abstract class CheckableAdapter<VH:RecyclerView.ViewHolder>
    : RecyclerView.Adapter<VH>() {

    companion object {
        const val CHOICE_MODE_NONE = 0      // 選択をしない
        const val CHOICE_MODE_SINGLE = 1    // 常に1つを選択
        const val CHOICE_MODE_MULTIPLE = 2  // 複数を選択
    }

    var choiceMode = CHOICE_MODE_SINGLE

    private var checkedItems = CheckedItems()
    private var attachedItems = mutableSetOf<RecyclerView.ViewHolder>()

    // --------------------------------------------------------------
    // Stable IDを使用
    // 　・デフォルトはポジションをIDにする
    // 　　（ただし、ポジションが変わるアイテムの操作は行わない前提）
    //
    init {
        setHasStableIds(true)
    }
    override fun getItemId(position: Int): Long = position.toLong()

    // --------------------------------------------------------------
    // 選択の状態を取得
    //
    fun isChecked(id: Long): Boolean = checkedItems.contains(id)
    fun getCheckedItems(): Set<Long> = checkedItems

    // --------------------------------------------------------------
    // 選択の状態の管理
    // 　・SINGLE、MULTIPLE、NONEの処理
    // 　・アイテムのクリックをトリガにして動作
    //
    fun createCheckableViewHolder(holder: VH): VH {
        holder.itemView.setOnClickListener{
            when(choiceMode) {
                CHOICE_MODE_SINGLE -> choiceSingleItem(holder)
                CHOICE_MODE_MULTIPLE -> choiceMultiItem(holder)
				CHOICE_MODE_NONE -> {}
                else -> {}
            }
        }
        return holder
    }
    private fun choiceSingleItem(holder: VH) {
        val _prevChecked = checkedItems.contains(holder.itemId)
        checkedItems.forEach() { _id ->     // 全てを未選択へ変更
            checkedItems.remove(_id)
            attachedItems.forEach {         // 表示中のアイテム⇒表示更新
                if(it.itemId == _id) notifyItemChanged(it.adapterPosition)
            }
        }
        if(! _prevChecked) {                // 状態のトグル
            checkedItems.add(holder.itemId)
            notifyItemChanged(holder.adapterPosition)
        }
    }
    private fun choiceMultiItem(holder: VH) {
        val _prevChecked = checkedItems.contains(holder.itemId)
        if(! _prevChecked)                  // 状態のトグル
            checkedItems.add(holder.itemId)
        else
            checkedItems.remove(holder.itemId)
        notifyItemChanged(holder.adapterPosition)
    }

    // --------------------------------------------------------------
    // Activityの再生成（端末の回転）時に状態を保存・復元
    //
    fun restoreInstanceState(inState: Bundle) {
        checkedItems = inState.getParcelable("CheckedItems")?: CheckedItems()
    }
    fun saveInstanceState(outState: Bundle) {
        outState.putParcelable("CheckedItems", checkedItems)
    }

    // --------------------------------------------------------------
    // 表示中のViewHolderをキャッシュ
    //
    override fun onViewAttachedToWindow(holder: VH) {
        attachedItems.add(holder)
    }
    override fun onViewDetachedFromWindow(holder: VH) {
        attachedItems.remove(holder)
    }

    // --------------------------------------------------------------
    // 選択の状態を格納
    // 　・状態はStable ID(Long型)で格納
    // 　・HashSetをBundleへ格納可能にするためParcelableを実装
    //
    private class CheckedItems(checkedItems: HashSet<Long>?)
        : HashSet<Long>(), Parcelable {
        constructor(): this(null)

        init {
            checkedItems?.let { addAll(checkedItems) }
        }

        // ----- Parcelableの定義
        override fun writeToParcel(dest: Parcel?, flags: Int) {
            dest?.writeLongArray(this.toLongArray())
        }
        override fun describeContents(): Int {
            return 0
        }

        companion object {
            @JvmField
            val CREATOR: Parcelable.Creator<CheckedItems>
            = object : Parcelable.Creator<CheckedItems> {
                override fun createFromParcel(src: Parcel): CheckedItems {
                    val _idsArray = longArrayOf().apply { src.readLongArray(this) }
                    return CheckedItems(_idsArray.toHashSet())
                }
                override fun newArray(size: Int): Array<CheckedItems?> {
                    return arrayOfNulls(size)
                }
            }
        }
    }
}

class ListAdapter(var items: MutableList<String>)
    : CheckableAdapter<ListAdapter.ListViewHolder>(){

    // ----- ビューホルダー ------------------------------------------
    class ListViewHolder(val view: View) : RecyclerView.ViewHolder(view) {
        val mesg = view.findViewById<CheckedTextView>(R.id.txtMesg)
    }

    // ----- アダプター本体 ------------------------------------------
    override fun getItemCount(): Int {
        return items.size
    }

    override fun getItemId(position: Int): Long {
        return items[position].hashCode().toLong()
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int)
            : ListViewHolder {
        Log.i("Adapter", "Create ViewHolder!! viewType = ${viewType}")
        val _view = LayoutInflater.from(parent.context).inflate(R.layout.item, parent, false)
        val _holder = ListViewHolder(_view)
        return createCheckableViewHolder(_holder)   // チェック可能なViewHolderを返す
    }

    override fun onBindViewHolder(holder: ListViewHolder, position: Int) {
        Log.i("Adapter", "  Bind   ViewHolder!! position = ${position}")
        holder.mesg.text = items[position]
        holder.mesg.isChecked = isChecked(getItemId(position))
    }
}

	...
	
    lateinit var items: MutableList<String>

    lateinit var adapter: ListAdapter
    lateinit var rcySample: RecyclerView

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

		...
        adapter = ListAdapter(items)
        val _manager = LinearLayoutManager(this)

        rcySample = findViewById<RecyclerView>(R.id.rcySample)
        rcySample.adapter = adapter
        rcySample.layoutManager = _manager

        savedInstanceState?.let {							// 復元
            adapter.restoreInstanceState(it)
        }
		
		...
	}
	
	override fun onSaveInstanceState(outState: Bundle) {	// 保存
        super.onSaveInstanceState(outState)
        adapter.saveInstanceState(outState)
    }
	
	...

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    ...>

    <CheckedTextView
        android:id="@+id/txtMesg"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:background="@drawable/check_item"
        android:textAlignment="gravity"
        android:gravity="center"
        android:text="Message"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@color/Checked" android:state_checked="true" />
    <item android:drawable="@color/Unchecked" android:state_checked="false" />
</selector>