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

        val _folder = "4000x3000"
        val _row = 4

        val _photos = assets.list(_folder)
        val _items = List(100) { PhotoData(it, "${_folder}/${_photos!![it]}") }

        val _adapter = PhotoAdapter(_items)
        val _manager = GridLayoutManager(this, _row)

        val _photoList = findViewById<RecyclerView>(R.id.rcyPhotoList)
        _photoList.adapter = _adapter
        _photoList.layoutManager = _manager
    }

data class PhotoData(
    val id: Int = 0,
    val filename: String = "default"
)

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:padding="1dp">

    <ImageView
        android:id="@+id/imgPhoto"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:scaleType="centerCrop"
        app:layout_constraintDimensionRatio="4:3"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:srcCompat="@android:drawable/ic_menu_gallery" />

</androidx.constraintlayout.widget.ConstraintLayout>

class PhotoAdapter(val items: List<PhotoData>)
    : RecyclerView.Adapter<PhotoAdapter0.PhotoViewHolder>() {

    // ----- ビューホルダー ------------------------------------------
    class PhotoViewHolder(val view: View) 
        : RecyclerView.ViewHolder(view) {
            val photo = view.findViewById<ImageView>(R.id.imgPhoto)
        }

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

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int)
            : PhotoViewHolder {
        val _view = LayoutInflater.from(parent.context)
            .inflate(R.layout.photo_item, parent, false)
        return PhotoViewHolder(_view)
    }

    override fun onBindViewHolder(holder: PhotoViewHolder, position: Int) {
        val _filename = items[position].filename
        val _iStream = holder.view.context.assets.open(_filename)
        val _bitmap = BitmapFactory.decodeStream(_iStream)
        holder.photo.setImageBitmap(_bitmap)
    }
}