ui: rework primitive backing data into sync

Rework PrimitiveBackingData into SyncBackingData.

Google says that you should not use DiffUtil on the main thread, as it
is far too slow. In practice, the time it takes to diff is negligable,
even more so when costly diffs are filled in with a replace call
instead. Not only that, but switching to a synced list differ fixees
the horrible nightmare that is the queue adapter. Generally a win-win
for most use-cases in Auxio.

app/src/main/java/org/oxycblt/auxio/home/list/AlbumListFragment.kt

@@ -28,8 +28,8 @@ import org.oxycblt.auxio.ui.DisplayMode
 import org.oxycblt.auxio.ui.Item
 import org.oxycblt.auxio.ui.MenuItemListener
 import org.oxycblt.auxio.ui.MonoAdapter
-import org.oxycblt.auxio.ui.PrimitiveBackingData
 import org.oxycblt.auxio.ui.Sort
+import org.oxycblt.auxio.ui.SyncBackingData
 import org.oxycblt.auxio.ui.newMenu
 import org.oxycblt.auxio.util.formatDuration
 import org.oxycblt.auxio.util.launch
@@ -50,7 +50,7 @@ class AlbumListFragment : HomeListFragment<Album>() {
             adapter = homeAdapter
         }
 
-        launch { homeModel.albums.collect(homeAdapter.data::submitList) }
+        launch { homeModel.albums.collect(homeAdapter.data::replaceList) }
     }
 
     override fun getPopup(pos: Int): String? {
@@ -89,7 +89,7 @@ class AlbumListFragment : HomeListFragment<Album>() {
 
     class AlbumAdapter(listener: MenuItemListener) :
         MonoAdapter<Album, MenuItemListener, AlbumViewHolder>(listener) {
-        override val data = PrimitiveBackingData<Album>(this)
+        override val data = SyncBackingData(this, AlbumViewHolder.DIFFER)
         override val creator = AlbumViewHolder.CREATOR
     }
 }

app/src/main/java/org/oxycblt/auxio/home/list/ArtistListFragment.kt

@@ -28,8 +28,8 @@ import org.oxycblt.auxio.ui.DisplayMode
 import org.oxycblt.auxio.ui.Item
 import org.oxycblt.auxio.ui.MenuItemListener
 import org.oxycblt.auxio.ui.MonoAdapter
-import org.oxycblt.auxio.ui.PrimitiveBackingData
 import org.oxycblt.auxio.ui.Sort
+import org.oxycblt.auxio.ui.SyncBackingData
 import org.oxycblt.auxio.ui.newMenu
 import org.oxycblt.auxio.util.formatDuration
 import org.oxycblt.auxio.util.launch
@@ -50,7 +50,7 @@ class ArtistListFragment : HomeListFragment<Artist>() {
             adapter = homeAdapter
         }
 
-        launch { homeModel.artists.collect(homeAdapter.data::submitList) }
+        launch { homeModel.artists.collect(homeAdapter.data::replaceList) }
     }
 
     override fun getPopup(pos: Int): String? {
@@ -83,7 +83,7 @@ class ArtistListFragment : HomeListFragment<Artist>() {
 
     class ArtistAdapter(listener: MenuItemListener) :
         MonoAdapter<Artist, MenuItemListener, ArtistViewHolder>(listener) {
-        override val data = PrimitiveBackingData<Artist>(this)
+        override val data = SyncBackingData(this, ArtistViewHolder.DIFFER)
         override val creator = ArtistViewHolder.CREATOR
     }
 }

app/src/main/java/org/oxycblt/auxio/home/list/GenreListFragment.kt

@@ -28,8 +28,8 @@ import org.oxycblt.auxio.ui.GenreViewHolder
 import org.oxycblt.auxio.ui.Item
 import org.oxycblt.auxio.ui.MenuItemListener
 import org.oxycblt.auxio.ui.MonoAdapter
-import org.oxycblt.auxio.ui.PrimitiveBackingData
 import org.oxycblt.auxio.ui.Sort
+import org.oxycblt.auxio.ui.SyncBackingData
 import org.oxycblt.auxio.ui.newMenu
 import org.oxycblt.auxio.util.formatDuration
 import org.oxycblt.auxio.util.launch
@@ -50,7 +50,7 @@ class GenreListFragment : HomeListFragment<Genre>() {
             adapter = homeAdapter
         }
 
