jellytau/docs/architecture/05-platform-backends.md

Platform-Specific Player Backends

Player Events System

Location: src-tauri/src/player/events.rs

The player uses a push-based event system to notify the frontend of state changes:

pub enum PlayerStatusEvent {
    /// Playback position updated (emitted periodically during playback)
    PositionUpdate { position: f64, duration: f64 },

    /// Player state changed
    StateChanged { state: String, media_id: Option<String> },

    /// Media has finished loading and is ready to play
    MediaLoaded { duration: f64 },

    /// Playback has ended naturally
    PlaybackEnded,

    /// Buffering state changed
    Buffering { percent: u8 },

    /// An error occurred during playback
    Error { message: String, recoverable: bool },

    /// Volume changed
    VolumeChanged { volume: f32, muted: bool },

    /// Sleep timer state changed
    SleepTimerChanged {
        mode: SleepTimerMode,
        remaining_seconds: u32,
    },

    /// Show next episode popup with countdown
    ShowNextEpisodePopup {
        current_episode: MediaItem,
        next_episode: MediaItem,
        countdown_seconds: u32,
        auto_advance: bool,
    },

    /// Countdown tick (emitted every second during autoplay countdown)
    CountdownTick { remaining_seconds: u32 },

    /// Queue changed (items added, removed, reordered, or playback mode changed)
    QueueChanged {
        items: Vec<MediaItem>,
        current_index: Option<usize>,
        shuffle: bool,
        repeat: RepeatMode,
        has_next: bool,
        has_previous: bool,
    },

    /// Media session changed (activity context changed: Audio/Movie/TvShow/Idle)
    SessionChanged { session: MediaSessionType },
}

Events are emitted via Tauri's event system:

flowchart LR
    subgraph Backend["Player Backend"]
        MPV["MPV/ExoPlayer"]
    end

    subgraph EventSystem["Event System"]
        Emitter["TauriEventEmitter<br/>emit()"]
        Bus["Tauri Event Bus<br/>'player-event'"]
    end

    subgraph Frontend["Frontend"]
        Listener["playerEvents.ts<br/>Frontend Listener"]
        Store["Player Store Update<br/>(position, state, etc)"]
    end

    MPV --> Emitter --> Bus --> Listener --> Store

Frontend Listener (src/lib/services/playerEvents.ts):

• Listens for player-event Tauri events
• Updates player/queue stores based on event type
• Auto-advances to next track on PlaybackEnded
• On StateChanged events, calls invoke("player_get_queue") to update appState.hasNext/hasPrevious -- this enables MiniPlayer skip button state

Important: The command is player_get_queue (returns QueueStatus with hasNext/hasPrevious). There is no player_get_queue_status command.

Video is always native (no webview <video>)

TRACES: UR-080 | DR-231, DR-235, DR-237

Every video renderer is a native player drawing behind the transparent webview, with the Svelte controls composited over it: ExoPlayer on Android, mpv on Linux and Windows. There is no webview <video> element, no hls.js, and no HTML5 adapter; the frontend has one video adapter, NativePlayerAdapter, and the backend performs every seek, track switch and quality change itself.

Why the webview path was deleted rather than kept as a fallback:

• The transcode was a decoder constraint. The <video> element decodes little beyond h264, so the desktop profile could only claim h264 and the server re-encoded almost everything (7% direct play on a real library, against 85% for the same library through ExoPlayer). The machine was never the limit — mpv was already running for audio. (The Linux/Windows profile still claims h264 until DR-234 widens it; see desktop-native-video.md.)
• Each renderer is another place for every bug. Three video renderers meant every seek strategy, track switch and lifecycle fix had three places to be got right; the webview path was also where the renderer choice itself went wrong (silent Linux video, a Windows soundtrack decoded twice — DR-237).
• A fallback that decodes less is not a fallback. Android showed it first (DR-293): an original-file download plays silent in the webview. With no webview path there is no silent downgrade to fall into; a failed mpv init emits backend-init-failed instead.

What survives of the webview reporting pipeline is audio-only: WebviewAudioBackend plays through a hidden <audio> element and reports back through the player_report_* commands (rustReportHost.ts), for a desktop with no mpv. No shipped platform uses it.

