LazyListState.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.foundation.ExperimentalFoundationApi
import androidx.compose.foundation.MutatePriority
import androidx.compose.foundation.gestures.Orientation
import androidx.compose.foundation.gestures.ScrollScope
import androidx.compose.foundation.gestures.ScrollableState
import androidx.compose.foundation.interaction.InteractionSource
import androidx.compose.foundation.interaction.MutableInteractionSource
import androidx.compose.foundation.lazy.layout.LazyLayoutPrefetchState
import androidx.compose.runtime.Composable
import androidx.compose.runtime.Stable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.saveable.Saver
import androidx.compose.runtime.saveable.listSaver
import androidx.compose.runtime.saveable.rememberSaveable
import androidx.compose.runtime.setValue
import androidx.compose.ui.layout.LayoutCoordinates
import androidx.compose.ui.layout.OnGloballyPositionedModifier
import androidx.compose.ui.layout.Remeasurement
import androidx.compose.ui.layout.RemeasurementModifier
import androidx.compose.ui.unit.Constraints
import androidx.compose.ui.unit.Density
import androidx.compose.ui.unit.IntSize
import kotlin.coroutines.Continuation
import kotlin.coroutines.resume
import kotlin.coroutines.suspendCoroutine
import kotlin.math.abs
/**
* Creates a [LazyListState] that is remembered across compositions.
*
* Changes to the provided initial values will **not** result in the state being recreated or
* changed in any way if it has already been created.
*
* @param initialFirstVisibleItemIndex the initial value for [LazyListState.firstVisibleItemIndex]
* @param initialFirstVisibleItemScrollOffset the initial value for
* [LazyListState.firstVisibleItemScrollOffset]
*/
@Composable
fun rememberLazyListState(
initialFirstVisibleItemIndex: Int = 0,
initialFirstVisibleItemScrollOffset: Int = 0
): LazyListState {
return rememberSaveable(saver = LazyListState.Saver) {
LazyListState(
initialFirstVisibleItemIndex,
initialFirstVisibleItemScrollOffset
)
}
}
/**
* A state object that can be hoisted to control and observe scrolling.
*
* In most cases, this will be created via [rememberLazyListState].
*
* @param firstVisibleItemIndex the initial value for [LazyListState.firstVisibleItemIndex]
* @param firstVisibleItemScrollOffset the initial value for
* [LazyListState.firstVisibleItemScrollOffset]
*/
@OptIn(ExperimentalFoundationApi::class)
@Stable
class LazyListState constructor(
firstVisibleItemIndex: Int = 0,
firstVisibleItemScrollOffset: Int = 0
) : ScrollableState {
/**
* The holder class for the current scroll position.
*/
private val scrollPosition =
LazyListScrollPosition(firstVisibleItemIndex, firstVisibleItemScrollOffset)
/**
* The index of the first item that is visible.
*
* Note that this property is observable and if you use it in the composable function it will
* be recomposed on every change causing potential performance issues.
*
* If you want to run some side effects like sending an analytics event or updating a state
* based on this value consider using "snapshotFlow":
* @sample androidx.compose.foundation.samples.UsingListScrollPositionForSideEffectSample
*
* If you need to use it in the composition then consider wrapping the calculation into a
* derived state in order to only have recompositions when the derived value changes:
* @sample androidx.compose.foundation.samples.UsingListScrollPositionInCompositionSample
*/
val firstVisibleItemIndex: Int get() = scrollPosition.index.value
/**
* The scroll offset of the first visible item. Scrolling forward is positive - i.e., the
* amount that the item is offset backwards.
*
* Note that this property is observable and if you use it in the composable function it will
* be recomposed on every scroll causing potential performance issues.
* @see firstVisibleItemIndex for samples with the recommended usage patterns.
*/
val firstVisibleItemScrollOffset: Int get() = scrollPosition.scrollOffset
/** Backing state for [layoutInfo] */
private val layoutInfoState = mutableStateOf<LazyListLayoutInfo>(EmptyLazyListLayoutInfo)
/**
* The object of [LazyListLayoutInfo] calculated during the last layout pass. For example,
* you can use it to calculate what items are currently visible.
*
* Note that this property is observable and is updated after every scroll or remeasure.
* If you use it in the composable function it will be recomposed on every change causing
* potential performance issues including infinity recomposition loop.
* Therefore, avoid using it in the composition.
*
* If you want to run some side effects like sending an analytics event or updating a state
* based on this value consider using "snapshotFlow":
* @sample androidx.compose.foundation.samples.UsingListLayoutInfoForSideEffectSample
*/
val layoutInfo: LazyListLayoutInfo get() = layoutInfoState.value
/**
* [InteractionSource] that will be used to dispatch drag events when this
* list is being dragged. If you want to know whether the fling (or animated scroll) is in
* progress, use [isScrollInProgress].
