components.kt

/**
 * Nice annotation instead of putting a comment
 * Only for readability purpose
 */
@Target(AnnotationTarget.PROPERTY)
@Retention(AnnotationRetention.SOURCE)
private annotation class Provides

/**
 * Nice annotation instead of putting a comment
 * Only for readability purpose
 */
@Target(AnnotationTarget.PROPERTY)
@Retention(AnnotationRetention.SOURCE)
private annotation class Requires

/**
 * Takes a function to construct a part of a composite component
 */
private fun <T: Any> part(builder: () -> T) = lazy(builder)

/**
 * Takes a function to construct a part of a composite component
 * and a function to be executed with the constructed component
 * as receiver to proceed to services binding
 */
private fun <T: Any> part(builder: () -> T, binder: T.() -> Unit) =
    lazy {
        val instance = builder()
        instance.binder()
        instance
    }

/**
 * Some client component type
 */
private interface ClientType {
    var service: () -> Unit
}

/**
 * Some client component implementation
 */
private class ClientImpl: ClientType {
    @Requires
    override lateinit var service: () -> Unit
}

/**
 * Some server component type
 */
private interface ServerType {
    val serverService: () -> Unit
}

/**
 * Some server component implementation
 */
private class ServerImpl: ServerType {
    @Provides
    override val serverService = { println("Helloworld!") }
}

/**
 * Some composite component type
 */
private abstract class CompositeType {

    private val client: ClientType by part(::createClient) {
        service = server.serverService
    }

    private val server: ServerType by part(::createServer)

    @Provides
    val service = client.service

    abstract fun createClient(): ClientType

    abstract fun createServer(): ServerType
}

/**
 * Some composite component implementation
 */
private class CompositeImpl: CompositeType() {
    override fun createClient() = ClientImpl()
    override fun createServer() = ServerImpl()
}

fun main() {
    val component = CompositeImpl()
    component.service()
}