| /* |
| * 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.list |
| |
| import androidx.compose.foundation.lazy.LazyListScope |
| import androidx.compose.foundation.lazy.LazyListState |
| import androidx.compose.foundation.lazy.getDefaultLazyKeyFor |
| import androidx.compose.foundation.lazy.layout.IntervalHolder |
| import androidx.compose.foundation.lazy.layout.IntervalList |
| import androidx.compose.foundation.lazy.layout.intervalForIndex |
| import androidx.compose.foundation.lazy.layout.intervalIndexForItemIndex |
| import androidx.compose.runtime.Composable |
| import androidx.compose.runtime.LaunchedEffect |
| import androidx.compose.runtime.State |
| import androidx.compose.runtime.derivedStateOf |
| import androidx.compose.runtime.mutableStateOf |
| import androidx.compose.runtime.remember |
| import androidx.compose.runtime.rememberUpdatedState |
| import androidx.compose.runtime.snapshotFlow |
| import androidx.compose.ui.node.Ref |
| import kotlinx.coroutines.flow.collect |
| |
| @Composable |
| internal fun rememberStateOfItemsProvider( |
| state: LazyListState, |
| content: LazyListScope.() -> Unit, |
| itemScope: Ref<LazyItemScopeImpl> |
| ): State<LazyListItemsProvider> { |
| val latestContent = rememberUpdatedState(content) |
| val nearestItemsRangeState = remember(state) { |
| mutableStateOf( |
| calculateNearestItemsRange(state.firstVisibleItemIndexNonObservable.value) |
| ) |
| } |
| LaunchedEffect(nearestItemsRangeState) { |
| snapshotFlow { calculateNearestItemsRange(state.firstVisibleItemIndex) } |
| // MutableState's SnapshotMutationPolicy will make sure the provider is only |
| // recreated when the state is updated with a new range. |
| .collect { nearestItemsRangeState.value = it } |
| } |
| return remember(nearestItemsRangeState) { |
| derivedStateOf<LazyListItemsProvider> { |
| val listScope = LazyListScopeImpl().apply(latestContent.value) |
| LazyListItemsProviderImpl( |
| itemScope, |
| listScope.intervals, |
| listScope.headerIndexes, |
| nearestItemsRangeState.value |
| ) |
| } |
| } |
| } |
| |
| internal class LazyListItemsProviderImpl( |
| private val itemScope: Ref<LazyItemScopeImpl>, |
| private val list: IntervalList<LazyListIntervalContent>, |
| override val headerIndexes: List<Int>, |
| nearestItemsRange: IntRange |
| ) : LazyListItemsProvider { |
| /** |
| * Caches the last interval we binary searched for. We might not need to recalculate |
| * for subsequent queries, as they tend to be localised. |
| */ |
| private var lastInterval: IntervalHolder<LazyListIntervalContent>? = null |
| |
| override val itemsCount get() = list.totalSize |
| |
| override fun getKey(index: Int): Any { |
| val interval = getIntervalForIndex(index) |
| val localIntervalIndex = index - interval.startIndex |
| val key = interval.content.key?.invoke(localIntervalIndex) |
| return key ?: getDefaultLazyKeyFor(index) |
| } |
| |
| override fun getContent(index: Int): @Composable () -> Unit { |
| val interval = getIntervalForIndex(index) |
| val localIntervalIndex = index - interval.startIndex |
| return interval.content.content.invoke(itemScope.value!!, localIntervalIndex) |
| } |
| |
| override val keyToIndexMap: Map<Any, Int> = generateKeyToIndexMap(nearestItemsRange, list) |
| |
| private fun getIntervalForIndex(itemIndex: Int) = lastInterval.let { |
| if (it != null && itemIndex in it.startIndex until it.startIndex + it.size) { |
| it |
| } else { |
| list.intervalForIndex(itemIndex).also { lastInterval = it } |
| } |
| } |
| } |
| |
| /** |
| * Traverses the interval [list] in order to create a mapping from the key to the index for all |
| * the indexes in the passed [range]. |
| * The returned map will not contain the values for intervals with no key mapping provided. |
| */ |
| internal fun generateKeyToIndexMap( |
| range: IntRange, |
| list: IntervalList<LazyListIntervalContent> |
| ): Map<Any, Int> { |
| val first = range.first |
| check(first >= 0) |
| val last = minOf(range.last, list.totalSize - 1) |
| return if (last < first) { |
| emptyMap() |
| } else { |
| hashMapOf<Any, Int>().also { map -> |
| var intervalIndex = list.intervalIndexForItemIndex(first) |
| var itemIndex = first |
| while (itemIndex <= last) { |
| val interval = list.intervals[intervalIndex] |
| val keyFactory = interval.content.key |
| if (keyFactory != null) { |
| val localItemIndex = itemIndex - interval.startIndex |
| if (localItemIndex == interval.size) { |
| intervalIndex++ |
| } else { |
| map[keyFactory(localItemIndex)] = itemIndex |
| itemIndex++ |
| } |
| } else { |
| intervalIndex++ |
| itemIndex = interval.startIndex + interval.size |
| } |
| } |
| } |
| } |
| } |
| |
| /** |
| * Returns a range of indexes which contains at least [ExtraItemsNearTheSlidingWindow] items near |
| * the first visible item. It is optimized to return the same range for small changes in the |
| * firstVisibleItem value so we do not regenerate the map on each scroll. |
| */ |
| private fun calculateNearestItemsRange(firstVisibleItem: Int): IntRange { |
| val slidingWindowStart = VisibleItemsSlidingWindowSize * |
| (firstVisibleItem / VisibleItemsSlidingWindowSize) |
| |
| val start = maxOf(slidingWindowStart - ExtraItemsNearTheSlidingWindow, 0) |
| val end = slidingWindowStart + VisibleItemsSlidingWindowSize + ExtraItemsNearTheSlidingWindow |
| return start until end |
| } |
| |
| /** |
| * We use the idea of sliding window as an optimization, so user can scroll up to this number of |
| * items until we have to regenerate the key to index map. |
| */ |
| private val VisibleItemsSlidingWindowSize = 30 |
| |
| /** |
| * The minimum amount of items near the current first visible item we want to have mapping for. |
| */ |
| private val ExtraItemsNearTheSlidingWindow = 100 |
