LazyMeasuredItemProvider.kt

/*
 * 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.runtime.Composable
import androidx.compose.ui.layout.Placeable
import androidx.compose.ui.layout.SubcomposeMeasureScope
import androidx.compose.ui.unit.Constraints
import androidx.compose.ui.util.fastMap

/**
 * Abstracts away the subcomposition from the measuring logic.
 */
internal class LazyMeasuredItemProvider(
    constraints: Constraints,
    isVertical: Boolean,
    private val scope: SubcomposeMeasureScope,
    private val itemContentFactory: (Int) -> @Composable () -> Unit,
    private val measuredItemFactory: (DataIndex, List<Placeable>) -> LazyMeasuredItem
) {
    // the constraints we will measure child with. the main axis is not restricted
    private val childConstraints = Constraints(
        maxWidth = if (isVertical) constraints.maxWidth else Constraints.Infinity,
        maxHeight = if (!isVertical) constraints.maxHeight else Constraints.Infinity
    )

    /**
     * Used to subcompose items of lazy lists. Composed placeables will be measured with the
     * correct constraints and wrapped into [LazyMeasuredItem].
     * This method can be called only once with each [index] per the measure pass.
     */
    fun getAndMeasure(index: DataIndex): LazyMeasuredItem {
        val placeables = scope.subcompose(index, itemContentFactory(index.value)).fastMap {
            it.measure(childConstraints)
        }
        return measuredItemFactory(index, placeables)
    }
}