Handle nested scroll state in shelf

app/src/main/java/org/koitharu/kotatsu/base/ui/list/NestedScrollStateHandle.kt

@@ -0,0 +1,64 @@
+package org.koitharu.kotatsu.base.ui.list
+
+import android.os.Bundle
+import android.os.Parcelable
+import android.util.SparseArray
+import androidx.core.os.BundleCompat
+import androidx.core.view.doOnNextLayout
+import androidx.recyclerview.widget.RecyclerView
+import java.util.Collections
+import java.util.WeakHashMap
+
+class NestedScrollStateHandle(
+	savedInstanceState: Bundle?,
+	private val key: String,
+) {
+
+	private val storage: SparseArray<Parcelable?> = savedInstanceState?.let {
+		BundleCompat.getSparseParcelableArray(it, key, Parcelable::class.java)
+	} ?: SparseArray<Parcelable?>()
+	private val controllers = Collections.newSetFromMap<Controller>(WeakHashMap())
+
+	fun attach(recycler: RecyclerView) = Controller(recycler).also(controllers::add)
+
+	fun onSaveInstanceState(outState: Bundle) {
+		controllers.forEach {
+			it.saveState()
+		}
+		outState.putSparseParcelableArray(key, storage)
+	}
+
+	inner class Controller(
+		private val recycler: RecyclerView
+	) {
+
+		private var lastPosition: Int = -1
+
+		fun onBind(position: Int) {
+			if (position != lastPosition) {
+				saveState()
+				lastPosition = position
+				storage[position]?.let {
+					restoreState(it)
+				}
+			}
+		}
+
+		fun onRecycled() {
+			saveState()
+			lastPosition = -1
+		}
+
+		fun saveState() {
+			if (lastPosition != -1) {
+				storage[lastPosition] = recycler.layoutManager?.onSaveInstanceState()
+			}
+		}
+
+		private fun restoreState(state: Parcelable) {
+			recycler.doOnNextLayout {
+				recycler.layoutManager?.onRestoreInstanceState(state)
+			}
+		}
+	}
+}

app/src/main/java/org/koitharu/kotatsu/shelf/ui/ShelfFragment.kt

@@ -14,6 +14,7 @@ import androidx.recyclerview.widget.RecyclerView
 import coil.ImageLoader
 import dagger.hilt.android.AndroidEntryPoint
 import org.koitharu.kotatsu.base.ui.BaseFragment
+import org.koitharu.kotatsu.base.ui.list.NestedScrollStateHandle
 import org.koitharu.kotatsu.base.ui.list.SectionedSelectionController
 import org.koitharu.kotatsu.base.ui.util.RecyclerViewOwner
 import org.koitharu.kotatsu.base.ui.util.ReversibleActionObserver
@@ -47,6 +48,7 @@ class ShelfFragment :
 	@Inject
 	lateinit var settings: AppSettings
 
+	private var nestedScrollStateHandle: NestedScrollStateHandle? = null
 	private val viewModel by viewModels<ShelfViewModel>()
 	private var adapter: ShelfAdapter? = null
 	private var selectionController: SectionedSelectionController<ShelfSectionModel>? = null
@@ -60,6 +62,7 @@ class ShelfFragment :
 
 	override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
 		super.onViewCreated(view, savedInstanceState)
+		nestedScrollStateHandle = NestedScrollStateHandle(savedInstanceState, KEY_NESTED_SCROLL)
 		val sizeResolver = ItemSizeResolver(resources, settings)
 		selectionController = SectionedSelectionController(
 			activity = requireActivity(),
@@ -72,6 +75,7 @@ class ShelfFragment :
 			listener = this,
 			sizeResolver = sizeResolver,
 			selectionController = checkNotNull(selectionController),
+			nestedScrollStateHandle = checkNotNull(nestedScrollStateHandle),
 		)
 		binding.recyclerView.adapter = adapter
 		binding.recyclerView.setHasFixedSize(true)
@@ -82,10 +86,16 @@ class ShelfFragment :
 		viewModel.onActionDone.observe(viewLifecycleOwner, ReversibleActionObserver(binding.recyclerView))
 	}
 
