[Nested RecyclerView State] - Scrolling vertically

NestedRecyclerViewStateRecoverAdapter.kt

abstract class NestedRecyclerViewStateRecoverAdapter<T, VH : RecyclerView.ViewHolder>(
    diffUtil: DiffUtil.ItemCallback<T>
) : ListAdapter<T, VH>(diffUtil) {
    private val layoutManagerStates = hashMapOf<String, Parcelable?>()

    @CallSuper
    override fun onViewRecycled(holder: VH) {
        if (holder is NestedRecyclerViewViewHolder) {
            // Save the scroll position state (LayoutManager state)
            val state = holder.getLayoutManager()?.onSaveInstanceState()
            layoutManagerStates[holder.getId()] = state
        }
        super.onViewRecycled(holder)
    }

    @CallSuper
    override fun onBindViewHolder(holder: VH, position: Int) {
        if (holder is NestedRecyclerViewViewHolder) {
            holder.getLayoutManager()?.let {
                // Retrieve and set the saved LayoutManager state
                val state: Parcelable? = layoutManagerStates[holder.getId()]
                if (state != null) {
                    it.onRestoreInstanceState(state)
                } else {
                    // We need to reset the scroll position when no state needs to be restored
                    it.scrollToPosition(0)
                }
            }
        }
    }
}

// ViewHolders containing a RecyclerView should inherit this interface.
// An alternative solution could be manually searching if the view constains a RecyclerView
interface NestedRecyclerViewViewHolder {
    fun getId(): String
    fun getLayoutManager(): RecyclerView.LayoutManager?
}