector
    }

    private fun addChildFragment(@IdRes viewId: Int, fragment: Fragment) {
        getChildFragmentManager()
            .beginTransaction()
            .add(viewId, fragment)
            .commit()
    }
}

// MainFragmentModule.kt
@Module
abstract class MainFragmentModule {
    @PerChildFragment
    @ContributesAndroidInjector(modules = [MainChildFragmentModule::class])
    abstract fun mainChildFragmentInjector(): MainChildFragment
}

// MainChildFragment.kt
class MainChildFragment : Fragment() {

    @Inject
    lateinit var appDependency: AppDependency // same object from App

    @Inject
    lateinit var activityDependency: ActivityDependency // same object from MainActivity

    @Inject
    lateinit var fragmentDependency: FragmentDependency // same object from MainFragment

    @Inject
    lateinit var childFragmentDependency: ChildFragmentDependency

    fun onAttach(context: Context?) {
        AndroidInjection.inject(this)
        super.onAttach(context)
    }

    fun onCreateView(
        inflater: LayoutInflater, @Nullable container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        return inflater.inflate(R.layout.main_child_fragment, container, false)
    }
    
}

// MainChildFragmentModule.kt
@Module
abstract class MainChildFragmentModule {
}

// PerActivity.kt
@Scope
@Retention(RetentionPolicy.RUNTIME)
annotation class PerActivity

// PerFragment.kt
@Scope
@Retention(RetentionPolicy.RUNTIME)
annotation class PerFragment

// PerChildFragment.kt
@Scope
@Retention(RetentionPolicy.RUNTIME)
annotation class PerChildFragment

// AppDependency.kt
@Singleton
class AppDependency @Inject internal constructor()

// ActivityDependency.kt
@PerActivity
class ActivityDependency @Inject internal constructor()

// FragmentDependency.kt
@PerFragment
class FragmentDependency @Inject internal constructor() 

// ChildFragmentDependency.kt
@PerChildFragment
class ChildFragmentDependency @Inject internal constructor()