music: rework loading management

Rework loading management into a new object, Indexer, which should act
as the base for all future loading changes.

Indexer tries to solve a two issues with music loading:
1. The issue of what occurs when tasks must be restarted or cancelled,
which is common with music loading.
2. Trying to find a good strategy to mediate between service and
ViewModel components in a sensible manner.

Indexer also rolls in a lot of the universal music loading code
alongside this, as much of MusicStore's loading state went unused
by app components and only raised technical challenges when reworking
it.

app/src/main/java/org/oxycblt/auxio/MainFragment.kt

@@ -26,8 +26,8 @@ import androidx.fragment.app.activityViewModels
 import androidx.navigation.findNavController
 import androidx.navigation.fragment.findNavController
 import org.oxycblt.auxio.databinding.FragmentMainBinding
+import org.oxycblt.auxio.music.IndexerViewModel
 import org.oxycblt.auxio.music.Music
-import org.oxycblt.auxio.music.MusicViewModel
 import org.oxycblt.auxio.music.Song
 import org.oxycblt.auxio.playback.PlaybackViewModel
 import org.oxycblt.auxio.ui.MainNavigationAction
@@ -43,7 +43,7 @@ import org.oxycblt.auxio.util.launch
 class MainFragment : ViewBindingFragment<FragmentMainBinding>() {
     private val playbackModel: PlaybackViewModel by activityViewModels()
     private val navModel: NavigationViewModel by activityViewModels()
-    private val musicModel: MusicViewModel by activityViewModels()
+    private val musicModel: IndexerViewModel by activityViewModels()
     private var callback: DynamicBackPressedCallback? = null
 
     override fun onCreateBinding(inflater: LayoutInflater) = FragmentMainBinding.inflate(inflater)
@@ -73,7 +73,7 @@ class MainFragment : ViewBindingFragment<FragmentMainBinding>() {
         // Initialize music loading. Do it here so that it shows on every fragment that this
         // one contains.
         // TODO: Move this to a service [automatic rescanning]
-        musicModel.loadMusic(requireContext())
+        musicModel.index(requireContext())
 
         launch { navModel.mainNavigationAction.collect(::handleMainNavigation) }
         launch { navModel.exploreNavigationItem.collect(::handleExploreNavigation) }

app/src/main/java/org/oxycblt/auxio/home/HomeFragment.kt

@@ -44,9 +44,9 @@ import org.oxycblt.auxio.home.list.SongListFragment
 import org.oxycblt.auxio.music.Album
 import org.oxycblt.auxio.music.Artist
 import org.oxycblt.auxio.music.Genre
+import org.oxycblt.auxio.music.Indexer
+import org.oxycblt.auxio.music.IndexerViewModel
 import org.oxycblt.auxio.music.Music
-import org.oxycblt.auxio.music.MusicStore
-import org.oxycblt.auxio.music.MusicViewModel
 import org.oxycblt.auxio.music.Song
 import org.oxycblt.auxio.playback.PlaybackViewModel
 import org.oxycblt.auxio.ui.DisplayMode
@@ -70,7 +70,7 @@ class HomeFragment : ViewBindingFragment<FragmentHomeBinding>(), Toolbar.OnMenuI
     private val playbackModel: PlaybackViewModel by activityViewModels()
     private val navModel: NavigationViewModel by activityViewModels()
     private val homeModel: HomeViewModel by activityViewModels()
-    private val musicModel: MusicViewModel by activityViewModels()
+    private val indexerModel: IndexerViewModel by activityViewModels()
 
     private var storagePermissionLauncher: ActivityResultLauncher<String>? = null
     private var sortItem: MenuItem? = null
@@ -81,7 +81,7 @@ class HomeFragment : ViewBindingFragment<FragmentHomeBinding>(), Toolbar.OnMenuI
         // Build the permission launcher here as you can only do it in onCreateView/onCreate
         storagePermissionLauncher =
             registerForActivityResult(ActivityResultContracts.RequestPermission()) {
-                musicModel.reloadMusic(requireContext())
+                indexerModel.reindex(requireContext())
             }
 
         binding.homeToolbar.apply {
@@ -123,7 +123,7 @@ class HomeFragment : ViewBindingFragment<FragmentHomeBinding>(), Toolbar.OnMenuI
         launch { homeModel.isFastScrolling.collect(::updateFastScrolling) }
         launch { homeModel.currentTab.collect(::updateCurrentTab) }
         launch { homeModel.recreateTabs.collect(::handleRecreateTabs) }
-        launch { musicModel.loadState.collect(::handleLoadEvent) }
+        launch { indexerModel.state.collect(::handleIndexerState) }
         launch { navModel.exploreNavigationItem.collect(::handleNavigation) }
     }
 
