From 799619557bad032c6bb50b2c6228788e46d01c49 Mon Sep 17 00:00:00 2001
From: "Timo K." <toger5@hotmail.de>
Date: Thu, 8 Oct 2026 16:35:01 +0200
Subject: [PATCH] [SDK] make stats$ a polled method and add audioLevel$ to
 audio tracks

---
 sdk/SdkArchitecture.md                        | 16 ++--
 sdk/dev/main.ts                               |  2 +-
 sdk/src/media-api.ts                          | 25 +++---
 .../livekit/LivekitMediaTrack.test.ts         | 85 +++++++++++++++++--
 .../livekit/LivekitMediaTrack.ts              | 34 +++++---
 .../livekit/LivekitMemberMedia.ts             |  4 +-
 sdk/src/utils/LazyBehavior.test.ts            | 52 ------------
 sdk/src/utils/LazyBehavior.ts                 | 53 ------------
 8 files changed, 127 insertions(+), 144 deletions(-)
 delete mode 100644 sdk/src/utils/LazyBehavior.test.ts
 delete mode 100644 sdk/src/utils/LazyBehavior.ts

diff --git a/sdk/SdkArchitecture.md b/sdk/SdkArchitecture.md
index 8ad211853..a1a7623f4 100644
--- a/sdk/SdkArchitecture.md
+++ b/sdk/SdkArchitecture.md
@@ -599,11 +599,11 @@ export interface MediaTrack {
   /** False when the SFU reports the track as unencrypted. Fixed for the life of the track. */
   encrypted: boolean;
   /**
-   * Inbound statistics for a remote track, outbound for a local one. Polled
-   * once a second, but only while someone is subscribed, so a statistics
-   * panel costs nothing while closed.
+   * Inbound statistics for a remote track, outbound for a local one, polled
+   * every `intervalMs` while the observable is subscribed and not at all
+   * otherwise, so a statistics panel costs nothing while closed.
    */
-  stats$: Behavior<MediaStreamStats>;
+  stats$(intervalMs: number): Observable<MediaStreamStats>;
   /**
    * Rendering. The view hands its <video> or <audio> element over; the SDK
    * sets its stream and, for video, registers the size and on-screen
@@ -624,6 +624,12 @@ export interface AudioMediaTrack extends MediaTrack {
    * member reports the same value.
    */
   isActive$: Behavior<boolean>;
+  /**
+   * How loud the track is right now, 0 to 1, as the backend measures it. 0
+   * while muted. Not what decides `isActive$`: the backend applies its own
+   * threshold and hysteresis to that.
+   */
+  audioLevel$: Behavior<number>;
   /** Route playback through Web Audio, for earpiece pan and gain. Undefined resets. */
   setAudioContext(ctx: AudioContext | undefined, plugins?: AudioNode[]): void;
   /** No-op for our own audio, which is never played back. */
@@ -721,7 +727,7 @@ sdk/
                       metadata, transports, discovery, the js-sdk join
   src/media-backend/  the MediaBackend interface in api.ts; livekit/ is the one
                       backend: connections, publisher, JWT, keys, the tracks
-  src/utils/          LazyBehavior, mapScoped, network retry, display names, test helpers
+  src/utils/          mapScoped, network retry, display names, test helpers
   dev/                the harness (below)
   SdkArchitecture.md  this document
   SdkMigration.md     how Element Call gets from CallViewModel to the client
diff --git a/sdk/dev/main.ts b/sdk/dev/main.ts
index 37db72bf0..774676361 100644
--- a/sdk/dev/main.ts
+++ b/sdk/dev/main.ts
@@ -269,7 +269,7 @@ function render(
     "active",
     of$((t) => (t.kind === "audio" ? t.isActive$ : of(undefined))),
   );
-  const stats$ = of$((t) => t.stats$);
+  const stats$ = of$((t) => t.stats$(1000));
