commonMain.androidx.compose.foundation.lazy.layout.LazyLayoutNearestRangeState.kt Maven / Gradle / Ivy

Go to download

Show more of this group Show more artifacts with this name
Show all versions of foundation-desktop Show documentation

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

The newest version!

/*
 * Copyright 2023 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.layout

import androidx.compose.runtime.State
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.setValue
import androidx.compose.runtime.structuralEqualityPolicy

internal class LazyLayoutNearestRangeState(
    firstVisibleItem: Int,
    private val slidingWindowSize: Int,
    private val extraItemCount: Int
) : State {

    override var value: IntRange by
        mutableStateOf(
            calculateNearestItemsRange(firstVisibleItem, slidingWindowSize, extraItemCount),
            structuralEqualityPolicy()
        )
        private set

    private var lastFirstVisibleItem = firstVisibleItem

    fun update(firstVisibleItem: Int) {
        if (firstVisibleItem != lastFirstVisibleItem) {
            lastFirstVisibleItem = firstVisibleItem
            value = calculateNearestItemsRange(firstVisibleItem, slidingWindowSize, extraItemCount)
        }
    }

    private companion object {
        /**
         * Returns a range of indexes which contains at least [extraItemCount] items near the first
         * visible item. It is optimized to return the same range for small changes in the
         * firstVisibleItem value so we do not regenerate the map on each scroll.
         */
        private fun calculateNearestItemsRange(
            firstVisibleItem: Int,
            slidingWindowSize: Int,
            extraItemCount: Int
        ): IntRange {
            val slidingWindowStart = slidingWindowSize * (firstVisibleItem / slidingWindowSize)

            val start = maxOf(slidingWindowStart - extraItemCount, 0)
            val end = slidingWindowStart + slidingWindowSize + extraItemCount
            return start until end
        }
    }
}