@@ -178,8 +178,8 @@ class HomeFragment : ViewBindingFragment<FragmentHomeBinding>(), Toolbar.OnMenuI
 
         // Make sure an update here doesn't mess up the FAB state when it comes to the
         // loader response.
-        val state = musicModel.loadState.value
+        val state = indexerModel.state.value
-        if (!(state is MusicStore.LoadState.Complete && state.response is MusicStore.Response.Ok)) {
+        if (!(state is Indexer.State.Complete && state.response is Indexer.Response.Ok)) {
             return
         }
 
@@ -257,18 +257,18 @@ class HomeFragment : ViewBindingFragment<FragmentHomeBinding>(), Toolbar.OnMenuI
         }
     }
 
-    private fun handleLoadEvent(state: MusicStore.LoadState?) {
+    private fun handleIndexerState(state: Indexer.State?) {
         val binding = requireBinding()
 
-        if (state is MusicStore.LoadState.Complete) {
+        if (state is Indexer.State.Complete) {
             handleLoaderResponse(binding, state.response)
         } else {
-            handleLoadEvent(binding, state)
+            handleLoadingState(binding, state)
         }
     }
 
-    private fun handleLoaderResponse(binding: FragmentHomeBinding, response: MusicStore.Response) {
+    private fun handleLoaderResponse(binding: FragmentHomeBinding, response: Indexer.Response) {
-        if (response is MusicStore.Response.Ok) {
+        if (response is Indexer.Response.Ok) {
             binding.homeFab.show()
             binding.homeLoadingContainer.visibility = View.INVISIBLE
             binding.homePager.visibility = View.VISIBLE
@@ -280,30 +280,29 @@ class HomeFragment : ViewBindingFragment<FragmentHomeBinding>(), Toolbar.OnMenuI
             logD("Received non-ok response $response")
 
             when (response) {
-                is MusicStore.Response.Ok -> error("Unreachable")
+                is Indexer.Response.Ok -> error("Unreachable")
-                is MusicStore.Response.Err -> {
+                is Indexer.Response.Err -> {
-                    logD("Received Response.Err")
                     binding.homeLoadingProgress.visibility = View.INVISIBLE
                     binding.homeLoadingStatus.textSafe = getString(R.string.err_load_failed)
                     binding.homeLoadingAction.apply {
                         visibility = View.VISIBLE
                         text = getString(R.string.lbl_retry)
-                        setOnClickListener { musicModel.reloadMusic(requireContext()) }
+                        setOnClickListener { indexerModel.reindex(requireContext()) }
                     }
                 }
-                is MusicStore.Response.NoMusic -> {
+                is Indexer.Response.NoMusic -> {
                     binding.homeLoadingProgress.visibility = View.INVISIBLE
                     binding.homeLoadingStatus.textSafe = getString(R.string.err_no_music)
                     binding.homeLoadingAction.apply {
                         visibility = View.VISIBLE
                         text = getString(R.string.lbl_retry)
-                        setOnClickListener { musicModel.reloadMusic(requireContext()) }
+                        setOnClickListener { indexerModel.reindex(requireContext()) }
                     }
                 }
-                is MusicStore.Response.NoPerms -> {
+                is Indexer.Response.NoPerms -> {
                     val launcher =
                         requireNotNull(storagePermissionLauncher) {
-                            "Cannot access permission launcher while in non-view state"
+                            "Cannot access permission launcher while detached"
                         }
 
                     binding.homeLoadingProgress.visibility = View.INVISIBLE
@@ -320,14 +319,14 @@ class HomeFragment : ViewBindingFragment<FragmentHomeBinding>(), Toolbar.OnMenuI
         }
     }
 
-    private fun handleLoadEvent(binding: FragmentHomeBinding, event: MusicStore.LoadState?) {
+    private fun handleLoadingState(binding: FragmentHomeBinding, event: Indexer.State?) {
         binding.homeFab.hide()
         binding.homePager.visibility = View.INVISIBLE
         binding.homeLoadingContainer.visibility = View.VISIBLE
         binding.homeLoadingProgress.visibility = View.VISIBLE
         binding.homeLoadingAction.visibility = View.INVISIBLE
 
