Creating a grid RecyclerView with quick drag and drop item swapping, Room / LiveData support, and more!

ItemAdapter.kt

class ItemAdapter(
    private val itemClickListener: (OwnedItem) -> Unit,
    private val itemSaver: (List<OwnedItem>) -> Unit
) : RecyclerView.Adapter<ItemViewHolder>() {

    val items = ArrayList<OwnedItem>()

    fun setItems(newItems: List<OwnedItem>) {
        val result = calculateDiff(newItems)
        items.clear()
        items.addAll(newItems)
        result.dispatchUpdatesTo(this)
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ItemViewHolder {
        val binding: BoardItemBinding =
            BoardItemBinding.inflate(LayoutInflater.from(parent.context), parent, false)
        return ItemViewHolder(binding, itemClickListener, itemTouchHelper::startDrag)
    }

    override fun getItemCount(): Int = items.size

    override fun onBindViewHolder(holder: ItemViewHolder, position: Int) =
        holder.bind(items[position])

    private fun calculateDiff(newItems: List<OwnedItem>) = DiffUtil.calculateDiff(object :
        DiffUtil.Callback() {

        override fun getOldListSize() = items.size

        override fun getNewListSize() = newItems.size

        override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
            return items[oldItemPosition] == newItems[newItemPosition]
        }

        override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
            val newProduct = newItems[newItemPosition]
            val oldProduct = items[oldItemPosition]
            return newProduct.id == oldProduct.id
                    && newProduct.item == oldProduct.item
                    && newProduct.board == oldProduct.board
                    && newProduct.position == oldProduct.position
        }
    })

    val itemTouchHelper = ItemTouchHelper(object : ItemTouchHelper.SimpleCallback(
        ItemTouchHelper.UP or ItemTouchHelper.DOWN or
                ItemTouchHelper.LEFT or ItemTouchHelper.RIGHT, 0
    ) {
        var oldPosition = -1
        var newPosition = -1

        override fun onMove(
            recyclerView: RecyclerView,
            viewHolder: RecyclerView.ViewHolder,
            target: RecyclerView.ViewHolder
        ): Boolean {
            newPosition = target.adapterPosition
            return false
        }

        override fun onSwiped(viewHolder: RecyclerView.ViewHolder, direction: Int) {}

        override fun onSelectedChanged(viewHolder: RecyclerView.ViewHolder?, actionState: Int) {
            super.onSelectedChanged(viewHolder, actionState)
            when (actionState) {
                ItemTouchHelper.ACTION_STATE_DRAG -> {
                    viewHolder?.adapterPosition?.let { oldPosition = it }
                }
                ItemTouchHelper.ACTION_STATE_IDLE -> {
                    if (oldPosition != -1 && newPosition != -1 && oldPosition != newPosition) {
                        val old = items[oldPosition]
                        val new = items[newPosition]

                        old.position = newPosition
                        new.position = oldPosition
                        items[oldPosition] = new
                        items[newPosition] = old

                        itemSaver.invoke(listOf(old, new))
                        notifyDataSetChanged()
                        oldPosition = -1
                        newPosition = -1
                    }
                }
            }
        }
    })
}

class ItemViewHolder(
    private val itemBinding: BoardItemBinding,
    private val itemClickListener: (OwnedItem) -> Unit,
    private val itemTouchListener: (ItemViewHolder) -> Unit
) : RecyclerView.ViewHolder(itemBinding.root) {

    private lateinit var ownedItem: OwnedItem

    fun bind(ownedItem: OwnedItem) {
        this.ownedItem = ownedItem
        Glide.with(itemBinding.root)
            .load(ownedItem.item.image)
            .into(itemBinding.image)
        if (ownedItem.item != Item.NONE) {
            itemBinding.root.setOnTouchListener { v, event ->
                if (event.actionMasked == MotionEvent.ACTION_DOWN) {
                    itemTouchListener.invoke(this)
                }
                false
            }
            itemBinding.root.setOnClickListener {
                itemClickListener.invoke(ownedItem)
            }
            itemBinding.tier.text = "Pos: ${ownedItem.position}"
        } else {
            itemBinding.tier.text = ""
        }
    }

}

table_fragment.xml

        <androidx.recyclerview.widget.RecyclerView
            android:id="@+id/itemGrid"
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:nestedScrollingEnabled="false"
            android:overScrollMode="never"
            app:layoutManager="androidx.recyclerview.widget.GridLayoutManager"
            app:layout_constraintBottom_toTopOf="@id/testButton"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@id/reputation"
            app:spanCount="5"
            tools:listitem="@layout/board_item" />

TableFragment.kt

@AndroidEntryPoint
class TableFragment : Fragment() {

    private var binding: TableFragmentBinding by autoCleared()
    private val viewModel: TableViewModel by viewModels()
    private lateinit var adapter: ItemAdapter

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        binding = TableFragmentBinding.inflate(inflater, container, false)
        binding.viewModel = viewModel
        binding.lifecycleOwner = viewLifecycleOwner
        return binding.root
    }

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

        viewModel.items.observe(viewLifecycleOwner) {
            adapter.setItems(it)
        }
    }

    private fun setupRecyclerView() {
        adapter = ItemAdapter(
            viewModel::handleItemClick,
            viewModel::saveItems
        )
        adapter.itemTouchHelper.attachToRecyclerView(binding.itemGrid)
        binding.itemGrid.setHasFixedSize(true)
        binding.itemGrid.adapter = adapter
    }

}

TableViewModel.kt

    fun handleItemClick(ownedItem: OwnedItem) {
        _textToShow.postValue(String.format("That's a %s at position %d!", ownedItem.item.name, ownedItem.position))
    }

    fun saveItems(items: List<OwnedItem>) {
        viewModelScope.launch(Dispatchers.IO) {
            itemRepository.insertItems(items)
        }
    }

Great gist. I am also looking on the merge algorithm shown in the video but not here in the gist. Can you release it, I try to figure out how to play with ItemTouchHelper tp implement it but I do not find a simple way.