Androidで複数ViewHolder, SpanSizeしたときのItemDecoration

/**
 * 1,2,3カラムの [RecyclerView.ViewHolder] を表示する [GridLayoutManager]
 */
class GridDecorationLayoutManager(context: Context, val adapter: GridItemAdapter)
    : GridLayoutManager(context, 6, GridLayout.VERTICAL, false) {

    init {
        spanSizeLookup = object : SpanSizeLookup() {
            override fun getSpanSize(position: Int): Int {
                // 1カラム→6, 2カラム→3, 3カラム→2
                return adapter.getSpanSize(position)
            }
        }
    }
}

/**
 * [RecyclerView.ViewHolder] とSpanSizeによって余白を分ける [RecyclerView.ItemDecoration]
 */
class GridItemDecoration(context: Context) : RecyclerView.ItemDecoration() {

    private val margin = context.resources.getDimensionPixelSize(R.dimen.grid_decoration_margin)

    override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) {
        val params = view.layoutParams as? GridLayoutManager.LayoutParams
        val manager = parent.layoutManager as? GridLayoutManager
        val spanIndex = params?.spanIndex ?: 0
        val spanSize = params?.spanSize ?: 1
        val spanCount = manager?.spanCount ?: 1

        val isTop = parent.getChildAdapterPosition(view) == 0
        val isStart = spanIndex == 0
        val isEnd = spanIndex + spanSize == spanCount

        when (parent.findContainingViewHolder(view)) {
            is LargeViewHolder -> {
                // 1カラムで表示するViewHolder、最上部のとき上方向に余白をつける
                outRect.set(margin, if (isTop) margin else 0, margin, margin)
            }
            is MediumViewHolder -> {
                // 2カラムで表示するViewHolder、最上部のとき上方向に余白をつける、端かどうかで余白を変える
                outRect.set(
                        if (isStart) margin else margin / 2,
                        if (isTop) margin else 0,
                        if (isEnd) margin else margin / 2,
                        margin)
            }
            is SmallViewHolder -> {
                // 3カラムで表示するViewHolder、最上部のとき上方向に余白をつける、端かどうかで余白を変える
                outRect.set(
                        if (isStart) margin else if (isEnd) margin / 3 else margin * 2 / 3,
                        if (isTop) margin else 0,
                        if (isStart) margin / 3 else if (isEnd) margin else margin * 2 / 3,
                        margin)
            }
        }
    }
}