-        launch { homeModel.genres.collect(homeAdapter.data::submitList) }
+        launch { homeModel.genres.collect(homeAdapter.data::replaceList) }
     }
 
     override fun getPopup(pos: Int): String? {
@@ -83,7 +83,7 @@ class GenreListFragment : HomeListFragment<Genre>() {
 
     class GenreAdapter(listener: MenuItemListener) :
         MonoAdapter<Genre, MenuItemListener, GenreViewHolder>(listener) {
-        override val data = PrimitiveBackingData<Genre>(this)
+        override val data = SyncBackingData(this, GenreViewHolder.DIFFER)
         override val creator = GenreViewHolder.CREATOR
     }
 }

app/src/main/java/org/oxycblt/auxio/home/list/SongListFragment.kt

@@ -26,9 +26,9 @@ import org.oxycblt.auxio.ui.DisplayMode
 import org.oxycblt.auxio.ui.Item
 import org.oxycblt.auxio.ui.MenuItemListener
 import org.oxycblt.auxio.ui.MonoAdapter
-import org.oxycblt.auxio.ui.PrimitiveBackingData
 import org.oxycblt.auxio.ui.SongViewHolder
 import org.oxycblt.auxio.ui.Sort
+import org.oxycblt.auxio.ui.SyncBackingData
 import org.oxycblt.auxio.ui.newMenu
 import org.oxycblt.auxio.util.formatDuration
 import org.oxycblt.auxio.util.launch
@@ -49,7 +49,7 @@ class SongListFragment : HomeListFragment<Song>() {
             adapter = homeAdapter
         }
 
-        launch { homeModel.songs.collect(homeAdapter.data::submitList) }
+        launch { homeModel.songs.collect(homeAdapter.data::replaceList) }
     }
 
     override fun getPopup(pos: Int): String? {
@@ -90,7 +90,7 @@ class SongListFragment : HomeListFragment<Song>() {
 
     inner class SongsAdapter(listener: MenuItemListener) :
         MonoAdapter<Song, MenuItemListener, SongViewHolder>(listener) {
-        override val data = PrimitiveBackingData<Song>(this)
+        override val data = SyncBackingData(this, SongViewHolder.DIFFER)
         override val creator = SongViewHolder.CREATOR
     }
 }

app/src/main/java/org/oxycblt/auxio/music/excluded/ExcludedAdapter.kt

@@ -19,9 +19,9 @@ package org.oxycblt.auxio.music.excluded
 
 import android.content.Context
 import org.oxycblt.auxio.databinding.ItemExcludedDirBinding
+import org.oxycblt.auxio.ui.BackingData
 import org.oxycblt.auxio.ui.BindingViewHolder
 import org.oxycblt.auxio.ui.MonoAdapter
-import org.oxycblt.auxio.ui.PrimitiveBackingData
 import org.oxycblt.auxio.util.inflater
 import org.oxycblt.auxio.util.textSafe
 
@@ -31,12 +31,42 @@ import org.oxycblt.auxio.util.textSafe
  */
 class ExcludedAdapter(listener: Listener) :
     MonoAdapter<ExcludedDirectory, ExcludedAdapter.Listener, ExcludedViewHolder>(listener) {
-    override val data = PrimitiveBackingData<ExcludedDirectory>(this)
+    override val data = ExcludedBackingData(this)
     override val creator = ExcludedViewHolder.CREATOR
 
     interface Listener {
         fun onRemoveDirectory(dir: ExcludedDirectory)
     }
+
+    class ExcludedBackingData(private val adapter: ExcludedAdapter) :
+        BackingData<ExcludedDirectory>() {
+        private val _currentList = mutableListOf<ExcludedDirectory>()
+        val currentList: List<ExcludedDirectory> = _currentList
+
+        override fun getItemCount(): Int = _currentList.size
+        override fun getItem(position: Int): ExcludedDirectory = _currentList[position]
+
+        fun add(dir: ExcludedDirectory) {
+            if (_currentList.contains(dir)) {
+                return
+            }
+
+            _currentList.add(dir)
+            adapter.notifyItemInserted(_currentList.lastIndex)
+        }
+
+        fun addAll(dirs: List<ExcludedDirectory>) {
+            val oldLastIndex = dirs.lastIndex
+            _currentList.addAll(dirs)
+            adapter.notifyItemRangeInserted(oldLastIndex, dirs.size)
+        }
+
+        fun remove(dir: ExcludedDirectory) {
+            val idx = _currentList.indexOf(dir)
+            _currentList.removeAt(idx)
+            adapter.notifyItemRemoved(idx)
+        }
+    }
 }
 
