Remove unsound participant casts

By tagging participant behaviors with a type (local vs. remote) we can now tell what kind of participant it will be in a completely type-safe manner.

src/state/CallViewModel/remoteMembers/MatrixLivekitMembers.ts

@@ -5,10 +5,7 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
-import {
-  type LocalParticipant as LocalLivekitParticipant,
-  type RemoteParticipant as RemoteLivekitParticipant,
-} from "livekit-client";
+import { type LocalParticipant, type RemoteParticipant } from "livekit-client";
 import {
   type LivekitTransport,
   type CallMembership,
@@ -24,16 +21,24 @@ import { generateItemsWithEpoch } from "../../../utils/observable";
 
 const logger = rootLogger.getChild("[MatrixLivekitMembers]");
 
+/**
+ * A dynamic participant value with a static tag to tell what kind of
+ * participant it can be (local vs. remote).
+ */
+export type TaggedParticipant =
+  | { type: "local"; value$: Behavior<LocalParticipant | null> }
+  | { type: "remote"; value$: Behavior<RemoteParticipant | null> };
+
 /**
  * Represents a Matrix call member and their associated LiveKit participation.
  * `livekitParticipant` can be undefined if the member is not yet connected to the livekit room
  * or if it has no livekit transport at all.
  */
-export interface MatrixLivekitMember {
+export interface MatrixLivekitMember<
+  ParticipantType extends TaggedParticipant["type"],
+> {
   membership$: Behavior<CallMembership>;
-  participant$: Behavior<
-    LocalLivekitParticipant | RemoteLivekitParticipant | null
-  >;
+  participant: TaggedParticipant & { type: ParticipantType };
   connection$: Behavior<Connection | null>;
   // participantId: string; We do not want a participantId here since it will be generated by the jwt
   // TODO decide if we can also drop the userId. Its in the matrix membership anyways.
@@ -61,7 +66,7 @@ export function createMatrixLivekitMembers$({
   scope,
   membershipsWithTransport$,
   connectionManager,
-}: Props): Behavior<Epoch<MatrixLivekitMember[]>> {
+}: Props): Behavior<Epoch<MatrixLivekitMember<"remote">[]>> {
   /**
    * Stream of all the call members and their associated livekit data (if available).
    */
@@ -110,12 +115,14 @@ export function createMatrixLivekitMembers$({
           logger.debug(
             `Updating data$ for participantId: ${participantId}, userId: ${userId}`,
           );
+          const { participant$, ...rest } = scope.splitBehavior(data$);
           // will only get called once per `participantId, userId` pair.
           // updates to data$ and as a result to displayName$ and mxcAvatarUrl$ are more frequent.
           return {
             participantId,
             userId,
-            ...scope.splitBehavior(data$),
+            participant: { type: "remote" as const, value$: participant$ },
+            ...rest,
           };
         },
       ),