RecyclerView Endless Scroll Listener

EndlessScrollListener.kt

import android.support.v7.widget.GridLayoutManager
import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.RecyclerView
import android.support.v7.widget.StaggeredGridLayoutManager

class EndlessScrollListener(private val layoutManager: RecyclerView.LayoutManager,
    private val callback: EndlessScrollCallback) :
    RecyclerView.OnScrollListener() {

  companion object {
    private const val VISIBLE_ITEM_THRESHOLD = 10
  }

  private val visibleThreshold: Int

  private val lastLoadedPosition
    get() = layoutManager.itemCount - 1

  init {
    visibleThreshold = when (layoutManager) {
      is StaggeredGridLayoutManager -> VISIBLE_ITEM_THRESHOLD * layoutManager.spanCount
      is GridLayoutManager -> VISIBLE_ITEM_THRESHOLD * layoutManager.spanCount
      is LinearLayoutManager -> VISIBLE_ITEM_THRESHOLD
      else -> throw IllegalArgumentException()
    }
  }

  override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
    when {
      isScrollingDown(dy) -> if (shouldLoadNext()) callback.loadNextPage()
      isScrollingUp(dy) -> if (shouldLoadPrev()) callback.loadPreviousPage()
    }
  }

  private fun isScrollingDown(dy: Int) = dy > 0
  private fun isScrollingUp(dy: Int) = dy < 0

  private fun shouldLoadNext() = getLastVisiblePosition() >= lastLoadedPosition - visibleThreshold
  private fun shouldLoadPrev() = getFirstVisiblePosition() <= visibleThreshold

  private fun getFirstVisiblePosition(): Int {
    return when (layoutManager) {
      is StaggeredGridLayoutManager -> layoutManager.findFirstVisibleItemPositions(null).min() ?: 0
      is GridLayoutManager -> layoutManager.findFirstVisibleItemPosition()
      is LinearLayoutManager -> layoutManager.findFirstVisibleItemPosition()
      else -> throw IllegalArgumentException()
    }
  }

  private fun getLastVisiblePosition(): Int {
    return when (layoutManager) {
      is StaggeredGridLayoutManager -> layoutManager.findLastVisibleItemPositions(null).max() ?: 0
      is GridLayoutManager -> layoutManager.findLastVisibleItemPosition()
      is LinearLayoutManager -> layoutManager.findLastVisibleItemPosition()
      else -> throw IllegalArgumentException()
    }
  }

  interface EndlessScrollCallback {
    fun loadNextPage()
    fun loadPreviousPage()
  }
}