commonMain.androidx.compose.foundation.lazy.LazyListMeasureResult.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 2020 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.gestures.Orientation
import androidx.compose.ui.layout.MeasureResult
import androidx.compose.ui.unit.Constraints
import androidx.compose.ui.unit.Density
import androidx.compose.ui.unit.IntSize
import androidx.compose.ui.util.fastForEach
import kotlinx.coroutines.CoroutineScope

/** The result of the measure pass for lazy list layout. */
internal class LazyListMeasureResult(
    // properties defining the scroll position:
    /** The new first visible item. */
    val firstVisibleItem: LazyListMeasuredItem?,
    /** The new value for [LazyListState.firstVisibleItemScrollOffset]. */
    val firstVisibleItemScrollOffset: Int,
    /** True if there is some space available to continue scrolling in the forward direction. */
    val canScrollForward: Boolean,
    /** The amount of scroll consumed during the measure pass. */
    val consumedScroll: Float,
    /** MeasureResult defining the layout. */
    private val measureResult: MeasureResult,
    /** The amount of scroll-back that happened due to reaching the end of the list. */
    val scrollBackAmount: Float,
    /** True when extra remeasure is required. */
    val remeasureNeeded: Boolean,
    /** Scope for animations. */
    val coroutineScope: CoroutineScope,
    /** Density of the last measure. */
    val density: Density,
    /** Constraints used to measure children. */
    val childConstraints: Constraints,
    // properties representing the info needed for LazyListLayoutInfo:
    /** see [LazyListLayoutInfo.visibleItemsInfo] */
    override val visibleItemsInfo: List,
    /** see [LazyListLayoutInfo.viewportStartOffset] */
    override val viewportStartOffset: Int,
    /** see [LazyListLayoutInfo.viewportEndOffset] */
    override val viewportEndOffset: Int,
    /** see [LazyListLayoutInfo.totalItemsCount] */
    override val totalItemsCount: Int,
    /** see [LazyListLayoutInfo.reverseLayout] */
    override val reverseLayout: Boolean,
    /** see [LazyListLayoutInfo.orientation] */
    override val orientation: Orientation,
    /** see [LazyListLayoutInfo.afterContentPadding] */
    override val afterContentPadding: Int,
    /** see [LazyListLayoutInfo.mainAxisItemSpacing] */
    override val mainAxisItemSpacing: Int,
) : LazyListLayoutInfo, MeasureResult by measureResult {

    val canScrollBackward
        get() = (firstVisibleItem?.index ?: 0) != 0 || firstVisibleItemScrollOffset != 0

    override val viewportSize: IntSize
        get() = IntSize(width, height)

    override val beforeContentPadding: Int
        get() = -viewportStartOffset

    /**
     * Creates a new layout info with applying a scroll [delta] for this layout info. In some cases
     * we can apply small scroll deltas by just changing the offsets for each [visibleItemsInfo].
     * But we can only do so if after applying the delta we would not need to compose a new item or
     * dispose an item which is currently visible. In this case this function will not apply the
     * [delta] and return null.
     *
     * @return new layout info if we can safely apply a passed scroll [delta] to this layout info.
     *   If If new layout info is returned, only the placement phase is needed to apply new offsets.
     *   If null is returned, it means we have to rerun the full measure phase to apply the [delta].
     */
    fun copyWithScrollDeltaWithoutRemeasure(
        delta: Int,
        updateAnimations: Boolean
    ): LazyListMeasureResult? {
        if (
            remeasureNeeded ||
                visibleItemsInfo.isEmpty() ||
                firstVisibleItem == null ||
                // applying this delta will change firstVisibleItem
                (firstVisibleItemScrollOffset - delta) !in
                    0 until firstVisibleItem.mainAxisSizeWithSpacings
        ) {
            return null
        }
        val first = visibleItemsInfo.first()
        val last = visibleItemsInfo.last()
        if (first.nonScrollableItem || last.nonScrollableItem) {
            // non scrollable items like headers require special handling in the measurement.
            return null
        }
        val canApply =
            if (delta < 0) {
                // scrolling forward
                val deltaToFirstItemChange =
                    first.offset + first.mainAxisSizeWithSpacings - viewportStartOffset
                val deltaToLastItemChange =
                    last.offset + last.mainAxisSizeWithSpacings - viewportEndOffset
                minOf(deltaToFirstItemChange, deltaToLastItemChange) > -delta
            } else {
                // scrolling backward
                val deltaToFirstItemChange = viewportStartOffset - first.offset
                val deltaToLastItemChange = viewportEndOffset - last.offset
                minOf(deltaToFirstItemChange, deltaToLastItemChange) > delta
            }
        return if (canApply) {
            visibleItemsInfo.fastForEach { it.applyScrollDelta(delta, updateAnimations) }
            LazyListMeasureResult(
                firstVisibleItem = firstVisibleItem,
                firstVisibleItemScrollOffset = firstVisibleItemScrollOffset - delta,
                canScrollForward =
                    canScrollForward ||
                        delta > 0, // we scrolled backward, so now we can scroll forward
                consumedScroll = delta.toFloat(),
                measureResult = measureResult,
                scrollBackAmount = scrollBackAmount,
                remeasureNeeded = remeasureNeeded,
                coroutineScope = coroutineScope,
                density = density,
                childConstraints = childConstraints,
                visibleItemsInfo = visibleItemsInfo,
                viewportStartOffset = viewportStartOffset,
                viewportEndOffset = viewportEndOffset,
                totalItemsCount = totalItemsCount,
                reverseLayout = reverseLayout,
                orientation = orientation,
                afterContentPadding = afterContentPadding,
                mainAxisItemSpacing = mainAxisItemSpacing
            )
        } else {
            null
        }
    }
}