   label("frameWidth", stats$.pipe(switchMap((s) => of(frames(s)?.frameWidth))));
   label("frames", stats$.pipe(switchMap((s) => of(frames(s)?.count))));
 }
diff --git a/sdk/src/media-api.ts b/sdk/src/media-api.ts
index 5e341914e..27ddcb0d0 100644
--- a/sdk/src/media-api.ts
+++ b/sdk/src/media-api.ts
@@ -94,13 +94,12 @@ export interface MediaTrack {
    * of the track: a publisher that changes its encryption publishes anew.
    */
   encrypted: boolean;
-  /** Polled while subscribed. */
-  // REVIEW: we want the perf gain of not having it always run.
-  //
-  // proposals
-  //  - obs -> on subscribe start polling timer
-  //  - add public method the MediaTrack: for example: actiavteStatsPolling(ms)
-  stats$: Behavior<MediaStreamStats>;
+  /**
+   * Inbound statistics for a remote track, outbound for a local one, polled
+   * every `intervalMs` while the observable is subscribed and not at all
+   * otherwise. Each subscription runs its own poll, so a view shares one.
+   */
+  stats$(intervalMs: number): Observable<MediaStreamStats>;
   /**
    * Rendering. The view hands its `<video>` or `<audio>` element over; the SDK
    * sets its stream and, for video, registers the size and on-screen observers
@@ -117,11 +116,17 @@ export interface AudioMediaTrack extends MediaTrack {
   /**
    * Whether the track carries sound right now, as the backend measures it.
    * False while muted. A call reads this on the microphone track and calls it
-   * "speaking". LiveKit measures it per member, so every audio track of a
-   * member reports the same value.
+   * "speaking".
+   * This is different to audioLevel = 0. The backend might run some hysteresis on it.
    */
-  // REVIEW: audio Level (could even super-seed isActive "audioLevel != 0")
   isActive$: Behavior<boolean>;
+  /**
+   * How loud the track is right now, 0 to 1, as the backend measures it. 0
+   * while muted. Not what decides `isActive$`: the backend applies its own
+   * threshold and hysteresis to that, so a level above 0 may not be active and
+   * an active track may briefly sit at 0.
+   */
+  audioLevel$: Behavior<number>;
   /** Route playback through Web Audio, for earpiece pan and gain. Undefined resets. */
   setAudioContext(ctx: AudioContext | undefined, plugins?: AudioNode[]): void;
   setVolume(volume: number): void;
diff --git a/sdk/src/media-backend/livekit/LivekitMediaTrack.test.ts b/sdk/src/media-backend/livekit/LivekitMediaTrack.test.ts
index 4cd66bed1..dcce056ef 100644
--- a/sdk/src/media-backend/livekit/LivekitMediaTrack.test.ts
+++ b/sdk/src/media-backend/livekit/LivekitMediaTrack.test.ts
@@ -12,6 +12,7 @@ import {
   RemoteAudioTrack,
   type RemoteParticipant,
   type Room as LivekitRoom,
+  RoomEvent,
   Track,
   TrackEvent,
   type TrackPublication,
@@ -30,14 +31,18 @@ describe("createLivekitMediaTrack", () => {
   beforeEach(() => {
     scope = new ObservableScope();
   });
-  afterEach(() => scope.end());
+  afterEach(() => {
+    scope.end();
+    vi.useRealTimers();
+  });
 
   it("describes the publication", () => {
-    const { participant, publication } = fakes();
+    const { participant, publication, room } = fakes();
     const track = createLivekitMediaTrack(
       scope,
       participant,
       publication as unknown as TrackPublication,
+      room,
     );
     expect(track.kind).toBe("video");
     expect(track.source).toBe("camera");
@@ -46,7 +51,7 @@ describe("createLivekitMediaTrack", () => {
   });
 