 /** The viewholder for [ExcludedAdapter]. Not intended for use in other adapters. */

app/src/main/java/org/oxycblt/auxio/music/excluded/ExcludedDialog.kt

@@ -97,7 +97,7 @@ class ExcludedDialog :
                 ?.mapNotNull(ExcludedDirectory::fromString)
                 ?: settingsManager.excludedDirs
 
-        excludedAdapter.data.submitList(dirs)
+        excludedAdapter.data.addAll(dirs)
         requireBinding().excludedEmpty.isVisible = dirs.isEmpty()
     }
 

app/src/main/java/org/oxycblt/auxio/playback/PlaybackViewModel.kt

@@ -192,28 +192,20 @@ class PlaybackViewModel : ViewModel(), PlaybackStateManager.Callback, MusicStore
         playbackManager.prev()
     }
 
-    /**
-     * Remove a queue item using it's recyclerview adapter index. If the indices are valid, [apply]
-     * is called just before the change is committed so that the adapter can be updated.
-     */
-    fun removeQueueDataItem(adapterIndex: Int, apply: () -> Unit) {
+    /** Remove a queue item using it's recyclerview adapter index. */
+    fun removeQueueDataItem(adapterIndex: Int) {
         val index =
             adapterIndex + (playbackManager.queue.size - unlikelyToBeNull(_nextUp.value).size)
         if (index in playbackManager.queue.indices) {
-            apply()
             playbackManager.removeQueueItem(index)
         }
     }
-    /**
-     * Move queue items using their recyclerview adapter indices. If the indices are valid, [apply]
-     * is called just before the change is committed so that the adapter can be updated.
-     */
-    fun moveQueueDataItems(adapterFrom: Int, adapterTo: Int, apply: () -> Unit): Boolean {
+    /** Move queue items using their recyclerview adapter indices. */
+    fun moveQueueDataItems(adapterFrom: Int, adapterTo: Int): Boolean {
         val delta = (playbackManager.queue.size - unlikelyToBeNull(_nextUp.value).size)
         val from = adapterFrom + delta
         val to = adapterTo + delta
         if (from in playbackManager.queue.indices && to in playbackManager.queue.indices) {
-            apply()
             playbackManager.moveQueueItem(from, to)
             return true
         }

app/src/main/java/org/oxycblt/auxio/playback/queue/QueueAdapter.kt

@@ -23,17 +23,15 @@ import android.graphics.drawable.ColorDrawable
 import android.view.MotionEvent
 import android.view.View
 import androidx.core.view.isInvisible
-import androidx.recyclerview.widget.AsyncListDiffer
-import androidx.recyclerview.widget.DiffUtil
 import androidx.recyclerview.widget.RecyclerView
 import com.google.android.material.shape.MaterialShapeDrawable
 import org.oxycblt.auxio.IntegerTable
 import org.oxycblt.auxio.databinding.ItemQueueSongBinding
 import org.oxycblt.auxio.music.Song
-import org.oxycblt.auxio.ui.BackingData
 import org.oxycblt.auxio.ui.BindingViewHolder
 import org.oxycblt.auxio.ui.MonoAdapter
 import org.oxycblt.auxio.ui.SongViewHolder
+import org.oxycblt.auxio.ui.SyncBackingData
 import org.oxycblt.auxio.util.context
 import org.oxycblt.auxio.util.disableDropShadowCompat
 import org.oxycblt.auxio.util.inflater
@@ -42,7 +40,7 @@ import org.oxycblt.auxio.util.textSafe
 
 class QueueAdapter(listener: QueueItemListener) :
     MonoAdapter<Song, QueueItemListener, QueueSongViewHolder>(listener) {
-    override val data = HybridBackingData(this, QueueSongViewHolder.DIFFER)
+    override val data = SyncBackingData(this, QueueSongViewHolder.DIFFER)
     override val creator = QueueSongViewHolder.CREATOR
 }
 
