LazyListScrollDirection.kt

import androidx.compose.foundation.lazy.LazyListState
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue
import kotlin.math.abs

enum class ScrollDirection {
    FORWARD, BACKWARD, IDLE,
}

/**
 * LazyListStateのスクロール方向を保持、参照できるState Composable
 *
 * @param state LazyListState
 * @param playOffset スクロールoffsetの遊びの領域。遊びの範囲内の場合はIDLEが返る。
 * @return スクロール方向を返却する
 */
@Composable
fun rememberLazyListScrollDirection(state: LazyListState, playOffset: Int = 0): ScrollDirection {
    var prevFirstVisibleItemScrollOffset by remember { mutableStateOf(0) }
    var prevFirstVisibleItemIndex by remember { mutableStateOf(0) }

    val offset = state.firstVisibleItemScrollOffset
    val index = state.firstVisibleItemIndex

    // indexが同じ場合はoffsetの比較
    // indexが違う場合はその方向で判定
    val direction = when {
        prevFirstVisibleItemIndex == index -> {
            when {
                prevFirstVisibleItemScrollOffset == offset -> ScrollDirection.IDLE
                abs(prevFirstVisibleItemScrollOffset - offset) <= playOffset -> ScrollDirection.IDLE
                prevFirstVisibleItemScrollOffset < offset -> ScrollDirection.FORWARD
                else -> ScrollDirection.BACKWARD
            }
        }
        prevFirstVisibleItemIndex < index -> ScrollDirection.FORWARD
        else -> ScrollDirection.BACKWARD
    }

    prevFirstVisibleItemScrollOffset = offset
    prevFirstVisibleItemIndex = index

    return direction
}