   it("attaches elements to the track, even one that arrives later", () => {
-    const { participant, publication, emitter } = fakes({
+    const { participant, publication, room, emitter } = fakes({
       track: undefined,
     });
     const element = document.createElement("video");
@@ -54,6 +59,7 @@ describe("createLivekitMediaTrack", () => {
       scope,
       participant,
       publication as unknown as TrackPublication,
+      room,
     );
     track.attach(element);
     track.attach(element);
@@ -69,13 +75,14 @@ describe("createLivekitMediaTrack", () => {
   });
 
   it("detaches everything when its scope ends", () => {
-    const { participant, publication } = fakes();
+    const { participant, publication, room } = fakes();
     const element = document.createElement("video");
     const scope = new ObservableScope();
     const track = createLivekitMediaTrack(
       scope,
       participant,
       publication as unknown as TrackPublication,
+      room,
     );
     track.attach(element);
     scope.end();
@@ -83,11 +90,12 @@ describe("createLivekitMediaTrack", () => {
   });
 
   it("follows the mute state", () => {
-    const { participant, publication, emitter } = fakes();
+    const { participant, publication, room, emitter } = fakes();
     const track = createLivekitMediaTrack(
       scope,
       participant,
       publication as unknown as TrackPublication,
+      room,
     );
     expect(track.muted$.value).toBe(false);
     publication.isMuted = true;
@@ -96,26 +104,28 @@ describe("createLivekitMediaTrack", () => {
   });
 
   it("scales a remote member's volume on the participant", () => {
-    const { participant, publication } = fakes({
+    const { participant, publication, room } = fakes({
       kind: Track.Kind.Audio,
     });
     const track = createLivekitMediaTrack(
       scope,
       participant,
       publication as unknown as TrackPublication,
+      room,
     ) as AudioMediaTrack;
     track.setVolume(0.5);
     expect(publication.track!.setVolume).toHaveBeenCalledWith(0.5);
   });
 
   it("is active while the member speaks and the track is not muted", () => {
-    const { participant, publication, emitter } = fakes({
+    const { participant, publication, room, emitter } = fakes({
       kind: Track.Kind.Audio,
     });
     const track = createLivekitMediaTrack(
       scope,
       participant,
       publication as unknown as TrackPublication,
+      room,
     ) as AudioMediaTrack;
     expect(track.isActive$.value).toBe(false);
     participant.isSpeaking = true;
@@ -126,6 +136,57 @@ describe("createLivekitMediaTrack", () => {
     expect(track.isActive$.value).toBe(false);
   });
 
+  it("reports the audio level the room announces, 0 while muted", () => {
+    const { participant, publication, room, emitter } = fakes({
+      kind: Track.Kind.Audio,
+    });
+    const track = createLivekitMediaTrack(
+      scope,
+      participant,
+      publication as unknown as TrackPublication,
+      room,
+    ) as AudioMediaTrack;
+    expect(track.audioLevel$.value).toBe(0);
+    participant.audioLevel = 0.4;
+    (room as unknown as EventEmitter).emit(RoomEvent.ActiveSpeakersChanged, [
+      participant,
+    ]);
+    expect(track.audioLevel$.value).toBe(0.4);
+    publication.isMuted = true;
+    emitter.emit(ParticipantEvent.TrackMuted, publication);
+    expect(track.audioLevel$.value).toBe(0);
+  });
+
+  it("polls statistics at the given interval, only while subscribed", async () => {
+    vi.useFakeTimers();
+    const { participant, publication, room } = fakes();
+    const stats = { type: "inbound-rtp" } as RTCInboundRtpStreamStats;
+    publication.track!.getRTCStatsReport.mockResolvedValue(
+      new Map([["s", stats]]),
+    );
+    const track = createLivekitMediaTrack(
+      scope,
+      participant,
+      publication as unknown as TrackPublication,
+      room,
+    );
+    const stats$ = track.stats$(500);
+    await vi.advanceTimersByTimeAsync(1000);
+    expect(publication.track!.getRTCStatsReport).not.toHaveBeenCalled();
+
+    const seen = vi.fn();
+    const subscription = stats$.subscribe(seen);
+    await vi.advanceTimersByTimeAsync(0);
+    expect(publication.track!.getRTCStatsReport).toHaveBeenCalledTimes(1);
+    expect(seen).toHaveBeenLastCalledWith(stats);
+    await vi.advanceTimersByTimeAsync(1000);
+    expect(publication.track!.getRTCStatsReport).toHaveBeenCalledTimes(3);
+
+    subscription.unsubscribe();
+    await vi.advanceTimersByTimeAsync(1000);
+    expect(publication.track!.getRTCStatsReport).toHaveBeenCalledTimes(3);
+  });
+
   it("gives our own track its controls", async () => {
     const { participant, publication, room } = fakes({
       kind: Track.Kind.Audio,
@@ -152,6 +213,7 @@ type FakeTrack = Track & {
   attach: ReturnType<typeof vi.fn>;
   detach: ReturnType<typeof vi.fn>;
   setVolume: ReturnType<typeof vi.fn>;
+  getRTCStatsReport: ReturnType<typeof vi.fn>;
 };
 
