feat(android): cached-indicator dot on track rows (#38 slice 2)

Ports Flutter's CachedIndicator (circle_check_big, size 14, accent, 4px
left pad) to all 5 track-row screens (Album, History, Liked, Playlist,
Search). CachedTrackIds exposes a reactive Set of trackIds with bytes in
Media3's SimpleCache — true on-disk residency (keyed per track via the
custom cache key from slice 1), so an evicted track loses its dot. Read
through a LocalCachedTrackIds CompositionLocal provided at the app root,
mirroring the existing LocalDetailSeedCache pattern, so leaf rows need no
per-screen plumbing. The persisted cache is read eagerly, so dots paint
for previously-cached tracks even when the library is opened offline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/MainActivity.kt

@@ -18,9 +18,11 @@ import androidx.hilt.navigation.compose.hiltViewModel
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import androidx.navigation.compose.rememberNavController
 import com.fabledsword.minstrel.auth.ui.AuthGateViewModel
+import com.fabledsword.minstrel.cache.CachedTrackIds
 import com.fabledsword.minstrel.nav.DetailSeedCache
 import com.fabledsword.minstrel.nav.LocalDetailSeedCache
 import com.fabledsword.minstrel.nav.MinstrelNavGraph
+import com.fabledsword.minstrel.shared.widgets.LocalCachedTrackIds
 import com.fabledsword.minstrel.theme.MinstrelTheme
 import com.fabledsword.minstrel.theme.ThemePreferenceViewModel
 import dagger.hilt.android.AndroidEntryPoint
@@ -29,23 +31,29 @@ import javax.inject.Inject
 @AndroidEntryPoint
 class MainActivity : ComponentActivity() {
     @Inject lateinit var seedCache: DetailSeedCache
+    @Inject lateinit var cachedTrackIds: CachedTrackIds
 
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         enableEdgeToEdge()
-        setContent { App(seedCache = seedCache) }
+        setContent { App(seedCache = seedCache, cachedTrackIds = cachedTrackIds) }
     }
 }
 
 @Composable
 private fun App(
     seedCache: DetailSeedCache,
+    cachedTrackIds: CachedTrackIds,
     themeVm: ThemePreferenceViewModel = hiltViewModel(),
     gate: AuthGateViewModel = hiltViewModel(),
 ) {
     val theme by themeVm.themeMode.collectAsStateWithLifecycle()
+    val cached by cachedTrackIds.ids.collectAsStateWithLifecycle()
     MinstrelTheme(darkOverride = theme.toDarkOverride()) {
-        CompositionLocalProvider(LocalDetailSeedCache provides seedCache) {
+        CompositionLocalProvider(
+            LocalDetailSeedCache provides seedCache,
+            LocalCachedTrackIds provides cached,
+        ) {
             val startDestination by gate.startDestination.collectAsStateWithLifecycle()
             val resolved = startDestination
             if (resolved == null) {

android/app/src/main/java/com/fabledsword/minstrel/cache/CachedTrackIds.kt

@@ -0,0 +1,60 @@
+package com.fabledsword.minstrel.cache
+
+import com.fabledsword.minstrel.di.ApplicationScope
+import com.fabledsword.minstrel.player.PlayerController
+import com.fabledsword.minstrel.player.PlayerFactory
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.SharingStarted
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.distinctUntilChanged
+import kotlinx.coroutines.flow.flowOn
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.stateIn
+import kotlinx.coroutines.launch
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * Reactive set of track ids that have audio bytes in the Media3
+ * [SimpleCache][PlayerFactory.simpleCache], used to drive the cached
+ * indicator dot on track rows.
+ *
+ * The SimpleCache (keyed per track via the MediaItem custom cache key)
+ * is the source of truth for true on-disk residency — querying it
+ * directly, rather than trusting an index row, means an evicted track
+ * loses its dot. The key set is read on the IO dispatcher (it scans
+ * cache metadata) and re-read on every track change, since playback is
+ * what grows the incidental cache.
+ *
+ * The persisted SimpleCache survives process death, so the eager
+ * initial read paints dots for previously-cached tracks even when the
+ * library is opened offline with no active playback.
+ */
+@Singleton
+class CachedTrackIds @Inject constructor(
+    playerController: PlayerController,
+    private val playerFactory: PlayerFactory,
+    @ApplicationScope scope: CoroutineScope,
+) {
+    private val refresh = MutableStateFlow(0L)
+
+    val ids: StateFlow<Set<String>> =
+        refresh
+            .map { readKeys() }
+            .flowOn(Dispatchers.IO)
+            .stateIn(scope, SharingStarted.Eagerly, emptySet())
+
+    init {
+        scope.launch {
+            playerController.uiState
+                .map { it.currentTrack?.id }
+                .distinctUntilChanged()
+                .collect { refresh.value += 1 }
+        }
+    }
+
+    private fun readKeys(): Set<String> =
+        runCatching { playerFactory.simpleCache.keys.toSet() }.getOrDefault(emptySet())
+}

android/app/src/main/java/com/fabledsword/minstrel/history/ui/HistoryTab.kt

@@ -27,6 +27,7 @@ import com.fabledsword.minstrel.history.data.HistoryEntry
 import com.fabledsword.minstrel.history.data.HistoryRepository
 import com.fabledsword.minstrel.models.TrackRef
 import com.fabledsword.minstrel.player.PlayerController
+import com.fabledsword.minstrel.shared.widgets.CachedDot
 import com.fabledsword.minstrel.shared.widgets.EmptyState
 import com.fabledsword.minstrel.shared.widgets.LoadingCentered
 import com.fabledsword.minstrel.shared.widgets.PullToRefreshScaffold
@@ -200,6 +201,7 @@ private fun HistoryRow(
                 overflow = TextOverflow.Ellipsis,
             )
         }
+        CachedDot(entry.track.id)
         Text(
             text = relativeTime(entry.playedAt),
             style = MaterialTheme.typography.bodySmall,

android/app/src/main/java/com/fabledsword/minstrel/library/ui/AlbumDetailScreen.kt