*/
val interactionSource: InteractionSource get() = internalInteractionSource
internal val internalInteractionSource: MutableInteractionSource = MutableInteractionSource()
/**
* The amount of scroll to be consumed in the next layout pass. Scrolling forward is negative
* - that is, it is the amount that the items are offset in y
*/
internal var scrollToBeConsumed = 0f
private set
/**
* Needed for [animateScrollToItem]. Updated on every measure.
*/
internal var density: Density by mutableStateOf(Density(1f, 1f))
/**
* The ScrollableController instance. We keep it as we need to call stopAnimation on it once
* we reached the end of the list.
*/
private val scrollableState = ScrollableState { -onScroll(-it) }
/**
* Only used for testing to confirm that we're not making too many measure passes
*/
/*@VisibleForTesting*/
internal var numMeasurePasses: Int = 0
private set
/**
* Only used for testing to disable prefetching when needed to test the main logic.
*/
/*@VisibleForTesting*/
internal var prefetchingEnabled: Boolean = true
/**
* The index scheduled to be prefetched (or the last prefetched index if the prefetch is done).
*/
private var indexToPrefetch = -1
/**
* The handle associated with the current index from [indexToPrefetch].
*/
private var currentPrefetchHandle: LazyLayoutPrefetchState.PrefetchHandle? = null
/**
* Keeps the scrolling direction during the previous calculation in order to be able to
* detect the scrolling direction change.
*/
private var wasScrollingForward = false
/**
* The [Remeasurement] object associated with our layout. It allows us to remeasure
* synchronously during scroll.
*/
internal var remeasurement: Remeasurement? by mutableStateOf(null)
private set
/**
* The modifier which provides [remeasurement].
*/
internal val remeasurementModifier = object : RemeasurementModifier {
override fun onRemeasurementAvailable(remeasurement: Remeasurement) {
this@LazyListState.remeasurement = remeasurement
}
}
/**
* Provides a modifier which allows to delay some interactions (e.g. scroll)
* until layout is ready.
*/
internal val awaitLayoutModifier = AwaitFirstLayoutModifier()
internal var placementAnimator by mutableStateOf<LazyListItemPlacementAnimator?>(null)
/**
* Constraints passed to the prefetcher for premeasuring the prefetched items.
*/
internal var premeasureConstraints by mutableStateOf(Constraints())
/**
* Instantly brings the item at [index] to the top of the viewport, offset by [scrollOffset]
* pixels.
*
* @param index the index to which to scroll. Must be non-negative.
* @param scrollOffset the offset that the item should end up after the scroll. Note that
* positive offset refers to forward scroll, so in a top-to-bottom list, positive offset will
* scroll the item further upward (taking it partly offscreen).
*/
suspend fun scrollToItem(
/*@IntRange(from = 0)*/
index: Int,
scrollOffset: Int = 0
) {
scroll {
snapToItemIndexInternal(index, scrollOffset)
}
}
internal fun snapToItemIndexInternal(index: Int, scrollOffset: Int) {
scrollPosition.requestPosition(DataIndex(index), scrollOffset)
// placement animation is not needed because we snap into a new position.
placementAnimator?.reset()
remeasurement?.forceRemeasure()
}
/**
* Call this function to take control of scrolling and gain the ability to send scroll events
* via [ScrollScope.scrollBy]. All actions that change the logical scroll position must be
* performed within a [scroll] block (even if they don't call any other methods on this
* object) in order to guarantee that mutual exclusion is enforced.
*
* If [scroll] is called from elsewhere, this will be canceled.
