不多說 先上圖

RecycleView的項目，如果要作到左右滑動以及上下移動位置
可以透過ItemTouchHelper的ItemTouchHelper.Callback來實作。

今天先來講上下拖曳的部分。

先宣告一個class 繼承 ItemTouchHelper.Callback

程式碼

public class ItemTouchHelperCallback extends ItemTouchHelper.Callback {

    private ItemTouchHelperAdapter mAdapter;

    public ItemTouchHelperCallback(ItemTouchHelperAdapter adapter) {
        mAdapter = adapter;
    }

    @Override
    public int getMovementFlags(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
        int dragFlags = ItemTouchHelper.UP | ItemTouchHelper.DOWN;
        int swipeFlags = ItemTouchHelper.ACTION_STATE_IDLE;
        return makeMovementFlags(dragFlags, swipeFlags);
    }

    @Override
    public boolean isLongPressDragEnabled() {
        return true;
    }

    @Override
    public boolean isItemViewSwipeEnabled() {
        return false;
    }

    @Override
    public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
        mAdapter.onItemMove(viewHolder.getAdapterPosition(), target.getAdapterPosition());
        return true;
    }

    @Override
    public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction) {

    }
}

其中getMovementFlags函式可以指定項目移動的方向

int dragFlags = ItemTouchHelper.UP | ItemTouchHelper.DOWN;
指定拖曳方向往上還是往下，使用|符號表示兩者任一都成立
int swipeFlags = ItemTouchHelper.LEFT| ItemTouchHelper.RIGHT;
指定滑動方向往左還是往右，使用|符號表示兩者任一都成立
這邊設定

int swipeFlags = ItemTouchHelper.ACTION_STATE_IDLE;

onMove函式可以處理上下拖曳時的動作
onSwiped函式可以處理左右滑動時的動作

之後宣告一個interfece 來作為 RecycleView Adapter與ItemTouchHelper.Callback的接口

interface ItemTouchHelperAdapter {
    fun onItemMove( fromPosition : Int, toPosition :Int)
    fun onItemDissmiss(position: Int)
}

接下來實作RecyclerView.Adapter

open class recycleViewAdapter(val context: Context, val recordList: MutableList<String>
) :
    RecyclerView.Adapter<recycleViewAdapter.ViewHolder>()
    , ItemTouchHelperAdapter
{

    lateinit var mViewHolder : recycleViewAdapter.ViewHolder
    override fun onItemDissmiss(position: Int) {
        recordList.removeAt(position)
        notifyItemRemoved(position)
    }

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

由於我們在adapter引入了interface ItemTouchHelperAdapter，
所以要override ItemTouchHelperAdapter的兩個方法
onItemDissmiss: 處理滑動時adapter動作
onItemMove: 處理拖曳時adapter動作

之後在activity中，將ItemTouchHelperCallback、ItemTouchHelper與adapter綁定

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
...
        var adapter = recycleViewAdapter(this,mlist)
        rv_this.adapter =adapter
        val layoutManager = LinearLayoutManager(this!!)
        rv_this.layoutManager = layoutManager
        
        val callback = ItemTouchHelperCallback(adapter)
        val touchHelper = ItemTouchHelper(callback)
        touchHelper.attachToRecyclerView(rv_this)
    }
}

宣告callback變數將adaptert傳入ItemTouchHelperCallback，並交由touchHelper綁定MainActivity的RecycleView元件

到這邊就可以上下拖曳的實作就完成了