Give the local tracks their controls

Our own tracks are LocalAudioMediaTrack and LocalVideoMediaTrack, with setEnabled and setDevice, and on video facingMode$, switchFacingMode and setProcessor. switchCamera on the local media goes, as do the device and processor behaviors in LocalMediaInputs: a host calls the track when its setting changes instead of the SDK watching behaviors it does not own. The input device ids and the processor stay as initial values for the tracks created at the join. setMicrophoneEnabled and setCameraEnabled stay for the case of no track yet.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

sdk/SdkArchitecture.md

@@ -198,20 +198,20 @@ export interface MatrixRTCClientOptions {
 }
 
 /**
- * What the local member publishes. The enabled flags are the initial state;
- * from then on `setMicrophoneEnabled` and `setCameraEnabled` on the client
- * change it, and report what the device allowed. Device enumeration,
- * permission prompts and the lobby preview stay with the host.
+ * What the local member publishes when it joins. Initial values: from then on
+ * `setMicrophoneEnabled` and `setCameraEnabled` on the client, and the
+ * controls on the local tracks, change it. Device enumeration, permission
+ * prompts and the lobby preview stay with the host.
  */
 export interface LocalMediaInputs {
   microphoneEnabled: boolean;
   cameraEnabled: boolean;
-  audioInputDeviceId$: Behavior<string | undefined>;
-  videoInputDeviceId$: Behavior<string | undefined>;
+  audioInputDeviceId?: string;
+  videoInputDeviceId?: string;
+  /** Background blur and the like. */
+  videoProcessor?: TrackProcessor<Track.Kind.Video>;
   /** Undefined where the host routes audio itself, or to leave the browser's choice. */
   audioOutputDeviceId$: Behavior<string | undefined>;
-  /** Background blur and the like. */
-  videoProcessor$: Behavior<TrackProcessor<Track.Kind.Video> | undefined>;
 }
 
 /**
@@ -500,8 +500,28 @@ export interface AudioMediaTrack extends MediaTrack {
 
 export interface VideoMediaTrack extends MediaTrack {
   kind: "video";
-  /** For mirroring and switching cameras. Undefined for remote tracks. */
-  facingMode$?: Behavior<"user" | "environment" | undefined>;
+}
+
+/** The controls a member has over a track it publishes itself. */
+export interface LocalMediaTrack {
+  /** Mutes or unmutes. Resolves with the state that resulted. */
+  setEnabled(enabled: boolean): Promise<boolean>;
+  /** Captures from another device. Rejects for a screen share, which has none. */
+  setDevice(deviceId: string): Promise<void>;
+}
+
+export interface LocalAudioMediaTrack
+  extends AudioMediaTrack, LocalMediaTrack {}
+
+export interface LocalVideoMediaTrack extends VideoMediaTrack, LocalMediaTrack {
+  /** For mirroring; undefined where the camera does not say which way it faces. */
+  facingMode$: Behavior<"user" | "environment" | undefined>;
+  /** Restarts the camera facing the other way; resolves with the device now in use. */
+  switchFacingMode(): Promise<string | undefined>;
+  /** Background blur and the like; undefined removes the processor. */
+  setProcessor(
+    processor: TrackProcessor<Track.Kind.Video> | undefined,
+  ): Promise<void>;
 }
 
 export type EncryptionError = "MissingKey" | "InvalidKey";
@@ -532,12 +552,7 @@ export function trackBySource$<S extends MediaSource>(
 
 export interface LocalMemberMedia extends MemberMedia {
   local: true;
-  /**
-   * Restarts the camera facing the other way, on devices with a front and a
-   * back camera, and resolves with the id of the device now in use. Does
-   * nothing without a camera track or where the facing mode is unknown.
-   */
-  switchCamera(): Promise<string | undefined>;
+  tracks$: Behavior<(LocalAudioMediaTrack | LocalVideoMediaTrack)[]>;
 }
 ```
 
@@ -630,11 +645,11 @@ public API only.
 
 ## Open decisions
 
-- **`LocalMediaInputs` shape.** The enabled flags are initial values and the
-  setters on the client change them, because the setters have to report what
-  the device allowed; the device ids and the processor stay behaviors. Creating
-  the tracks before `join()` for a preview is still missing: today they are
-  created at the join.
+- **`LocalMediaInputs` shape.** Everything in it is an initial value; the
+  setters on the client and the controls on the local tracks change it from
+  then on, because they have to report what the device allowed. Creating the
+  tracks before `join()` for a preview is still missing: today they are created
+  at the join.
 - **`applicationData` after the join.** The option is read once. The client
   still updates `m.call.intent` from the camera state, as Element Call did; that
   moves to the host once the data can be updated.