LazyListScrolling.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

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.ui.unit.dp
import androidx.compose.ui.util.fastFirstOrNull
import kotlin.coroutines.cancellation.CancellationException

private class ItemFoundInScroll(val item: LazyListItemInfo) : 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()}")
    }
}

internal suspend fun LazyListState.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
            )
            val target = (itemFound.item.offset + 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)
        }
    }
}