-        if (event is MusicStore.LoadState.Indexing) {
+        if (event is Indexer.State.Loading) {
             binding.homeLoadingStatus.textSafe =
                 getString(R.string.fmt_indexing, event.current, event.total)
             binding.homeLoadingProgress.apply {

app/src/main/java/org/oxycblt/auxio/home/HomeViewModel.kt

@@ -134,9 +134,8 @@ class HomeViewModel : ViewModel(), SettingsManager.Callback, MusicStore.Callback
 
     // --- OVERRIDES ---
 
-    override fun onMusicUpdate(response: MusicStore.Response) {
+    override fun onLibraryChanged(library: MusicStore.Library?) {
-        if (response is MusicStore.Response.Ok) {
+        if (library != null) {
-            val library = response.library
             _songs.value = settingsManager.libSongSort.songs(library.songs)
             _albums.value = settingsManager.libAlbumSort.albums(library.albums)
             _artists.value = settingsManager.libArtistSort.artists(library.artists)

app/src/main/java/org/oxycblt/auxio/music/Indexer.kt

@@ -15,18 +15,23 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
  
-package org.oxycblt.auxio.music.indexer
+package org.oxycblt.auxio.music
 
+import android.Manifest
 import android.content.Context
+import android.content.pm.PackageManager
 import android.database.Cursor
 import android.os.Build
-import org.oxycblt.auxio.music.Album
+import androidx.core.content.ContextCompat
-import org.oxycblt.auxio.music.Artist
+import kotlinx.coroutines.Dispatchers
-import org.oxycblt.auxio.music.Genre
+import kotlinx.coroutines.delay
-import org.oxycblt.auxio.music.MusicStore
+import kotlinx.coroutines.withContext
-import org.oxycblt.auxio.music.Song
+import org.oxycblt.auxio.music.backend.Api21MediaStoreBackend
+import org.oxycblt.auxio.music.backend.Api30MediaStoreBackend
+import org.oxycblt.auxio.music.backend.ExoPlayerBackend
 import org.oxycblt.auxio.ui.Sort
 import org.oxycblt.auxio.util.logD
+import org.oxycblt.auxio.util.logE
 
 /**
  * Auxio's media indexer.
@@ -40,13 +45,93 @@ import org.oxycblt.auxio.util.logD
  * 3. Using the songs to build the library, which primarily involves linking up all data objects
  * with their corresponding parents/children.
  *
- * This class in particular handles 3 primarily. For the code that handles 1 and 2, see the other
+ * This class in particular handles 3 primarily. For the code that handles 1 and 2, see the
- * files in the module.
+ * [Backend] implementations.
+ *
+ * This class also fulfills the role of maintaining the current music loading state, which seems
+ * like a job for [MusicStore] but in practice is only really leveraged by the components that
+ * directly work with music loading, making such redundant.
  *
  * @author OxygenCobalt
  */
-object Indexer {
+class Indexer {
-    fun index(context: Context, callback: MusicStore.LoadCallback): MusicStore.Library? {
+    private var state: State? = null
+    private var currentGeneration: Long = 0
+    private val callbacks = mutableListOf<Callback>()
+
+    fun addCallback(callback: Callback) {
+        callback.onIndexerStateChanged(state)
+        callbacks.add(callback)
+    }
+
+    fun removeCallback(callback: Callback) {
+        callbacks.remove(callback)
+    }
+
+    suspend fun index(context: Context) {
+        val generation = synchronized(this) { ++currentGeneration }
+
+        val notGranted =
+            ContextCompat.checkSelfPermission(context, Manifest.permission.READ_EXTERNAL_STORAGE) ==
+                PackageManager.PERMISSION_DENIED
+
+        if (notGranted) {
+            emitState(State.Complete(Response.NoPerms), generation)
+        }
+
+        val response =
+            try {
+                val start = System.currentTimeMillis()
+                val library = withContext(Dispatchers.IO) { indexImpl(context, generation) }
+                if (library != null) {
+                    logD(
+                        "Music load completed successfully in " +
+                            "${System.currentTimeMillis() - start}ms")
+                    Response.Ok(library)
+                } else {
+                    logE("No music found")
+                    Response.NoMusic
+                }
+            } catch (e: Exception) {
+                logE("Music loading failed.")
+                logE(e.stackTraceToString())
+                Response.Err(e)
+            }
+
+        emitState(State.Complete(response), generation)
+    }
+
+    /**
+     * "Cancel" the last job by making it unable to send further state updates. This should be
+     * called if an object that called [index] is about to be destroyed and thus will have it's task
+     * canceled, in which it would be useful for any ongoing loading process to not accidentally
+     * corrupt the current state.
+     */
+    fun cancelLast() {
+        synchronized(this) {
+            currentGeneration++
+            emitState(null, currentGeneration)
+        }
+    }
+
+    private fun emitState(newState: State?, generation: Long) {
+        synchronized(this) {
+            if (currentGeneration == generation) {
+                state = newState
+                for (callback in callbacks) {
+                    callback.onIndexerStateChanged(newState)
+                }
+            }
+        }
+    }
+
+    /**
+     * Run the proper music loading process. [generation] must be a truthful value of the generation
+     * calling this function.
+     */
+    private fun indexImpl(context: Context, generation: Long): MusicStore.Library? {
+        emitState(State.Query, generation)
+
         // Establish the backend to use when initially loading songs.
         val mediaStoreBackend =
             when {
@@ -54,7 +139,7 @@ object Indexer {
                 else -> Api21MediaStoreBackend()
             }
 
