EndlessScrollListener implementation

EndlessScrollListener.kt

package com.upmc.enterprises.myupmc

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

abstract class EndlessRecyclerViewScrollListener(
        private val layoutManager: RecyclerView.LayoutManager,
        private val indexDelta: Int = 20
) : RecyclerView.OnScrollListener() {
    // The minimum amount of items to have below your scroll position before loading more
    private var visibleThreshold = 5

    // The current offset index of data you have loaded
    private var currentIndex = 0

    // The total number of items in the data set after the last load
    private var previousTotalItemCount = 0

    // True if we are still waiting for the last set of data to load
    private var loading = true

    // Sets the starting page index
    private val startingPageIndex = 0

    init {
        when (layoutManager) {
            is StaggeredGridLayoutManager -> {
                visibleThreshold *= layoutManager.spanCount
            }
            is GridLayoutManager -> {
                visibleThreshold *= layoutManager.spanCount
            }
        }
    }

    abstract fun doneLoading()

    abstract fun onLoadMore(currentIndex: Int, totalItemsCount: Int, view: RecyclerView)

    // This happens many times a second during a scroll, so be wary of the code you place here.
    // We are given a few useful parameters to help us work out if we need to load some more data,
    // but first we check if we are waiting for the previous load to finish.
    override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
        // ignore scrolling up
        if (dy <= 0) return

        val totalItemCount = this.layoutManager.itemCount

        val lastVisibleItemPosition = when (layoutManager) {
            is StaggeredGridLayoutManager -> {
                val lastVisibleItemPositions = layoutManager.findLastVisibleItemPositions(null)
                // get maximum element within the list
                getLastVisibleItem(lastVisibleItemPositions)
            }
            is GridLayoutManager -> layoutManager.findLastVisibleItemPosition()
            is LinearLayoutManager -> layoutManager.findLastVisibleItemPosition()
            else -> 0
        }

        // If the total item count is zero and the previous isn't, assume the
        // list is invalidated and should be reset back to initial state
        if (totalItemCount < previousTotalItemCount) {
            this.currentIndex = this.startingPageIndex
            this.previousTotalItemCount = totalItemCount

            if (totalItemCount == 0) {
                this.loading = true
            }
        }

        // If it’s still loading, we check to see if the dataset count has
        // changed, if so we conclude it has finished loading and update the current page
        // number and total item count.
        if (loading && (totalItemCount > previousTotalItemCount)) {
            loading = false

            doneLoading()

            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 (!loading && (lastVisibleItemPosition + visibleThreshold) > totalItemCount) {
            currentIndex += indexDelta

            onLoadMore(currentIndex, totalItemCount, recyclerView)

            loading = true
        }
    }

    fun getLastVisibleItem(lastVisibleItemPositions: IntArray): Int {
        var maxSize = 0

        lastVisibleItemPositions.forEach {
            if (it == 0) {
                maxSize = lastVisibleItemPositions[it]
            } else if (lastVisibleItemPositions[it] > maxSize) {
                maxSize = lastVisibleItemPositions[it]
            }
        }

        return maxSize
    }

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

RecyclerViewFragment.kt

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        val layoutManager = LinearLayoutManager(context)
        recyclerView?.addItemDecoration(DividerItemDecoration(recyclerView?.context, DividerItemDecoration.VERTICAL))
        recyclerView?.adapter = adapter
        recyclerView?.layoutManager = layoutManager
        recyclerView?.addOnScrollListener(object : EndlessRecyclerViewScrollListener(layoutManager) {
            override fun doneLoading() {
                Logger.d("Finished loading more")
            }

            override fun onLoadMore(currentIndex: Int, totalItemsCount: Int, view: RecyclerView) {
                Logger.d("On load more $currentIndex, $totalItemsCount")

                // load more with the current index
            }
        })
    }