Draft double reader implementation

app/build.gradle

@@ -81,7 +81,7 @@ afterEvaluate {
 }
 dependencies {
 	//noinspection GradleDependency
-	implementation('com.github.KotatsuApp:kotatsu-parsers:3d7e62d2fe') {
+	implementation('com.github.KotatsuApp:kotatsu-parsers:400a90464e') {
 		exclude group: 'org.json', module: 'json'
 	}
 
@@ -90,7 +90,7 @@ dependencies {
 
 	implementation 'androidx.appcompat:appcompat:1.6.1'
 	implementation 'androidx.core:core-ktx:1.12.0'
-	implementation 'androidx.activity:activity-ktx:1.8.0-rc01'
+	implementation 'androidx.activity:activity-ktx:1.8.0'
 	implementation 'androidx.fragment:fragment-ktx:1.6.1'
 	implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.6.2'
 	implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.6.2'
@@ -102,7 +102,7 @@ dependencies {
 	implementation 'androidx.viewpager2:viewpager2:1.1.0-beta02'
 	implementation 'androidx.preference:preference-ktx:1.2.1'
 	implementation 'androidx.biometric:biometric-ktx:1.2.0-alpha05'
-	implementation 'com.google.android.material:material:1.9.0'
+	implementation 'com.google.android.material:material:1.10.0'
 	implementation 'androidx.lifecycle:lifecycle-common-java8:2.6.2'
 
 	// TODO https://issuetracker.google.com/issues/254846063
@@ -125,8 +125,8 @@ dependencies {
 	implementation 'com.hannesdorfmann:adapterdelegates4-kotlin-dsl:4.3.2'
 	implementation 'com.hannesdorfmann:adapterdelegates4-kotlin-dsl-viewbinding:4.3.2'
 
-	implementation 'com.google.dagger:hilt-android:2.48'
-	kapt 'com.google.dagger:hilt-compiler:2.48'
+	implementation 'com.google.dagger:hilt-android:2.48.1'
+	kapt 'com.google.dagger:hilt-compiler:2.48.1'
 	implementation 'androidx.hilt:hilt-work:1.0.0'
 	kapt 'androidx.hilt:hilt-compiler:1.0.0'
 
@@ -155,6 +155,6 @@ dependencies {
 	androidTestImplementation 'androidx.room:room-testing:2.5.2'
 	androidTestImplementation 'com.squareup.moshi:moshi-kotlin:1.15.0'
 
-	androidTestImplementation 'com.google.dagger:hilt-android-testing:2.48'
-	kaptAndroidTest 'com.google.dagger:hilt-android-compiler:2.48'
+	androidTestImplementation 'com.google.dagger:hilt-android-testing:2.48.1'
+	kaptAndroidTest 'com.google.dagger:hilt-android-compiler:2.48.1'
 }

app/src/main/kotlin/org/koitharu/kotatsu/core/parser/MangaLoaderContextImpl.kt

@@ -17,7 +17,7 @@ import org.koitharu.kotatsu.parsers.MangaLoaderContext
 import org.koitharu.kotatsu.parsers.config.MangaSourceConfig
 import org.koitharu.kotatsu.parsers.model.MangaSource
 import java.lang.ref.WeakReference
-import java.util.*
+import java.util.Locale
 import javax.inject.Inject
 import javax.inject.Singleton
 import kotlin.coroutines.resume
@@ -50,7 +50,7 @@ class MangaLoaderContextImpl @Inject constructor(
 	}
 
 	override fun encodeBase64(data: ByteArray): String {
-		return Base64.encodeToString(data, Base64.NO_PADDING)
+		return Base64.encodeToString(data, Base64.NO_WRAP)
 	}
 
 	override fun decodeBase64(data: String): ByteArray {

app/src/main/kotlin/org/koitharu/kotatsu/reader/domain/DetectReaderModeUseCase.kt