Native video on the desktop (mpv)

TRACES: UR-080 | DR-231 … DR-237, DR-298, DR-299

The mpv half is shared; only the surface differs per platform (mpv_backend::video_output):

Platform	Output	Where the picture goes
Linux	vo=libmpv (render API)	An FBO drawn in the main window's own GtkBox draw handler, which GTK paints before its children — so beneath the webview, with no widget reparenting (video_surface.rs; a GtkOverlay aborts the process on the first click, see that module)
Windows	vo=gpu-next,gpu, wid=<HWND>	mpv renders as a child of the app window, beneath the transparent WebView2 — the arrangement tauri-plugin-libmpv ships. wid only takes effect before initialisation, so MpvBackend::new takes the handle; with no handle it draws nothing rather than open a window of its own. mpv's controller, bindings and cursor handling are off

In both, the page clears its opaque backgrounds while a video is on screen (data-native-video, the same CSS Android uses).

Tracks (mpv_tracks.rs): subtitles are the WebVTT list the play request carries, queued on sub-files before the load and selected by position in that list — the same meaning ExoPlayer gives player_set_subtitle_track. Selection starts off (the menu opens on "Off") and sid/aid are reset before every load, so a choice made for one item cannot leak into the next. Audio tracks are selected by position in the file; a transcode carries one track and is re-opened instead.

Two rules for every libmpv handle (mpv_command.rs):

• Commands go through mpv_command::command, an argv built for mpv_command, never the pinned crate's Mpv::command, which joins its arguments into a string that mpv parses — ; chains a second command, so a track title in a downloaded file's path could run run … (DR-298).
• Every handle is hardened before its first load: tls-verify=yes (mpv's default is no, and its URLs carry the ApiKey) and ytdl=no (DR-299).

MpvBackend (Linux)

Location: src-tauri/src/player/mpv/

The MPV backend uses libmpv for audio and video playback on Linux and Windows (video: see Native video on the desktop above). Since MPV handles are not Send, all operations occur on a dedicated thread.

flowchart TB
    subgraph MainThread["Main Thread"]
        MpvBackend["MpvBackend<br/>- command_tx<br/>- shared_state<br/>- shutdown"]
        Commands["Commands:<br/>Load, Play, Pause<br/>Stop, Seek, SetVolume"]
    end

    subgraph EventLoopThread["MPV Event Loop Thread"]
        EventLoop["event_loop.rs<br/>- MPV Handle<br/>- command_rx<br/>- Event Emitter"]
        TauriEmitter["TauriEventEmitter"]
    end

    MpvBackend -->|"MpvCommand"| EventLoop
    MpvBackend <-->|"Arc<Mutex<>>"| EventLoop
    EventLoop -->|"Events"| TauriEmitter
    TauriEmitter --> FrontendStore["Frontend Store"]

Key Components:

// Command enum sent to event loop thread
pub enum MpvCommand {
    Load { url: String, media: MediaItem },
    Play,
    Pause,
    Stop,
    Seek(f64),
    SetVolume(f32),
    Quit,
}

// Shared state between main thread and event loop
pub struct MpvSharedState {
    pub state: PlayerState,
    pub position: f64,
    pub duration: Option<f64>,
    pub volume: f32,
    pub is_loaded: bool,
    pub current_media: Option<MediaItem>,
}

Event Loop (event_loop.rs):

• Initializes MPV with audio-only config (vo=null, video=false)
• Observes properties: time-pos, duration, pause, volume
• Emits position updates every 250ms during playback
• Processes commands from channel (non-blocking)
• Handles MPV events: FileLoaded, EndFile, PropertyChange

ExoPlayerBackend (Android)

Location: src-tauri/src/player/android/ and Kotlin sources

The ExoPlayer backend uses Android's Media3/ExoPlayer library via JNI.

flowchart TB
    subgraph RustNative["Rust (Native)"]
        ExoBackend["ExoPlayerBackend<br/>- player_ref<br/>- shared_state"]
        NativeFuncs["JNI Callbacks<br/>nativeOnPosition...<br/>nativeOnState...<br/>nativeOnMediaLoaded<br/>nativeOnPlaybackEnd"]
        TauriEmitter2["TauriEventEmitter"]
    end

    subgraph KotlinJVM["Kotlin (JVM)"]
        JellyTauPlayer["JellyTauPlayer<br/>- ExoPlayer<br/>- Player.Listener"]
    end

    ExoBackend -->|"JNI Calls"| JellyTauPlayer
    JellyTauPlayer -->|"Callbacks"| NativeFuncs
    NativeFuncs --> TauriEmitter2
    TauriEmitter2 --> FrontendStore2["Frontend Store"]

Kotlin Player (JellyTauPlayer.kt):

class JellyTauPlayer(context: Context) {
    private val exoPlayer: ExoPlayer
    private var positionUpdateJob: Job?

