/*
* 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.OverscrollEffect
import androidx.compose.foundation.checkScrollableContainerConstraints
import androidx.compose.foundation.clipScrollableContainer
import androidx.compose.foundation.gestures.FlingBehavior
import androidx.compose.foundation.gestures.Orientation
import androidx.compose.foundation.gestures.ScrollableDefaults
import androidx.compose.foundation.gestures.scrollable
import androidx.compose.foundation.layout.Arrangement
import androidx.compose.foundation.layout.PaddingValues
import androidx.compose.foundation.layout.calculateEndPadding
import androidx.compose.foundation.layout.calculateStartPadding
import androidx.compose.foundation.lazy.layout.LazyLayout
import androidx.compose.foundation.lazy.layout.LazyLayoutMeasureScope
import androidx.compose.foundation.overscroll
import androidx.compose.runtime.Composable
import androidx.compose.runtime.remember
import androidx.compose.runtime.rememberCoroutineScope
import androidx.compose.runtime.snapshots.Snapshot
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.layout.MeasureResult
import androidx.compose.ui.platform.LocalLayoutDirection
import androidx.compose.ui.unit.Constraints
import androidx.compose.ui.unit.IntOffset
import androidx.compose.ui.unit.constrainHeight
import androidx.compose.ui.unit.constrainWidth
import androidx.compose.ui.unit.offset
@OptIn(ExperimentalFoundationApi::class)
@Composable
internal fun LazyList(
/** Modifier to be applied for the inner layout */
modifier: Modifier,
/** State controlling the scroll position */
state: LazyListState,
/** The inner padding to be added for the whole content(not for each individual item) */
contentPadding: PaddingValues,
/** reverse the direction of scrolling and layout */
reverseLayout: Boolean,
/** The layout orientation of the list */
isVertical: Boolean,
/** fling behavior to be used for flinging */
flingBehavior: FlingBehavior,
/** Whether scrolling via the user gestures is allowed. */
userScrollEnabled: Boolean,
/** The alignment to align items horizontally. Required when isVertical is true */
horizontalAlignment: Alignment.Horizontal? = null,
/** The vertical arrangement for items. Required when isVertical is true */
verticalArrangement: Arrangement.Vertical? = null,
/** The alignment to align items vertically. Required when isVertical is false */
verticalAlignment: Alignment.Vertical? = null,
/** The horizontal arrangement for items. Required when isVertical is false */
horizontalArrangement: Arrangement.Horizontal? = null,
/** The content of the list */
content: LazyListScope.() -> Unit
) {
val overscrollEffect = ScrollableDefaults.overscrollEffect()
val itemProvider = rememberLazyListItemProvider(state, content)
val beyondBoundsInfo = remember { LazyListBeyondBoundsInfo() }
val scope = rememberCoroutineScope()
val placementAnimator = remember(state, isVertical) {
LazyListItemPlacementAnimator(scope, isVertical)
}
state.placementAnimator = placementAnimator
val measurePolicy = rememberLazyListMeasurePolicy(
itemProvider,
state,
beyondBoundsInfo,
overscrollEffect,
contentPadding,
reverseLayout,
isVertical,
horizontalAlignment,
verticalAlignment,
horizontalArrangement,
verticalArrangement,
placementAnimator
)
ScrollPositionUpdater(itemProvider, state)
val orientation = if (isVertical) Orientation.Vertical else Orientation.Horizontal
LazyLayout(
modifier = modifier
.then(state.remeasurementModifier)
.then(state.awaitLayoutModifier)
.lazyListSemantics(
itemProvider = itemProvider,
state = state,
coroutineScope = scope,
isVertical = isVertical,
reverseScrolling = reverseLayout,
userScrollEnabled = userScrollEnabled
)
.clipScrollableContainer(orientation)
.lazyListBeyondBoundsModifier(state, beyondBoundsInfo, reverseLayout)
.lazyListPinningModifier(state, beyondBoundsInfo)
.overscroll(overscrollEffect)
.scrollable(
orientation = orientation,
reverseDirection = ScrollableDefaults.reverseDirection(
LocalLayoutDirection.current,
orientation,
reverseLayout
),
interactionSource = state.internalInteractionSource,
flingBehavior = flingBehavior,
state = state,
overscrollEffect = overscrollEffect,
enabled = userScrollEnabled
),
prefetchState = state.prefetchState,
measurePolicy = measurePolicy,
itemProvider = itemProvider
)
}
/** Extracted to minimize the recomposition scope */
@ExperimentalFoundationApi
