/*
* 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.layout.Arrangement
import androidx.compose.ui.layout.MeasureResult
import androidx.compose.ui.layout.Placeable
import androidx.compose.ui.unit.Constraints
import androidx.compose.ui.unit.Density
import androidx.compose.ui.unit.LayoutDirection
import androidx.compose.ui.unit.constrainHeight
import androidx.compose.ui.unit.constrainWidth
import androidx.compose.ui.util.fastForEach
import kotlin.math.abs
import kotlin.math.roundToInt
import kotlin.math.sign
/**
* Measures and calculates the positions for the currently visible items. The result is produced
* as a [LazyListMeasureResult] which contains all the calculations.
*/
internal fun measureLazyList(
itemsCount: Int,
itemProvider: LazyMeasuredItemProvider,
mainAxisMaxSize: Int,
startContentPadding: Int,
endContentPadding: Int,
firstVisibleItemIndex: DataIndex,
firstVisibleItemScrollOffset: Int,
scrollToBeConsumed: Float,
constraints: Constraints,
isVertical: Boolean,
headerIndexes: List<Int>,
verticalArrangement: Arrangement.Vertical?,
horizontalArrangement: Arrangement.Horizontal?,
reverseLayout: Boolean,
density: Density,
layoutDirection: LayoutDirection,
layout: (Int, Int, Placeable.PlacementScope.() -> Unit) -> MeasureResult
): LazyListMeasureResult {
require(startContentPadding >= 0)
require(endContentPadding >= 0)
if (itemsCount <= 0) {
// empty data set. reset the current scroll and report zero size
return LazyListMeasureResult(
firstVisibleItem = null,
firstVisibleItemScrollOffset = 0,
canScrollForward = false,
consumedScroll = 0f,
measureResult = layout(constraints.minWidth, constraints.minHeight) {},
visibleItemsInfo = emptyList(),
viewportStartOffset = -startContentPadding,
viewportEndOffset = endContentPadding,
totalItemsCount = 0,
)
} else {
var currentFirstItemIndex = firstVisibleItemIndex
var currentFirstItemScrollOffset = firstVisibleItemScrollOffset
if (currentFirstItemIndex.value >= itemsCount) {
// the data set has been updated and now we have less items that we were
// scrolled to before
currentFirstItemIndex = DataIndex(itemsCount - 1)
currentFirstItemScrollOffset = 0
}
// represents the real amount of scroll we applied as a result of this measure pass.
var scrollDelta = scrollToBeConsumed.roundToInt()
// applying the whole requested scroll offset. we will figure out if we can't consume
// all of it later
currentFirstItemScrollOffset -= scrollDelta
// if the current scroll offset is less than minimally possible
if (currentFirstItemIndex == DataIndex(0) && currentFirstItemScrollOffset < 0) {
scrollDelta += currentFirstItemScrollOffset
currentFirstItemScrollOffset = 0
}
// this will contain all the MeasuredItems representing the visible items
val visibleItems = mutableListOf<LazyMeasuredItem>()
// include the start padding so we compose items in the padding area. before starting
// scrolling forward we would remove it back
currentFirstItemScrollOffset -= startContentPadding
// define min and max offsets (min offset currently includes startPadding)
val minOffset = -startContentPadding
val maxOffset = mainAxisMaxSize
// max of cross axis sizes of all visible items
var maxCrossAxis = 0
// we had scrolled backward or we compose items in the start padding area, which means
// items before current firstItemScrollOffset should be visible. compose them and update
// firstItemScrollOffset
while (currentFirstItemScrollOffset < 0 && currentFirstItemIndex > DataIndex(0)) {
val previous = DataIndex(currentFirstItemIndex.value - 1)
val measuredItem = itemProvider.getAndMeasure(previous)
visibleItems.add(0, measuredItem)
maxCrossAxis = maxOf(maxCrossAxis, measuredItem.crossAxisSize)
currentFirstItemScrollOffset += measuredItem.sizeWithSpacings
currentFirstItemIndex = previous
}
// if we were scrolled backward, but there were not enough items before. this means
// not the whole scroll was consumed
if (currentFirstItemScrollOffset < minOffset) {
scrollDelta += currentFirstItemScrollOffset
currentFirstItemScrollOffset = minOffset
}
// neutralize previously added start padding as we stopped filling the start padding area
currentFirstItemScrollOffset += startContentPadding
var index = currentFirstItemIndex
val maxMainAxis = maxOffset + endContentPadding
var mainAxisUsed = -currentFirstItemScrollOffset
// first we need to skip items we already composed while composing backward
visibleItems.fastForEach {
index++
mainAxisUsed += it.sizeWithSpacings
}
// then composing visible items forward until we fill the whole viewport
while (mainAxisUsed <= maxMainAxis && index.value < itemsCount) {
val measuredItem = itemProvider.getAndMeasure(index)
mainAxisUsed += measuredItem.sizeWithSpacings
if (mainAxisUsed <= minOffset) {
// this item is offscreen and will not be placed. advance firstVisibleItemIndex
currentFirstItemIndex = index + 1
currentFirstItemScrollOffset -= measuredItem.sizeWithSpacings
} else {
maxCrossAxis = maxOf(maxCrossAxis, measuredItem.crossAxisSize)
visibleItems.add(measuredItem)
}
index++
}
// we didn't fill the whole viewport with items starting from firstVisibleItemIndex.