    // Methods callable from Rust via JNI
    fun load(url: String, mediaId: String)
    fun play()
    fun pause()
    fun stop()
    fun seek(positionSeconds: Double)
    fun setVolume(volume: Float)

    // Native callbacks to Rust
    private external fun nativeOnPositionUpdate(position: Double, duration: Double)
    private external fun nativeOnStateChanged(state: String, mediaId: String?)
    private external fun nativeOnMediaLoaded(duration: Double)
    private external fun nativeOnPlaybackEnded()
}

JNI Callbacks (Rust):

#[no_mangle]
pub extern "system" fn Java_com_dtourolle_jellytau_player_JellyTauPlayer_nativeOnPositionUpdate(
    _env: JNIEnv, _class: JClass, position: jdouble, duration: jdouble
) {
    // Update shared state
    // Emit PlayerStatusEvent::PositionUpdate
}

Audio settings on ExoPlayer

TRACES: UR-027, UR-032, UR-033 | DR-030, DR-035, DR-036

PlayerBackend declares set_audio_settings with a default Ok(()) body. For a long time ExoPlayerBackend took that default, so Settings › Audio rendered controls that silently did nothing on Android — the parity gap recorded in requirements.md, now closed.

The settings cross to Kotlin as JSON over JNI, not as a wide signature, so new fields do not change the method signature — the same approach load() uses for subtitles:

fn set_audio_settings(&mut self, settings: &AudioSettings) -> Result<(), PlayerError> {
    let json = audio_settings_jni_payload(settings)?;
    env.call_method(&self.player_ref, "setAudioSettings", "(Ljava/lang/String;)V", …)?;
    // Store the sanitised form, so audio_settings() reflects what was applied.
    self.shared_state.lock_safe().audio_settings =
        settings.clone().with_crossfade_clamped().with_equalizer_normalised();
}

Kotlin owns the mechanics — attaching AudioEffects to the audio session — while the canonical band layout and preset curves stay in Rust:

Feature	Android mechanism	Notes
Gapless	pauseAtEndOfMediaItems
Volume normalization	LoudnessEnhancer	A gain stage — approximate next to MPV's dynaudnorm
Equalizer	android.media.audiofx.Equalizer	The canonical 10 bands are resampled onto the device's own band centres
Crossfade	—	Unimplemented on every platform (DR-034), architecturally blocked on MPV. Building it on Android alone would invert the parity gap

Two things are deliberately still open: the effects are not yet verified on a physical device (AudioEffect availability and band layouts are device-specific), and the trait default is still a silent Ok(()) rather than an error, so a backend that omits the method still reports success. Flipping that default waits on the device verification.

Licensed audio codecs: the FFmpeg extension

TRACES: UR-004, UR-071 | DR-293

Android does not ship AC-3, E-AC-3, DTS or TrueHD decoders — they are licensed codecs, present only where a vendor paid for them. The ROD2-W09 test tablet has a vendor DTS decoder and no AC-3/E-AC-3 at all. ExoPlayer has no decoders of its own, so on such a device those tracks are undecodable, and before this every film with Dolby audio was re-encoded by the server — for streaming and for download.

JellyTauPlayer builds ExoPlayer with DefaultRenderersFactory in EXTENSION_RENDERER_MODE_ON: the platform's decoders are tried first (a vendor DTS decoder stays in charge where there is one) and the FFmpeg audio renderer takes what they cannot decode. CodecDetector reports the extension's codecs beside the MediaCodecList ones, asking FfmpegLibrary.supportsFormat per MIME type rather than assuming, so a build whose native library failed to load reports only what the platform decodes. Rust's device profile and download policy read that list, which is what keeps "what we tell the server" and "what actually decodes" in step.