+	override fun onSaveInstanceState(outState: Bundle) {
+		super.onSaveInstanceState(outState)
+		nestedScrollStateHandle?.onSaveInstanceState(outState)
+	}
+
 	override fun onDestroyView() {
 		super.onDestroyView()
 		adapter = null
 		selectionController = null
+		nestedScrollStateHandle = null
 	}
 
 	override fun onItemClick(item: Manga, section: ShelfSectionModel, view: View) {
@@ -133,6 +143,8 @@ class ShelfFragment :
 
 	companion object {
 
+		private const val KEY_NESTED_SCROLL = "nested_scroll"
+
 		fun newInstance() = ShelfFragment()
 	}
 }

app/src/main/java/org/koitharu/kotatsu/shelf/ui/adapter/ScrollKeepObserver.kt

@@ -11,14 +11,16 @@ class ScrollKeepObserver(
 		get() = recyclerView.layoutManager as LinearLayoutManager
 
 	override fun onItemRangeMoved(fromPosition: Int, toPosition: Int, itemCount: Int) {
+		val firstVisiblePosition = layoutManager.findFirstVisibleItemPosition()
 		val position = minOf(toPosition, fromPosition) // if items are swapping positions may be swapped too
-		if (position == 0 || position < layoutManager.findFirstVisibleItemPosition()) {
+		if (firstVisiblePosition != RecyclerView.NO_POSITION && (position == 0 || position < firstVisiblePosition)) {
 			postScroll(position)
 		}
 	}
 
 	override fun onItemRangeInserted(positionStart: Int, itemCount: Int) {
-		if (positionStart == 0 || positionStart < layoutManager.findFirstVisibleItemPosition()) {
+		val firstVisiblePosition = layoutManager.findFirstVisibleItemPosition()
+		if (firstVisiblePosition != RecyclerView.NO_POSITION && (positionStart == 0 || positionStart < firstVisiblePosition)) {
 			postScroll(positionStart)
 		}
 	}

app/src/main/java/org/koitharu/kotatsu/shelf/ui/adapter/ShelfAdapter.kt

@@ -6,6 +6,7 @@ import androidx.recyclerview.widget.DiffUtil
 import androidx.recyclerview.widget.RecyclerView
 import coil.ImageLoader
 import com.hannesdorfmann.adapterdelegates4.AsyncListDifferDelegationAdapter
+import org.koitharu.kotatsu.base.ui.list.NestedScrollStateHandle
 import org.koitharu.kotatsu.base.ui.list.SectionedSelectionController
 import org.koitharu.kotatsu.base.ui.list.fastscroll.FastScroller
 import org.koitharu.kotatsu.list.ui.ItemSizeResolver
@@ -24,6 +25,7 @@ class ShelfAdapter(
 	listener: ShelfListEventListener,
 	sizeResolver: ItemSizeResolver,
 	selectionController: SectionedSelectionController<ShelfSectionModel>,
+	nestedScrollStateHandle: NestedScrollStateHandle,
 ) : AsyncListDifferDelegationAdapter<ListModel>(DiffCallback()), FastScroller.SectionIndexer {
 
 	init {
@@ -37,6 +39,7 @@ class ShelfAdapter(
 					sizeResolver = sizeResolver,
 					selectionController = selectionController,
 					listener = listener,
+					nestedScrollStateHandle = nestedScrollStateHandle,
 				),
 			)
 			.addDelegate(loadingStateAD())

