specific call tracks -> generic tracks

sdk/src/utils/tracks.ts

@@ -0,0 +1,44 @@
+/*
+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 { distinctUntilChanged, map } from "rxjs";
+
+import { type Behavior } from "../reactive/Behavior";
+import { type ObservableScope } from "../reactive/ObservableScope";
+import {
+  type AudioMediaTrack,
+  type MediaSource,
+  type MediaTrack,
+  type VideoMediaTrack,
+} from "../api";
+
+export type TrackOfSource<S extends MediaSource> = S extends
+  | "microphone"
+  | "screenShareAudio"
+  ? AudioMediaTrack
+  : S extends "camera" | "screenShare"
+    ? VideoMediaTrack
+    : AudioMediaTrack | VideoMediaTrack;
+
+/** The member's first track of a source; undefined while there is none. */
+export function trackBySource$<S extends MediaSource>(
+  scope: ObservableScope,
+  tracks$: Behavior<MediaTrack[]>,
+  source: S,
+): Behavior<TrackOfSource<S> | undefined> {
+  return scope.behavior(
+    tracks$.pipe(
+      map(
+        (tracks) =>
+          tracks.find((t) => t.source === source) as
+            | TrackOfSource<S>
+            | undefined,
+      ),
+      distinctUntilChanged(),
+    ),
+  );
+}