@@ -107,67 +105,3 @@ private constructor(
         val DIFFER = SongViewHolder.DIFFER
     }
 }
-
-/**
- * A list-backed [BackingData] that can be modified with both adapter primitives and
- * [AsyncListDiffer]. This is incredibly dangerous and can probably crash the app if you look at it
- * the wrong way, so please don't use it outside of the queue module.
- */
-class HybridBackingData<T>(
-    private val adapter: RecyclerView.Adapter<*>,
-    diffCallback: DiffUtil.ItemCallback<T>
-) : BackingData<T>() {
-    private var _currentList = mutableListOf<T>()
-    val currentList: List<T>
-        get() = _currentList
-
-    private val differ = AsyncListDiffer(adapter, diffCallback)
-
-    override fun getItem(position: Int): T = _currentList[position]
-    override fun getItemCount(): Int = _currentList.size
-
-    fun submitList(newData: List<T>, onDone: () -> Unit = {}) {
-        if (newData != _currentList) {
-            _currentList = newData.toMutableList()
-            differ.submitList(newData, onDone)
-        }
-    }
-
-    fun moveItems(from: Int, to: Int) {
-        _currentList.add(to, _currentList.removeAt(from))
-        differ.rewriteListUnsafe(_currentList)
-        adapter.notifyItemMoved(from, to)
-    }
-
-    fun removeItem(at: Int) {
-        _currentList.removeAt(at)
-        differ.rewriteListUnsafe(_currentList)
-        adapter.notifyItemRemoved(at)
-    }
-
-    /**
-     * Rewrites the AsyncListDiffer's internal list, cancelling any diffs that are currently in
-     * progress. I cannot describe in words how dangerous this is, but it's also the only thing I
-     * can do to marry the adapter primitives with DiffUtil.
-     */
-    private fun <T> AsyncListDiffer<T>.rewriteListUnsafe(newList: List<T>) {
-        differMaxGenerationsField.set(this, (differMaxGenerationsField.get(this) as Int) + 1)
-        differListField.set(this, newList.toMutableList())
-        differImmutableListField.set(this, newList)
-    }
-
-    companion object {
-        private val differListField =
-            AsyncListDiffer::class.java.getDeclaredField("mList").apply { isAccessible = true }
-
-        private val differImmutableListField =
-            AsyncListDiffer::class.java.getDeclaredField("mReadOnlyList").apply {
-                isAccessible = true
-            }
-
-        private val differMaxGenerationsField =
-            AsyncListDiffer::class.java.getDeclaredField("mMaxScheduledGeneration").apply {
-                isAccessible = true
-            }
-    }
-}

app/src/main/java/org/oxycblt/auxio/playback/queue/QueueDragCallback.kt

@@ -148,17 +148,12 @@ class QueueDragCallback(
         recyclerView: RecyclerView,
         viewHolder: RecyclerView.ViewHolder,
         target: RecyclerView.ViewHolder
-    ): Boolean {
-        val from = viewHolder.bindingAdapterPosition
-        val to = target.bindingAdapterPosition
-
-        return playbackModel.moveQueueDataItems(from, to) { queueAdapter.data.moveItems(from, to) }
-    }
+    ): Boolean =
+        playbackModel.moveQueueDataItems(
+            viewHolder.bindingAdapterPosition, target.bindingAdapterPosition)
 
     override fun onSwiped(viewHolder: RecyclerView.ViewHolder, direction: Int) {
-        playbackModel.removeQueueDataItem(viewHolder.bindingAdapterPosition) {
-            queueAdapter.data.removeItem(viewHolder.bindingAdapterPosition)
-        }
+        playbackModel.removeQueueDataItem(viewHolder.bindingAdapterPosition)
     }
 
     override fun isLongPressDragEnabled(): Boolean = false

app/src/main/java/org/oxycblt/auxio/ui/RecyclerFramework.kt

@@ -20,8 +20,8 @@ package org.oxycblt.auxio.ui
 import android.content.Context
 import android.view.View
 import android.view.ViewGroup
-import android.widget.Adapter
 import androidx.annotation.StringRes
+import androidx.recyclerview.widget.AdapterListUpdateCallback
 import androidx.recyclerview.widget.AsyncListDiffer
 import androidx.recyclerview.widget.DiffUtil
 import androidx.recyclerview.widget.RecyclerView
@@ -183,50 +183,128 @@ abstract class BackingData<T> {
 }
 
 /**
- * A list-backed [BackingData] that is modified using adapter primitives. Useful in cases where
- * [AsyncBackingData] is not preferable due to bugs involving diffing.
+ * A list-backed [BackingData] that is modified synchronously. This is generally the recommended
+ * option for most adapters.
  * @author OxygenCobalt
  */
-class PrimitiveBackingData<T>(private val adapter: RecyclerView.Adapter<*>) : BackingData<T>() {
-    private var _currentList = mutableListOf<T>()
-
+class SyncBackingData<T>(adapter: RecyclerView.Adapter<*>, diffCallback: DiffUtil.ItemCallback<T>) :
+    BackingData<T>() {
+    private var differ = SyncListDiffer(adapter, diffCallback)
     /** The current list backing this adapter. */
     val currentList: List<T>
-        get() = _currentList
+        get() = differ.currentList
 