@@ -13,11 +13,11 @@ import org.koitharu.kotatsu.core.parser.MangaDataRepository
 import org.koitharu.kotatsu.core.parser.MangaRepository
 import org.koitharu.kotatsu.core.prefs.AppSettings
 import org.koitharu.kotatsu.core.prefs.ReaderMode
+import org.koitharu.kotatsu.core.util.ext.printStackTraceDebug
 import org.koitharu.kotatsu.parsers.model.Manga
 import org.koitharu.kotatsu.parsers.model.MangaPage
 import org.koitharu.kotatsu.parsers.util.runCatchingCancellable
 import org.koitharu.kotatsu.reader.ui.ReaderState
-import org.koitharu.kotatsu.core.util.ext.printStackTraceDebug
 import java.io.InputStream
 import java.util.zip.ZipFile
 import javax.inject.Inject

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

@@ -103,7 +103,7 @@ class ReaderActivity :
 	override fun onCreate(savedInstanceState: Bundle?) {
 		super.onCreate(savedInstanceState)
 		setContentView(ActivityReaderBinding.inflate(layoutInflater))
-		readerManager = ReaderManager(supportFragmentManager, R.id.container)
+		readerManager = ReaderManager(supportFragmentManager, viewBinding.container)
 		supportActionBar?.setDisplayHomeAsUpEnabled(true)
 		touchHelper = GridTouchHelper(this, this)
 		scrollTimer = scrollTimerFactory.create(this, this)

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

@@ -1,10 +1,12 @@
 package org.koitharu.kotatsu.reader.ui
 
-import androidx.annotation.IdRes
+import androidx.fragment.app.FragmentContainerView
 import androidx.fragment.app.FragmentManager
 import androidx.fragment.app.commit
+import org.koitharu.kotatsu.R
 import org.koitharu.kotatsu.core.prefs.ReaderMode
 import org.koitharu.kotatsu.reader.ui.pager.BaseReaderFragment
+import org.koitharu.kotatsu.reader.ui.pager.doublepage.DoublePageReaderFragment
 import org.koitharu.kotatsu.reader.ui.pager.reversed.ReversedReaderFragment
 import org.koitharu.kotatsu.reader.ui.pager.standard.PagerReaderFragment
 import org.koitharu.kotatsu.reader.ui.pager.webtoon.WebtoonReaderFragment
@@ -12,19 +14,24 @@ import java.util.EnumMap
 
 class ReaderManager(
 	private val fragmentManager: FragmentManager,
-	@IdRes private val containerResId: Int,
+	private val container: FragmentContainerView,
 ) {
 
 	private val modeMap = EnumMap<ReaderMode, Class<out BaseReaderFragment<*>>>(ReaderMode::class.java)
 
 	init {
-		modeMap[ReaderMode.STANDARD] = PagerReaderFragment::class.java
+		val isTablet = container.resources.getBoolean(R.bool.is_tablet)
+		modeMap[ReaderMode.STANDARD] = if (isTablet) {
+			DoublePageReaderFragment::class.java
+		} else {
+			PagerReaderFragment::class.java
+		}
 		modeMap[ReaderMode.REVERSED] = ReversedReaderFragment::class.java
 		modeMap[ReaderMode.WEBTOON] = WebtoonReaderFragment::class.java
 	}
 
 	val currentReader: BaseReaderFragment<*>?
-		get() = fragmentManager.findFragmentById(containerResId) as? BaseReaderFragment<*>
+		get() = fragmentManager.findFragmentById(container.id) as? BaseReaderFragment<*>
 
 	val currentMode: ReaderMode?
 		get() {
@@ -36,14 +43,14 @@ class ReaderManager(
 		val readerClass = requireNotNull(modeMap[newMode])
 		fragmentManager.commit {
 			setReorderingAllowed(true)
-			replace(containerResId, readerClass, null, null)
+			replace(container.id, readerClass, null, null)
 		}
 	}
 
