Improve webtoon reader performance

app/src/main/java/org/koitharu/kotatsu/reader/ui/pager/webtoon/WebtoonLayoutManager.kt

@@ -0,0 +1,41 @@
+package org.koitharu.kotatsu.reader.ui.pager.webtoon
+
+import android.content.Context
+import android.util.AttributeSet
+import androidx.recyclerview.widget.LinearLayoutManager
+import androidx.recyclerview.widget.RecyclerView
+import kotlin.math.sign
+
+class WebtoonLayoutManager : LinearLayoutManager {
+
+	private var scrollDirection: Int = 0
+
+	constructor(context: Context) : super(context)
+	constructor(
+		context: Context,
+		orientation: Int,
+		reverseLayout: Boolean,
+	) : super(context, orientation, reverseLayout)
+
+	constructor(
+		context: Context,
+		attrs: AttributeSet?,
+		defStyleAttr: Int,
+		defStyleRes: Int,
+	) : super(context, attrs, defStyleAttr, defStyleRes)
+
+	override fun scrollVerticallyBy(dy: Int, recycler: RecyclerView.Recycler?, state: RecyclerView.State): Int {
+		scrollDirection = dy.sign
+		return super.scrollVerticallyBy(dy, recycler, state)
+	}
+
+	override fun calculateExtraLayoutSpace(state: RecyclerView.State, extraLayoutSpace: IntArray) {
+		if (state.hasTargetScrollPosition()) {
+			super.calculateExtraLayoutSpace(state, extraLayoutSpace)
+			return
+		}
+		val pageSize = height
+		extraLayoutSpace[0] = if (scrollDirection < 0) pageSize else 0
+		extraLayoutSpace[1] = if (scrollDirection < 0) 0 else pageSize
+	}
+}

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

@@ -12,7 +12,6 @@ import org.koitharu.kotatsu.reader.ui.ReaderState
 import org.koitharu.kotatsu.reader.ui.pager.BaseReader
 import org.koitharu.kotatsu.reader.ui.pager.BaseReaderAdapter
 import org.koitharu.kotatsu.reader.ui.pager.ReaderPage
-import org.koitharu.kotatsu.utils.ext.doOnCurrentItemChanged
 import org.koitharu.kotatsu.utils.ext.findCenterViewPosition
 import org.koitharu.kotatsu.utils.ext.firstVisibleItemPosition
 import org.koitharu.kotatsu.utils.ext.viewLifecycleScope
@@ -33,7 +32,7 @@ class WebtoonReaderFragment : BaseReader<FragmentReaderWebtoonBinding>() {
 		with(binding.recyclerView) {
 			setHasFixedSize(true)
 			adapter = webtoonAdapter
-			doOnCurrentItemChanged(::notifyPageChanged)
+			addOnPageScrollListener(PageScrollListener())
 		}
 	}
 
@@ -93,4 +92,12 @@ class WebtoonReaderFragment : BaseReader<FragmentReaderWebtoonBinding>() {
 	override fun switchPageTo(position: Int, smooth: Boolean) {
 		binding.recyclerView.firstVisibleItemPosition = position
 	}
+
+	private inner class PageScrollListener : WebtoonRecyclerView.OnPageScrollListener() {
+
+		override fun onPageChanged(recyclerView: WebtoonRecyclerView, index: Int) {
+			super.onPageChanged(recyclerView, index)
+			notifyPageChanged(index)
+		}
+	}
 }

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

@@ -2,25 +2,27 @@ package org.koitharu.kotatsu.reader.ui.pager.webtoon
 
 import android.content.Context
 import android.util.AttributeSet
-import androidx.core.view.ViewCompat
+import androidx.core.view.ViewCompat.TYPE_TOUCH
 import androidx.recyclerview.widget.RecyclerView
+import org.koitharu.kotatsu.utils.ext.findCenterViewPosition
+import java.util.*
 
 class WebtoonRecyclerView @JvmOverloads constructor(
 	context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0
 ) : RecyclerView(context, attrs, defStyleAttr) {
 
