/*
* 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.grid
import androidx.compose.foundation.ExperimentalFoundationApi
import androidx.compose.foundation.gestures.Orientation
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.min
import kotlin.math.roundToInt
import kotlin.math.sign
/**
* Measures and calculates the positions for the currently visible items. The result is produced
* as a [LazyGridMeasureResult] which contains all the calculations.
*/
@OptIn(ExperimentalFoundationApi::class)
internal fun measureLazyGrid(
itemsCount: Int,
measuredLineProvider: LazyMeasuredLineProvider,
measuredItemProvider: LazyMeasuredItemProvider,
mainAxisAvailableSize: Int,
slotsPerLine: Int,
beforeContentPadding: Int,
afterContentPadding: Int,
firstVisibleLineIndex: LineIndex,
firstVisibleLineScrollOffset: Int,
scrollToBeConsumed: Float,
constraints: Constraints,
isVertical: Boolean,
verticalArrangement: Arrangement.Vertical?,
horizontalArrangement: Arrangement.Horizontal?,
reverseLayout: Boolean,
density: Density,
layoutDirection: LayoutDirection,
placementAnimator: LazyGridItemPlacementAnimator,
layout: (Int, Int, Placeable.PlacementScope.() -> Unit) -> MeasureResult
): LazyGridMeasureResult {
require(beforeContentPadding >= 0)
require(afterContentPadding >= 0)
if (itemsCount <= 0) {
// empty data set. reset the current scroll and report zero size
return LazyGridMeasureResult(
firstVisibleLine = null,
firstVisibleLineScrollOffset = 0,
canScrollForward = false,
consumedScroll = 0f,
measureResult = layout(constraints.minWidth, constraints.minHeight) {},
visibleItemsInfo = emptyList(),
viewportStartOffset = -beforeContentPadding,
viewportEndOffset = mainAxisAvailableSize + afterContentPadding,
totalItemsCount = 0,
reverseLayout = reverseLayout,
orientation = if (isVertical) Orientation.Vertical else Orientation.Horizontal,
afterContentPadding = afterContentPadding
)
} else {
var currentFirstLineIndex = firstVisibleLineIndex
var currentFirstLineScrollOffset = firstVisibleLineScrollOffset
// 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
currentFirstLineScrollOffset -= scrollDelta
// if the current scroll offset is less than minimally possible
if (currentFirstLineIndex == LineIndex(0) && currentFirstLineScrollOffset < 0) {
scrollDelta += currentFirstLineScrollOffset
currentFirstLineScrollOffset = 0
}
// this will contain all the MeasuredItems representing the visible lines
val visibleLines = mutableListOf<LazyMeasuredLine>()
// include the start padding so we compose items in the padding area. before starting
// scrolling forward we would remove it back
currentFirstLineScrollOffset -= beforeContentPadding
// define min and max offsets (min offset currently includes beforeContentPadding)
val minOffset = -beforeContentPadding
val maxOffset = mainAxisAvailableSize
// we had scrolled backward or we compose items in the start padding area, which means
// items before current firstLineScrollOffset should be visible. compose them and update
// firstLineScrollOffset
while (currentFirstLineScrollOffset < 0 && currentFirstLineIndex > LineIndex(0)) {
val previous = LineIndex(currentFirstLineIndex.value - 1)
val measuredLine = measuredLineProvider.getAndMeasure(previous)
visibleLines.add(0, measuredLine)
currentFirstLineScrollOffset += measuredLine.mainAxisSizeWithSpacings
currentFirstLineIndex = previous
}
// if we were scrolled backward, but there were not enough lines before. this means
// not the whole scroll was consumed
if (currentFirstLineScrollOffset < minOffset) {
scrollDelta += currentFirstLineScrollOffset
currentFirstLineScrollOffset = minOffset
}
// neutralize previously added start padding as we stopped filling the before content padding
currentFirstLineScrollOffset += beforeContentPadding
var index = currentFirstLineIndex
val maxMainAxis = (maxOffset + afterContentPadding).coerceAtLeast(0)
var currentMainAxisOffset = -currentFirstLineScrollOffset
// first we need to skip lines we already composed while composing backward
visibleLines.fastForEach {
index++
currentMainAxisOffset += it.mainAxisSizeWithSpacings
}
// then composing visible lines forward until we fill the whole viewport.
// we want to have at least one line in visibleItems even if in fact all the items are
// offscreen, this can happen if the content padding is larger than the available size.
while (currentMainAxisOffset <= maxMainAxis || visibleLines.isEmpty()) {
val measuredLine = measuredLineProvider.getAndMeasure(index)
if (measuredLine.isEmpty()) {
--index
break
}
currentMainAxisOffset += measuredLine.mainAxisSizeWithSpacings
if (currentMainAxisOffset <= minOffset &&
measuredLine.items.last().index.value != itemsCount - 1) {
// this line is offscreen and will not be placed. advance firstVisibleLineIndex
currentFirstLineIndex = index + 1
currentFirstLineScrollOffset -= measuredLine.mainAxisSizeWithSpacings
} else {
visibleLines.add(measuredLine)
}
index++
}
// we didn't fill the whole viewport with lines starting from firstVisibleLineIndex.
