【Android】＜Kotlin＞ItemTouchHelperを使って、ドラッグ&ドロップでリスト表示の項目を並び替える方法を徹底解説！

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerView"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

class MainActivity : AppCompatActivity() {

    private lateinit var recyclerView: RecyclerView
    private lateinit var adapter: ItemAdapter

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

        recyclerView = findViewById(R.id.recyclerView)
        val items = mutableListOf("Item 1", "Item 2", "Item 3", "Item 4")

        adapter = ItemAdapter(items)
        recyclerView.layoutManager = LinearLayoutManager(this)
        recyclerView.adapter = adapter

        val callback = ItemMoveCallback(adapter)
        val itemTouchHelper = ItemTouchHelper(callback)
        itemTouchHelper.attachToRecyclerView(recyclerView)
    }
}

class ItemMoveCallback(private val adapter: ItemAdapter) : ItemTouchHelper.Callback() {

    override fun getMovementFlags(recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder): Int {
        val dragFlags = ItemTouchHelper.UP or ItemTouchHelper.DOWN
        return makeMovementFlags(dragFlags, 0)
    }

    override fun onMove(
        recyclerView: RecyclerView,
        viewHolder: RecyclerView.ViewHolder,
        target: RecyclerView.ViewHolder
    ): Boolean {
        val fromPosition = viewHolder.adapterPosition
        val toPosition = target.adapterPosition
        adapter.onItemMove(fromPosition, toPosition)
        return true
    }
}

class ItemAdapter(private val items: MutableList<String>) : RecyclerView.Adapter<ItemAdapter.ViewHolder>() {

    class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        val textView: TextView = itemView.findViewById(R.id.itemText)
    }

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

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.textView.text = items[position]
    }

    override fun getItemCount(): Int = items.size

    fun onItemMove(fromPosition: Int, toPosition: Int) {
        Collections.swap(items, fromPosition, toPosition)
        notifyItemMoved(fromPosition, toPosition)
    }
}

override fun onSelectedChanged(viewHolder: RecyclerView.ViewHolder?, actionState: Int) {
    super.onSelectedChanged(viewHolder, actionState)
    if (actionState == ItemTouchHelper.ACTION_STATE_DRAG) {
        viewHolder?.itemView?.setBackgroundColor(Color.LTGRAY)
    }
}

override fun clearView(recyclerView: RecyclerView, viewHolder: RecyclerView.ViewHolder) {
    super.clearView(recyclerView, viewHolder)
    viewHolder.itemView.setBackgroundColor(Color.WHITE)
}