 type FakePublication = Omit<TrackPublication, "track" | "isMuted"> & {
@@ -166,6 +228,7 @@ function fakeTrack(): FakeTrack {
     setVolume: vi.fn(),
     setAudioContext: vi.fn(),
     setWebAudioPlugins: vi.fn(),
+    getRTCStatsReport: vi.fn().mockResolvedValue(undefined),
   }) as FakeTrack;
 }
 
@@ -173,7 +236,7 @@ function fakes({
   track = fakeTrack(),
   kind = Track.Kind.Video,
 }: { track?: FakeTrack | undefined; kind?: Track.Kind } = {}): {
-  participant: RemoteParticipant & { isSpeaking: boolean };
+  participant: RemoteParticipant & { isSpeaking: boolean; audioLevel: number };
   publication: FakePublication;
   room: LivekitRoom;
   emitter: EventEmitter;
@@ -182,10 +245,14 @@ function fakes({
   const participant = Object.assign(emitter, {
     isLocal: false,
     isSpeaking: false,
+    audioLevel: 0,
     identity: "@alice:example.org:DEVICE",
     getTrackPublication: (): TrackPublication =>
       publication as unknown as TrackPublication,
-  }) as unknown as RemoteParticipant & { isSpeaking: boolean };
+  }) as unknown as RemoteParticipant & {
+    isSpeaking: boolean;
+    audioLevel: number;
+  };
   const publication = Object.assign(emitter, {
     kind,
     source:
diff --git a/sdk/src/media-backend/livekit/LivekitMediaTrack.ts b/sdk/src/media-backend/livekit/LivekitMediaTrack.ts
index a0111eda7..dd1b81c93 100644
--- a/sdk/src/media-backend/livekit/LivekitMediaTrack.ts
+++ b/sdk/src/media-backend/livekit/LivekitMediaTrack.ts
@@ -8,6 +8,7 @@ Please see LICENSE in the repository root for full details.
 import {
   observeParticipantEvents,
   observeParticipantMedia,
+  roomEventSelector,
 } from "@livekit/components-core";
 import {
   facingModeFromLocalTrack,
@@ -19,6 +20,7 @@ import {
   RemoteAudioTrack,
   RemoteTrack,
   type Room as LivekitRoom,
+  RoomEvent,
   Track,
   TrackEvent,
   type TrackPublication,
@@ -27,13 +29,13 @@ import {
   combineLatest,
   distinctUntilChanged,
   fromEvent,
-  interval,
   map,
   merge,
+  type Observable,
   of,
-  share,
   startWith,
   switchMap,
+  timer,
 } from "rxjs";
 
 import { type Behavior } from "../../reactive/Behavior";
@@ -48,7 +50,6 @@ import {
   type VideoMediaTrack,
 } from "../../media-api";
 import { MatrixRTCError } from "../../errors";
-import { LazyBehavior } from "../../utils/LazyBehavior";
 import { convertToLivekitProcessor } from "./videoProcessor";
 
 export const mediaSources: Record<Track.Source, MediaSource> = {
@@ -67,10 +68,6 @@ export const livekitSources: Record<MediaSource, Track.Source> = {
   unknown: Track.Source.Unknown,
 };
 
