HiltUserComponent.kt

@DefineComponent(parent = ApplicationComponent::class)
internal interface UserComponent

@DefineComponent.Builder
internal interface UserComponentBuilder {
    fun withUserId(@BindsInstance userId: Int): UserComponentBuilder
    fun build(): UserComponent
}

@DefineComponent(parent = UserComponent::class)
internal interface ActivityUserComponent

@DefineComponent.Builder
internal interface ActivityUserComponentBuilder {
    fun withActivity(@BindsInstance activity: Activity): ActivityUserComponentBuilder
    fun build(): ActivityUserComponent
}

@EntryPoint
@InstallIn(UserComponent::class)
internal interface ActivityUserComponentBuilderEntryPoint {
    fun activityUserComponentBuilder(): ActivityUserComponentBuilder
}

@EntryPoint
@InstallIn(ActivityUserComponent::class)
interface CalendarInjector {
    fun inject(calendarActivity: CalendarActivity)
}

internal var currentUser: UserComponent? = null

internal fun createUserComponent(application: Application, userId: Int = 1) =
    (application as CalendarApp).userComponentBuilder.withUserId(userId)
        .build()

internal fun createActivityComponent(activity: Activity): ActivityUserComponent {
    val userComponent: UserComponent = currentUser ?: createUserComponent(activity.application)
    val buildEntryPoint = userComponent as ActivityUserComponentBuilderEntryPoint
    return buildEntryPoint
        .activityUserComponentBuilder()
        .withActivity(activity)
        .build()
}

@HiltAndroidApp
class CalendarApp : Application(){
   @Inject
   internal lateinit var userComponentBuilder: UserComponentBuilder
}

@EntryPoint
@InstallIn(ActivityUserComponent::class)
interface CalendarInjector {
    fun inject(calendarActivity: CalendarActivity)
}

private fun calendarInjector(calendarActivity: CalendarActivity) {
    val activityComponent = createActivityComponent(calendarActivity)
    EntryPoints.get(activityComponent, CalendarInjector::class.java).inject(calendarActivity)
}
class CalendarActivity : AppCompatActivity() {
    @Inject
    lateinit var userId: Integer

    @Inject
    lateinit var activity: Activity
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        calendarInjector(this)
    }
}