我想在加载recyclerview中的元素（大约要填充150个元素）时显示活动中的progressbar（在subitemadapter.kt中），但现在这个progressbar根本不显示。这是我的密码：
currencylistfragment.kt

class CurrencyListFragment : Fragment(), MainContract.View {

    companion object {
        private val TAG = CurrencyListFragment::class.qualifiedName
    }

    private val restModel: RestModel = RestModel()
    private val handler: Handler = Handler(Looper.getMainLooper())
    private lateinit var mainPresenter: MainPresenter
    private lateinit var itemAdapter: ItemAdapter
    private lateinit var _layoutManager: LinearLayoutManager
    private lateinit var onChangeFragment: OnChangeFragment
    private lateinit var currentDate: String
    private var isLoading: Boolean = false
    private var apiResponseList: MutableList<ApiResponse> = arrayListOf()
    private var listSize: Int = 0

    override fun onAttach(context: Context) {
        super.onAttach(context)
        try {
            if (activity is OnChangeFragment) onChangeFragment = activity as OnChangeFragment
        } catch (error: ClassCastException) {
            error.message?.let { Log.e(TAG, it) }
        }
    }

    // @formatter:off
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.currency_list_fragment, container, false)
    }
    // @formatter:on

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        _layoutManager = LinearLayoutManager(activity)
        mainPresenter = MainPresenter(this, restModel, SharedPreferencesModel(activity as Activity))
        currentDate = mainPresenter.convertCurrentDate()
        if (mainPresenter.checkIfSuchDateExistsinSp(currentDate)) {
            Log.i(TAG, "Date $currentDate already exists in SharedPreferences")
            mainPresenter.processDateWithoutMakingACall(currentDate)
        } else {
            mainPresenter.makeACall(currentDate)
            Log.i(TAG, "Date $currentDate does not exist in SharedPreferences. Retrofit call made")
        }
        mainPresenter.saveNumberOfMinusDaysIntoSp(0)
        addScrollerListener()
    }

    override fun showProgressBarOnLoadingCurrencies() {
        progress_bar.visibility = View.VISIBLE
    }

    override fun hideProgressBarOnFinishedLoadingCurrencies() {
        progress_bar.visibility = View.GONE
    }

    override fun setRecyclerViewStateToLoading() {
        if (apiResponseList.size > 0) {
            apiResponseList.add(ApiResponse("", "", listOf(Currency("", 0f)), true))
            itemAdapter.notifyItemInserted(apiResponseList.size - 1)
        }
    }

    override fun removeRecyclerViewStetOfLoading() {
        if (apiResponseList.size > 1) {
            apiResponseList.removeAt(apiResponseList.size - 1)
            listSize = apiResponseList.size
            itemAdapter.notifyItemRemoved(listSize)
        }
        isLoading = false
    }

    override fun getApiResponseList(): List<ApiResponse> {
        return apiResponseList
    }

    override fun showLogAboutExistingDateInSp(date: String) {
        Log.i(TAG, "Date $date already exists in SharedPreferences (new element)")
    }

    override fun showLogAboutNotExistingDateInSp(date: String) {
        Log.i(TAG, "Date $date does not exist in SharedPreferences. Retrofit call made (new element)")
    }

    override fun assignResponseToRecyclerview(apiResponse: ApiResponse?) {
        rv_item.apply {
            layoutManager = _layoutManager
            apiResponseList.add(apiResponse!!)
            itemAdapter = activity?.let { ItemAdapter(apiResponseList, it) }!!
            adapter = itemAdapter
        }
    }

    private fun addScrollerListener() {
        rv_item.addOnScrollListener(object : RecyclerView.OnScrollListener() {
            override fun onScrollStateChanged(rvItem: RecyclerView, newState: Int) {
                super.onScrollStateChanged(rvItem, newState)
                mainPresenter.processRvitemOnScroll(isLoading, rvItem, newState)
            }
        })
    }

    private fun loadMore() {
        setRecyclerViewStateToLoading()
        var numberOfDays = mainPresenter.getNumberOfMinusDays()
        numberOfDays++
        mainPresenter.saveNumberOfMinusDaysIntoSp(numberOfDays)
        val dateMinusXDays = mainPresenter.currentDateMinusXDaysToStr(numberOfDays)