-	fun replace(reader: BaseReaderFragment<*>) {
+	/*fun replace(reader: BaseReaderFragment<*>) {
 		fragmentManager.commit {
 			setReorderingAllowed(true)
 			replace(containerResId, reader)
 		}
-	}
+	}*/
 }

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

@@ -0,0 +1,48 @@
+package org.koitharu.kotatsu.reader.ui.pager.doublepage
+
+import android.graphics.PointF
+import android.view.Gravity
+import android.widget.FrameLayout
+import androidx.lifecycle.LifecycleOwner
+import com.davemorrissey.labs.subscaleview.SubsamplingScaleImageView
+import org.koitharu.kotatsu.core.exceptions.resolve.ExceptionResolver
+import org.koitharu.kotatsu.core.os.NetworkState
+import org.koitharu.kotatsu.databinding.ItemPageBinding
+import org.koitharu.kotatsu.reader.domain.PageLoader
+import org.koitharu.kotatsu.reader.ui.config.ReaderSettings
+import org.koitharu.kotatsu.reader.ui.pager.ReaderPage
+import org.koitharu.kotatsu.reader.ui.pager.standard.PageHolder
+
+class DoublePageHolder(
+	owner: LifecycleOwner,
+	binding: ItemPageBinding,
+	loader: PageLoader,
+	settings: ReaderSettings,
+	networkState: NetworkState,
+	exceptionResolver: ExceptionResolver,
+) : PageHolder(owner, binding, loader, settings, networkState, exceptionResolver) {
+
+	private val isEven: Boolean
+		get() = bindingAdapterPosition and 1 == 0
+
+	override fun onBind(data: ReaderPage) {
+		super.onBind(data)
+		(binding.textViewNumber.layoutParams as FrameLayout.LayoutParams)
+			.gravity = (if (isEven) Gravity.START else Gravity.END) or Gravity.BOTTOM
+	}
+
+	override fun onImageShowing(settings: ReaderSettings) {
+		with(binding.ssiv) {
+			maxScale = 2f * maxOf(
+				width / sWidth.toFloat(),
+				height / sHeight.toFloat(),
+			)
+			binding.ssiv.colorFilter = settings.colorFilter?.toColorFilter()
+			minimumScaleType = SubsamplingScaleImageView.SCALE_TYPE_CENTER_INSIDE
+			setScaleAndCenter(
+				minScale,
+				PointF(if (isEven) sWidth.toFloat() else 0f, 0f),
+			)
+		}
+	}
+}

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

@@ -0,0 +1,19 @@
+package org.koitharu.kotatsu.reader.ui.pager.doublepage
+
+import android.content.Context
+import android.util.AttributeSet
+import androidx.recyclerview.widget.LinearLayoutManager
+import androidx.recyclerview.widget.RecyclerView
+
+class DoublePageLayoutManager(
+	context: Context,
+	attrs: AttributeSet?,
+	defStyleAttr: Int,
+	defStyleRes: Int,
+) : LinearLayoutManager(context, attrs, defStyleAttr, defStyleRes) {
+
+	override fun checkLayoutParams(lp: RecyclerView.LayoutParams?): Boolean {
+		lp?.width = width / 2
+		return super.checkLayoutParams(lp)
+	}
+}

