Pagination on a nested scroll view!

extension.kt

fun NestedScrollView.addPagination(listener: NSVPaginationListener, rv: RecyclerView){
    rv.isNestedScrollingEnabled = false
    this.setOnScrollChangeListener (listener)

}

NSVPaginationListener.kt

import android.view.View
import androidx.core.widget.NestedScrollView
import timber.log.Timber

/**
 * when a recycler view can be in nested scroll view, its ability to recycle views is lost as its
 * linear layout manager loads all the views at one go. and since all views are loaded at once,
 * the layout manager or (recycelr view) don't fire the last child callbacks properly.
 *
 * For pagination, the basic principle is:
 * - X(20) items are loaded in rv, user sees Y(8).
 * - while user is scrolling, the recycling logic of RV and LM are firing events which helps
 *   us determine when the user reaches the last position.
 * - when user reaches last position, we fire an api call, which further loads Z(10) more items.
 * - now user is at Xth(20) item, and sees Y(8) more it, while X+Z(30) items are loaded in RV
 * - user scrolls and cycle continues.
 *
 * But what to do in case of NSV+RV? add scroll listener to NSV!. so logic becomes
 * -  X(20) items are loaded in rv, user sees p1+Y(8)+p2,where p1,p2 are more views of
 *   nested scroll view
 * - while user is scrolling, the NSV is now firing events which helps us determine when the
 *   user reaches the last VIEW of NS.
 * - when user reaches last position, we fire an api call, which further loads Z(10) more items.
 * - now user is at Xth(20)+p1+p2 item, and sees Y(8) more it, while X+Z(30)+p1+p2 items are loaded
 *   in NSV
 * - user scrolls and cycle continues.
 *
 * make sure to use this listener on NSV after disabling nested scroll  of recycler
 *
 * */
class NSVPaginationListener(

    private val isLastPage: () -> Boolean,
    private val isLoading: () -> Boolean,
    private val loadMoreItems: () -> Unit
) : NestedScrollView.OnScrollChangeListener {
    private val log by lazy {Timber.tag("HLAdp")}

    override fun onScrollChange(v: NestedScrollView, scrollX: Int, scrollY: Int, oldScrollX: Int, oldScrollY: Int) {
        val lastChild: View? = v.getChildAt(v.childCount - 1)
        val loading = isLoading.invoke()
        val lastPage = isLastPage.invoke()
        val lastItemHeight = (lastChild?.measuredHeight?:0) - v.measuredHeight
        //log.d("onScrollChange() called with: nsv = ${v.hashCode()}, lastChild = ${lastChild?.hashCode()}, loading = $loading, lastPage = $lastPage, lastItemHeight = $lastItemHeight, scrollX = $scrollX, oldScrollX = $oldScrollX, scrollY = $scrollY, oldScrollY = $oldScrollY")
        if (lastChild != null
            && (scrollY >= (lastItemHeight))
            && scrollY > oldScrollY
            && !loading
            && !lastPage
        ) {
            //log.d("onScrollChange:calling scroll")
            loadMoreItems()
        }
    }


}

usage.kt

@AndroidEntryPoint
class MyFragment : BaseFragment() {
    private lateinit var binding: ViewBinding
    private lateinit var rvAdapter: RvAdapter
    private val viewModel by viewModels<ViewModel>()
    private var request: ApiRequest? = null
    private var currentPageNum = 1
    private var nextPageNum = 0
    private var isLoadingMoreResults = false
    private var paginationListener: NSVPaginationListener? = null
    private var shouldReplace = false



    @SuppressLint("ClickableViewAccessibility")
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        viewModel.livedata.observe(viewLifecycleOwner, ::onApiResp)
        
        binding.rvList.apply {
            adapter = RvAdapter(::onClick).also { rvAdapter = it }
        }
        NSVPaginationListener(::isLastResult,::isLoadingResults,::onLoadMoreItems).also {
            paginationListener = it
            binding.nsvList.addPagination(it,binding.rvList)
        }

    }

    

    private fun onLoadMoreItems() {
        shouldReplace = false
        makeApiRequest()
    }

    private fun isLoadingResults(): Boolean {
        return isLoadingMoreResults
    }

    private fun isLastResult(): Boolean {
        return nextPageNum==0
    }
    
    private fun onApiResp(resp: Resource<ApiResponse>?) {
        customProgressBar.toggle(resp?.status==ResourceState.LOADING)
        isLoadingMoreResults = resp.status == ResourceState.LOADING
        when (resp?.status) {
            ResourceState.SUCCESS -> {
                customProgressBar.hide()
                val newEntries =  resp.data?.toList().orEmpty()
                nextPageNum = resp.data?.next_page?:0
                currentPageNum = resp.data?.current_page_no?:0
                if(shouldReplace){
                    rvAdapter.replace(newEntries)
                }else{
                    rvAdapter.addMore(newEntries)
                }

            }
            else -> {}
        }
    }

    private fun onClick(item: Item) {

    }


    private fun makeApiRequest() {
        request?.let {
            it.pageNo = nextPageNum.toString()
            viewModel.fetchData(it)
        }

    }


}