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:

#![allow(unused)]
fn main() {
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.

HTML5 Video Adapter (webview-rendered video)

Location: src/lib/player/html5Adapter.ts, src/lib/player/index.ts, report commands in src-tauri/src/commands/player/timers.rs

Video on desktop (Linux WebKitGTK) — and, per current interim behavior, Android — is rendered by an HTML5 <video>/HLS element inside the webview. libmpv is initialized audio-only (vo=null, video=false), so the native backend cannot render or observe this element. The <video> is therefore the real player, living outside Rust's reach.

To keep the PlayerController the single source of truth (matching the audio path), the HTML5 element is treated as a dumb output device that reports back into Rust, rather than an independent state authority:

flowchart LR
    subgraph Webview["Webview"]
        Video["HTML5 <video> / HLS.js"]
        Adapter["html5Adapter.ts<br/>(reports DOM events)"]
    end
    subgraph Backend["Rust"]
        Cmds["player_report_state<br/>player_report_position<br/>player_report_media_loaded"]
        Controller["PlayerController"]
        Emitter["TauriEventEmitter"]
    end
    subgraph Frontend["Frontend"]
        Events["playerEvents.ts"]
        Store["player store"]
    end

    Video -->|DOM events| Adapter --> Cmds --> Controller --> Emitter --> Events --> Store

Key points:

• The adapter re-emits the same PlayerStatusEvents (StateChanged, PositionUpdate, MediaLoaded) the native backends emit, so playerEvents.ts needs no HTML5-specific branch — HTML5 is just another event source feeding the existing pipeline.
• Position reports are throttled (~250ms) to match the MPV cadence and avoid flooding IPC from the 60fps RAF loop.
• Boundary rule: UI components never touch the report commands or videoElement state directly. Playback control goes through the unified facade src/lib/player/index.ts (playerController); HTML5 state reporting goes through html5Adapter.ts. This restores the documented invariant ("frontend only displays state and invokes commands") for the video path.

MpvBackend (Linux)

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

The MPV backend uses libmpv for audio playback on Linux. 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:

#![allow(unused)]
fn main() {
// 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):

#![allow(unused)]
fn main() {
#[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:

#![allow(unused)]
fn main() {
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:

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.

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.

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. An Android WebView <video> does not keep playing audio once the app is backgrounded — the system throttles the WebView and media pauses.
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. Transcoded HLS tracks time as videoElement.currentTime + seekOffset (the element resets to 0 after each transcode reload). computeHandoffPosition sums both terms; using the element time alone rewinds by the offset.
• 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 restart the renderer that is actually on screen (DR-196). The two paths resume by different means — the webview <video> reloads off its stream URL, watched by an $effect; ExoPlayer owns no element and nothing watches the URL for it, so it needs an explicit re-issue. Doing only the URL assignment restarted nothing on the native path 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. This is on by default; the HTML5 <video> path remains the fallback and is not being removed. The default has been flipped and reverted twice and each revert has 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 can lift on a path with no <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):

#![allow(unused)]
fn main() {
/// 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:

#![allow(unused)]
fn main() {
// 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:

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:

#![allow(unused)]
fn main() {
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())
}
}