-        val songs = buildSongs(context, ExoPlayerBackend(mediaStoreBackend), callback)
+        val songs = buildSongs(context, ExoPlayerBackend(mediaStoreBackend), generation)
         if (songs.isEmpty()) {
             return null
         }
@@ -88,11 +173,7 @@ object Indexer {
      * [buildGenres] functions must be called with the returned list so that all songs are properly
      * linked up.
      */
-    private fun buildSongs(
+    private fun buildSongs(context: Context, backend: Backend, generation: Long): List<Song> {
-        context: Context,
-        backend: Backend,
-        callback: MusicStore.LoadCallback
-    ): List<Song> {
         val start = System.currentTimeMillis()
 
         var songs =
@@ -101,7 +182,9 @@ object Indexer {
                     "Successfully queried media database " +
                         "in ${System.currentTimeMillis() - start}ms")
 
-                backend.loadSongs(context, cursor, callback)
+                backend.loadSongs(context, cursor) { count, total ->
+                    emitState(State.Loading(count, total), generation)
+                }
             }
 
         // Deduplicate songs to prevent (most) deformed music clones
@@ -213,6 +296,25 @@ object Indexer {
         return genres
     }
 
+    /** Represents the current indexer state. */
+    sealed class State {
+        object Query : State()
+        data class Loading(val current: Int, val total: Int) : State()
+        data class Complete(val response: Response) : State()
+    }
+
+    /** Represents the possible outcomes of a loading process. */
+    sealed class Response {
+        data class Ok(val library: MusicStore.Library) : Response()
+        data class Err(val throwable: Throwable) : Response()
+        object NoMusic : Response()
+        object NoPerms : Response()
+    }
+
+    interface Callback {
+        fun onIndexerStateChanged(state: State?)
+    }
+
     /** Represents a backend that metadata can be extracted from. */
     interface Backend {
         /** Query the media database for a basic cursor. */
@@ -222,7 +324,26 @@ object Indexer {
         fun loadSongs(
             context: Context,
             cursor: Cursor,
-            callback: MusicStore.LoadCallback
+            onAddSong: (count: Int, total: Int) -> Unit
         ): Collection<Song>
     }
+
+    companion object {
+        @Volatile private var INSTANCE: Indexer? = null
+
+        /** Get the process-level instance of [Indexer]. */
+        fun getInstance(): Indexer {
+            val currentInstance = INSTANCE
+
+            if (currentInstance != null) {
+                return currentInstance
+            }
+
+            synchronized(this) {
+                val newInstance = Indexer()
+                INSTANCE = newInstance
+                return newInstance
+            }
+        }
+    }
 }

app/src/main/java/org/oxycblt/auxio/music/IndexerViewModel.kt

@@ -25,45 +25,52 @@ import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.launch
 import org.oxycblt.auxio.util.logD
 