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

@@ -0,0 +1,128 @@
+package org.koitharu.kotatsu.reader.ui.pager.doublepage
+
+import android.os.Bundle
+import android.view.LayoutInflater
+import android.view.ViewGroup
+import androidx.recyclerview.widget.LinearLayoutManager
+import androidx.recyclerview.widget.RecyclerView
+import com.google.android.material.snackbar.Snackbar
+import dagger.hilt.android.AndroidEntryPoint
+import kotlinx.coroutines.async
+import kotlinx.coroutines.coroutineScope
+import kotlinx.coroutines.yield
+import org.koitharu.kotatsu.R
+import org.koitharu.kotatsu.core.os.NetworkState
+import org.koitharu.kotatsu.core.util.ext.firstVisibleItemPosition
+import org.koitharu.kotatsu.databinding.FragmentReaderDoubleBinding
+import org.koitharu.kotatsu.parsers.util.toIntUp
+import org.koitharu.kotatsu.reader.domain.PageLoader
+import org.koitharu.kotatsu.reader.ui.ReaderState
+import org.koitharu.kotatsu.reader.ui.pager.BaseReaderAdapter
+import org.koitharu.kotatsu.reader.ui.pager.BaseReaderFragment
+import org.koitharu.kotatsu.reader.ui.pager.ReaderPage
+import javax.inject.Inject
+import kotlin.math.absoluteValue
+
+@AndroidEntryPoint
+class DoublePageReaderFragment : BaseReaderFragment<FragmentReaderDoubleBinding>() {
+
+	@Inject
+	lateinit var networkState: NetworkState
+
+	@Inject
+	lateinit var pageLoader: PageLoader
+
+	override fun onCreateViewBinding(
+		inflater: LayoutInflater,
+		container: ViewGroup?,
+	) = FragmentReaderDoubleBinding.inflate(inflater, container, false)
+
+	override fun onViewBindingCreated(
+		binding: FragmentReaderDoubleBinding,
+		savedInstanceState: Bundle?,
+	) {
+		super.onViewBindingCreated(binding, savedInstanceState)
+		with(binding.recyclerView) {
+			adapter = readerAdapter
+			addOnScrollListener(PageScrollListener())
+			DoublePageSnapHelper().attachToRecyclerView(this)
+		}
+	}
+
+	override fun onDestroyView() {
+		requireViewBinding().recyclerView.adapter = null
+		super.onDestroyView()
+	}
+
+	override suspend fun onPagesChanged(pages: List<ReaderPage>, pendingState: ReaderState?) =
+		coroutineScope {
+			val items = async {
+				requireAdapter().setItems(pages)
+				yield()
+			}
+			if (pendingState != null) {
+				val position = pages.indexOfFirst {
+					it.chapterId == pendingState.chapterId && it.index == pendingState.page
+				}
+				items.await()
+				if (position != -1) {
+					requireViewBinding().recyclerView.firstVisibleItemPosition = position or 1
+					notifyPageChanged(position)
+				} else {
+					Snackbar.make(requireView(), R.string.not_found_404, Snackbar.LENGTH_SHORT)
+						.show()
+				}
+			} else {
+				items.await()
+			}
+		}
+
+	override fun onCreateAdapter() = DoublePagesAdapter(
+		lifecycleOwner = viewLifecycleOwner,
+		loader = pageLoader,
+		settings = viewModel.readerSettings,
+		networkState = networkState,
+		exceptionResolver = exceptionResolver,
+	)
+
+	override fun switchPageBy(delta: Int) {
+		switchPageTo((requireViewBinding().recyclerView.currentItem() + delta) or 1, delta.absoluteValue > 1)
+	}
+
+	override fun switchPageTo(position: Int, smooth: Boolean) {
+		requireViewBinding().recyclerView.firstVisibleItemPosition = position or 1
+	}
+
+	override fun getCurrentState(): ReaderState? = viewBinding?.run {
+		val adapter = recyclerView.adapter as? BaseReaderAdapter<*>
+		val page = adapter?.getItemOrNull(recyclerView.currentItem()) ?: return@run null
+		ReaderState(
+			chapterId = page.chapterId,
+			page = page.index,
+			scroll = 0,
+		)
+	}
+
+	private fun notifyPageChanged(page: Int) {
+		viewModel.onCurrentPageChanged(page)
+	}
+
+	private fun RecyclerView.currentItem(): Int {
+		val lm = layoutManager as LinearLayoutManager
+		return ((lm.findFirstVisibleItemPosition() + lm.findLastVisibleItemPosition()) / 2f).toIntUp()
+	}
+
+	private inner class PageScrollListener : RecyclerView.OnScrollListener() {
+
+		private var lastPage = RecyclerView.NO_POSITION
+
+		override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
+			super.onScrolled(recyclerView, dx, dy)
+			val page = recyclerView.currentItem()
+			if (page != lastPage) {
+				lastPage = page
+				notifyPageChanged(page)
+			}
+		}
+	}
+}