*/
override suspend fun scroll(
scrollPriority: MutatePriority,
block: suspend ScrollScope.() -> Unit
) {
awaitLayoutModifier.waitForFirstLayout()
scrollableState.scroll(scrollPriority, block)
}
override fun dispatchRawDelta(delta: Float): Float =
scrollableState.dispatchRawDelta(delta)
override val isScrollInProgress: Boolean
get() = scrollableState.isScrollInProgress
private var canScrollBackward: Boolean = false
internal var canScrollForward: Boolean = false
private set
// TODO: Coroutine scrolling APIs will allow this to be private again once we have more
// fine-grained control over scrolling
/*@VisibleForTesting*/
internal fun onScroll(distance: Float): Float {
if (distance < 0 && !canScrollForward || distance > 0 && !canScrollBackward) {
return 0f
}
check(abs(scrollToBeConsumed) <= 0.5f) {
"entered drag with non-zero pending scroll: $scrollToBeConsumed"
}
scrollToBeConsumed += distance
// scrollToBeConsumed will be consumed synchronously during the forceRemeasure invocation
// inside measuring we do scrollToBeConsumed.roundToInt() so there will be no scroll if
// we have less than 0.5 pixels
if (abs(scrollToBeConsumed) > 0.5f) {
val preScrollToBeConsumed = scrollToBeConsumed
remeasurement?.forceRemeasure()
if (prefetchingEnabled) {
notifyPrefetch(preScrollToBeConsumed - scrollToBeConsumed)
}
}
// here scrollToBeConsumed is already consumed during the forceRemeasure invocation
if (abs(scrollToBeConsumed) <= 0.5f) {
// We consumed all of it - we'll hold onto the fractional scroll for later, so report
// that we consumed the whole thing
return distance
} else {
val scrollConsumed = distance - scrollToBeConsumed
// We did not consume all of it - return the rest to be consumed elsewhere (e.g.,
// nested scrolling)
scrollToBeConsumed = 0f // We're not consuming the rest, give it back
return scrollConsumed
}
}
private fun notifyPrefetch(delta: Float) {
if (!prefetchingEnabled) {
return
}
val info = layoutInfo
if (info.visibleItemsInfo.isNotEmpty()) {
// check(isActive)
val scrollingForward = delta < 0
val indexToPrefetch = if (scrollingForward) {
info.visibleItemsInfo.last().index + 1
} else {
info.visibleItemsInfo.first().index - 1
}
if (indexToPrefetch != this.indexToPrefetch &&
indexToPrefetch in 0 until info.totalItemsCount
) {
if (wasScrollingForward != scrollingForward) {
// the scrolling direction has been changed which means the last prefetched
// is not going to be reached anytime soon so it is safer to dispose it.
// if this item is already visible it is safe to call the method anyway
// as it will be no-op
currentPrefetchHandle?.cancel()
}
this.wasScrollingForward = scrollingForward
this.indexToPrefetch = indexToPrefetch
currentPrefetchHandle = prefetchState.schedulePrefetch(
indexToPrefetch, premeasureConstraints
)
}
}
}
internal val prefetchState = LazyLayoutPrefetchState()
/**
* Animate (smooth scroll) to the given item.
*
* @param index the index to which to scroll. Must be non-negative.
* @param scrollOffset the offset that the item should end up after the scroll. Note that
* positive offset refers to forward scroll, so in a top-to-bottom list, positive offset will
* scroll the item further upward (taking it partly offscreen).
*/
suspend fun animateScrollToItem(
/*@IntRange(from = 0)*/
index: Int,
scrollOffset: Int = 0
) {
doSmoothScrollToItem(index, scrollOffset)
}
/**
* Updates the state with the new calculated scroll position and consumed scroll.
*/
internal fun applyMeasureResult(result: LazyListMeasureResult) {
scrollPosition.updateFromMeasureResult(result)
scrollToBeConsumed -= result.consumedScroll
layoutInfoState.value = result
canScrollForward = result.canScrollForward
canScrollBackward = (result.firstVisibleItem?.index ?: 0) != 0 ||
result.firstVisibleItemScrollOffset != 0
numMeasurePasses++
}
/**
* When the user provided custom keys for the items we can try to detect when there were
* items added or removed before our current first visible item and keep this item
* as the first visible one even given that its index has been changed.
*/
internal fun updateScrollPositionIfTheFirstItemWasMoved(itemProvider: LazyListItemProvider) {
scrollPosition.updateScrollPositionIfTheFirstItemWasMoved(itemProvider)
}
companion object {
/**
* The default [Saver] implementation for [LazyListState].
*/
val Saver: Saver<LazyListState, *> = listSaver(
save = { listOf(it.firstVisibleItemIndex, it.firstVisibleItemScrollOffset) },
restore = {
LazyListState(
firstVisibleItemIndex = it[0],
firstVisibleItemScrollOffset = it[1]
)
}
)
}
}
private object EmptyLazyListLayoutInfo : LazyListLayoutInfo {
override val visibleItemsInfo = emptyList<LazyListItemInfo>()
override val viewportStartOffset = 0
override val viewportEndOffset = 0
override val totalItemsCount = 0
override val viewportSize = IntSize.Zero
override val orientation = Orientation.Vertical
override val reverseLayout = false
override val beforeContentPadding = 0
override val afterContentPadding = 0
}
internal class AwaitFirstLayoutModifier : OnGloballyPositionedModifier {
private var wasPositioned = false
private var continuation: Continuation<Unit>? = null
suspend fun waitForFirstLayout() {
if (!wasPositioned) {
val oldContinuation = continuation
suspendCoroutine<Unit> { continuation = it }
oldContinuation?.resume(Unit)
}
}
override fun onGloballyPositioned(coordinates: LayoutCoordinates) {
if (!wasPositioned) {
wasPositioned = true
continuation?.resume(Unit)
continuation = null
}
}
}