-// One timer for every track so that a large session does not keep hundreds of
-// them, each firing a statistics request, in the event loop.
-const refreshStats$ = interval(1000).pipe(startWith(0), share());
-
 /**
  * One publication of a participant as a `MediaTrack`. The publication is
  * fixed; the track behind it may come and go (a remote track arrives on
@@ -80,6 +77,7 @@ export function createLivekitMediaTrack(
   scope: ObservableScope,
   participant: Participant,
   publication: TrackPublication,
+  room: LivekitRoom,
 ): AudioMediaTrack | VideoMediaTrack {
   const mediaChanged$ = observeParticipantMedia(participant);
   const track$ = publicationTrack$(scope, participant, publication);
@@ -117,13 +115,11 @@ export function createLivekitMediaTrack(
     id: publication.trackSid,
     muted$,
     encrypted: publication.isEncrypted,
-    stats$: new LazyBehavior<MediaStreamStats>(
-      refreshStats$.pipe(
+    stats$: (intervalMs): Observable<MediaStreamStats> =>
+      timer(0, intervalMs).pipe(
         switchMap(async () => rtpStreamStats(publication, participant.isLocal)),
         scope.bind(),
       ),
-      undefined,
-    ),
     attach: (element) => {
       if (attached.has(element)) return;
       attached.add(element);
@@ -154,6 +150,20 @@ export function createLivekitMediaTrack(
         ),
         participant.isSpeaking && !publication.isMuted,
       ),
+      // The level has no event of its own: it is written to the participant
+      // with each speaker update, which the room announces
+      audioLevel$: scope.behavior(
+        combineLatest(
+          [
+            roomEventSelector(room, RoomEvent.ActiveSpeakersChanged).pipe(
+              map(() => participant.audioLevel),
+              startWith(participant.audioLevel),
+            ),
+            muted$,
+          ],
+          (level, muted) => (muted ? 0 : level),
+        ),
+      ),
       setAudioContext: (ctx, plugins = []) => {
         audioContext = ctx;
         audioPlugins = plugins;
@@ -181,7 +191,7 @@ export function createLocalLivekitMediaTrack(
   room: LivekitRoom,
   setEnabled: (source: MediaSource, enabled: boolean) => Promise<boolean>,
 ): LocalAudioMediaTrack | LocalVideoMediaTrack {
-  const base = createLivekitMediaTrack(scope, participant, publication);
+  const base = createLivekitMediaTrack(scope, participant, publication, room);
   const local = {
     setEnabled: async (enabled: boolean) => setEnabled(base.source, enabled),
     setDevice: async (deviceId: string): Promise<void> => {
diff --git a/sdk/src/media-backend/livekit/LivekitMemberMedia.ts b/sdk/src/media-backend/livekit/LivekitMemberMedia.ts
index 32b99b8c3..f4e19eb72 100644
--- a/sdk/src/media-backend/livekit/LivekitMemberMedia.ts
+++ b/sdk/src/media-backend/livekit/LivekitMemberMedia.ts
@@ -54,8 +54,8 @@ export function createLivekitMemberMedia(
     tracks$: memberTracks$(
       scope,
       source$,
-      (trackScope, participant, publication) =>
-        createLivekitMediaTrack(trackScope, participant, publication),
+      (trackScope, participant, publication, room) =>
+        createLivekitMediaTrack(trackScope, participant, publication, room),
     ),
     encryptionError$: encryptionErrors$(source$, encryptionSystem),
   };
diff --git a/sdk/src/utils/LazyBehavior.test.ts b/sdk/src/utils/LazyBehavior.test.ts
deleted file mode 100644
index 091d57fdc..000000000
--- a/sdk/src/utils/LazyBehavior.test.ts
+++ /dev/null
@@ -1,52 +0,0 @@