-/** A [ViewModel] that represents the current music indexing state. */
+/** A ViewModel representing the current music indexing state. */
-class MusicViewModel : ViewModel(), MusicStore.LoadCallback {
+class IndexerViewModel : ViewModel(), Indexer.Callback {
+    private val indexer = Indexer.getInstance()
     private val musicStore = MusicStore.getInstance()
 
-    private val _loadState = MutableStateFlow<MusicStore.LoadState?>(null)
+    private val _state = MutableStateFlow<Indexer.State?>(null)
-    val loadState: StateFlow<MusicStore.LoadState?> = _loadState
+    val state: StateFlow<Indexer.State?> = _state
 
-    private var isBusy = false
+    init {
+        indexer.addCallback(this)
+    }
 
-    /**
+    /** Initiate the indexing process. */
-     * Initiate the loading process. This is done here since HomeFragment will be the first fragment
+    fun index(context: Context) {
-     * navigated to and because SnackBars will have the best UX here.
+        if (state.value != null) {
-     */
+            logD("Loader is already loading/is completed, not reloading")
-    fun loadMusic(context: Context) {
-        if (_loadState.value != null || isBusy) {
-            logD("Loader is busy/already completed, not reloading")
             return
         }
 
-        isBusy = true
+        indexImpl(context)
-        _loadState.value = null
-
-        viewModelScope.launch {
-            musicStore.load(context, this@MusicViewModel)
-            isBusy = false
-        }
     }
 
     /**
      * Reload the music library. Note that this call will result in unexpected behavior in the case
      * that music is reloaded after a loading process has already exceeded.
      */
-    fun reloadMusic(context: Context) {
+    fun reindex(context: Context) {
         logD("Reloading music library")
-        _loadState.value = null
+        indexImpl(context)
-        loadMusic(context)
     }
 
-    override fun onLoadStateChanged(state: MusicStore.LoadState?) {
+    private fun indexImpl(context: Context) {
-        _loadState.value = state
+        viewModelScope.launch { indexer.index(context) }
+    }
+
+    override fun onIndexerStateChanged(state: Indexer.State?) {
+        _state.value = state
+
+        if (state is Indexer.State.Complete && state.response is Indexer.Response.Ok) {
+            musicStore.library = state.response.library
+        }
+    }
+
+    override fun onCleared() {
+        super.onCleared()
+        indexer.cancelLast()
+        indexer.removeCallback(this)
     }
 }

app/src/main/java/org/oxycblt/auxio/music/Music.kt

@@ -23,8 +23,8 @@ import android.content.Context
 import android.net.Uri
 import android.provider.MediaStore
 import org.oxycblt.auxio.R
-import org.oxycblt.auxio.music.indexer.id3v2GenreName
+import org.oxycblt.auxio.music.backend.id3v2GenreName
-import org.oxycblt.auxio.music.indexer.withoutArticle
+import org.oxycblt.auxio.music.backend.withoutArticle
 import org.oxycblt.auxio.ui.Item
 import org.oxycblt.auxio.util.unlikelyToBeNull
 

app/src/main/java/org/oxycblt/auxio/music/MusicStore.kt

@@ -17,20 +17,11 @@
  
 package org.oxycblt.auxio.music
 
-import android.Manifest
 import android.content.Context
-import android.content.pm.PackageManager
 import android.net.Uri
 import android.provider.OpenableColumns
-import androidx.core.content.ContextCompat
+import org.oxycblt.auxio.music.backend.useQuery
-import java.lang.Exception
-import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.withContext
-import org.oxycblt.auxio.music.indexer.Indexer
-import org.oxycblt.auxio.music.indexer.useQuery
 import org.oxycblt.auxio.util.contentResolverSafe
-import org.oxycblt.auxio.util.logD
-import org.oxycblt.auxio.util.logE
 
 /**
  * The main storage for music items. Getting an instance of this object is more complicated as it
@@ -40,22 +31,19 @@ import org.oxycblt.auxio.util.logE
  * @author OxygenCobalt
  */
 class MusicStore private constructor() {
-    private var response: Response? = null
-    val library: Library?
-        get() =
-            response?.let { currentResponse ->
-                if (currentResponse is Response.Ok) {
-                    currentResponse.library
-                } else {
-                    null
-                }
-            }
-
     private val callbacks = mutableListOf<Callback>()
 