@@ -56,6 +56,7 @@ import com.fabledsword.minstrel.models.AlbumRef
 import com.fabledsword.minstrel.models.TrackRef
 import com.fabledsword.minstrel.nav.AlbumDetail
 import com.fabledsword.minstrel.nav.ArtistDetail
+import com.fabledsword.minstrel.shared.widgets.CachedDot
 import com.fabledsword.minstrel.shared.widgets.EmptyState
 import com.fabledsword.minstrel.shared.widgets.LikeButton
 import com.fabledsword.minstrel.shared.widgets.PullToRefreshScaffold
@@ -349,6 +350,7 @@ private fun TrackRow(
                 )
             }
         }
+        CachedDot(track.id)
         Text(
             text = formatDuration(track.durationSec),
             style = MaterialTheme.typography.bodySmall,

android/app/src/main/java/com/fabledsword/minstrel/likes/ui/LikedTab.kt

@@ -37,6 +37,7 @@ import com.fabledsword.minstrel.models.TrackRef
 import com.fabledsword.minstrel.nav.AlbumDetail
 import com.fabledsword.minstrel.nav.ArtistDetail
 import com.fabledsword.minstrel.player.PlayerController
+import com.fabledsword.minstrel.shared.widgets.CachedDot
 import com.fabledsword.minstrel.shared.widgets.EmptyState
 import com.fabledsword.minstrel.shared.widgets.HorizontalScrollRow
 import com.fabledsword.minstrel.shared.widgets.LikeButton
@@ -288,6 +289,7 @@ private fun LikedTrackRow(
                 overflow = TextOverflow.Ellipsis,
             )
         }
+        CachedDot(track.id)
         LikeButton(liked = true, onToggle = onUnlike)
         TrackActionsButton(
             track = track,

android/app/src/main/java/com/fabledsword/minstrel/playlists/ui/PlaylistDetailScreen.kt

@@ -72,6 +72,7 @@ import com.fabledsword.minstrel.player.PlayerController
 import com.fabledsword.minstrel.likes.data.LikesRepository
 import com.fabledsword.minstrel.playlists.data.PlaylistDetailRef
 import com.fabledsword.minstrel.playlists.data.PlaylistsRepository
+import com.fabledsword.minstrel.shared.widgets.CachedDot
 import com.fabledsword.minstrel.shared.widgets.LikeButton
 import com.fabledsword.minstrel.shared.widgets.PullToRefreshScaffold
 import com.fabledsword.minstrel.shared.widgets.SkeletonTrackRow
@@ -528,6 +529,7 @@ private fun TrackRow(
                 overflow = TextOverflow.Ellipsis,
             )
         }
+        row.trackId?.let { CachedDot(it) }
         Text(
             text = formatDuration(row.durationSec),
             style = MaterialTheme.typography.bodySmall,

android/app/src/main/java/com/fabledsword/minstrel/search/ui/SearchScreen.kt

@@ -59,6 +59,7 @@ import com.fabledsword.minstrel.models.TrackRef
 import com.fabledsword.minstrel.nav.AlbumDetail
 import com.fabledsword.minstrel.nav.ArtistDetail
 import com.fabledsword.minstrel.nav.Search as SearchRoute
+import com.fabledsword.minstrel.shared.widgets.CachedDot
 import com.fabledsword.minstrel.shared.widgets.LoadingCentered
 import com.fabledsword.minstrel.shared.widgets.MainAppBarActions
 import com.fabledsword.minstrel.shared.widgets.TrackCoverThumb
@@ -311,6 +312,7 @@ private fun TrackResultRow(
                 )
             }
         }
+        CachedDot(track.id)
         TrackActionsButton(
             track = track,
             onNavigateToAlbum = onNavigateToAlbum,

android/app/src/main/java/com/fabledsword/minstrel/shared/widgets/CachedDot.kt

@@ -0,0 +1,40 @@
+package com.fabledsword.minstrel.shared.widgets
+
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.material3.Icon
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.staticCompositionLocalOf
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import com.composables.icons.lucide.Circle_check_big
+import com.composables.icons.lucide.Lucide
+import com.fabledsword.minstrel.theme.LocalFabledSwordTheme
+
+/**
+ * Track ids with audio cached locally, provided once at the app root
+ * from `CachedTrackIds`. Defaults to empty so any unwrapped preview /
+ * test simply shows no dots rather than crashing.
+ */
+val LocalCachedTrackIds = staticCompositionLocalOf { emptySet<String>() }
+
+/**
+ * Small accent download glyph shown next to a track row when the track
+ * is cached locally. Ports Flutter's `CachedIndicator`
+ * (`circle_check_big`, size 14, accent, 4px left pad) — renders nothing
+ * when the track isn't cached. Reads the reactive set from
+ * [LocalCachedTrackIds], so the dot appears/disappears as the cache
+ * fills and evicts without per-row queries.
+ */
+@Composable
+fun CachedDot(trackId: String, modifier: Modifier = Modifier) {
+    if (trackId !in LocalCachedTrackIds.current) return
+    Icon(
+        imageVector = Lucide.Circle_check_big,
+        contentDescription = "Downloaded",
+        tint = LocalFabledSwordTheme.current.accent,
+        modifier = modifier
+            .padding(start = 4.dp)
+            .size(14.dp),
+    )
+}