LazyBeyondBoundsModifier.kt
/*
* Copyright 2022 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.gestures.Orientation
import androidx.compose.foundation.lazy.LazyListBeyondBoundsInfo.Interval
import androidx.compose.runtime.Composable
import androidx.compose.runtime.remember
import androidx.compose.ui.Modifier
import androidx.compose.ui.layout.BeyondBoundsLayout
import androidx.compose.ui.layout.BeyondBoundsLayout.BeyondBoundsScope
import androidx.compose.ui.layout.BeyondBoundsLayout.LayoutDirection.Companion.Above
import androidx.compose.ui.layout.BeyondBoundsLayout.LayoutDirection.Companion.After
import androidx.compose.ui.layout.BeyondBoundsLayout.LayoutDirection.Companion.Before
import androidx.compose.ui.layout.BeyondBoundsLayout.LayoutDirection.Companion.Below
import androidx.compose.ui.layout.BeyondBoundsLayout.LayoutDirection.Companion.Left
import androidx.compose.ui.layout.BeyondBoundsLayout.LayoutDirection.Companion.Right
import androidx.compose.ui.layout.ModifierLocalBeyondBoundsLayout
import androidx.compose.ui.modifier.ModifierLocalProvider
import androidx.compose.ui.modifier.ProvidableModifierLocal
import androidx.compose.ui.platform.LocalLayoutDirection
import androidx.compose.ui.unit.LayoutDirection
import androidx.compose.ui.unit.LayoutDirection.Ltr
import androidx.compose.ui.unit.LayoutDirection.Rtl
/**
* This modifier is used to measure and place additional items when the lazyList receives a
* request to layout items beyond the visible bounds.
*/
@Suppress("ComposableModifierFactory")
@Composable
internal fun Modifier.lazyListBeyondBoundsModifier(
state: LazyListState,
beyondBoundsInfo: LazyListBeyondBoundsInfo,
reverseLayout: Boolean,
orientation: Orientation
): Modifier {
val layoutDirection = LocalLayoutDirection.current
return this then remember(
state,
beyondBoundsInfo,
reverseLayout,
layoutDirection,
orientation
) {
LazyListBeyondBoundsModifierLocal(
state,
beyondBoundsInfo,
reverseLayout,
layoutDirection,
orientation
)
}
}
private class LazyListBeyondBoundsModifierLocal(
private val state: LazyListState,
private val beyondBoundsInfo: LazyListBeyondBoundsInfo,
private val reverseLayout: Boolean,
private val layoutDirection: LayoutDirection,
private val orientation: Orientation
) : ModifierLocalProvider<BeyondBoundsLayout?>, BeyondBoundsLayout {
override val key: ProvidableModifierLocal<BeyondBoundsLayout?>
get() = ModifierLocalBeyondBoundsLayout
override val value: BeyondBoundsLayout
get() = this
override fun <T> layout(
direction: BeyondBoundsLayout.LayoutDirection,
block: BeyondBoundsScope.() -> T?
): T? {
// We use a new interval each time because this function is re-entrant.
var interval = beyondBoundsInfo.addInterval(
state.firstVisibleItemIndex,
state.layoutInfo.visibleItemsInfo.last().index
)
var found: T? = null
while (found == null && interval.hasMoreContent(direction)) {
// Add one extra beyond bounds item.
interval = addNextInterval(interval, direction).also {
beyondBoundsInfo.removeInterval(interval)
}
state.remeasurement?.forceRemeasure()
// When we invoke this block, the beyond bounds items are present.
found = block.invoke(
object : BeyondBoundsScope {
override val hasMoreContent: Boolean
get() = interval.hasMoreContent(direction)
}
)
}
// Dispose the items that are beyond the visible bounds.
beyondBoundsInfo.removeInterval(interval)
state.remeasurement?.forceRemeasure()
return found
}
private fun addNextInterval(
currentInterval: Interval,
direction: BeyondBoundsLayout.LayoutDirection
): Interval {
var start = currentInterval.start
var end = currentInterval.end
when (direction) {
Before -> start--
After -> end++
Above -> if (reverseLayout) end++ else start--
Below -> if (reverseLayout) start-- else end++
Left -> when (layoutDirection) {
Ltr -> if (reverseLayout) end++ else start--
Rtl -> if (reverseLayout) start-- else end++
}
Right -> when (layoutDirection) {
Ltr -> if (reverseLayout) start-- else end++
Rtl -> if (reverseLayout) end++ else start--
}
else -> unsupportedDirection()
}
return beyondBoundsInfo.addInterval(start, end)
}
private fun Interval.hasMoreContent(direction: BeyondBoundsLayout.LayoutDirection): Boolean {
fun hasMoreItemsBefore() = start > 0
fun hasMoreItemsAfter() = end < state.layoutInfo.totalItemsCount - 1
if (direction.isOppositeToOrientation()) return false
return when (direction) {
Before -> hasMoreItemsBefore()
After -> hasMoreItemsAfter()
Above -> if (reverseLayout) hasMoreItemsAfter() else hasMoreItemsBefore()
Below -> if (reverseLayout) hasMoreItemsBefore() else hasMoreItemsAfter()
Left -> when (layoutDirection) {
Ltr -> if (reverseLayout) hasMoreItemsAfter() else hasMoreItemsBefore()
Rtl -> if (reverseLayout) hasMoreItemsBefore() else hasMoreItemsAfter()
}
Right -> when (layoutDirection) {
Ltr -> if (reverseLayout) hasMoreItemsBefore() else hasMoreItemsAfter()
Rtl -> if (reverseLayout) hasMoreItemsAfter() else hasMoreItemsBefore()
}
else -> unsupportedDirection()
}
}
private fun BeyondBoundsLayout.LayoutDirection.isOppositeToOrientation(): Boolean {
return when (this) {
Above, Below -> orientation == Orientation.Horizontal
Left, Right -> orientation == Orientation.Vertical
Before, After -> false
else -> unsupportedDirection()
}
}
}
private fun unsupportedDirection(): Nothing = error(
"Lazy list does not support beyond bounds layout for the specified direction"
)