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

@@ -0,0 +1,280 @@
+package org.koitharu.kotatsu.reader.ui.pager.doublepage
+
+import android.util.DisplayMetrics
+import android.view.View
+import android.view.animation.Interpolator
+import android.widget.Scroller
+import androidx.core.view.ViewCompat
+import androidx.recyclerview.widget.GridLayoutManager
+import androidx.recyclerview.widget.LinearLayoutManager
+import androidx.recyclerview.widget.LinearSmoothScroller
+import androidx.recyclerview.widget.OrientationHelper
+import androidx.recyclerview.widget.RecyclerView
+import androidx.recyclerview.widget.RecyclerView.SmoothScroller.ScrollVectorProvider
+import androidx.recyclerview.widget.SnapHelper
+import kotlin.math.abs
+import kotlin.math.max
+import kotlin.math.roundToInt
+
+class DoublePageSnapHelper : SnapHelper() {
+
+	private lateinit var recyclerView: RecyclerView
+
+	// Total number of items in a block of view in the RecyclerView
+	private var blockSize = 2
+
+	// Maximum number of positions to move on a fling.
+	private var maxPositionsToMove = 0
+
+	// Width of a RecyclerView item if orientation is horizontal; height of the item if vertical
+	private var itemDimension = 0
+
+	// Maxim blocks to move during most vigorous fling.
+	private val maxFlingBlocks = 2
+
+	// When snapping, used to determine direction of snap.
+	private var priorFirstPosition = RecyclerView.NO_POSITION
+
+	// Our private scroller
+	private var scroller: Scroller? = null
+
+	// Horizontal/vertical layout helper
+	private lateinit var orientationHelper: OrientationHelper
+
+	// LTR/RTL helper
+	private lateinit var layoutDirectionHelper: LayoutDirectionHelper
+
+	private val snapInterpolator = Interpolator { input ->
+		var t = input
+		t -= 1.0f
+		t * t * t + 1.0f
+	}
+
+	@Throws(IllegalStateException::class)
+	override fun attachToRecyclerView(target: RecyclerView?) {
+		if (target != null) {
+			recyclerView = target
+			val layoutManager = recyclerView.layoutManager as LinearLayoutManager
+			check(layoutManager.canScrollHorizontally()) { "RecyclerView must be scrollable" }
+			orientationHelper = OrientationHelper.createHorizontalHelper(layoutManager)
+			layoutDirectionHelper = LayoutDirectionHelper(ViewCompat.getLayoutDirection(recyclerView))
+			scroller = Scroller(target.context, snapInterpolator)
+			initItemDimensionIfNeeded(layoutManager)
+		}
+		super.attachToRecyclerView(recyclerView)
+	}
+
+	override fun calculateDistanceToFinalSnap(
+		layoutManager: RecyclerView.LayoutManager,
+		targetView: View
+	): IntArray {
+		val out = IntArray(2)
+		if (layoutManager.canScrollHorizontally()) {
+			out[0] = layoutDirectionHelper.getScrollToAlignView(targetView)
+		}
+		if (layoutManager.canScrollVertically()) {
+			out[1] = layoutDirectionHelper.getScrollToAlignView(targetView)
+		}
+		return out
+	}
+
+	// We are flinging and need to know where we are heading.
+	override fun findTargetSnapPosition(
+		layoutManager: RecyclerView.LayoutManager,
+		velocityX: Int, velocityY: Int
+	): Int {
+		val lm = layoutManager as LinearLayoutManager
+		initItemDimensionIfNeeded(layoutManager)
+		scroller!!.fling(0, 0, velocityX, velocityY, Int.MIN_VALUE, Int.MAX_VALUE, Int.MIN_VALUE, Int.MAX_VALUE)
+		if (velocityX != 0) {
+			return layoutDirectionHelper
+				.getPositionsToMove(lm, scroller!!.finalX, itemDimension)
+		}
+		return if (velocityY != 0) {
+			layoutDirectionHelper
+				.getPositionsToMove(lm, scroller!!.finalY, itemDimension)
+		} else RecyclerView.NO_POSITION
+	}
+
