Kotlin DSL example

KotlinDsl.kt

@DslMarker
annotation class HorizontalSelectorGroups

interface HorizontalSelectorHelper {
  fun groups(block: GroupsBuilder.() -> Unit): List<Pair<Header, List<Item>>> = GroupsBuilder().apply(block).build()

  @HorizontalSelectorGroups
  class GroupsBuilder {
    private val groups = mutableListOf<Group>()

    fun group(block: GroupBuilder.() -> Unit) {
      groups.add(GroupBuilder().apply(block).build())
    }

    fun build(): List<Pair<Header, List<Item>>> = groups.map { group -> Pair(group.header, group.items) }
  }

  class Group(
    val header: Header,
    val items: List<Item>
  )

  @HorizontalSelectorGroups
  class GroupBuilder {
    var header = Header()
    private val items = mutableListOf<Item>()

    fun items(block: ItemsBuilder.() -> Unit) {
      items.addAll(ItemsBuilder().apply(block).build())
    }

    fun build(): Group =
      Group(header = header, items = items)
  }

  @HorizontalSelectorGroups
  class ItemsBuilder {
    private val items = mutableListOf<Item>()

    operator fun Item.unaryPlus() {
      items += this
    }

    operator fun Item.unaryMinus() {
      items += this
    }

    fun item(block: ItemBuilder.() -> Unit) {
      items.add(ItemBuilder().apply(block).build())
    }

    fun build() = items
  }

  @HorizontalSelectorGroups
  class ItemBuilder {
    var uuid: String = String.EMPTY
    var title: String = String.EMPTY

    fun build() = Item(uuid = uuid, title = title)
  }
}