FixedScrollThumbLinearLayoutManager.kt

/*
 * Copyright 2023 Tusky Contributors
 *
 * This file is a part of Tusky.
 *
 * This program is free software; you can redistribute it and/or modify it under the terms of the
 * GNU General Public License as published by the Free Software Foundation; either version 3 of the
 * License, or (at your option) any later version.
 *
 * Tusky is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
 * the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
 * Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along with Tusky; if not,
 * see <http://www.gnu.org/licenses>.
 */

package com.keylesspalace.tusky.view

import android.content.Context
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import kotlin.math.abs

/**
 * [LinearLayoutManager] with a fixed size scroll bar thumb.
 *
 * The normal [LinearLayoutManager] handles scrolling through items of different heights by
 * varying the size of the scrollbar thumb.
 *
 * This code maintains a fixed thumb size (relative to the number of items in the list), and
 * adjusts the thumb position to compensate.
 *
 * Based on https://medium.com/dowjones/scrolling-in-android-custom-scroll-behavior-for-a-list-of-varying-height-7426f0f9046
 */
class FixedScrollThumbLinearLayoutManager(context: Context?) : LinearLayoutManager(context) {
    init {
        isSmoothScrollbarEnabled = false
    }

    override fun computeVerticalScrollExtent(state: RecyclerView.State): Int {
        return if (childCount > 0) SMOOTH_VALUE * 3 else 0
    }

    override fun computeVerticalScrollRange(state: RecyclerView.State): Int {
        return ((itemCount - 1) * SMOOTH_VALUE).coerceAtLeast(0)
    }

    override fun computeVerticalScrollOffset(state: RecyclerView.State): Int {
        val count = childCount

        if (count <= 0) return 0

        if (findLastCompletelyVisibleItemPosition() == itemCount - 1) {
            return ((itemCount - 1) * SMOOTH_VALUE).coerceAtLeast(0)
        }

        val firstPosition = findFirstVisibleItemPosition()
        if (firstPosition == RecyclerView.NO_POSITION) return 0

        val view = findViewByPosition(firstPosition) ?: return 0

        val top = getDecoratedTop(view)
        val height = getDecoratedMeasuredHeight(view)
        val heightOfScreen = if (height <= 0) 0 else abs(SMOOTH_VALUE * top / height)
        if (heightOfScreen == 0 && firstPosition > 0) return SMOOTH_VALUE * firstPosition - 1
        return (SMOOTH_VALUE * firstPosition) + heightOfScreen
    }

    companion object {
        private const val SMOOTH_VALUE = 100
    }
}