/*
* 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.grid
import androidx.compose.animation.core.AnimationSpec
import androidx.compose.animation.core.AnimationState
import androidx.compose.animation.core.animateTo
import androidx.compose.animation.core.spring
import androidx.compose.foundation.ExperimentalFoundationApi
import androidx.compose.foundation.lazy.LazyGridItemInfo
import androidx.compose.foundation.lazy.LazyGridState
import androidx.compose.ui.unit.dp
import androidx.compose.ui.util.fastFirstOrNull
import kotlin.coroutines.cancellation.CancellationException
@OptIn(ExperimentalFoundationApi::class)
private class ItemFoundInScroll(val item: LazyGridItemInfo) : CancellationException()
private val TargetDistance = 2500.dp
private val BoundDistance = 1500.dp
private const val DEBUG = false
private inline fun debugLog(generateMsg: () -> String) {
if (DEBUG) {
println("LazyListScrolling: ${generateMsg()}")
}
}
@OptIn(ExperimentalFoundationApi::class)
internal suspend fun LazyGridState.doSmoothScrollToItem(
index: Int,
scrollOffset: Int
) {
val animationSpec: AnimationSpec<Float> = spring()
fun getTargetItem() = layoutInfo.visibleItemsInfo.fastFirstOrNull {
it.index == index
}
scroll {
val targetDistancePx = with(density) { TargetDistance.toPx() }
val boundDistancePx = with(density) { BoundDistance.toPx() }
var loop = true
var prevVelocity = 0f
try {
val targetItemInitialInfo = getTargetItem()
if (targetItemInitialInfo != null) {
// It's already visible, just animate directly
throw ItemFoundInScroll(targetItemInitialInfo)
}
val forward = index > firstVisibleItemIndex
val target = if (forward) targetDistancePx else -targetDistancePx
var loops = 1
while (loop) {
val anim = AnimationState(
initialValue = 0f,
initialVelocity = prevVelocity
)
var prevValue = 0f
anim.animateTo(
target,
animationSpec = animationSpec,
sequentialAnimation = (anim.velocity != 0f)
) {
// If we haven't found the item yet, check if it's visible.
var targetItem = getTargetItem()
if (targetItem == null) {
// Springs can overshoot their target, clamp to the desired range
val coercedValue = if (target > 0) {
value.coerceAtMost(target)
} else {
value.coerceAtLeast(target)
}
val delta = coercedValue - prevValue
debugLog {
"Scrolling by $delta (target: $target, coercedValue: $coercedValue)"
}
val consumed = scrollBy(delta)
targetItem = getTargetItem()
if (targetItem != null) {
debugLog { "Found the item after performing scrollBy()" }
} else {
if (delta != consumed) {
debugLog { "Hit end without finding the item" }
cancelAnimation()
loop = false
return@animateTo
}
prevValue += delta
if (forward) {
if (value > boundDistancePx) {
debugLog { "Struck bound going forward" }
cancelAnimation()
}
} else {
if (value < -boundDistancePx) {
debugLog { "Struck bound going backward" }
cancelAnimation()
}
}
// Magic constants for teleportation chosen arbitrarily by experiment
if (forward) {
if (
loops >= 2 &&
index - layoutInfo.visibleItemsInfo.last().index > 100
) {
// Teleport
debugLog { "Teleport forward" }
snapToItemIndexInternal(index = index - 100, scrollOffset = 0)
}
} else {
if (
loops >= 2 &&
layoutInfo.visibleItemsInfo.first().index - index > 100
) {
// Teleport
debugLog { "Teleport backward" }
snapToItemIndexInternal(index = index + 100, scrollOffset = 0)
}
}
}
}
// Did we scroll past the item?
@Suppress("RedundantIf") // It's way easier to understand the logic this way
val overshot = if (forward) {
if (firstVisibleItemIndex > index) {
true
} else if (
firstVisibleItemIndex == index &&
firstVisibleItemScrollOffset > scrollOffset
) {
true
} else {
false
}
} else { // backward
if (firstVisibleItemIndex < index) {
true
} else if (
firstVisibleItemIndex == index &&
firstVisibleItemScrollOffset < scrollOffset
) {
true
} else {
false
}
}
// We don't throw ItemFoundInScroll when we snap, because once we've snapped to
// the final position, there's no need to animate to it.
if (overshot) {
debugLog { "Overshot" }
snapToItemIndexInternal(index = index, scrollOffset = scrollOffset)
loop = false
cancelAnimation()
return@animateTo
} else if (targetItem != null) {
debugLog { "Found item" }
throw ItemFoundInScroll(targetItem)
}
}
prevVelocity = anim.velocity
loops++
}
} catch (itemFound: ItemFoundInScroll) {
// We found it, animate to it
// Bring to the requested position - will be automatically stopped if not possible
val anim = AnimationState(
initialValue = 0f,
initialVelocity = prevVelocity
)
// TODO(popam): hardcoded .y
val target = (itemFound.item.offset.y + scrollOffset).toFloat()
var prevValue = 0f
debugLog {
"Seeking by $target at velocity $prevVelocity, sequential: ${anim.velocity != 0f}"
}
anim.animateTo(target, sequentialAnimation = (anim.velocity != 0f)) {
// Springs can overshoot their target, clamp to the desired range
val coercedValue = when {
target > 0 -> {
value.coerceAtMost(target)
}
target < 0 -> {
value.coerceAtLeast(target)
}
else -> {
debugLog { "WARNING: somehow ended up seeking 0px, this shouldn't happen" }
0f
}
}
val delta = coercedValue - prevValue
debugLog { "Seeking by $delta (coercedValue = $coercedValue)" }
val consumed = scrollBy(delta)
if (delta != consumed /* hit the end, stop */ ||
coercedValue != value /* would have overshot, stop */
) {
cancelAnimation()
}
prevValue += delta
}
// Once we're finished the animation, snap to the exact position to account for
// rounding error (otherwise we tend to end up with the previous item scrolled the
// tiniest bit onscreen)
// TODO: prevent temporarily scrolling *past* the item
snapToItemIndexInternal(index = index, scrollOffset = scrollOffset)
}
}
}