@Composable
private fun ScrollPositionUpdater(
itemProvider: LazyListItemProvider,
state: LazyListState
) {
if (itemProvider.itemCount > 0) {
state.updateScrollPositionIfTheFirstItemWasMoved(itemProvider)
}
}
@ExperimentalFoundationApi
@Composable
private fun rememberLazyListMeasurePolicy(
/** Items provider of the list. */
itemProvider: LazyListItemProvider,
/** The state of the list. */
state: LazyListState,
/** Keeps track of the number of items we measure and place that are beyond visible bounds. */
beyondBoundsInfo: LazyListBeyondBoundsInfo,
/** The overscroll controller. */
overscrollEffect: OverscrollEffect,
/** The inner padding to be added for the whole content(nor for each individual item) */
contentPadding: PaddingValues,
/** reverse the direction of scrolling and layout */
reverseLayout: Boolean,
/** The layout orientation of the list */
isVertical: Boolean,
/** The alignment to align items horizontally. Required when isVertical is true */
horizontalAlignment: Alignment.Horizontal? = null,
/** The alignment to align items vertically. Required when isVertical is false */
verticalAlignment: Alignment.Vertical? = null,
/** The horizontal arrangement for items. Required when isVertical is false */
horizontalArrangement: Arrangement.Horizontal? = null,
/** The vertical arrangement for items. Required when isVertical is true */
verticalArrangement: Arrangement.Vertical? = null,
/** Item placement animator. Should be notified with the measuring result */
placementAnimator: LazyListItemPlacementAnimator
) = remember<LazyLayoutMeasureScope.(Constraints) -> MeasureResult>(
state,
beyondBoundsInfo,
overscrollEffect,
contentPadding,
reverseLayout,
isVertical,
horizontalAlignment,
verticalAlignment,
horizontalArrangement,
verticalArrangement,
placementAnimator
) {
{ containerConstraints ->
checkScrollableContainerConstraints(
containerConstraints,
if (isVertical) Orientation.Vertical else Orientation.Horizontal
)
// resolve content paddings
val startPadding =
if (isVertical) {
contentPadding.calculateLeftPadding(layoutDirection).roundToPx()
} else {
// in horizontal configuration, padding is reversed by placeRelative
contentPadding.calculateStartPadding(layoutDirection).roundToPx()
}
val endPadding =
if (isVertical) {
contentPadding.calculateRightPadding(layoutDirection).roundToPx()
} else {
// in horizontal configuration, padding is reversed by placeRelative
contentPadding.calculateEndPadding(layoutDirection).roundToPx()
}
val topPadding = contentPadding.calculateTopPadding().roundToPx()
val bottomPadding = contentPadding.calculateBottomPadding().roundToPx()
val totalVerticalPadding = topPadding + bottomPadding
val totalHorizontalPadding = startPadding + endPadding
val totalMainAxisPadding = if (isVertical) totalVerticalPadding else totalHorizontalPadding
val beforeContentPadding = when {
isVertical && !reverseLayout -> topPadding
isVertical && reverseLayout -> bottomPadding
!isVertical && !reverseLayout -> startPadding
else -> endPadding // !isVertical && reverseLayout
}
val afterContentPadding = totalMainAxisPadding - beforeContentPadding
val contentConstraints =
containerConstraints.offset(-totalHorizontalPadding, -totalVerticalPadding)
state.updateScrollPositionIfTheFirstItemWasMoved(itemProvider)
// Update the state's cached Density
state.density = this
// this will update the scope used by the item composables
itemProvider.itemScope.setMaxSize(
width = contentConstraints.maxWidth,
height = contentConstraints.maxHeight
)
val spaceBetweenItemsDp = if (isVertical) {
requireNotNull(verticalArrangement).spacing
} else {
requireNotNull(horizontalArrangement).spacing
}
val spaceBetweenItems = spaceBetweenItemsDp.roundToPx()
val itemsCount = itemProvider.itemCount
// can be negative if the content padding is larger than the max size from constraints
val mainAxisAvailableSize = if (isVertical) {
containerConstraints.maxHeight - totalVerticalPadding
} else {
containerConstraints.maxWidth - totalHorizontalPadding
}
val visualItemOffset = if (!reverseLayout || mainAxisAvailableSize > 0) {
IntOffset(startPadding, topPadding)
} else {
// When layout is reversed and paddings together take >100% of the available space,
// layout size is coerced to 0 when positioning. To take that space into account,
// we offset start padding by negative space between paddings.
