LazyListItemProvider.kt
/*
* 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.lazy.layout.DelegatingLazyLayoutItemProvider
import androidx.compose.foundation.lazy.layout.IntervalList
import androidx.compose.foundation.lazy.layout.LazyLayoutItemProvider
import androidx.compose.foundation.lazy.layout.rememberLazyNearestItemsRangeState
import androidx.compose.runtime.Composable
import androidx.compose.runtime.derivedStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.rememberUpdatedState
@ExperimentalFoundationApi
internal interface LazyListItemProvider : LazyLayoutItemProvider {
/** The list of indexes of the sticky header items */
val headerIndexes: List<Int>
/** The scope used by the item content lambdas */
val itemScope: LazyItemScopeImpl
}
@ExperimentalFoundationApi
@Composable
internal fun rememberLazyListItemProvider(
state: LazyListState,
content: LazyListScope.() -> Unit
): LazyListItemProvider {
val latestContent = rememberUpdatedState(content)
val nearestItemsRangeState = rememberLazyNearestItemsRangeState(
firstVisibleItemIndex = { state.firstVisibleItemIndex },
slidingWindowSize = { NearestItemsSlidingWindowSize },
extraItemCount = { NearestItemsExtraItemCount }
)
return remember(nearestItemsRangeState) {
val itemScope = LazyItemScopeImpl()
val itemProviderState = derivedStateOf {
val listScope = LazyListScopeImpl().apply(latestContent.value)
LazyListItemProviderImpl(
listScope.intervals,
nearestItemsRangeState.value,
listScope.headerIndexes,
itemScope
)
}
object : LazyListItemProvider,
LazyLayoutItemProvider by DelegatingLazyLayoutItemProvider(itemProviderState) {
override val headerIndexes: List<Int> get() = itemProviderState.value.headerIndexes
override val itemScope: LazyItemScopeImpl get() = itemProviderState.value.itemScope
}
}
}
@ExperimentalFoundationApi
private class LazyListItemProviderImpl(
intervals: IntervalList<LazyListIntervalContent>,
nearestItemsRange: IntRange,
override val headerIndexes: List<Int>,
override val itemScope: LazyItemScopeImpl
) : LazyListItemProvider,
LazyLayoutItemProvider by LazyLayoutItemProvider(
intervals = intervals,
nearestItemsRange = nearestItemsRange,
itemContent = { interval: LazyListIntervalContent, index: Int ->
interval.item.invoke(itemScope, index)
}
)
/**
* 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 const val NearestItemsSlidingWindowSize = 30
/**
* The minimum amount of items near the current first visible item we want to have mapping for.
*/
private const val NearestItemsExtraItemCount = 100