LazyMeasurePolicy.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.layout
import androidx.compose.runtime.Stable
import androidx.compose.ui.layout.MeasureScope
import androidx.compose.ui.layout.SubcomposeMeasureScope
import androidx.compose.ui.unit.Constraints
/**
* Defines the measure and layout behaviour of a [LazyLayout].
*/
@Stable
internal fun interface LazyMeasurePolicy {
fun MeasureScope.measure(
placeablesProvider: LazyLayoutPlaceablesProvider,
constraints: Constraints
): LazyLayoutMeasureResult
}
@Stable
internal class LazyLayoutPlaceablesProvider internal constructor(
private val itemsProvider: LazyLayoutItemsProvider,
private val itemContentFactory: LazyLayoutItemContentFactory,
private val subcomposeMeasureScope: SubcomposeMeasureScope
) {
/**
* A cache of the previously composed items. It allows us to support [get]
* re-executions with the same index during the same measure pass.
*/
private val placeablesCache = hashMapOf<Int, Array<LazyLayoutPlaceable>>()
/**
* Used to subcompose and measure the items of lazy layout.
*/
fun getAndMeasure(index: Int, constraints: Constraints): Array<LazyLayoutPlaceable> {
val cachedPlaceable = placeablesCache[index]
return if (cachedPlaceable != null) {
cachedPlaceable
} else {
val key = itemsProvider.getKey(index)
val itemContent = itemContentFactory.getContent(index, key)
val measurables = subcomposeMeasureScope.subcompose(key, itemContent)
Array(measurables.size) { i ->
val measurable = measurables[i]
LazyLayoutPlaceable(measurable.measure(constraints), measurable.parentData)
}.also {
placeablesCache[index] = it
}
}
}
}