+    var library: Library? = null
+        set(value) {
+            field = value
+            for (callback in callbacks) {
+                callback.onLibraryChanged(library)
+            }
+        }
+
     /** Add a callback to this instance. Make sure to remove it when done. */
     fun addCallback(callback: Callback) {
-        response?.let(callback::onMusicUpdate)
+        callback.onLibraryChanged(library)
         callbacks.add(callback)
     }
 
@@ -64,53 +52,7 @@ class MusicStore private constructor() {
         callbacks.remove(callback)
     }
 
-    /** Load/Sort the entire music library. Should always be ran on a coroutine. */
+    /** Represents a library of music owned by [MusicStore]. */
-    suspend fun load(context: Context, callback: LoadCallback): Response {
-        logD("Starting initial music load")
-
-        callback.onLoadStateChanged(null)
-        val newResponse =
-            withContext(Dispatchers.IO) { loadImpl(context, callback) }.also { response = it }
-
-        callback.onLoadStateChanged(LoadState.Complete(newResponse))
-        for (responseCallbacks in callbacks) {
-            responseCallbacks.onMusicUpdate(newResponse)
-        }
-
-        return newResponse
-    }
-
-    private fun loadImpl(context: Context, callback: LoadCallback): Response {
-        val notGranted =
-            ContextCompat.checkSelfPermission(context, Manifest.permission.READ_EXTERNAL_STORAGE) ==
-                PackageManager.PERMISSION_DENIED
-
-        if (notGranted) {
-            return Response.NoPerms
-        }
-
-        val response =
-            try {
-                val start = System.currentTimeMillis()
-                val library = Indexer.index(context, callback)
-                if (library != null) {
-                    logD(
-                        "Music load completed successfully in " +
-                            "${System.currentTimeMillis() - start}ms")
-                    Response.Ok(library)
-                } else {
-                    logE("No music found")
-                    Response.NoMusic
-                }
-            } catch (e: Exception) {
-                logE("Music loading failed.")
-                logE(e.stackTraceToString())
-                Response.Err(e)
-            }
-
-        return response
-    }
-
     data class Library(
         val genres: List<Genre>,
         val artists: List<Artist>,
@@ -138,35 +80,9 @@ class MusicStore private constructor() {
             }
     }
 
-    /** Represents the current state of the loading process. */
-    sealed class LoadState {
-        data class Indexing(val current: Int, val total: Int) : LoadState()
-        data class Complete(val response: Response) : LoadState()
-    }
-
-    /**
-     * A callback for events that occur during the loading process. This is used by [load] in order
-     * to have a separate callback interface that is more efficient for rapid-fire updates.
-     */
-    interface LoadCallback {
-        /**
-         * Called when the state of the loading process changes. A value of null represents the
-         * beginning of a loading process.
-         */
-        fun onLoadStateChanged(state: LoadState?)
-    }
-
-    /** Represents the possible outcomes of a loading process. */
-    sealed class Response {
-        data class Ok(val library: Library) : Response()
-        data class Err(val throwable: Throwable) : Response()
-        object NoMusic : Response()
-        object NoPerms : Response()
-    }
-
     /** A callback for awaiting the loading of music. */
     interface Callback {
-        fun onMusicUpdate(response: Response)
+        fun onLibraryChanged(library: Library?)
     }
 
     companion object {

app/src/main/java/org/oxycblt/auxio/music/backend/ExoPlayerBackend.kt

@@ -15,7 +15,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
  
-package org.oxycblt.auxio.music.indexer
+package org.oxycblt.auxio.music.backend
 
 import android.content.Context
 import android.database.Cursor
@@ -31,12 +31,12 @@ import java.util.concurrent.ConcurrentLinkedQueue
 import java.util.concurrent.Future
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.asExecutor
-import org.oxycblt.auxio.music.MusicStore
+import org.oxycblt.auxio.music.Indexer
 import org.oxycblt.auxio.music.Song
 import org.oxycblt.auxio.util.logW
 