-	override fun startNestedScroll(axes: Int) = startNestedScroll(axes, ViewCompat.TYPE_TOUCH)
+	private var onPageScrollListeners: MutableList<OnPageScrollListener>? = null
 
-	override fun startNestedScroll(axes: Int, type: Int): Boolean {
-		return true
-	}
+	override fun startNestedScroll(axes: Int) = startNestedScroll(axes, TYPE_TOUCH)
+
+	override fun startNestedScroll(axes: Int, type: Int): Boolean = true
 
 	override fun dispatchNestedPreScroll(
 		dx: Int,
 		dy: Int,
 		consumed: IntArray?,
 		offsetInWindow: IntArray?
-	) = dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, ViewCompat.TYPE_TOUCH)
+	) = dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow, TYPE_TOUCH)
 
 	override fun dispatchNestedPreScroll(
 		dx: Int,
@@ -34,6 +36,7 @@ class WebtoonRecyclerView @JvmOverloads constructor(
 			consumed[0] = 0
 			consumed[1] = consumedY
 		}
+		notifyScrollChanged(dy)
 		return consumedY != 0 || dy == 0
 	}
 
@@ -75,4 +78,39 @@ class WebtoonRecyclerView @JvmOverloads constructor(
 		}
 		return 0
 	}
+
+	fun addOnPageScrollListener(listener: OnPageScrollListener) {
+		val list = onPageScrollListeners ?: LinkedList<OnPageScrollListener>().also { onPageScrollListeners = it }
+		list.add(listener)
+	}
+
+	fun removeOnPageScrollListener(listener: OnPageScrollListener) {
+		onPageScrollListeners?.remove(listener)
+	}
+
+	private fun notifyScrollChanged(dy: Int) {
+		val listeners = onPageScrollListeners
+		if (listeners.isNullOrEmpty()) {
+			return
+		}
+		val centerPosition = findCenterViewPosition()
+		listeners.forEach { it.dispatchScroll(this, dy, centerPosition) }
+	}
+
+	abstract class OnPageScrollListener {
+
+		private var lastPosition = NO_POSITION
+
+		fun dispatchScroll(recyclerView: WebtoonRecyclerView, dy: Int, centerPosition: Int) {
+			onScroll(recyclerView, dy)
+			if (centerPosition != NO_POSITION && centerPosition != lastPosition) {
+				lastPosition = centerPosition
+				onPageChanged(recyclerView, centerPosition)
+			}
+		}
+
+		open fun onScroll(recyclerView: WebtoonRecyclerView, dy: Int) = Unit
+
+		open fun onPageChanged(recyclerView: WebtoonRecyclerView, index: Int) = Unit
+	}
 }

app/src/main/java/org/koitharu/kotatsu/utils/ext/ViewExt.kt

@@ -91,22 +91,6 @@ fun View.resetTransformations() {
 	rotationY = 0f
 }
 
-inline fun RecyclerView.doOnCurrentItemChanged(crossinline callback: (Int) -> Unit) {
-	addOnScrollListener(object : RecyclerView.OnScrollListener() {
-
-		private var lastItem = -1
-
-		override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
-			super.onScrolled(recyclerView, dx, dy)
-			val item = recyclerView.findCenterViewPosition()
-			if (item != RecyclerView.NO_POSITION && item != lastItem) {
-				lastItem = item
-				callback(item)
-			}
-		}
-	})
-}
-
 fun RecyclerView.findCenterViewPosition(): Int {
 	val centerX = width / 2f
 	val centerY = height / 2f

app/src/main/res/layout/fragment_reader_webtoon.xml

@@ -6,4 +6,4 @@
 	android:layout_width="match_parent"
 	android:layout_height="match_parent"
 	android:orientation="vertical"
-	app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager" />
+	app:layoutManager="org.koitharu.kotatsu.reader.ui.pager.webtoon.WebtoonLayoutManager" />