-    override fun getItem(position: Int): T = _currentList[position]
-    override fun getItemCount(): Int = _currentList.size
+    override fun getItem(position: Int): T = differ.currentList[position]
+    override fun getItemCount(): Int = differ.currentList.size
+
+    /** Submit a list normally, doing a diff synchronously. */
+    fun submitList(newList: List<T>) {
+        differ.currentList = newList
+    }
 
     /**
-     * Update the list with a [newList]. This does a basic animation that removes all items and
-     * replaces them with the new ones.
+     * Replace this list with a new list. This is useful for very large list diffs that would
+     * generally be too chaotic and slow to provide a good UX.
      */
-    @Suppress("NotifyDatasetChanged")
-    fun submitList(newList: List<T>) {
-        if (_currentList.isNotEmpty()) {
-            val oldListSize = _currentList.size
-            _currentList.clear()
-            adapter.notifyItemRangeRemoved(0, oldListSize)
+    fun replaceList(newList: List<T>) {
+        if (newList == differ.currentList) {
+            return
         }
 
-        _currentList = newList.toMutableList()
-        adapter.notifyItemRangeInserted(0, _currentList.size)
+        differ.currentList = emptyList()
+        differ.currentList = newList
     }
+}
 
-    /** Add an item to the list. */
-    fun add(item: T) {
-        _currentList.add(item)
-        adapter.notifyItemInserted(_currentList.lastIndex)
-    }
+/**
+ * Like [AsyncListDiffer], but synchronous. This may seem like it would be inefficient, but in
+ * practice Auxio's lists tend to be small enough to the point where this does not matter,
+ * and situations that would be inefficient are ruled out with [SyncBackingData.replaceList].
+ */
+private class SyncListDiffer<T>(
+    adapter: RecyclerView.Adapter<*>,
+    private val diffCallback: DiffUtil.ItemCallback<T>
+) {
+    private val updateCallback = AdapterListUpdateCallback(adapter)
 
-    /** Remove an item from the list. */
-    fun remove(item: T) {
-        val idx = _currentList.indexOf(item)
-        if (idx != -1) {
-            _currentList.removeAt(idx)
-            adapter.notifyItemRemoved(idx)
+    private var _currentList: List<T> = emptyList()
+    var currentList: List<T>
+        get() = _currentList
+        set(newList) {
+            if (newList === _currentList || newList.isEmpty() && _currentList.isEmpty()) {
+                return
+            }
+
+            if (newList.isEmpty()) {
+                val oldListSize = _currentList.size
+                _currentList = emptyList()
+                updateCallback.onRemoved(0, oldListSize)
+                return
+            }
+
+            if (_currentList.isEmpty()) {
+                _currentList = newList
+                updateCallback.onInserted(0, newList.size)
+                return
+            }
+
+            val oldList = _currentList
+            val result =
+                DiffUtil.calculateDiff(
+                    object : DiffUtil.Callback() {
+                        override fun getOldListSize(): Int {
+                            return oldList.size
+                        }
+
+                        override fun getNewListSize(): Int {
+                            return newList.size
+                        }
+
+                        override fun areItemsTheSame(
+                            oldItemPosition: Int,
+                            newItemPosition: Int
+                        ): Boolean {
+                            val oldItem: T? = oldList[oldItemPosition]
+                            val newItem: T? = newList[newItemPosition]
+                            return if (oldItem != null && newItem != null) {
+                                diffCallback.areItemsTheSame(oldItem, newItem)
+                            } else {
+                                oldItem == null && newItem == null
+                            }
+                        }
+
+                        override fun areContentsTheSame(
+                            oldItemPosition: Int,
+                            newItemPosition: Int
+                        ): Boolean {
+                            val oldItem: T? = oldList[oldItemPosition]
+                            val newItem: T? = newList[newItemPosition]
+                            return if (oldItem != null && newItem != null) {
+                                diffCallback.areContentsTheSame(oldItem, newItem)
+                            } else if (oldItem == null && newItem == null) {
+                                true
+                            } else {
+                                throw AssertionError()
+                            }
+                        }
+
+                        override fun getChangePayload(
+                            oldItemPosition: Int,
+                            newItemPosition: Int
+                        ): Any? {
+                            val oldItem: T? = oldList[oldItemPosition]
+                            val newItem: T? = newList[newItemPosition]
+                            return if (oldItem != null && newItem != null) {
+                                diffCallback.getChangePayload(oldItem, newItem)
+                            } else {
+                                throw AssertionError()
+                            }
+                        }
+                    })
+            
+            _currentList = newList
+            result.dispatchUpdatesTo(updateCallback)
         }
-    }
 }
 
 /**