RecyclerViewPagingListener.kt

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

/**
 * RecyclerViewのページングリスナー
 */
// 参考:https://thetopsites.net/article/50618951.shtml
abstract class RecyclerViewPagingListener(
    layoutManager: RecyclerView.LayoutManager,
    direction: LoadOnScrollDirection
) : RecyclerView.OnScrollListener() {

    /**
     * 現在読み込んだページ数
     */
    private var pendingCurrentPage = 0

    /**
     * 現在読み込んだアイテム数
     */
    private var pendingTotalItemCount = 0

    /**
     * ロード中フラグ
     */
    private var loading = true

    /**
     * 検知する方向
     */
    private val pendingDirection: LoadOnScrollDirection

    /**
     * LinearLayoutManager
     */
    private var pendingLinearLayoutManager: LinearLayoutManager? = null

    /**
     * GridLayoutManager
     */
    private var pendingGridLayoutManager: GridLayoutManager? = null

    /**
     * StaggeredGridLayoutManager
     */
    private var pendingStaggeredGridLayoutManager: StaggeredGridLayoutManager? = null

    init {
        // パフォーマンスのためonScrolledでキャストしない
        when (layoutManager) {
            is LinearLayoutManager -> {
                pendingLinearLayoutManager = layoutManager
            }
            is GridLayoutManager -> {
                pendingGridLayoutManager = layoutManager
            }
            is StaggeredGridLayoutManager -> {
                pendingStaggeredGridLayoutManager = layoutManager
            }
            else -> {
                throw IllegalArgumentException("unsupported this layout manager.")
            }
        }
        pendingDirection = direction
    }

    override fun onScrolled(view: RecyclerView, dx: Int, dy: Int) {
        var lastVisibleItemPosition = 0
        var firstVisibleItemPosition = 0
        var totalItemCount = 0
        when {
            pendingLinearLayoutManager != null -> {
                val layoutManager = requireNotNull(pendingLinearLayoutManager)
                totalItemCount = layoutManager.itemCount
                lastVisibleItemPosition = layoutManager.findLastVisibleItemPosition()
                firstVisibleItemPosition = layoutManager.findFirstVisibleItemPosition()
            }
            pendingGridLayoutManager != null -> {
                val layoutManager = requireNotNull(pendingGridLayoutManager)
                totalItemCount = layoutManager.itemCount
                lastVisibleItemPosition = layoutManager.findLastVisibleItemPosition()
                firstVisibleItemPosition = layoutManager.findFirstVisibleItemPosition()
            }
            pendingStaggeredGridLayoutManager != null -> {
                val layoutManager = requireNotNull(pendingStaggeredGridLayoutManager)
                val lastVisibleItemPositions = layoutManager.findLastVisibleItemPositions(null)
                val firstVisibleItemPositions = layoutManager.findFirstVisibleItemPositions(null)
                totalItemCount = layoutManager.itemCount
                lastVisibleItemPosition = getLastVisibleItem(lastVisibleItemPositions)
                firstVisibleItemPosition = getFirstVisibleItem(firstVisibleItemPositions)
            }
        }
        when (pendingDirection) {
            LoadOnScrollDirection.BOTTOM -> {
                if (totalItemCount < pendingTotalItemCount) {
                    pendingCurrentPage = STARTING_PAGE_INDEX
                    pendingTotalItemCount = totalItemCount
                    if (totalItemCount == 0) {
                        loading = true
                    }
                }
                if (loading && totalItemCount > pendingTotalItemCount) {
                    loading = false
                    pendingTotalItemCount = totalItemCount
                }
                if (!loading && lastVisibleItemPosition + PREFETCH_ITEM_POSITION > totalItemCount) {
                    pendingCurrentPage++
                    onLoadMore(pendingCurrentPage, totalItemCount)
                    loading = true
                }
            }
            LoadOnScrollDirection.TOP -> {
                if (totalItemCount < pendingTotalItemCount) {
                    pendingCurrentPage = STARTING_PAGE_INDEX
                    pendingTotalItemCount = totalItemCount
                    if (totalItemCount == 0) {
                        loading = true
                    }
                }
                if (loading && totalItemCount > pendingTotalItemCount) {
                    loading = false
                    pendingTotalItemCount = totalItemCount
                }
                if (!loading && firstVisibleItemPosition < PREFETCH_ITEM_POSITION) {
                    pendingCurrentPage++
                    onLoadMore(pendingCurrentPage, totalItemCount)
                    loading = true
                }
            }
        }
    }

    private 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
    }

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

    /**
     * 追加読み込み
     *
     * @param page 現在のページ数
     * @param totalItemsCount 現在のアイテム数
     */
    abstract fun onLoadMore(page: Int, totalItemsCount: Int)

    /**
     * スクロール検知方向
     */
    enum class LoadOnScrollDirection {
        TOP, BOTTOM
    }

    companion object {
        /**
         * 開始ページ
         */
        private const val STARTING_PAGE_INDEX = 0

        /**
         * プリフェッチする位置
         *
         * "5"に設定した場合
         * "100"アイテムあると"95"が見えた時にロード開始する
         */
        private const val PREFETCH_ITEM_POSITION = 10
    }
}