remember the library tab across restarts

The selected tab lived in rememberSaveable, which survives a rotation or a
process-death restore but not a cold start, so the app always came back on
Albums. It goes through DataStore now, like the per-tab sort and filters, which
were already durable - landing on a different tab than the one you had filtered
is what made the filters look reset.

app/src/main/kotlin/com/subtracks/data/prefs/UserPreferences.kt

@@ -134,6 +134,14 @@ class UserPreferences(
 
     private fun listQueryKey(tab: LibraryListTab) = stringPreferencesKey("list_query_${tab.key}")
 
+    fun libraryTab(): Flow<String> = store.data.map { prefs -> prefs[libraryTabKey] ?: LibraryListTab.Albums.name }
+
+    suspend fun setLibraryTab(tab: String) {
+        store.edit { prefs -> prefs[libraryTabKey] = tab }
+    }
+
+    private val libraryTabKey = stringPreferencesKey("library_tab")
+
     private fun encode(query: ListQuery) =
         "${query.sort}|${if (query.descending) 1 else 0}|${query.starred.ordinal}|${if (query.downloaded) 1 else 0}"
 

app/src/main/kotlin/com/subtracks/ui/library/LibraryScreen.kt

@@ -195,9 +195,8 @@ fun LibraryRoute(
     viewModel: LibraryViewModel = koinViewModel(),
     playbackController: PlaybackController = koinInject(),
 ) {
-    var selectedTabName by rememberSaveable { mutableStateOf(LibraryTab.Albums.name) }
-    var previousTabName by rememberSaveable { mutableStateOf(LibraryTab.Albums.name) }
-    val selectedTab = LibraryTab.entries.firstOrNull { it.name == selectedTabName } ?: LibraryTab.Albums
+    val selectedTab by viewModel.selectedTab.collectAsStateWithLifecycle()
+    var previousTabName by rememberSaveable { mutableStateOf(selectedTab.name) }
     val previousTab = LibraryTab.entries.firstOrNull { it.name == previousTabName } ?: LibraryTab.Albums
     val syncing by viewModel.syncing.collectAsStateWithLifecycle()
     val listTab = selectedTab.listTab()
@@ -242,7 +241,7 @@ fun LibraryRoute(
         )
     LibraryScreen(
         selectedTab = selectedTab,
-        onTabSelected = { selectedTabName = it.name },
+        onTabSelected = { viewModel.selectTab(it) },
         syncing = syncing,
         bottomInset = bottomInset,
         artwork = artwork,

app/src/main/kotlin/com/subtracks/ui/library/LibraryViewModel.kt

@@ -135,6 +135,16 @@ class LibraryViewModel(
 
     fun listQuery(tab: LibraryListTab): StateFlow<ListQuery> = listQueries.getValue(tab)
 
+    val selectedTab: StateFlow<LibraryTab> =
+        userPreferences
+            .libraryTab()
+            .map { name -> LibraryTab.entries.firstOrNull { it.name == name } ?: LibraryTab.Albums }
+            .stateIn(viewModelScope, SharingStarted.Eagerly, LibraryTab.Albums)
+
+    fun selectTab(tab: LibraryTab) {
+        viewModelScope.launch { userPreferences.setLibraryTab(tab.name) }
+    }
+
     fun setListQuery(
         tab: LibraryListTab,
         query: ListQuery,

app/src/test/kotlin/com/subtracks/data/prefs/UserPreferencesTest.kt

@@ -45,6 +45,22 @@ class UserPreferencesTest {
             file.delete()
         }
 
+    @Test
+    fun theSelectedLibraryTabRoundTrips() =
+        runTest {
+            val file = File.createTempFile("user-prefs", ".preferences_pb").apply { delete() }
+            val store = PreferenceDataStoreFactory.create(scope = backgroundScope) { file }
+            val prefs = UserPreferences(store)
+
+            assertEquals("Albums", prefs.libraryTab().first())
+
+            prefs.setLibraryTab("Playlists")
+
+            assertEquals("Playlists", prefs.libraryTab().first())
+
+            file.delete()
+        }
+
     @Test
     fun aListQueryStoredBeforeTheDownloadedFilterStillDecodes() =
         runTest {