Fix webtoon scroll dispatching

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/ReaderViewModel.kt

@@ -263,7 +263,7 @@ class ReaderViewModel @Inject constructor(
 	}
 
 	@MainThread
-	fun onCurrentPageChanged(position: Int) {
+	fun onCurrentPageChanged(lowerPos: Int, upperPos: Int) {
 		val prevJob = stateChangeJob
 		val pages = content.value.pages // capture immediately
 		stateChangeJob = launchJob(Dispatchers.Default) {
@@ -272,7 +272,8 @@ class ReaderViewModel @Inject constructor(
 			if (pages.size != content.value.pages.size) {
 				return@launchJob // TODO
 			}
-			pages.getOrNull(position)?.let { page ->
+			val centerPos = (lowerPos + upperPos) / 2
+			pages.getOrNull(centerPos)?.let { page ->
 				currentState.update { cs ->
 					cs?.copy(chapterId = page.chapterId, page = page.index)
 				}
@@ -282,14 +283,14 @@ class ReaderViewModel @Inject constructor(
 				return@launchJob
 			}
 			ensureActive()
-			if (position >= pages.lastIndex - BOUNDS_PAGE_OFFSET) {
+			if (upperPos >= pages.lastIndex - BOUNDS_PAGE_OFFSET) {
 				loadPrevNextChapter(pages.last().chapterId, isNext = true)
 			}
-			if (position <= BOUNDS_PAGE_OFFSET) {
+			if (lowerPos <= BOUNDS_PAGE_OFFSET) {
 				loadPrevNextChapter(pages.first().chapterId, isNext = false)
 			}
 			if (pageLoader.isPrefetchApplicable()) {
-				pageLoader.prefetch(pages.trySublist(position + 1, position + PREFETCH_LIMIT))
+				pageLoader.prefetch(pages.trySublist(upperPos + 1, upperPos + PREFETCH_LIMIT))
 			}
 		}
 	}

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/pager/BasePagerReaderFragment.kt

@@ -179,7 +179,7 @@ abstract class BasePagerReaderFragment : BaseReaderFragment<FragmentReaderPagerB
 	}
 
 	protected open fun notifyPageChanged(page: Int) {
-		viewModel.onCurrentPageChanged(page)
+		viewModel.onCurrentPageChanged(page, page)
 	}
 
 	companion object {

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/pager/reversed/ReversedReaderFragment.kt

@@ -32,7 +32,8 @@ class ReversedReaderFragment : BasePagerReaderFragment() {
 	}
 
 	override fun notifyPageChanged(page: Int) {
-		viewModel.onCurrentPageChanged(reversed(page))
+		val pos = reversed(page)
+		viewModel.onCurrentPageChanged(pos, pos)
 	}
 
 	private fun reversed(position: Int): Int {

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/pager/webtoon/WebtoonReaderFragment.kt

@@ -24,7 +24,8 @@ import org.koitharu.kotatsu.reader.ui.pager.ReaderPage
 import javax.inject.Inject
 
 @AndroidEntryPoint
-class WebtoonReaderFragment : BaseReaderFragment<FragmentReaderWebtoonBinding>() {
+class WebtoonReaderFragment : BaseReaderFragment<FragmentReaderWebtoonBinding>(),
+	WebtoonRecyclerView.OnWebtoonScrollListener {
 
 	@Inject
 	lateinit var networkState: NetworkState
@@ -46,7 +47,7 @@ class WebtoonReaderFragment : BaseReaderFragment<FragmentReaderWebtoonBinding>()
 		with(binding.recyclerView) {
 			setHasFixedSize(true)
 			adapter = readerAdapter
-			addOnPageScrollListener(PageScrollListener())
+			addOnPageScrollListener(this@WebtoonReaderFragment)
 			recyclerLifecycleDispatcher = RecyclerViewLifecycleDispatcher().also {
 				addOnScrollListener(it)
 			}
@@ -70,6 +71,15 @@ class WebtoonReaderFragment : BaseReaderFragment<FragmentReaderWebtoonBinding>()
 		exceptionResolver = exceptionResolver,
 	)
 
+	override fun onScrollChanged(
+		recyclerView: WebtoonRecyclerView,
+		dy: Int,
+		firstVisiblePosition: Int,
+		lastVisiblePosition: Int,
+	) {
+		viewModel.onCurrentPageChanged(firstVisiblePosition, lastVisiblePosition)
+	}
+
 	override suspend fun onPagesChanged(pages: List<ReaderPage>, pendingState: ReaderState?) = coroutineScope {
 		val setItems = launch {
 			requireAdapter().setItems(pages)
@@ -91,7 +101,7 @@ class WebtoonReaderFragment : BaseReaderFragment<FragmentReaderWebtoonBinding>()
 							?.restoreScroll(pendingState.scroll)
 					}
 				}
-				notifyPageChanged(position)
+				viewModel.onCurrentPageChanged(position, position)
 			} else {
 				Snackbar.make(requireView(), R.string.not_found_404, Snackbar.LENGTH_SHORT)
 					.show()
@@ -121,10 +131,6 @@ class WebtoonReaderFragment : BaseReaderFragment<FragmentReaderWebtoonBinding>()
 		viewBinding?.frame?.onZoomOut()
 	}
 