        val nextLimit = listSize + 1
        for (i in listSize until nextLimit) {

            if (mainPresenter.checkIfSuchDateExistsinSp(dateMinusXDays)) {
                Log.i(TAG, "Date $dateMinusXDays already exists in SharedPreferences (new element)")
                handler.postDelayed({
                    mainPresenter.processDateWithoutMakingACall(dateMinusXDays)
                }, 2000)
            } else {
                Log.i(TAG, "Date $dateMinusXDays does not exist in SharedPreferences. Retrofit call made (new element)")
                mainPresenter.makeACall(dateMinusXDays)
            }
        }
        itemAdapter.notifyDataSetChanged()
    }

    override fun notifyChangedItemAdapter() {
        itemAdapter.notifyDataSetChanged()
    }

    override fun onDestroy() {
        super.onDestroy()
        restModel.cancelJob()
    }
}

itemadapter.kt

class ItemAdapter(private var items: MutableList<ApiResponse>, private val activity: Activity) : RecyclerView.Adapter<RecyclerView.ViewHolder>() {

    companion object {
        private const val VIEW_TYPE_DATA = 0
        private const val VIEW_TYPE_PROGRESS = 1
    }

    override fun onCreateViewHolder(parent: ViewGroup, p1: Int): RecyclerView.ViewHolder {
        return when (p1) {
            VIEW_TYPE_DATA -> {
                val view = LayoutInflater.from(parent.context).inflate(R.layout.item, parent, false)
                DataViewHolder(view, activity)
            }
            VIEW_TYPE_PROGRESS -> {
                val view = LayoutInflater.from(parent.context).inflate(R.layout.progress_bar_layout, parent, false)
                ProgressViewHolder(view)
            }
            else -> throw IllegalArgumentException("Different View type")
        }
    }

    override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
        if (holder is DataViewHolder)
            holder.bind(items[position])
    }

    override fun getItemCount() = items.size

    override fun getItemViewType(position: Int): Int {
        val viewtype = items[position]
        return when (viewtype.isLoading) {//if data is load, returns PROGRESSBAR viewtype.
            true -> VIEW_TYPE_PROGRESS
            false -> VIEW_TYPE_DATA
        }
    }

    class DataViewHolder(view: View, activity: Activity) : RecyclerView.ViewHolder(view) {
        private var isRvSubitemVisible = false
        private val tvDate = view.tv_date
        private val rvSubitem = view.rv_subitem
        private val activity = activity

        fun bind(apiResponse: ApiResponse) {
            tvDate.text = String.format(itemView.context.getString(R.string.day_x), apiResponse.date)
            tvDate.setOnClickListener {
                if (isRvSubitemVisible) {
                    rvSubitem.visibility = View.GONE
                    isRvSubitemVisible = false
                } else {
                    rvSubitem.visibility = View.VISIBLE
                    isRvSubitemVisible = true
                }
            }
            rvSubitem.apply {
                layoutManager = LinearLayoutManager(itemView.context)
                adapter = SubitemAdapter(apiResponse.rates, apiResponse.date, activity)
            }
        }
    }

    inner class ProgressViewHolder(view: View) : RecyclerView.ViewHolder(view)
}

子项适配器.kt

class SubitemAdapter(private val subitems: List<Currency>, private val day: String, private val activity: Activity) : RecyclerView.Adapter<SubitemAdapter.SubitemViewHolder>() {

    override fun onCreateViewHolder(parent: ViewGroup, p1: Int): SubitemViewHolder {
        val view = LayoutInflater.from(parent.context).inflate(R.layout.subitem, parent, false)
        return SubitemViewHolder(view, day, activity)
    }

    override fun onBindViewHolder(holder: SubitemViewHolder, position: Int) {
        if(position < subitems.size - 1) {
            activity.progress_bar.visibility = View.VISIBLE
        }
        else
            activity.progress_bar.visibility = View.GONE
        holder.bind(subitems[position], position)
    }

    override fun getItemCount() = subitems.size

    class SubitemViewHolder(view: View, day: String, activity: Activity) : RecyclerView.ViewHolder(view) {
        private val subitemRootView = view.subitem_root
        private val tvCurrencyName = view.tv_currency_name
        private val tvCurrencyValue = view.tv_currency_value
        private val day = day
        private val activity = activity
        fun bind(currency: Currency, position: Int) {
            subitemRootView.setOnClickListener { v ->
                activity as OnChangeFragment
                activity.changeFragment(SpecificCurrencyFragment(), ChangeFragmentData(hashMapOf(currency.currencyName to currency.currencyValue.toString()), day))
            }
            tvCurrencyName.text = currency.currencyName
            tvCurrencyValue.text = currency.currencyValue.toString()
        }
    }
}

这是我认为一切可以帮助我的。但是如果你需要更多的东西，那就用aks。任何帮助我们都将不胜感激。提前谢谢你！