The decoder is org.jellyfin.media3:media3-ffmpeg-decoder — Jellyfin's build of media3's FFmpeg extension, versioned <media3 version>+N. Bump it in the same commit as media3. It is GPL-3.0: the distributed APK carries those terms, the source stays MIT (see THIRD_PARTY_NOTICES.md). Its JNI methods are covered by the AAR's own consumer rules and by -keep class androidx.media3.** { *; } in proguard-jellytau.pro, which also keeps the renderer ExoPlayer loads reflectively.

Rejected: re-encoding a download's audio on the device after it lands (a remux). It costs minutes of CPU and twice the disk per film, needs a pipeline state of its own, and does nothing for streaming. Decoding at playback fixes both paths with no extra step.

Why the webview could not stay a fallback on Android

The webview decodes none of the codecs above — so with the original file now downloaded as-is (DR-293), the old experimentalNativeVideo switch would have played every such download as a silent film. That is what first removed the webview video path on Android; DR-235 then removed it everywhere.

The equalizer, and where its vocabulary lives

TRACES: UR-027 | DR-030, IR-020

The canonical band layout (EQ_BANDS) and the preset curves live in settings.rs, not in either backend and not in the UI: a preset is a gain curve defined by the band layout, and the layout is a property of the audio engine rather than of the picker that renders it. Presets are Flat, Rock, Pop, Jazz, Classical, Bass Boost, Treble Boost and Vocal, all conservative (within ±8 dB) so they stack safely with volume normalization.

Platform	Mechanism
Linux	One ffmpeg two-pole peaking equalizer filter per band, composed by build_af_filter into MPV's af property alongside the normalization filter: equalizer=f=31:width_type=o:width=1:g=5
Android	android.media.audiofx.Equalizer, with the canonical 10 bands resampled onto whatever band centres the device actually has

Gains are normalised (with_equalizer_normalised) before use, and bands beyond EQ_BANDS are ignored, so a malformed settings payload cannot produce a filter chain of unbounded length.

Background Audio Handoff (Android)

TRACES: UR-040 | IR-025, DR-051, DR-052, DR-178 … DR-180, DR-196, DR-203

Keeping a video's audio alive when the app is backgrounded or the screen locks, while video decode stops. Two verified facts drive the whole design:

1. Nothing in the WebView keeps playing once the app is backgrounded — the system throttles it.
2. Keeping audio alive in the background requires a native foreground media service, which already exists for music (JellyTauPlaybackService + JellyTauPlayer + MediaSessionCompat).

So this is a handoff, not "keep the WebView alive": on background, tear down the current renderer and play the same item audio-only through the native service; on foreground, hand back. In the project's one-directional playback model this is a change of which player is authoritative, and the position must transfer cleanly across it.

sequenceDiagram
    participant App as App backgrounded
    participant FE as VideoPlayer
    participant Rust as player_enter/exit_background_audio
    participant Exo as Native audio service

    App-->>FE: jellytau-background (DOM CustomEvent)
    FE->>Rust: enter(item, position, audioStreamIndex)
    Rust->>Exo: play audio-only at position
    Note over Exo: lockscreen + notification, existing MediaSession
    App-->>FE: jellytau-foreground
    FE->>Rust: exit() -> final position
    Rust-->>FE: position
    FE->>FE: restart the renderer that is on screen

Details that were each a shipped defect:

• Position is absolute — the position on the item's timeline the player reports, never an offset within a re-opened transcode (computeHandoffPosition).
• A downloaded episode takes no base URL and an ordinary seek (DR-180); a stream takes the base and no seek; a handoff at 0:00 takes neither.
• The return must re-issue the load (DR-196). The native player owns no element and nothing watches the stream URL for it, so reassigning the URL — how the deleted webview path came back — restarted nothing and left a black screen with a play button that did nothing.
• wasPlaying is captured on the way out so play/pause survives the round trip, and the handoff does not silently rewind (DR-203).
• Mutually exclusive with PiP. Toggle on → setAutoEnterEnabled(false); toggle off → PiP on background, the status quo. The frontend re-asserts the value whenever the toggle changes and on unmount, so a stale setting cannot leak into the next player.
• The pure arithmetic and state transitions live in backgroundAudioHandoff.ts, free of Svelte and the DOM, so they are testable without mounting the player.

