Double page reader for reversed mode

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

@@ -266,6 +266,10 @@ class ReaderActivity :
 		viewModel.switchMode(mode)
 	}
 
+	override fun onDoubleModeChanged(isEnabled: Boolean) {
+		readerManager.setDoubleReaderMode(isEnabled)
+	}
+
 	private fun onPageSaved(uri: Uri?) {
 		if (uri != null) {
 			Snackbar.make(viewBinding.container, R.string.page_saved, Snackbar.LENGTH_LONG)

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

@@ -6,8 +6,10 @@ import androidx.fragment.app.FragmentManager
 import androidx.fragment.app.commit
 import org.koitharu.kotatsu.core.prefs.AppSettings
 import org.koitharu.kotatsu.core.prefs.ReaderMode
+import org.koitharu.kotatsu.core.util.ext.findKeyByValue
 import org.koitharu.kotatsu.reader.ui.pager.BaseReaderFragment
 import org.koitharu.kotatsu.reader.ui.pager.doublepage.DoubleReaderFragment
+import org.koitharu.kotatsu.reader.ui.pager.doublereversed.ReversedDoubleReaderFragment
 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.vertical.VerticalReaderFragment
@@ -23,14 +25,9 @@ class ReaderManager(
 	private val modeMap = EnumMap<ReaderMode, Class<out BaseReaderFragment<*>>>(ReaderMode::class.java)
 
 	init {
-		modeMap[ReaderMode.STANDARD] = if (useDoublePages()) {
-			DoubleReaderFragment::class.java
-		} else {
-			PagerReaderFragment::class.java
-		}
-		modeMap[ReaderMode.REVERSED] = ReversedReaderFragment::class.java
-		modeMap[ReaderMode.WEBTOON] = WebtoonReaderFragment::class.java
-		modeMap[ReaderMode.VERTICAL] = VerticalReaderFragment::class.java
+		val useDoublePages = container.resources.configuration.orientation == Configuration.ORIENTATION_LANDSCAPE
+			&& settings.isReaderDoubleOnLandscape
+		invalidateTypesMap(useDoublePages)
 	}
 
 	val currentReader: BaseReaderFragment<*>?
@@ -39,7 +36,7 @@ class ReaderManager(
 	val currentMode: ReaderMode?
 		get() {
 			val readerClass = currentReader?.javaClass ?: return null
-			return modeMap.entries.find { it.value == readerClass }?.key
+			return modeMap.findKeyByValue(readerClass)
 		}
 
 	fun replace(newMode: ReaderMode) {
@@ -50,13 +47,27 @@ class ReaderManager(
 		}
 	}
 
-	private fun useDoublePages() = container.resources.configuration.orientation == Configuration.ORIENTATION_LANDSCAPE
-		&& settings.isReaderDoubleOnLandscape
-
-	/*fun replace(reader: BaseReaderFragment<*>) {
-		fragmentManager.commit {
-			setReorderingAllowed(true)
-			replace(containerResId, reader)
+	fun setDoubleReaderMode(isEnabled: Boolean) {
+		val prevMode = currentMode
+		invalidateTypesMap(isEnabled)
+		val newMode = currentMode ?: return
+		if (newMode != prevMode) {
+			replace(newMode)
 		}
-	}*/
+	}
+
+	private fun invalidateTypesMap(useDoublePages: Boolean) {
+		modeMap[ReaderMode.STANDARD] = if (useDoublePages) {
+			DoubleReaderFragment::class.java
+		} else {
+			PagerReaderFragment::class.java
+		}
+		modeMap[ReaderMode.REVERSED] = if (useDoublePages) {
+			ReversedDoubleReaderFragment::class.java
+		} else {
+			ReversedReaderFragment::class.java
+		}
+		modeMap[ReaderMode.WEBTOON] = WebtoonReaderFragment::class.java
+		modeMap[ReaderMode.VERTICAL] = VerticalReaderFragment::class.java
+	}
 }

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

@@ -145,7 +145,7 @@ class ReaderConfigSheet :
 
 			R.id.switch_double_reader -> {
 				settings.isReaderDoubleOnLandscape = isChecked
-				findCallback()?.onReaderModeChanged(mode)
+				findCallback()?.onDoubleModeChanged(isChecked)
 			}
 		}
 	}
@@ -165,7 +165,7 @@ class ReaderConfigSheet :
 			R.id.button_vertical -> ReaderMode.VERTICAL
 			else -> return
 		}
-		viewBinding?.switchDoubleReader?.isEnabled = newMode == ReaderMode.STANDARD
+		viewBinding?.switchDoubleReader?.isEnabled = newMode == ReaderMode.STANDARD || newMode == ReaderMode.REVERSED
 		if (newMode == mode) {
 			return
 		}
@@ -212,6 +212,8 @@ class ReaderConfigSheet :
 		var isAutoScrollEnabled: Boolean
 
