/*
* 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.assertNotNestingScrollableContainers
import androidx.compose.foundation.clipScrollableContainer
import androidx.compose.foundation.gestures.FlingBehavior
import androidx.compose.foundation.gestures.Orientation
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.layout.padding
import androidx.compose.runtime.Composable
import androidx.compose.runtime.State
import androidx.compose.runtime.remember
import androidx.compose.runtime.rememberCoroutineScope
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.layout.SubcomposeLayout
import androidx.compose.ui.layout.SubcomposeLayoutState
import androidx.compose.ui.platform.LocalLayoutDirection
import androidx.compose.ui.unit.Density
import androidx.compose.ui.unit.LayoutDirection
@Composable
internal fun LazyList(
/** State object containing the latest item provider */
stateOfItemsProvider: State<LazyListItemsProvider>,
/** 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(nor 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,
/** 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
) {
val isRtl = LocalLayoutDirection.current == LayoutDirection.Rtl
// reverse scroll by default, to have "natural" gesture that goes reversed to layout
// if rtl and horizontal, do not reverse to make it right-to-left
val reverseScrollDirection = if (!isVertical && isRtl) reverseLayout else !reverseLayout
val itemContentFactory = rememberItemContentFactory(stateOfItemsProvider, state)
val subcomposeLayoutState = remember { SubcomposeLayoutState(MaxItemsToRetainForReuse) }
LazyListPrefetcher(state, stateOfItemsProvider, itemContentFactory, subcomposeLayoutState)
SubcomposeLayout(
subcomposeLayoutState,
modifier
.lazyListSemantics(
stateOfItemsProvider = stateOfItemsProvider,
state = state,
coroutineScope = rememberCoroutineScope(),
isVertical = isVertical
)
.scrollable(
orientation = if (isVertical) Orientation.Vertical else Orientation.Horizontal,
reverseDirection = reverseScrollDirection,
interactionSource = state.internalInteractionSource,
flingBehavior = flingBehavior,
state = state
)
.clipScrollableContainer(isVertical)
.padding(contentPadding)
.then(state.remeasurementModifier)
) { constraints ->
constraints.assertNotNestingScrollableContainers(isVertical)
val itemsProvider = stateOfItemsProvider.value
state.updateScrollPositionIfTheFirstItemWasMoved(itemsProvider)
// Update the state's cached Density
state.density = Density(density, fontScale)
// this will update the scope object if the constrains have been changed
itemContentFactory.updateItemScope(this, constraints)
val startContentPadding = if (isVertical) {
contentPadding.calculateTopPadding()
} else {
contentPadding.calculateStartPadding(layoutDirection)
}.roundToPx()
val endContentPadding = if (isVertical) {
contentPadding.calculateBottomPadding()
} else {
contentPadding.calculateEndPadding(layoutDirection)
}.roundToPx()
val mainAxisMaxSize = (if (isVertical) constraints.maxHeight else constraints.maxWidth)
val spaceBetweenItemsDp = if (isVertical) {
requireNotNull(verticalArrangement).spacing
} else {
requireNotNull(horizontalArrangement).spacing
}
val spaceBetweenItems = spaceBetweenItemsDp.roundToPx()
val itemsCount = itemsProvider.itemsCount
val itemProvider = LazyMeasuredItemProvider(
constraints,
isVertical,
this,
itemsProvider,
itemContentFactory
) { 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,
startContentPadding = startContentPadding,
endContentPadding = endContentPadding,
spacing = spacing,
key = key
)
}
val measureResult = measureLazyList(
itemsCount = itemsCount,
itemProvider = itemProvider,
mainAxisMaxSize = mainAxisMaxSize,
startContentPadding = if (reverseLayout) endContentPadding else startContentPadding,
endContentPadding = if (reverseLayout) startContentPadding else endContentPadding,
firstVisibleItemIndex = state.firstVisibleItemIndexNonObservable,
firstVisibleItemScrollOffset = state.firstVisibleItemScrollOffsetNonObservable,
scrollToBeConsumed = state.scrollToBeConsumed,
constraints = constraints,
isVertical = isVertical,
headerIndexes = itemsProvider.headerIndexes,
verticalArrangement = verticalArrangement,
horizontalArrangement = horizontalArrangement,
reverseLayout = reverseLayout,
density = this,
layoutDirection = layoutDirection
)
state.applyMeasureResult(measureResult)
state.onPostMeasureListener?.apply {
onPostMeasure(itemProvider.childConstraints, measureResult)
}
layout(
width = measureResult.layoutWidth,
height = measureResult.layoutHeight,
placementBlock = measureResult.placementBlock
)
}
}
private const val MaxItemsToRetainForReuse = 2
/**
* Platform specific implementation of lazy list prefetching - precomposing next items in
* advance during the scrolling.
*/
@Composable
internal expect fun LazyListPrefetcher(
lazyListState: LazyListState,
stateOfItemsProvider: State<LazyListItemsProvider>,
itemContentFactory: LazyListItemContentFactory,
subcomposeLayoutState: SubcomposeLayoutState
)