-/*
-Copyright 2026 Element Creations Ltd.
-
-SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
-Please see LICENSE in the repository root for full details.
-*/
-
-import { describe, expect, it, vi } from "vitest";
-import { Observable, Subject } from "rxjs";
-
-import { LazyBehavior } from "./LazyBehavior";
-
-describe("LazyBehavior", () => {
-  it("runs its source only while subscribed", () => {
-    const teardown = vi.fn();
-    let starts = 0;
-    const values$ = new Subject<number>();
-    const source$ = new Observable<number>((subscriber) => {
-      starts++;
-      const subscription = values$.subscribe(subscriber);
-      return (): void => {
-        teardown();
-        subscription.unsubscribe();
-      };
-    });
-    const lazy$ = new LazyBehavior(source$, 0);
-    expect(starts).toBe(0);
-    expect(lazy$.value).toBe(0);
-
-    const seenByFirst: number[] = [];
-    const first = lazy$.subscribe((v) => seenByFirst.push(v));
-    expect(starts).toBe(1);
-    values$.next(1);
-    expect(seenByFirst).toEqual([0, 1]);
-    expect(lazy$.value).toBe(1);
-
-    const seenBySecond: number[] = [];
-    const second = lazy$.subscribe((v) => seenBySecond.push(v));
-    expect(starts).toBe(1);
-    expect(seenBySecond).toEqual([1]);
-
-    first.unsubscribe();
-    expect(teardown).not.toHaveBeenCalled();
-    second.unsubscribe();
-    expect(teardown).toHaveBeenCalledTimes(1);
-
-    values$.next(2);
-    expect(lazy$.value).toBe(1);
-    lazy$.subscribe().unsubscribe();
-    expect(starts).toBe(2);
-  });
-});
diff --git a/sdk/src/utils/LazyBehavior.ts b/sdk/src/utils/LazyBehavior.ts
deleted file mode 100644
index 2fe8ccfc0..000000000
--- a/sdk/src/utils/LazyBehavior.ts
+++ /dev/null
@@ -1,53 +0,0 @@
-/*
-Copyright 2026 Element Creations Ltd.
-
-SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
-Please see LICENSE in the repository root for full details.
-*/
-
-import {
-  BehaviorSubject,
-  type Observable,
-  type Subscriber,
-  type Subscription,
-} from "rxjs";
-
-/**
- * A behavior that only runs its source while someone is subscribed. The source
- * is subscribed with the first subscriber and unsubscribed with the last, so a
- * value that is expensive to keep current, such as polled statistics, costs
- * nothing while nobody looks at it. `value` is whatever the source last
- * produced, or the initial value.
- */
-export class LazyBehavior<T> extends BehaviorSubject<T> {
-  private subscribers = 0;
-  private upstream: Subscription | undefined;
-
-  public constructor(
-    private readonly source$: Observable<T>,
-    initialValue: T,
-  ) {
-    super(initialValue);
-  }
-
-  // Every subscription of an Observable goes through this hook, which rxjs
-  // declares as internal and so leaves out of its types. The base class's
-  // version replays the current value and registers the subscriber; it is
-  // wrapped rather than replaced.
-  protected _subscribe(subscriber: Subscriber<T>): Subscription {
-    if (this.subscribers++ === 0)
-      this.upstream = this.source$.subscribe((value) => this.next(value));
-    const subscription = (
-      BehaviorSubject.prototype as unknown as {
-        _subscribe(this: BehaviorSubject<T>, s: Subscriber<T>): Subscription;
-      }
-    )._subscribe.call(this, subscriber);
-    subscription.add(() => {
-      if (--this.subscribers === 0) {
-        this.upstream?.unsubscribe();
-        this.upstream = undefined;
-      }
-    });
-    return subscription;
-  }
-}