+	// We have scrolled to the neighborhood where we will snap. Determine the snap position.
+	override fun findSnapView(layoutManager: RecyclerView.LayoutManager): View? {
+		// Snap to a view that is either 1) toward the bottom of the data and therefore on screen,
+		// or, 2) toward the top of the data and may be off-screen.
+		val snapPos: Int = calcTargetPosition(layoutManager as LinearLayoutManager)
+		return if (snapPos == RecyclerView.NO_POSITION) null else layoutManager.findViewByPosition(snapPos)
+	}
+
+	// Does the heavy lifting for findSnapView.
+	private fun calcTargetPosition(layoutManager: LinearLayoutManager): Int {
+		val snapPos: Int
+		val firstVisiblePos = layoutManager.findFirstVisibleItemPosition()
+		if (firstVisiblePos == RecyclerView.NO_POSITION) {
+			return RecyclerView.NO_POSITION
+		}
+		initItemDimensionIfNeeded(layoutManager)
+		if (firstVisiblePos >= priorFirstPosition) {
+			// Scrolling toward bottom of data
+			val firstCompletePosition = layoutManager.findFirstCompletelyVisibleItemPosition()
+			snapPos = if (firstCompletePosition != RecyclerView.NO_POSITION
+				&& firstCompletePosition % blockSize == 0
+			) {
+				firstCompletePosition
+			} else {
+				roundDownToBlockSize(firstVisiblePos + blockSize)
+			}
+		} else {
+			// Scrolling toward top of data
+			snapPos = roundDownToBlockSize(firstVisiblePos)
+			// Check to see if target view exists. If it doesn't, force a smooth scroll.
+			// SnapHelper only snaps to existing views and will not scroll to a non-existent one.
+			// If limiting fling to single block, then the following is not needed since the
+			// views are likely to be in the RecyclerView pool.
+			if (layoutManager.findViewByPosition(snapPos) == null) {
+				val toScroll: IntArray = layoutDirectionHelper.calculateDistanceToScroll(layoutManager, snapPos)
+				recyclerView.smoothScrollBy(toScroll[0], toScroll[1], snapInterpolator)
+			}
+		}
+		priorFirstPosition = firstVisiblePos
+		return snapPos
+	}
+
+	private fun initItemDimensionIfNeeded(layoutManager: RecyclerView.LayoutManager) {
+		if (itemDimension != 0) {
+			return
+		}
+		val child: View = layoutManager.getChildAt(0) ?: return
+		if (layoutManager.canScrollHorizontally()) {
+			itemDimension = child.width
+			blockSize = getSpanCount(layoutManager) * (recyclerView.width / itemDimension)
+		} else if (layoutManager.canScrollVertically()) {
+			itemDimension = child.height
+			blockSize = getSpanCount(layoutManager) * (recyclerView.height / itemDimension)
+		}
+		maxPositionsToMove = blockSize * maxFlingBlocks
+	}
+
+	private fun getSpanCount(layoutManager: RecyclerView.LayoutManager): Int {
+		return if (layoutManager is GridLayoutManager) layoutManager.spanCount else 1
+	}
+
+	private fun roundDownToBlockSize(trialPosition: Int): Int {
+		return trialPosition - trialPosition % blockSize
+	}
+
+	private fun roundUpToBlockSize(trialPosition: Int): Int {
+		return roundDownToBlockSize(trialPosition + blockSize - 1)
+	}
+
+	override fun createScroller(layoutManager: RecyclerView.LayoutManager): RecyclerView.SmoothScroller? {
+		return if (layoutManager !is ScrollVectorProvider) {
+			null
+		} else object : LinearSmoothScroller(recyclerView.context) {
+			override fun onTargetFound(targetView: View, state: RecyclerView.State, action: Action) {
+				val snapDistances = calculateDistanceToFinalSnap(
+					recyclerView.layoutManager!!,
+					targetView,
+				)
+				val dx = snapDistances[0]
+				val dy = snapDistances[1]
+				val time = calculateTimeForDeceleration(
+					max(abs(dx.toDouble()), abs(dy.toDouble()))