// lets try to scroll back if we have enough lines before firstVisibleLineIndex.
if (currentMainAxisOffset < maxOffset) {
val toScrollBack = maxOffset - currentMainAxisOffset
currentFirstLineScrollOffset -= toScrollBack
currentMainAxisOffset += toScrollBack
while (currentFirstLineScrollOffset < beforeContentPadding &&
currentFirstLineIndex > LineIndex(0)
) {
val previousIndex = LineIndex(currentFirstLineIndex.value - 1)
val measuredLine = measuredLineProvider.getAndMeasure(previousIndex)
visibleLines.add(0, measuredLine)
currentFirstLineScrollOffset += measuredLine.mainAxisSizeWithSpacings
currentFirstLineIndex = previousIndex
}
scrollDelta += toScrollBack
if (currentFirstLineScrollOffset < 0) {
scrollDelta += currentFirstLineScrollOffset
currentMainAxisOffset += currentFirstLineScrollOffset
currentFirstLineScrollOffset = 0
}
}
// report the amount of pixels we consumed. scrollDelta can be smaller than
// scrollToBeConsumed if there were not enough lines to fill the offered space or it
// can be larger if lines were resized, or if, for example, we were previously
// displaying the line 15, but now we have only 10 lines 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 lines from visibleLines list
val visibleLinesScrollOffset = -currentFirstLineScrollOffset
var firstLine = visibleLines.first()
// even if we compose lines to fill before content padding we should ignore lines fully
// located there for the state's scroll position calculation (first line + first offset)
if (beforeContentPadding > 0) {
for (i in visibleLines.indices) {
val size = visibleLines[i].mainAxisSizeWithSpacings
if (size <= currentFirstLineScrollOffset && i != visibleLines.lastIndex) {
currentFirstLineScrollOffset -= size
firstLine = visibleLines[i + 1]
} else {
break
}
}
}
val layoutWidth = if (isVertical) {
constraints.maxWidth
} else {
constraints.constrainWidth(currentMainAxisOffset)
}
val layoutHeight = if (isVertical) {
constraints.constrainHeight(currentMainAxisOffset)
} else {
constraints.maxHeight
}
val positionedItems = calculateItemsOffsets(
lines = visibleLines,
layoutWidth = layoutWidth,
layoutHeight = layoutHeight,
finalMainAxisOffset = currentMainAxisOffset,
maxOffset = maxOffset,
firstLineScrollOffset = visibleLinesScrollOffset,
isVertical = isVertical,
verticalArrangement = verticalArrangement,
horizontalArrangement = horizontalArrangement,
reverseLayout = reverseLayout,
density = density,
layoutDirection = layoutDirection
)
placementAnimator.onMeasured(
consumedScroll = consumedScroll.toInt(),
layoutWidth = layoutWidth,
layoutHeight = layoutHeight,
slotsPerLine = slotsPerLine,
reverseLayout = reverseLayout,
positionedItems = positionedItems,
measuredItemProvider = measuredItemProvider
)
return LazyGridMeasureResult(
firstVisibleLine = firstLine,
firstVisibleLineScrollOffset = currentFirstLineScrollOffset,
canScrollForward = currentMainAxisOffset > maxOffset,
consumedScroll = consumedScroll,
measureResult = layout(layoutWidth, layoutHeight) {
positionedItems.fastForEach { it.place(this) }
},
viewportStartOffset = -beforeContentPadding,
viewportEndOffset = mainAxisAvailableSize + afterContentPadding,
visibleItemsInfo = positionedItems,
totalItemsCount = itemsCount,
reverseLayout = reverseLayout,
orientation = if (isVertical) Orientation.Vertical else Orientation.Horizontal,
afterContentPadding = afterContentPadding
)
}
}
/**
* Calculates [LazyMeasuredLine]s offsets.
*/
@OptIn(ExperimentalFoundationApi::class)
private fun calculateItemsOffsets(
lines: List<LazyMeasuredLine>,
layoutWidth: Int,
layoutHeight: Int,
finalMainAxisOffset: Int,
maxOffset: Int,
firstLineScrollOffset: Int,
isVertical: Boolean,
verticalArrangement: Arrangement.Vertical?,
horizontalArrangement: Arrangement.Horizontal?,
reverseLayout: Boolean,
density: Density,
layoutDirection: LayoutDirection
): MutableList<LazyGridPositionedItem> {
val mainAxisLayoutSize = if (isVertical) layoutHeight else layoutWidth
val hasSpareSpace = finalMainAxisOffset < min(mainAxisLayoutSize, maxOffset)
if (hasSpareSpace) {
check(firstLineScrollOffset == 0)
}
val positionedItems = ArrayList<LazyGridPositionedItem>(lines.size)
if (hasSpareSpace) {
val linesCount = lines.size
val sizes = IntArray(linesCount) { index ->
val reverseLayoutAwareIndex = if (!reverseLayout) index else linesCount - index - 1
lines[reverseLayoutAwareIndex].mainAxisSize
}
val offsets = IntArray(linesCount) { 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 linesCount - index - 1
val line = lines[reverseLayoutAwareIndex]
val relativeOffset = if (reverseLayout) {
mainAxisLayoutSize - absoluteOffset - line.mainAxisSize
} else {
absoluteOffset
}
val addIndex = if (reverseLayout) 0 else positionedItems.size
positionedItems.addAll(
addIndex,
line.position(relativeOffset, layoutWidth, layoutHeight)
)
}
} else {
var currentMainAxis = firstLineScrollOffset
lines.fastForEach {
positionedItems.addAll(it.position(currentMainAxis, layoutWidth, layoutHeight))
currentMainAxis += it.mainAxisSizeWithSpacings
}
}
return positionedItems
}