commonMain.androidx.compose.foundation.lazy.LazyListScrollPosition.kt Maven / Gradle / Ivy

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Higher level abstractions of the Compose UI primitives. This library is design system agnostic, providing the high-level building blocks for both application and design-system developers
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/*
 * Copyright 2021 The Android Open Source Project
 *
 * 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
 *
 *      http://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 androidx.compose.foundation.lazy

import androidx.compose.foundation.ExperimentalFoundationApi
import androidx.compose.foundation.internal.checkPrecondition
import androidx.compose.foundation.internal.requirePrecondition
import androidx.compose.foundation.lazy.layout.LazyLayoutNearestRangeState
import androidx.compose.foundation.lazy.layout.findIndexByKey
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableIntStateOf
import androidx.compose.runtime.setValue

/**
 * Contains the current scroll position represented by the first visible item index and the first
 * visible item scroll offset.
 */
internal class LazyListScrollPosition(initialIndex: Int = 0, initialScrollOffset: Int = 0) {
    var index by mutableIntStateOf(initialIndex)

    var scrollOffset by mutableIntStateOf(initialScrollOffset)
        private set

    private var hadFirstNotEmptyLayout = false

    /** The last know key of the item at [index] position. */
    private var lastKnownFirstItemKey: Any? = null

    val nearestRangeState =
        LazyLayoutNearestRangeState(
            initialIndex,
            NearestItemsSlidingWindowSize,
            NearestItemsExtraItemCount
        )

    /** Updates the current scroll position based on the results of the last measurement. */
    fun updateFromMeasureResult(measureResult: LazyListMeasureResult) {
        lastKnownFirstItemKey = measureResult.firstVisibleItem?.key
        // we ignore the index and offset from measureResult until we get at least one
        // measurement with real items. otherwise the initial index and scroll passed to the
        // state would be lost and overridden with zeros.
        if (hadFirstNotEmptyLayout || measureResult.totalItemsCount > 0) {
            hadFirstNotEmptyLayout = true
            val scrollOffset = measureResult.firstVisibleItemScrollOffset
            checkPrecondition(scrollOffset >= 0f) { "scrollOffset should be non-negative" }

            val firstIndex = measureResult.firstVisibleItem?.index ?: 0
            update(firstIndex, scrollOffset)
        }
    }

    fun updateScrollOffset(scrollOffset: Int) {
        checkPrecondition(scrollOffset >= 0f) { "scrollOffset should be non-negative" }
        this.scrollOffset = scrollOffset
    }

    /**
     * Updates the scroll position - the passed values will be used as a start position for
     * composing the items during the next measure pass and will be updated by the real position
     * calculated during the measurement. This means that there is no guarantee that exactly this
     * index and offset will be applied as it is possible that: a) there will be no item at this
     * index in reality b) item at this index will be smaller than the asked scrollOffset, which
     * means we would switch to the next item c) there will be not enough items to fill the viewport
     * after the requested index, so we would have to compose few elements before the asked index,
     * changing the first visible item.
     */
    fun requestPositionAndForgetLastKnownKey(index: Int, scrollOffset: Int) {
        update(index, scrollOffset)
        // clear the stored key as we have a direct request to scroll to [index] position and the
        // next [checkIfFirstVisibleItemWasMoved] shouldn't override this.
        lastKnownFirstItemKey = null
    }

    /**
     * In addition to keeping the first visible item index we also store the key of this item. When
     * the user provided custom keys for the items this mechanism allows us to detect when there
     * were items added or removed before our current first visible item and keep this item as the
     * first visible one even given that its index has been changed.
     */
    @OptIn(ExperimentalFoundationApi::class)
    fun updateScrollPositionIfTheFirstItemWasMoved(
        itemProvider: LazyListItemProvider,
        index: Int
    ): Int {
        val newIndex = itemProvider.findIndexByKey(lastKnownFirstItemKey, index)
        if (index != newIndex) {
            this.index = newIndex
            nearestRangeState.update(index)
        }
        return newIndex
    }

    private fun update(index: Int, scrollOffset: Int) {
        requirePrecondition(index >= 0f) { "Index should be non-negative ($index)" }
        this.index = index
        nearestRangeState.update(index)
        this.scrollOffset = scrollOffset
    }
}

/**
 * We use the idea of sliding window as an optimization, so user can scroll up to this number of
 * items until we have to regenerate the key to index map.
 */
internal const val NearestItemsSlidingWindowSize = 30

/** The minimum amount of items near the current first visible item we want to have mapping for. */
internal const val NearestItemsExtraItemCount = 100