// lets try to scroll back if we have enough items before firstVisibleItemIndex.
if (mainAxisUsed < maxOffset) {
val toScrollBack = maxOffset - mainAxisUsed
currentFirstItemScrollOffset -= toScrollBack
mainAxisUsed += toScrollBack
while (currentFirstItemScrollOffset < 0 && currentFirstItemIndex > DataIndex(0)) {
val previousIndex = DataIndex(currentFirstItemIndex.value - 1)
val measuredItem = itemProvider.getAndMeasure(previousIndex)
visibleItems.add(0, measuredItem)
maxCrossAxis = maxOf(maxCrossAxis, measuredItem.crossAxisSize)
currentFirstItemScrollOffset += measuredItem.sizeWithSpacings
currentFirstItemIndex = previousIndex
}
scrollDelta += toScrollBack
if (currentFirstItemScrollOffset < 0) {
scrollDelta += currentFirstItemScrollOffset
mainAxisUsed += currentFirstItemScrollOffset
currentFirstItemScrollOffset = 0
}
}
// report the amount of pixels we consumed. scrollDelta can be smaller than
// scrollToBeConsumed if there were not enough items to fill the offered space or it
// can be larger if items were resized, or if, for example, we were previously
// displaying the item 15, but now we have only 10 items in total in the data set.
val consumedScroll = if (scrollToBeConsumed.roundToInt().sign == scrollDelta.sign &&
abs(scrollToBeConsumed.roundToInt()) >= abs(scrollDelta)
) {
scrollDelta.toFloat()
} else {
scrollToBeConsumed
}
// the initial offset for items from visibleItems list
val visibleItemsScrollOffset = -currentFirstItemScrollOffset
var firstItem = visibleItems.firstOrNull()
// even if we compose items to fill the start padding area we should ignore items fully
// located there for the state's scroll position calculation (first item + first offset)
if (startContentPadding > 0) {
for (i in visibleItems.indices) {
val size = visibleItems[i].sizeWithSpacings
if (size <= currentFirstItemScrollOffset && i != visibleItems.lastIndex) {
currentFirstItemScrollOffset -= size
firstItem = visibleItems[i + 1]
} else {
break
}
}
}
val layoutWidth =
constraints.constrainWidth(if (isVertical) maxCrossAxis else mainAxisUsed)
val layoutHeight =
constraints.constrainHeight(if (isVertical) mainAxisUsed else maxCrossAxis)
calculateItemsOffsets(
items = visibleItems,
mainAxisLayoutSize = if (isVertical) layoutHeight else layoutWidth,
usedMainAxisSize = mainAxisUsed,
itemsScrollOffset = visibleItemsScrollOffset,
isVertical = isVertical,
verticalArrangement = verticalArrangement,
horizontalArrangement = horizontalArrangement,
reverseLayout = reverseLayout,
density = density,
layoutDirection = layoutDirection
)
val headerItem = if (headerIndexes.isNotEmpty()) {
findOrComposeLazyListHeader(
composedVisibleItems = visibleItems,
itemProvider = itemProvider,
headerIndexes = headerIndexes,
startContentPadding = startContentPadding
)
} else {
null
}
val maximumVisibleOffset = minOf(mainAxisUsed, mainAxisMaxSize) + endContentPadding
return LazyListMeasureResult(
firstVisibleItem = firstItem,
firstVisibleItemScrollOffset = currentFirstItemScrollOffset,
canScrollForward = mainAxisUsed > maxOffset,
consumedScroll = consumedScroll,
measureResult = layout(layoutWidth, layoutHeight) {
visibleItems.fastForEach {
if (it !== headerItem) {
it.place(this, layoutWidth, layoutHeight)
}
}
// the header item should be placed (drawn) after all other items
headerItem?.place(this, layoutWidth, layoutHeight)
},
viewportStartOffset = -startContentPadding,
viewportEndOffset = maximumVisibleOffset,
visibleItemsInfo = visibleItems,
totalItemsCount = itemsCount,
)
}
}
/**
* Calculates [LazyMeasuredItem]s offsets.
*/
private fun calculateItemsOffsets(
items: List<LazyMeasuredItem>,
mainAxisLayoutSize: Int,
usedMainAxisSize: Int,
itemsScrollOffset: Int,
isVertical: Boolean,
verticalArrangement: Arrangement.Vertical?,
horizontalArrangement: Arrangement.Horizontal?,
reverseLayout: Boolean,
density: Density,
layoutDirection: LayoutDirection
) {
val hasSpareSpace = usedMainAxisSize < mainAxisLayoutSize
if (hasSpareSpace) {
check(itemsScrollOffset == 0)
}
if (hasSpareSpace) {
val itemsCount = items.size
val sizes = IntArray(itemsCount) { index ->
val reverseLayoutAwareIndex = if (!reverseLayout) index else itemsCount - index - 1
items[reverseLayoutAwareIndex].size
}
val offsets = IntArray(itemsCount) { 0 }
if (isVertical) {
with(requireNotNull(verticalArrangement)) {
density.arrange(mainAxisLayoutSize, sizes, offsets)
}
} else {
with(requireNotNull(horizontalArrangement)) {
density.arrange(mainAxisLayoutSize, sizes, layoutDirection, offsets)
}
}
offsets.forEachIndexed { index, absoluteOffset ->
val reverseLayoutAwareIndex = if (!reverseLayout) index else itemsCount - index - 1
val item = items[reverseLayoutAwareIndex]
val relativeOffset = if (reverseLayout) {
mainAxisLayoutSize - absoluteOffset - item.size
} else {
absoluteOffset
}
item.offset = relativeOffset
}
} else {
var currentMainAxis = itemsScrollOffset
items.fastForEach {
it.offset = currentMainAxis
currentMainAxis += it.sizeWithSpacings
}
}
}