Use a more suitable filter operator to compute local member

src/state/CallViewModel/CallViewModel.ts

@@ -52,7 +52,12 @@ import {
   ScreenShareViewModel,
   type UserMediaViewModel,
 } from "../MediaViewModel";
-import { accumulate, generateItems, pauseWhen } from "../../utils/observable";
+import {
+  accumulate,
+  filterBehavior,
+  generateItems,
+  pauseWhen,
+} from "../../utils/observable";
 import {
   duplicateTiles,
   MatrixRTCMode,
@@ -505,16 +510,13 @@ export function createCallViewModel$(
     ),
   );
 
-  const localMatrixLivekitMember$ =
-    scope.behavior<MatrixLivekitMember<"local"> | null>(
+  const localMatrixLivekitMember$: Behavior<MatrixLivekitMember<"local"> | null> =
+    scope.behavior(
       localRtcMembership$.pipe(
-        generateItems(
-          // Generate a local member when membership is non-null
-          function* (membership) {
-            if (membership !== null)
-              yield { keys: ["local"], data: membership };
-          },
-          (_scope, membership$) => ({
+        filterBehavior((membership) => membership !== null),
+        map((membership$) => {
+          if (membership$ === null) return null;
+          return {
             membership$,
             participant: {
               type: "local" as const,
@@ -522,9 +524,8 @@ export function createCallViewModel$(
             },
             connection$: localMembership.connection$,
             userId,
-          }),
-        ),
-        map(([localMember]) => localMember ?? null),
+          };
+        }),
       ),
     );
 

src/utils/observable.test.ts

@@ -5,11 +5,12 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
-import { test } from "vitest";
-import { Subject } from "rxjs";
+import { expect, test } from "vitest";
+import { type Observable, of, Subject, switchMap } from "rxjs";
 
 import { withTestScheduler } from "./test";
-import { generateItems, pauseWhen } from "./observable";
+import { filterBehavior, generateItems, pauseWhen } from "./observable";
+import { type Behavior } from "../state/Behavior";
 
 test("pauseWhen", () => {
   withTestScheduler(({ behavior, expectObservable }) => {
@@ -72,3 +73,31 @@ test("generateItems", () => {
     expectObservable(scope4$).toBe(scope4Marbles);
   });
 });
+
+test("filterBehavior", () => {
+  withTestScheduler(({ behavior, expectObservable }) => {
+    // Filtering the input should segment it into 2 modes of non-null behavior.
+    const inputMarbles = "   abcxabx";
+    const filteredMarbles = "a--xa-x";
+
+    const input$ = behavior(inputMarbles, {
+      a: "a",
+      b: "b",
+      c: "c",
+      x: null,
+    });
+    const filtered$: Observable<Behavior<string> | null> = input$.pipe(
+      filterBehavior((value) => typeof value === "string"),
+    );
+
+    expectObservable(filtered$).toBe(filteredMarbles, {
+      a: expect.any(Object),
+      x: null,
+    });
+    expectObservable(
+      filtered$.pipe(
+        switchMap((value$) => (value$ === null ? of(null) : value$)),
+      ),
+    ).toBe(inputMarbles, { a: "a", b: "b", c: "c", x: null });
+  });
+});

src/utils/observable.ts

@@ -22,6 +22,7 @@ import {
   withLatestFrom,
   BehaviorSubject,
   type OperatorFunction,
+  distinctUntilChanged,
 } from "rxjs";
 
 import { type Behavior } from "../state/Behavior";
@@ -185,6 +186,28 @@ export function generateItemsWithEpoch<
   );
 }
 
+/**
+ * Segments a behavior into periods during which its value matches the filter
+ * (outputting a behavior with a narrowed type) and periods during which it does
+ * not match (outputting null).
+ */
+export function filterBehavior<T, S extends T>(
+  predicate: (value: T) => value is S,
+): OperatorFunction<T, Behavior<S> | null> {
+  return (input$) =>
+    input$.pipe(
+      scan<T, BehaviorSubject<S> | null>((acc$, input) => {
+        if (predicate(input)) {
+          const output$ = acc$ ?? new BehaviorSubject(input);
+          output$.next(input);
+          return output$;
+        }
+        return null;
+      }, null),
+      distinctUntilChanged(),
+    );
+}
+
 function generateItemsInternal<
   Input,
   Keys extends [unknown, ...unknown[]],