 		fun onReaderModeChanged(mode: ReaderMode)
+
+		fun onDoubleModeChanged(isEnabled: Boolean)
 	}
 
 	companion object {

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

@@ -18,7 +18,6 @@ import org.koitharu.kotatsu.core.util.ext.firstVisibleItemPosition
 import org.koitharu.kotatsu.databinding.FragmentReaderDoubleBinding
 import org.koitharu.kotatsu.reader.domain.PageLoader
 import org.koitharu.kotatsu.reader.ui.ReaderState
-import org.koitharu.kotatsu.reader.ui.ReaderViewModel
 import org.koitharu.kotatsu.reader.ui.pager.BaseReaderAdapter
 import org.koitharu.kotatsu.reader.ui.pager.BaseReaderFragment
 import org.koitharu.kotatsu.reader.ui.pager.ReaderPage
@@ -26,7 +25,7 @@ import javax.inject.Inject
 import kotlin.math.absoluteValue
 
 @AndroidEntryPoint
-class DoubleReaderFragment : BaseReaderFragment<FragmentReaderDoubleBinding>() {
+open class DoubleReaderFragment : BaseReaderFragment<FragmentReaderDoubleBinding>() {
 
 	@Inject
 	lateinit var networkState: NetworkState
@@ -51,7 +50,7 @@ class DoubleReaderFragment : BaseReaderFragment<FragmentReaderDoubleBinding>() {
 			recyclerLifecycleDispatcher = RecyclerViewLifecycleDispatcher().also {
 				addOnScrollListener(it)
 			}
-			addOnScrollListener(PageScrollListener(viewModel))
+			addOnScrollListener(PageScrollListener())
 			DoublePageSnapHelper().attachToRecyclerView(this)
 		}
 	}
@@ -78,7 +77,7 @@ class DoubleReaderFragment : BaseReaderFragment<FragmentReaderDoubleBinding>() {
 			if (position != -1) {
 				position = position.toPagePosition()
 				requireViewBinding().recyclerView.firstVisibleItemPosition = position
-				viewModel.onCurrentPageChanged(position, position + 1)
+				notifyPageChanged(position, position + 1)
 			} else {
 				Snackbar.make(requireView(), R.string.not_found_404, Snackbar.LENGTH_SHORT)
 					.show()
@@ -132,14 +131,16 @@ class DoubleReaderFragment : BaseReaderFragment<FragmentReaderDoubleBinding>() {
 		)
 	}
 
+	protected open fun notifyPageChanged(lowerPos: Int, upperPos: Int) {
+		viewModel.onCurrentPageChanged(lowerPos, upperPos)
+	}
+
 	private fun getCurrentItem() = (requireViewBinding().recyclerView.layoutManager as LinearLayoutManager)
 		.findFirstCompletelyVisibleItemPosition().toPagePosition()
 
 	private fun Int.toPagePosition() = this and 1.inv()
 
-	private class PageScrollListener(
-		private val viewModel: ReaderViewModel,
-	) : RecyclerView.OnScrollListener() {
+	private inner class PageScrollListener : RecyclerView.OnScrollListener() {
 
 		private var firstPos = RecyclerView.NO_POSITION
 		private var lastPos = RecyclerView.NO_POSITION
@@ -157,7 +158,7 @@ class DoubleReaderFragment : BaseReaderFragment<FragmentReaderDoubleBinding>() {
 			if (newFirstPos != firstPos || newLastPos != lastPos) {
 				firstPos = newFirstPos
 				lastPos = newLastPos
-				viewModel.onCurrentPageChanged(newFirstPos, newLastPos)
+				notifyPageChanged(newFirstPos, newLastPos)
 			}
 		}
 	}

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

@@ -0,0 +1,28 @@
+package org.koitharu.kotatsu.reader.ui.pager.doublereversed
+
+import org.koitharu.kotatsu.reader.ui.ReaderState
+import org.koitharu.kotatsu.reader.ui.pager.ReaderPage
+import org.koitharu.kotatsu.reader.ui.pager.doublepage.DoubleReaderFragment
+
+class ReversedDoubleReaderFragment : DoubleReaderFragment() {
+
+	override fun switchPageBy(delta: Int) {
+		super.switchPageBy(-delta)
+	}
+
+	override fun switchPageTo(position: Int, smooth: Boolean) {
+		super.switchPageTo(reversed(position), smooth)
+	}
+
+	override suspend fun onPagesChanged(pages: List<ReaderPage>, pendingState: ReaderState?) {
+		super.onPagesChanged(pages.reversed(), pendingState)
+	}
+
+	override fun notifyPageChanged(lowerPos: Int, upperPos: Int) {
+		viewModel.onCurrentPageChanged(reversed(upperPos), reversed(lowerPos))
+	}
+
+	private fun reversed(position: Int): Int {
+		return ((readerAdapter?.itemCount ?: 0) - position - 1).coerceAtLeast(0)
+	}
+}