+						.toInt(),
+				)
+				if (time > 0) {
+					action.update(dx, dy, time, snapInterpolator)
+				}
+			}
+
+			override fun calculateSpeedPerPixel(displayMetrics: DisplayMetrics): Float {
+				return 40f / displayMetrics.densityDpi
+			}
+		}
+	}
+
+	/*
+		Helper class that handles calculations for LTR and RTL layouts.
+	 */
+	private inner class LayoutDirectionHelper(direction: Int) {
+
+		// Is the layout an RTL one?
+		private val mIsRTL: Boolean
+
+		init {
+			mIsRTL = direction == View.LAYOUT_DIRECTION_RTL
+		}
+
+		/*
+			Calculate the amount of scroll needed to align the target view with the layout edge.
+		 */
+		fun getScrollToAlignView(targetView: View?): Int {
+			return if (mIsRTL) orientationHelper.getDecoratedEnd(targetView) - recyclerView.width else orientationHelper.getDecoratedStart(
+				targetView,
+			)
+		}
+
+		/**
+		 * Calculate the distance to final snap position when the view corresponding to the snap
+		 * position is not currently available.
+		 *
+		 * @param layoutManager LinearLayoutManager or descendant class
+		 * @param targetPos     - Adapter position to snap to
+		 * @return int[2] {x-distance in pixels, y-distance in pixels}
+		 */
+		fun calculateDistanceToScroll(layoutManager: LinearLayoutManager, targetPos: Int): IntArray {
+			val out = IntArray(2)
+			val firstVisiblePos = layoutManager.findFirstVisibleItemPosition()
+			if (layoutManager.canScrollHorizontally()) {
+				if (targetPos <= firstVisiblePos) { // scrolling toward top of data
+					if (mIsRTL) {
+						val lastView = layoutManager.findViewByPosition(layoutManager.findLastVisibleItemPosition())
+						out[0] = (orientationHelper.getDecoratedEnd(lastView)
+							+ (firstVisiblePos - targetPos) * itemDimension)
+					} else {
+						val firstView = layoutManager.findViewByPosition(firstVisiblePos)
+						out[0] = (orientationHelper.getDecoratedStart(firstView)
+							- (firstVisiblePos - targetPos) * itemDimension)
+					}
+				}
+			}
+			if (layoutManager.canScrollVertically()) {
+				if (targetPos <= firstVisiblePos) { // scrolling toward top of data
+					val firstView = layoutManager.findViewByPosition(firstVisiblePos)
+					out[1] = firstView!!.top - (firstVisiblePos - targetPos) * itemDimension
+				}
+			}
+			return out
+		}
+
+		/*
+			Calculate the number of positions to move in the RecyclerView given a scroll amount
+			and the size of the items to be scrolled. Return integral multiple of mBlockSize not
+			equal to zero.
+		 */
+		fun getPositionsToMove(llm: LinearLayoutManager, scroll: Int, itemSize: Int): Int {
+			var positionsToMove: Int
+			positionsToMove = roundUpToBlockSize(abs((scroll.toDouble()) / itemSize).roundToInt())
+			if (positionsToMove < blockSize) {
+				// Must move at least one block
+				positionsToMove = blockSize
+			} else if (positionsToMove > maxPositionsToMove) {
+				// Clamp number of positions to move, so we don't get wild flinging.
+				positionsToMove = maxPositionsToMove
+			}
+			if (scroll < 0) {
+				positionsToMove *= -1
+			}
+			if (mIsRTL) {
+				positionsToMove *= -1
+			}
+			return if (layoutDirectionHelper.isDirectionToBottom(scroll < 0)) {
+				// Scrolling toward the bottom of data.
+				roundDownToBlockSize(llm.findFirstVisibleItemPosition()) + positionsToMove
+			} else roundDownToBlockSize(llm.findLastVisibleItemPosition()) + positionsToMove
+			// Scrolling toward the top of the data.
+		}
+
+		fun isDirectionToBottom(velocityNegative: Boolean): Boolean {
+			return if (mIsRTL) velocityNegative else !velocityNegative
+		}
+	}
+}

