PaginationScroller.kt

import androidx.recyclerview.widget.GridLayoutManager
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import androidx.recyclerview.widget.StaggeredGridLayoutManager

abstract class PaginationScrollListener : RecyclerView.OnScrollListener {
	private val mLayoutManager: RecyclerView.LayoutManager
	private val startingPageIndex = 0
	private var visibleThreshold = 5
	private var currentPage = 0
	private var previousTotalItemCount = 0
	private var loading = true
	private var hasMore: Boolean = false
	private var reverLay: Boolean = false // reverseLay is for reversing the direction of layout

	constructor(layoutManager: LinearLayoutManager) {
		this.mLayoutManager = layoutManager
		reverLay = layoutManager.reverseLayout
	}

	constructor(layoutManager: GridLayoutManager) {
		this.mLayoutManager = layoutManager
		visibleThreshold *= layoutManager.spanCount
		reverLay = layoutManager.reverseLayout
	}

	constructor(layoutManager: StaggeredGridLayoutManager) {
		this.mLayoutManager = layoutManager
		visibleThreshold *= layoutManager.spanCount
		reverLay = layoutManager.reverseLayout
	}

	private fun getLastVisibleItem(lastVisibleItemPositions: IntArray): Int {
		var maxSize = 0
		lastVisibleItemPositions.indices.forEach { i ->
			if (i == 0) {
				maxSize = lastVisibleItemPositions[i]
			} else if (lastVisibleItemPositions[i] > maxSize) {
				maxSize = lastVisibleItemPositions[i]
			}
		}
		return maxSize
	}

	override fun onScrolled(view: RecyclerView, dx: Int, dy: Int) {
		if (mLayoutManager.canScrollVertically() && (!reverLay && dy >= 0 || reverLay && dy <= 0)) {
			handlePagination(view)
		} else if (mLayoutManager.canScrollHorizontally() && (!reverLay && dx > 0 || reverLay && dx < 0)) {
			handlePagination(view)
		} else if (reverLay && dy < 0) {
			handlePagination(view)
		}
	}

	private fun handlePagination(view: RecyclerView) {
		var lastVisibleItemPosition = 0
		val totalItemCount = mLayoutManager.itemCount

		when (mLayoutManager) {
			is StaggeredGridLayoutManager -> {
				val lastVisibleItemPositions = mLayoutManager.findLastVisibleItemPositions(null)
				lastVisibleItemPosition = getLastVisibleItem(lastVisibleItemPositions)
			}
			is GridLayoutManager -> lastVisibleItemPosition =
				mLayoutManager.findLastVisibleItemPosition()
			is LinearLayoutManager -> lastVisibleItemPosition =
				mLayoutManager.findLastVisibleItemPosition()
		}

		if (totalItemCount < previousTotalItemCount) {
			this.currentPage = this.startingPageIndex
			this.previousTotalItemCount = totalItemCount
			if (totalItemCount == 0) {
				this.loading = true
			}
		}

		if (loading && totalItemCount > previousTotalItemCount) {
			loading = false
			previousTotalItemCount = totalItemCount
		}

		// If it isn’t currently loading, we check to see if we have breached
		// the visibleThreshold and need to reload more data.
		// If we do need to reload some more data, we execute onLoadMore to fetch the data.
		// threshold should reflect how many total columns there are too
		if (hasMore && !loading && lastVisibleItemPosition + visibleThreshold > totalItemCount) {
			currentPage++
			onLoadMore(currentPage, totalItemCount, view)
			loading = true
		}
	}

	// Call this method whenever performing new searches
	fun resetState() {
		this.currentPage = this.startingPageIndex
		this.previousTotalItemCount = 0
		this.loading = true
	}

	// use this only when network call fails.
	fun setLoading(loading: Boolean) {
		this.loading = loading
		if (!loading) currentPage--
	}

	fun setHasMore(hasMore: Boolean) {
		this.hasMore = hasMore
	}

	// Defines the process for actually loading more data based on page
	abstract fun onLoadMore(page: Int, totalItemsCount: Int, view: RecyclerView)
}