commonMain.com.tunjid.composables.lazy.LazyStateUtilities.kt Maven / Gradle / Ivy

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A collection of utility composable functions
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/*
 * Copyright 2021 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.tunjid.composables.lazy


import androidx.compose.foundation.gestures.ScrollableState
import kotlin.math.abs

/**
 * Linearly interpolates the index for the first item in [visibleItems] to smoothly match the
 * scroll rate of the backing [ScrollableState].
 *
 * @param visibleItems a list of items currently visible in the layout.
 * @param itemSize a lookup function for the size of an item in the layout.
 * @param offset a lookup function for the offset of an item relative to the start of the view port.
 * @param nextItemOnMainAxis a lookup function for the next item on the main axis in the direction
 * of the scroll.
 * @param itemIndex a lookup function for index of an item in the layout relative to
 * the total amount of items available.
 *
 * @return a [Float] in the range [firstItemPosition..nextItemPosition) where nextItemPosition
 * is the index of the consecutive item along the major axis.
 * */
inline fun  LazyState.interpolatedFirstItemIndex(
    visibleItems: List,
    crossinline itemSize: LazyState.(LazyStateItem) -> Int,
    crossinline offset: LazyState.(LazyStateItem) -> Int,
    crossinline nextItemOnMainAxis: LazyState.(LazyStateItem) -> LazyStateItem?,
    crossinline itemIndex: (LazyStateItem) -> Int,
): Float {
    if (visibleItems.isEmpty()) return 0f

    val firstItem = visibleItems.first()
    val firstItemIndex = itemIndex(firstItem)

    if (firstItemIndex < 0) return Float.NaN

    val firstItemSize = itemSize(firstItem)
    if (firstItemSize == 0) return Float.NaN

    val itemOffset = offset(firstItem).toFloat()
    val offsetPercentage = abs(itemOffset) / firstItemSize

    val nextItem = nextItemOnMainAxis(firstItem) ?: return firstItemIndex + offsetPercentage

    val nextItemIndex = itemIndex(nextItem)

    return firstItemIndex + ((nextItemIndex - firstItemIndex) * offsetPercentage)
}

/**
 * Returns the percentage of an item that is currently visible in the view port.
 * @param itemSize the size of the item
 * @param itemStartOffset the start offset of the item relative to the view port start
 * @param viewportStartOffset the start offset of the view port
 * @param viewportEndOffset the end offset of the view port
 */
internal fun itemVisibilityPercentage(
    itemSize: Int,
    itemStartOffset: Int,
    viewportStartOffset: Int,
    viewportEndOffset: Int,
): Float {
    if (itemSize == 0) return 0f
    val itemEnd = itemStartOffset + itemSize
    val startOffset = when {
        itemStartOffset > viewportStartOffset -> 0
        else -> abs(abs(viewportStartOffset) - abs(itemStartOffset))
    }
    val endOffset = when {
        itemEnd < viewportEndOffset -> 0
        else -> abs(abs(itemEnd) - abs(viewportEndOffset))
    }
    val size = itemSize.toFloat()
    return (size - startOffset - endOffset) / size
}