Expandable List Fragment

BrandItem.kt

import android.view.View
import com.xwray.groupie.ExpandableGroup
import com.xwray.groupie.ExpandableItem
import com.xwray.groupie.Item
import com.xwray.groupie.ViewHolder
import kotlinx.android.extensions.LayoutContainer
import kotlinx.android.synthetic.main.list_item_brand.*

class BrandItem(private val brand: String): Item<BrandItem.BrandViewHolder>(), ExpandableItem {

  private var expandableGroup: ExpandableGroup? = null

  override fun getLayout(): Int {
    return R.layout.list_item_brand
  }

  override fun createViewHolder(itemView: View): BrandViewHolder {
    val holder = BrandViewHolder(itemView)
    holder.expandCollapse.setOnClickListener {
      expandableGroup?.onToggleExpanded()
    }
    return holder
  }

  override fun bind(holder: BrandViewHolder, position: Int) {
    holder.brandName.text = brand
  }

  override fun setExpandableGroup(group: ExpandableGroup) {
    expandableGroup = group
  }

  class BrandViewHolder(override val containerView: View): ViewHolder(containerView), LayoutContainer
}

ExpandableListFragment.kt

import android.os.Bundle
import android.support.v4.app.Fragment
import android.support.v7.widget.GridLayoutManager
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import com.xwray.groupie.*
import kotlinx.android.synthetic.main.fragment_main.*

class KotlinSyntheticFragment: Fragment() {

  companion object {
    val phones = mapOf(
      "Apple" to listOf(
        "iPhone",
        "iPhone 3G",
        "iPhone 3GS",
        "iPhone 4",
        "iPhone 4S",
        "iPhone 5",
        "iPhone 5c",
        "iPhone 5s",
        "iPhone 6",
        "iPhone 6 Plus",
        "iPhone 6s",
        "iPhone 6s Plus",
        "iPhone SE",
        "iPhone 7",
        "iPhone 7 Plus",
        "iPhone 8",
        "iPhone 8 Plus",
        "iPhone X"
      ),
      "Google" to listOf(
        "Pixel",
        "Pixel XL",
        "Pixel 2",
        "Pixel 2 XL"
      ),
      "HTC" to listOf(
        "Nexus One"
      ),
      "Huawei" to listOf(
        "Nexus 6P"
      ),
      "LG" to listOf(
        "Nexus 5",
        "Nexus 5X"
      ),
      "Motorola" to listOf(
        "Nexus 6"
      ),
      "Samsung" to listOf(
        "Nexus S",
        "Galaxy Nexus"
      )
    )
  }

  private val groupAdapter = GroupAdapter<ViewHolder>()

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

  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    val layoutManager = GridLayoutManager(activity, groupAdapter.spanCount)
    layoutManager.spanSizeLookup = groupAdapter.spanSizeLookup

    phones.forEach { (brand, phones) ->
      val brandItem = BrandItem(brand)
      val expandableGroup = ExpandableGroup(brandItem)
      phones.forEach { phone ->
        expandableGroup.add(PhoneItem(phone))
      }
      groupAdapter.add(expandableGroup)
    }

    recyclerView.layoutManager = layoutManager
    recyclerView.adapter = groupAdapter
  }
}

PhoneItem.kt

import android.view.View
import com.xwray.groupie.Item
import com.xwray.groupie.ViewHolder
import kotlinx.android.extensions.LayoutContainer
import kotlinx.android.synthetic.main.list_item_phone.*

class PhoneItem(private val phone: String): Item<PhoneItem.PhoneViewHolder>() {

  override fun getLayout(): Int {
    return R.layout.list_item_phone
  }

  override fun createViewHolder(itemView: View): PhoneViewHolder {
    return PhoneViewHolder(itemView)
  }

  override fun bind(holder: PhoneViewHolder, position: Int) {
    holder.phoneName.text = phone
  }

  class PhoneViewHolder(override val containerView: View): ViewHolder(containerView), LayoutContainer
}