这是我的recyclerview代码，我想在单击按钮plus时增加一个项目产品的数量。

class RecyclerAdapterMain(    
    val product: ArrayList<ModelProductMain>    
) :
    RecyclerView.Adapter<RecyclerAdapterMain.ViewHolder>() {    

    class ViewHolder(itemview: View) : RecyclerView.ViewHolder(itemview) {    
        val title: TextView = itemview.product_txt
        val price: TextView = itemview.price_product
        val imageproduct: ImageView = itemview.product_image
        val additem: Button = itemview.btn_icon_add
        val rl_add = itemview.rl_section_additive_items
        val amount: TextView = itemview.amount_value
        val btnadd: Button = itemview.button_add_product    
    }

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

    override fun getItemCount() = product.size    
    override fun onBindViewHolder(holder: ViewHolder, position: Int) {    

        val products = product[position]
        holder.title.text = products.title
        holder.price.text = products.price.toString()
        Picasso.get().load(products.image).into(holder.imageproduct)
        holder.amount.text = products.amount.toString()


        holder.itemView.setOnClickListener {

            val bundle = Bundle()
            val myfragment = ItemDetailsfragment()
            myfragment.arguments = bundle


            val activity = it.context as AppCompatActivity
            activity.supportFragmentManager.beginTransaction()
                .replace(R.id.homepage, myfragment)
                .commit()


            bundle.putString("title", products.title)
            bundle.putString("price", products.price.toString())
            bundle.putString("image", products.image.toString())


        }

        holder.rl_add.visibility = View.GONE

        holder.additem.setOnClickListener {

            holder.rl_add.visibility = View.VISIBLE
            holder.additem.visibility = View.GONE
        }

        holder.btnadd.setOnClickListener {    
            products.amount++    
        }    
    }    
}

如果我想说我想从这段代码中得到什么，那就是:当我点击加号按钮时，产品项目的数量会增加。我试了那么多，但不能增加产品的数量。我该怎么修好呢？