app/src/main/java/org/koitharu/kotatsu/shelf/ui/adapter/ShelfGroupAD.kt

@@ -7,16 +7,17 @@ import coil.ImageLoader
 import com.hannesdorfmann.adapterdelegates4.AsyncListDifferDelegationAdapter
 import com.hannesdorfmann.adapterdelegates4.dsl.adapterDelegateViewBinding
 import org.koitharu.kotatsu.R
+import org.koitharu.kotatsu.base.ui.list.NestedScrollStateHandle
 import org.koitharu.kotatsu.base.ui.list.OnListItemClickListener
 import org.koitharu.kotatsu.base.ui.list.SectionedSelectionController
 import org.koitharu.kotatsu.base.ui.list.decor.AbstractSelectionItemDecoration
 import org.koitharu.kotatsu.base.ui.list.decor.SpacingItemDecoration
 import org.koitharu.kotatsu.databinding.ItemListGroupBinding
-import org.koitharu.kotatsu.shelf.ui.model.ShelfSectionModel
 import org.koitharu.kotatsu.list.ui.ItemSizeResolver
 import org.koitharu.kotatsu.list.ui.adapter.mangaGridItemAD
 import org.koitharu.kotatsu.list.ui.model.ListModel
 import org.koitharu.kotatsu.parsers.model.Manga
+import org.koitharu.kotatsu.shelf.ui.model.ShelfSectionModel
 import org.koitharu.kotatsu.utils.ext.removeItemDecoration
 import org.koitharu.kotatsu.utils.ext.setTextAndVisible
 
@@ -27,6 +28,7 @@ fun shelfGroupAD(
 	sizeResolver: ItemSizeResolver,
 	selectionController: SectionedSelectionController<ShelfSectionModel>,
 	listener: ShelfListEventListener,
+	nestedScrollStateHandle: NestedScrollStateHandle,
 ) = adapterDelegateViewBinding<ShelfSectionModel, ListModel, ItemListGroupBinding>(
 	{ layoutInflater, parent -> ItemListGroupBinding.inflate(layoutInflater, parent, false) },
 ) {
@@ -48,21 +50,25 @@ fun shelfGroupAD(
 		MangaItemDiffCallback(),
 		mangaGridItemAD(coil, lifecycleOwner, listenerAdapter, sizeResolver),
 	)
+	adapter.stateRestorationPolicy = RecyclerView.Adapter.StateRestorationPolicy.PREVENT_WHEN_EMPTY
 	adapter.registerAdapterDataObserver(ScrollKeepObserver(binding.recyclerView))
 	binding.recyclerView.setRecycledViewPool(sharedPool)
 	binding.recyclerView.adapter = adapter
 	val spacingDecoration = SpacingItemDecoration(context.resources.getDimensionPixelOffset(R.dimen.grid_spacing))
 	binding.recyclerView.addItemDecoration(spacingDecoration)
 	binding.buttonMore.setOnClickListener(listenerAdapter)
+	val stateController = nestedScrollStateHandle.attach(binding.recyclerView)
 
 	bind {
 		selectionController.attachToRecyclerView(item, binding.recyclerView)
 		binding.textViewTitle.text = item.getTitle(context.resources)
 		binding.buttonMore.setTextAndVisible(item.showAllButtonText)
 		adapter.items = item.items
+		stateController.onBind(bindingAdapterPosition)
 	}
 
 	onViewRecycled {
+		stateController.onRecycled()
 		adapter.items = emptyList()
 		binding.recyclerView.removeItemDecoration(AbstractSelectionItemDecoration::class.java)
 	}

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

@@ -7,6 +7,8 @@ import android.os.Build
 import android.os.Bundle
 import android.os.Parcel
 import android.os.Parcelable
+import androidx.core.content.IntentCompat
+import androidx.core.os.BundleCompat
 import androidx.lifecycle.SavedStateHandle
 import org.koitharu.kotatsu.core.model.parcelable.ParcelableMangaTags
 import java.io.Serializable
@@ -14,11 +16,11 @@ import java.io.Serializable
 // https://issuetracker.google.com/issues/240585930
 
 inline fun <reified T : Parcelable> Bundle.getParcelableCompat(key: String): T? {
-	return getParcelable(key) as T?
+	return BundleCompat.getParcelable(this, key, T::class.java)
 }
 
 inline fun <reified T : Parcelable> Intent.getParcelableExtraCompat(key: String): T? {
-	return getParcelableExtra(key) as T?
+	return IntentCompat.getParcelableExtra(this, key, T::class.java)
 }
 
 inline fun <reified T : Serializable> Intent.getSerializableExtraCompat(key: String): T? {