 /**
- * A [Indexer.Backend] that leverages ExoPlayer's metadata retrieval system to index metadata.
+ * A [OldIndexer.Backend] that leverages ExoPlayer's metadata retrieval system to index metadata.
  *
  * Normally, leveraging ExoPlayer's metadata system would be a terrible idea, as it is horrifically
  * slow. However, if we parallelize it, we can get similar throughput to other metadata extractors,
@@ -62,7 +62,7 @@ class ExoPlayerBackend(private val inner: MediaStoreBackend) : Indexer.Backend {
     override fun loadSongs(
         context: Context,
         cursor: Cursor,
-        callback: MusicStore.LoadCallback
+        onAddSong: (count: Int, total: Int) -> Unit
     ): Collection<Song> {
         // Metadata retrieval with ExoPlayer is asynchronous, so a callback may at any point
         // add a completed song to the list. To prevent a crash in that case, we use the
@@ -90,8 +90,7 @@ class ExoPlayerBackend(private val inner: MediaStoreBackend) : Indexer.Backend {
                         AudioCallback(audio) {
                             runningTasks[index] = null
                             songs.add(it)
-                            callback.onLoadStateChanged(
+                            onAddSong(songs.size, cursor.count)
-                                MusicStore.LoadState.Indexing(songs.size, cursor.count))
                         },
                         // Normal JVM dispatcher will suffice here, as there is no IO work
                         // going on (and there is no cost from switching contexts with executors)

app/src/main/java/org/oxycblt/auxio/music/backend/IndexerUtil.kt

@@ -15,7 +15,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
  
-package org.oxycblt.auxio.music.indexer
+package org.oxycblt.auxio.music.backend
 
 import android.content.ContentResolver
 import android.content.ContentUris
@@ -41,11 +41,19 @@ fun <R> ContentResolver.useQuery(
     block: (Cursor) -> R
 ): R? = queryCursor(uri, projection, selector, args)?.use(block)
 
+/**
+ * For some reason the album art URI namespace does not have a member in [MediaStore], but it still
+ * works since at least API 21.
+ */
+private val EXTERNAL_ALBUM_ART_URI = Uri.parse("content://media/external/audio/albumart")
+
 /** Converts a [Long] Audio ID into a URI to that particular audio file. */
 val Long.audioUri: Uri
-    get() =
+    get() = ContentUris.withAppendedId(MediaStore.Audio.Media.EXTERNAL_CONTENT_URI, this)
-        ContentUris.withAppendedId(
+
-            MediaStore.Audio.Media.EXTERNAL_CONTENT_URI, requireNotNull(this))
+/** Converts a [Long] Album ID into a URI pointing to MediaStore-cached album art. */
+val Long.albumCoverUri: Uri
+    get() = ContentUris.withAppendedId(EXTERNAL_ALBUM_ART_URI, this)
 
 /**
  * Parse out the number field from an NN/TT string that is typically found in DISC_NUMBER and
@@ -105,7 +113,8 @@ val String.id3v2GenreName: String
 
             return substring(1 until lastIndex).toIntOrNull()?.run {
                 genreConstantTable.getOrNull(this)
-            } ?: this
+            }
+                ?: this
         }
 
         // Current name is fine.

app/src/main/java/org/oxycblt/auxio/music/backend/MediaStoreBackend.kt

@@ -15,18 +15,16 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
  
-package org.oxycblt.auxio.music.indexer
+package org.oxycblt.auxio.music.backend
 
-import android.content.ContentUris
 import android.content.Context
 import android.database.Cursor
-import android.net.Uri
 import android.os.Build
 import android.provider.MediaStore
 import androidx.annotation.RequiresApi
 import androidx.core.database.getIntOrNull
 import androidx.core.database.getStringOrNull
-import org.oxycblt.auxio.music.MusicStore
+import org.oxycblt.auxio.music.Indexer
 import org.oxycblt.auxio.music.Song
 import org.oxycblt.auxio.music.excluded.ExcludedDatabase
 import org.oxycblt.auxio.util.contentResolverSafe
@@ -90,8 +88,9 @@ import org.oxycblt.auxio.util.contentResolverSafe
  */
 