Native signals background/foreground to the frontend as DOM CustomEvents (jellytau-background / jellytau-foreground); the frontend carries the toggle state to native through the AndroidBackgroundAudio bridge. No-op on every non-Android platform.

Native Video Compositing (Android)

TRACES: UR-003, UR-004 | DR-150 … DR-152, DR-182 … DR-196

Android can render video on the native ExoPlayer surface behind a transparent Tauri WebView, with the Svelte controls drawn over it. It is the only video path (DR-235). Before that it was an opt-in whose default was flipped and reverted twice, each revert with a named cause — the per-defect record is in requirements.md (DR-150 … DR-196).

flowchart TB
    subgraph Window["One Android window"]
        Texture["TextureView (index 0)<br/>ExoPlayer video"]
        WebView["Tauri WebView (above)<br/>transparent, Svelte controls"]
    end
    Rust["ExoPlayerBackend"] -->|JNI| Player["JellyTauPlayer"]
    Player --> Texture
    MainActivity -->|"setTransparent(true)"| WebView
    VideoOverlayManager -->|"attach / detach"| Texture

Load-bearing details, each of which was a shipped defect:

• TextureView, not SurfaceView (DR-192). A SurfaceView renders on its own layer outside the app window and punches a transparent hole through it; everything drawn above that hole — for us the whole UI — depends on that composition path, which Android's own documentation says does not reliably work. A TextureView makes "behind" ordinary view z-order within one window.
• Attached at index 0 by VideoOverlayManager, and detached when the video goes (DR-184) — a surface left in the hierarchy outlives its player.
• Bridges are installed before the page that uses them (DR-183). addJavascriptInterface must run once per WebView instance and a call that lands after the page has loaded never reaches it, so setTransparent(true) could be dropped entirely.
• The app shell stops painting over the surface (DR-185). app.css clears its opaque backgrounds off [data-native-video]; before that, a CSS rule targeted an attribute nothing ever set, so the fix looked applied and was not.
• The poster card lifts without a <video> element (DR-182) — the native reveal fires on a playing state or a position tick carrying a position or duration, and on nothing else.
• Letterbox bars are painted, not left holding whatever was last in the framebuffer (DR-194).
• There is deliberately no audio-focus bridge: manual focus requests from the WebView competed with Chromium's AudioFocusDelegate and with ExoPlayer, and the resulting AUDIOFOCUS_LOSS paused playback.

Related Kotlin pieces in the same window: PictureInPictureManager (DR-160/161), ScreenWakeManager (DR-202 — Android counts its display timeout from touch events, which a playing video does not generate), ImmersiveModeBridge and WindowInsetsBridge (IR-031/DR-112 — see 02-svelte-frontend.md).

Android MediaSession & Remote Volume Control

Location: JellyTauPlaybackService.kt

JellyTau uses a dual MediaSession architecture for Android to support both Media3 playback controls and remote volume control:

flowchart TB
    subgraph Service["JellyTauPlaybackService"]
        MediaSession["Media3 MediaSession<br/>- Lockscreen controls<br/>- Media notifications<br/>- Play/Pause/Next/Previous"]

        MediaSessionCompat["MediaSessionCompat<br/>- Remote volume control<br/>- Hardware button interception"]

        VolumeProvider["VolumeProviderCompat<br/>- onSetVolumeTo()<br/>- onAdjustVolume()"]

        MediaSessionCompat --> VolumeProvider
    end

    subgraph Hardware["System"]
        VolumeButtons["Hardware Volume Buttons"]
        Lockscreen["Lockscreen Controls"]
        Notification["Media Notification"]
    end

    subgraph Rust["Rust Backend"]
        JNI["JNI Callbacks<br/>nativeOnRemoteVolumeChange()"]
        PlaybackMode["PlaybackModeManager<br/>send_remote_volume_command()"]
        JellyfinAPI["Jellyfin API<br/>session_set_volume()"]
    end

    VolumeButtons --> VolumeProvider
    Lockscreen --> MediaSession
    Notification --> MediaSession

    VolumeProvider --> JNI
    JNI --> PlaybackMode
    PlaybackMode --> JellyfinAPI

Architecture Rationale:

