Give value to textview inside listview with button

namespace fnb.Adapters
{
    class listMenuAdapters : BaseAdapter<listMenuModels>, IFilterable
    {
        private Context mContext;
        private List<listMenuModels> mItems;
        private List<listMenuModels> _originalData;
        private TextView txtNamaMenu;
        private TextView txtHargaMenu;
        private ImageView imageMenu;
        private TextView txtKodeMenu;
        private TextView txtQty;
        private ImageButton butPlusMenu;
        private ImageButton butMinMenu;

        public listMenuAdapters(Context context, List<listMenuModels> items)
        {
            this.mItems = items;
            this.mContext = context;

            Filter = new MenuFilter(this);
        }

        public override int Count
        {
            get
            {
                return mItems.Count;
            }
        }

        public override listMenuModels this[int position]
        {
            get
            {
                return mItems[position];
            }
        }
        

        public override long GetItemId(int position)
        {
            return position;
        }

        public override View GetView(int position, View convertView, ViewGroup parent)
        {
            View row = convertView;

            if (row == null)
            {
                row = LayoutInflater.From(mContext).Inflate(Resource.Layout.lvListMenuFnb_rowItem, null, false);
            }

            txtNamaMenu = row.FindViewById<TextView>(Resource.Id.textMenuName);
            txtNamaMenu.Text = mItems[position].namaMenu;
            txtHargaMenu = row.FindViewById<TextView>(Resource.Id.textMenuPrice);
            txtHargaMenu.Text = mItems[position].hargaJual.ToString("N0");
            txtKodeMenu = row.FindViewById<TextView>(Resource.Id.textKodeMenu);
            txtKodeMenu.Text = mItems[position].kodeMenu;
            butPlusMenu = row.FindViewById<ImageButton>(Resource.Id.ibPlusMenuFnb);
            butPlusMenu.Focusable = false;
            butPlusMenu.FocusableInTouchMode = false;
            //butPlusMenu.SetTag(position);

            butMinMenu = row.FindViewById<ImageButton>(Resource.Id.ibMinusMenuFnb);
            butMinMenu.FocusableInTouchMode = false;
            butMinMenu.Focusable = false;

            txtQty = row.FindViewById<TextView>(Resource.Id.txtQtyMenuFnb);
            //txtQty.Text = "0";

            txtKodeMenu.Visibility = ViewStates.Invisible;

            imageMenu = row.FindViewById<ImageView>(Resource.Id.imageMenu);
            imageMenu.Focusable = false;
            imageMenu.FocusableInTouchMode = false;

            if (mItems[position].pic != null)
            {
                imageMenu.SetImageBitmap(bytesToBitmap(mItems[position].pic));
            }
            else
            {
                imageMenu.SetImageResource(Resource.Drawable.noimage);
            }

            row.SetBackgroundColor(Color.Beige);

            return row;
        }

        public static Bitmap bytesToBitmap(byte[] imageBytes)
        {
            Bitmap bitmap = BitmapFactory.DecodeByteArray(imageBytes, 0, imageBytes.Length);
            return bitmap;
        }

        public Filter Filter { get; private set; }

        private class MenuFilter : Filter
        {
            private listMenuAdapters menuAdapter;

            public MenuFilter(listMenuAdapters menuAdapter)
            {
                this.menuAdapter = menuAdapter;
            }

            protected override FilterResults PerformFiltering(ICharSequence constraint)
            {
                var returnObj = new FilterResults();
                var results = new List<listMenuModels>();

                if (menuAdapter._originalData == null)
                    menuAdapter._originalData = menuAdapter.mItems;

                if (constraint == null) return returnObj;

                if (menuAdapter._originalData != null && menuAdapter._originalData.Any())
                {
                    results.AddRange(
                        menuAdapter._originalData.Where(
                            menu => menu.namaMenu.ToLower().Contains(constraint.ToString())));
                }

                returnObj.Values = FromArray(results.ToArray());

                return returnObj;
            }

            protected override void PublishResults(ICharSequence constraint, FilterResults results)
            {
                var res = new List<listMenuModels>(results.Values.ToArray<listMenuModels>());
                menuAdapter.mItems = res;
                menuAdapter.NotifyDataSetChanged();
            }
        }
    }