IntOffset(
if (isVertical) startPadding else startPadding + mainAxisAvailableSize,
if (isVertical) topPadding + mainAxisAvailableSize else topPadding
)
}
val measuredItemProvider = LazyMeasuredItemProvider(
contentConstraints,
isVertical,
itemProvider,
this
) { index, key, placeables ->
// we add spaceBetweenItems as an extra spacing for all items apart from the last one so
// the lazy list measuring logic will take it into account.
val spacing = if (index.value == itemsCount - 1) 0 else spaceBetweenItems
LazyMeasuredItem(
index = index.value,
placeables = placeables,
isVertical = isVertical,
horizontalAlignment = horizontalAlignment,
verticalAlignment = verticalAlignment,
layoutDirection = layoutDirection,
reverseLayout = reverseLayout,
beforeContentPadding = beforeContentPadding,
afterContentPadding = afterContentPadding,
spacing = spacing,
visualOffset = visualItemOffset,
key = key,
placementAnimator = placementAnimator
)
}
state.premeasureConstraints = measuredItemProvider.childConstraints
val firstVisibleItemIndex: DataIndex
val firstVisibleScrollOffset: Int
Snapshot.withoutReadObservation {
firstVisibleItemIndex = DataIndex(state.firstVisibleItemIndex)
firstVisibleScrollOffset = state.firstVisibleItemScrollOffset
}
measureLazyList(
itemsCount = itemsCount,
itemProvider = measuredItemProvider,
mainAxisAvailableSize = mainAxisAvailableSize,
beforeContentPadding = beforeContentPadding,
afterContentPadding = afterContentPadding,
firstVisibleItemIndex = firstVisibleItemIndex,
firstVisibleItemScrollOffset = firstVisibleScrollOffset,
scrollToBeConsumed = state.scrollToBeConsumed,
constraints = contentConstraints,
isVertical = isVertical,
headerIndexes = itemProvider.headerIndexes,
verticalArrangement = verticalArrangement,
horizontalArrangement = horizontalArrangement,
reverseLayout = reverseLayout,
density = this,
placementAnimator = placementAnimator,
beyondBoundsInfo = beyondBoundsInfo,
layout = { width, height, placement ->
layout(
containerConstraints.constrainWidth(width + totalHorizontalPadding),
containerConstraints.constrainHeight(height + totalVerticalPadding),
emptyMap(),
placement
)
}
).also {
state.applyMeasureResult(it)
refreshOverscrollInfo(overscrollEffect, it)
}
}
}
@OptIn(ExperimentalFoundationApi::class)
private fun refreshOverscrollInfo(
overscrollEffect: OverscrollEffect,
result: LazyListMeasureResult
) {
val canScrollForward = result.canScrollForward
val canScrollBackward = (result.firstVisibleItem?.index ?: 0) != 0 ||
result.firstVisibleItemScrollOffset != 0
overscrollEffect.isEnabled = canScrollForward || canScrollBackward
}