JellyTau maintains both MediaSession types because they serve different purposes:

1. Media3 MediaSession: Handles lockscreen/notification playback controls (play/pause/next/previous)
2. MediaSessionCompat: Intercepts hardware volume button presses for remote playback control

When in remote playback mode (controlling a Jellyfin session on another device):

• Volume buttons are routed through VolumeProviderCompat
• Volume changes are sent to the remote session via Jellyfin API
• System volume UI shows the remote session's volume level

Remote Volume Flow:

sequenceDiagram
    participant User
    participant VolumeButton as Hardware Volume Button
    participant VolumeProvider as VolumeProviderCompat
    participant JNI as nativeOnRemoteVolumeChange
    participant PlaybackMode as PlaybackModeManager
    participant Jellyfin as Jellyfin Server
    participant RemoteSession as Remote Session (TV/Browser)

    User->>VolumeButton: Press Volume Up
    VolumeButton->>VolumeProvider: onAdjustVolume(ADJUST_RAISE)
    VolumeProvider->>VolumeProvider: remoteVolumeLevel += 2
    VolumeProvider->>VolumeProvider: currentVolume = remoteVolumeLevel
    VolumeProvider->>JNI: nativeOnRemoteVolumeChange("VolumeUp", level)
    JNI->>PlaybackMode: send_remote_volume_command("VolumeUp", level)
    PlaybackMode->>Jellyfin: POST /Sessions/{id}/Command/VolumeUp
    Jellyfin->>RemoteSession: Set volume to new level
    RemoteSession-->>User: Volume changes on TV/Browser

Key Implementation Details:

Enabling Remote Volume (enableRemoteVolume()):

fun enableRemoteVolume(initialVolume: Int) {
    volumeProvider = object : VolumeProviderCompat(
        VolumeProviderCompat.VOLUME_CONTROL_ABSOLUTE,
        100,  // Max volume
        initialVolume
    ) {
        override fun onSetVolumeTo(volume: Int) {
            remoteVolumeLevel = volume.coerceIn(0, 100)
            nativeOnRemoteVolumeChange("SetVolume", remoteVolumeLevel)
        }

        override fun onAdjustVolume(direction: Int) {
            when (direction) {
                AudioManager.ADJUST_RAISE -> {
                    remoteVolumeLevel = (remoteVolumeLevel + 2).coerceAtMost(100)
                    nativeOnRemoteVolumeChange("VolumeUp", remoteVolumeLevel)
                    currentVolume = remoteVolumeLevel
                }
                AudioManager.ADJUST_LOWER -> {
                    remoteVolumeLevel = (remoteVolumeLevel - 2).coerceAtLeast(0)
                    nativeOnRemoteVolumeChange("VolumeDown", remoteVolumeLevel)
                    currentVolume = remoteVolumeLevel
                }
            }
        }
    }

    mediaSessionCompat.setPlaybackToRemote(volumeProvider)
}

Disabling Remote Volume (disableRemoteVolume()):

fun disableRemoteVolume() {
    mediaSessionCompat.setPlaybackToLocal(AudioManager.STREAM_MUSIC)
    volumeProvider = null
}

Rust Integration (src-tauri/src/player/android/mod.rs):

/// Enable remote volume control on Android
pub fn enable_remote_volume(initial_volume: i32) -> Result<(), String> {
    start_playback_service()?;
    let service_instance = get_playback_service_instance()?;
    env.call_method(&service_instance, "enableRemoteVolume", "(I)V",
        &[JValue::Int(initial_volume)])?;
    Ok(())
}

Dependencies (src-tauri/android/build.gradle.kts):

dependencies {
    implementation("androidx.media3:media3-session:1.5.1")  // Media3 MediaSession
    implementation("androidx.media:media:1.7.0")            // MediaSessionCompat
}

Integration with Playback Mode:

Remote volume is automatically enabled/disabled during playback mode transfers:

// In PlaybackModeManager::transfer_to_remote()
#[cfg(target_os = "android")]
{
    if let Err(e) = crate::player::enable_remote_volume(50) {
        log::warn!("Failed to enable remote volume: {}", e);
    }
}