-	private fun notifyPageChanged(page: Int) {
-		viewModel.onCurrentPageChanged(page)
-	}
-
 	override fun switchPageBy(delta: Int) {
 		with(requireViewBinding().recyclerView) {
 			if (isAnimationEnabled()) {
@@ -147,12 +153,4 @@ class WebtoonReaderFragment : BaseReaderFragment<FragmentReaderWebtoonBinding>()
 		}
 		return true
 	}
-
-	private inner class PageScrollListener : WebtoonRecyclerView.OnPageScrollListener() {
-
-		override fun onPageChanged(recyclerView: WebtoonRecyclerView, index: Int) {
-			super.onPageChanged(recyclerView, index)
-			notifyPageChanged(index)
-		}
-	}
 }

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/pager/webtoon/WebtoonRecyclerView.kt

@@ -6,8 +6,8 @@ import android.view.View
 import androidx.core.view.ViewCompat.TYPE_TOUCH
 import androidx.core.view.forEach
 import androidx.core.view.iterator
+import androidx.recyclerview.widget.LinearLayoutManager
 import androidx.recyclerview.widget.RecyclerView
-import org.koitharu.kotatsu.core.util.ext.findCenterViewPosition
 import java.util.LinkedList
 import java.util.WeakHashMap
 
@@ -15,7 +15,8 @@ class WebtoonRecyclerView @JvmOverloads constructor(
 	context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0
 ) : RecyclerView(context, attrs, defStyleAttr) {
 
-	private var onPageScrollListeners: MutableList<OnPageScrollListener>? = null
+	private var onPageScrollListeners = LinkedList<OnWebtoonScrollListener>()
+	private val scrollDispatcher = WebtoonScrollDispatcher()
 	private val detachedViews = WeakHashMap<View, Unit>()
 	private var isFixingScroll: Boolean = false
 
@@ -103,22 +104,20 @@ class WebtoonRecyclerView @JvmOverloads constructor(
 		return 0
 	}
 
-	fun addOnPageScrollListener(listener: OnPageScrollListener) {
-		val list = onPageScrollListeners ?: LinkedList<OnPageScrollListener>().also { onPageScrollListeners = it }
-		list.add(listener)
+	fun addOnPageScrollListener(listener: OnWebtoonScrollListener) {
+		onPageScrollListeners.add(listener)
 	}
 
-	fun removeOnPageScrollListener(listener: OnPageScrollListener) {
-		onPageScrollListeners?.remove(listener)
+	fun removeOnPageScrollListener(listener: OnWebtoonScrollListener) {
+		onPageScrollListeners.remove(listener)
 	}
 
 	private fun notifyScrollChanged(dy: Int) {
 		val listeners = onPageScrollListeners
-		if (listeners.isNullOrEmpty()) {
+		if (listeners.isEmpty()) {
 			return
 		}
-		val centerPosition = findCenterViewPosition()
-		listeners.forEach { it.dispatchScroll(this, dy, centerPosition) }
+		scrollDispatcher.dispatchScroll(this, dy)
 	}
 
 	fun relayoutChildren() {
@@ -162,20 +161,30 @@ class WebtoonRecyclerView @JvmOverloads constructor(
 		else -> false
 	}
 
-	abstract class OnPageScrollListener {
+	private class WebtoonScrollDispatcher {
 
-		private var lastPosition = NO_POSITION
+		private var firstPos = NO_POSITION
+		private var lastPos = NO_POSITION
 
-		fun dispatchScroll(recyclerView: WebtoonRecyclerView, dy: Int, centerPosition: Int) {
-			onScroll(recyclerView, dy)
-			if (centerPosition != NO_POSITION && centerPosition != lastPosition) {
-				lastPosition = centerPosition
-				onPageChanged(recyclerView, centerPosition)
+		fun dispatchScroll(rv: WebtoonRecyclerView, dy: Int) {
+			val lm = rv.layoutManager as? LinearLayoutManager
+			if (lm == null) {
+				firstPos = NO_POSITION
+				lastPos = NO_POSITION
+				return
+			}
+			val newFirstPos = lm.findFirstVisibleItemPosition()
+			val newLastPos = lm.findLastVisibleItemPosition()
+			if (newFirstPos != firstPos || newLastPos != lastPos) {
+				firstPos = newFirstPos
+				lastPos = newLastPos
+				rv.onPageScrollListeners.forEach { it.onScrollChanged(rv, dy, newFirstPos, newLastPos) }
 			}
 		}
+	}
 
-		open fun onScroll(recyclerView: WebtoonRecyclerView, dy: Int) = Unit
+	interface OnWebtoonScrollListener {
 
-		open fun onPageChanged(recyclerView: WebtoonRecyclerView, index: Int) = Unit
+		fun onScrollChanged(recyclerView: WebtoonRecyclerView, dy: Int, firstVisiblePosition: Int, lastVisiblePosition: Int)
 	}
 }