EndlessRecyclerViewScrollListener

EndlessRecyclerViewScrollListener.kt

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

/**
 * Created by oohyugi on 2019-09-03.
 * github: https://github.com/oohyugi
 */
abstract class EndlessRecyclerViewScrollListener : RecyclerView.OnScrollListener {
    // The minimum amount of items to have below your current scroll position
    // before loading more.
    private var visibleThreshold = 5
    // The current offset index of data you have loaded
    private var currentPage = 1
    // The total number of items in the dataset 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 = 1

    internal var mLayoutManager: RecyclerView.LayoutManager

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

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

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

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

    // 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(view: RecyclerView, dx: Int, dy: Int) {
        if (dy > 0) {
            var lastVisibleItemPosition = 0
            val totalItemCount = mLayoutManager.itemCount

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

            // 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 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 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 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.currentPage = 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
                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) {
                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
    }

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

}