// In PlaybackModeManager::transfer_to_local()
#[cfg(target_os = "android")]
{
    if let Err(e) = crate::player::disable_remote_volume() {
        log::warn!("Failed to disable remote volume: {}", e);
    }
}

Android Album Art Caching

Location: src-tauri/android/src/main/java/com/dtourolle/jellytau/player/AlbumArtCache.kt

Album art caching provides efficient bitmap storage for lock screen notifications with automatic LRU eviction and memory management.

flowchart TB
    subgraph JellyTauPlayer["JellyTauPlayer.kt"]
        LoadMedia["loadWithMetadata()<br/>- Store artworkUrl<br/>- Launch async download"]
        AsyncDownload["Coroutine<br/>- Non-blocking<br/>- Dispatchers.IO"]
    end

    subgraph Cache["AlbumArtCache.kt"]
        MemoryCache["LruCache<String, Bitmap><br/>- 1/8 of heap<br/>- ~12-16MB typical<br/>- 50-100 albums capacity"]
        Download["Download & Scale<br/>- 512x512 max<br/>- Exponential backoff"]
        ErrorHandle["Error Handling<br/>- Graceful fallback<br/>- Auto-retry"]
    end

    subgraph Service["JellyTauPlaybackService.kt"]
        UpdateMeta["updateMediaMetadata()<br/>- Accept Bitmap parameter<br/>- Add METADATA_KEY_ALBUM_ART"]
        Notification["Notification<br/>- setLargeIcon()<br/>- Lock screen display"]
    end

    LoadMedia --> AsyncDownload
    AsyncDownload --> MemoryCache
    MemoryCache --> Download
    Download --> ErrorHandle
    AsyncDownload --> UpdateMeta
    UpdateMeta --> Notification

AlbumArtCache Singleton:

class AlbumArtCache(context: Context) {
    private val memoryCache = object : LruCache<String, Bitmap>(cacheSize) {
        override fun sizeOf(key: String, bitmap: Bitmap): Int {
            return bitmap.byteCount / 1024  // Size in KB
        }
    }

    suspend fun getArtwork(url: String): Bitmap? {
        memoryCache.get(url)?.let { return it }
        return downloadAndCache(url)
    }

    private suspend fun downloadAndCache(url: String): Bitmap? =
        withContext(Dispatchers.IO) {
            // HTTP download with 5s timeout
            // Scale to 512x512 max
            // Auto-evict LRU if needed
        }
}

Integration Flow:

1. Track Load (loadWithMetadata()):

   • Store artwork URL in currentArtworkUrl
   • Reset bitmap to null
   • Start playback immediately (non-blocking)

2. Async Download (Background Coroutine):

   • Check cache: instant hit if available
   • Network miss: download, scale, cache
   • Auto-retry on network failure with exponential backoff
   • Graceful fallback if artwork unavailable

3. Notification Update:

   • Pass bitmap to updatePlaybackServiceNotification()
   • Add to MediaMetadataCompat with METADATA_KEY_ALBUM_ART
   • Display as large icon in notification
   • Show on lock screen

Memory Management:

Metric	Value
Cache Size	1/8 of heap (12-16MB typical)
Max Resolution	512x512 pixels
Capacity	~50-100 album arts
Eviction Policy	LRU (Least Recently Used)
Lifetime	In-memory only (app session)
Network Timeout	5 seconds per download

Performance Characteristics:

• Cache Hit: ~1ms (in-memory retrieval)
• Cache Miss: ~200-500ms (download + scale)
• Playback Impact: Zero (async downloads)
• Memory Overhead: Max 16MB (auto-eviction)
• Error Recovery: Automatic with exponential backoff

Backend Initialization

Location: src-tauri/src/lib.rs

Backend selection is platform-specific:

fn create_player_backend(app_handle: tauri::AppHandle) -> Box<dyn PlayerBackend> {
    let event_emitter = Arc::new(TauriEventEmitter::new(app_handle));

    #[cfg(target_os = "linux")]
    {
        match MpvBackend::new(event_emitter.clone()) {
            Ok(backend) => return Box::new(backend),
            Err(e) => eprintln!("MPV init failed: {}", e),
        }
    }

    #[cfg(target_os = "android")]
    {
        // ExoPlayer requires Activity context, initialized separately
    }

    // Fallback
    Box::new(NullBackend::new())
}