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

@@ -0,0 +1,35 @@
+package org.koitharu.kotatsu.reader.ui.pager.doublepage
+
+import android.view.LayoutInflater
+import android.view.ViewGroup
+import androidx.lifecycle.LifecycleOwner
+import org.koitharu.kotatsu.core.exceptions.resolve.ExceptionResolver
+import org.koitharu.kotatsu.core.os.NetworkState
+import org.koitharu.kotatsu.databinding.ItemPageBinding
+import org.koitharu.kotatsu.reader.domain.PageLoader
+import org.koitharu.kotatsu.reader.ui.config.ReaderSettings
+import org.koitharu.kotatsu.reader.ui.pager.BaseReaderAdapter
+
+class DoublePagesAdapter(
+	private val lifecycleOwner: LifecycleOwner,
+	loader: PageLoader,
+	settings: ReaderSettings,
+	networkState: NetworkState,
+	exceptionResolver: ExceptionResolver,
+) : BaseReaderAdapter<DoublePageHolder>(loader, settings, networkState, exceptionResolver) {
+
+	override fun onCreateViewHolder(
+		parent: ViewGroup,
+		loader: PageLoader,
+		settings: ReaderSettings,
+		networkState: NetworkState,
+		exceptionResolver: ExceptionResolver,
+	) = DoublePageHolder(
+		owner = lifecycleOwner,
+		binding = ItemPageBinding.inflate(LayoutInflater.from(parent.context), parent, false),
+		loader = loader,
+		settings = settings,
+		networkState = networkState,
+		exceptionResolver = exceptionResolver,
+	)
+}

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

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<androidx.recyclerview.widget.RecyclerView
+	xmlns:android="http://schemas.android.com/apk/res/android"
+	xmlns:app="http://schemas.android.com/apk/res-auto"
+	android:id="@+id/recyclerView"
+	android:layout_width="match_parent"
+	android:layout_height="match_parent"
+	android:defaultFocusHighlightEnabled="false"
+	android:orientation="horizontal"
+	app:layoutManager="org.koitharu.kotatsu.reader.ui.pager.doublepage.DoublePageLayoutManager" />

build.gradle

@@ -4,9 +4,9 @@ buildscript {
 		mavenCentral()
 	}
 	dependencies {
-		classpath 'com.android.tools.build:gradle:8.1.1'
+		classpath 'com.android.tools.build:gradle:8.1.2'
 		classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.9.10'
-		classpath 'com.google.dagger:hilt-android-gradle-plugin:2.48'
+		classpath 'com.google.dagger:hilt-android-gradle-plugin:2.48.1'
 		classpath 'com.google.devtools.ksp:symbol-processing-gradle-plugin:1.9.10-1.0.13'
 	}
 }