 /**
- * Represents a [Indexer.Backend] that loads music from the media database ([MediaStore]). This is
+ * Represents a [OldIndexer.Backend] that loads music from the media database ([MediaStore]). This
- * not a fully-featured class by itself, and it's API-specific derivatives should be used instead.
+ * is not a fully-featured class by itself, and it's API-specific derivatives should be used
+ * instead.
  * @author OxygenCobalt
  */
 abstract class MediaStoreBackend : Indexer.Backend {
@@ -131,7 +130,7 @@ abstract class MediaStoreBackend : Indexer.Backend {
     override fun loadSongs(
         context: Context,
         cursor: Cursor,
-        callback: MusicStore.LoadCallback
+        onAddSong: (count: Int, total: Int) -> Unit
     ): Collection<Song> {
         // Note: We do not actually update the callback with an Indexing state, this is because
         // loading music from MediaStore tends to be quite fast, with the only bottlenecks being
@@ -280,9 +279,7 @@ abstract class MediaStoreBackend : Indexer.Backend {
                 _year = year,
                 _albumName = requireNotNull(album) { "Malformed audio: No album name" },
                 _albumCoverUri =
-                    ContentUris.withAppendedId(
+                    requireNotNull(albumId) { "Malformed audio: No album id" }.albumCoverUri,
-                        EXTERNAL_ALBUM_ART_URI,
-                        requireNotNull(albumId) { "Malformed audio: No album id" }),
                 _artistName = artist,
                 _albumArtistName = albumArtist,
                 _genreName = genre)
@@ -304,12 +301,6 @@ abstract class MediaStoreBackend : Indexer.Backend {
          */
         @Suppress("InlinedApi") private const val VOLUME_EXTERNAL = MediaStore.VOLUME_EXTERNAL
 
-        /**
-         * For some reason the album art URI namespace does not have a member in [MediaStore], but
-         * it still works since at least API 21.
-         */
-        private val EXTERNAL_ALBUM_ART_URI = Uri.parse("content://media/external/audio/albumart")
-
         /**
          * The basic projection that works across all versions of android. Is incomplete, hence why
          * sub-implementations should be used instead.

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

@@ -318,11 +318,11 @@ class PlaybackViewModel : ViewModel(), PlaybackStateManager.Callback, MusicStore
         _repeatMode.value = repeatMode
     }
 
-    override fun onMusicUpdate(response: MusicStore.Response) {
+    override fun onLibraryChanged(library: MusicStore.Library?) {
-        if (response is MusicStore.Response.Ok) {
+        if (library != null) {
             val action = pendingDelayedAction
             if (action != null) {
-                performActionImpl(action, response.library)
+                performActionImpl(action, library)
                 pendingDelayedAction = null
             }
         }

app/src/main/java/org/oxycblt/auxio/search/SearchFragment.kt

@@ -35,10 +35,10 @@ import org.oxycblt.auxio.databinding.FragmentSearchBinding
 import org.oxycblt.auxio.music.Album
 import org.oxycblt.auxio.music.Artist
 import org.oxycblt.auxio.music.Genre
+import org.oxycblt.auxio.music.Indexer
+import org.oxycblt.auxio.music.IndexerViewModel
 import org.oxycblt.auxio.music.Music
 import org.oxycblt.auxio.music.MusicParent
-import org.oxycblt.auxio.music.MusicStore
-import org.oxycblt.auxio.music.MusicViewModel
 import org.oxycblt.auxio.music.Song
 import org.oxycblt.auxio.playback.PlaybackViewModel
 import org.oxycblt.auxio.ui.Header
@@ -64,7 +64,7 @@ class SearchFragment :
     private val searchModel: SearchViewModel by viewModels()
     private val playbackModel: PlaybackViewModel by activityViewModels()
     private val navModel: NavigationViewModel by activityViewModels()
-    private val musicModel: MusicViewModel by activityViewModels()
+    private val indexerModel: IndexerViewModel by activityViewModels()
 
     private val searchAdapter = SearchAdapter(this)
     private var imm: InputMethodManager? = null
@@ -109,7 +109,7 @@ class SearchFragment :
         // --- VIEWMODEL SETUP ---
 
         launch { searchModel.searchResults.collect(::updateResults) }
-        launch { musicModel.loadState.collect(::handleLoadState) }
+        launch { indexerModel.state.collect(::handleIndexerState) }
         launch { navModel.exploreNavigationItem.collect(::handleNavigation) }
     }
 
@@ -171,8 +171,8 @@ class SearchFragment :
         requireImm().hide()
     }
 
-    private fun handleLoadState(state: MusicStore.LoadState?) {
+    private fun handleIndexerState(state: Indexer.State?) {
-        if (state is MusicStore.LoadState.Complete && state.response is MusicStore.Response.Ok) {
+        if (state is Indexer.State.Complete && state.response is Indexer.Response.Ok) {
             searchModel.refresh(requireContext())
         }
     }