finish up most of our helper classes. there are no lint issues in the

new classes. The CallViewModel is not done yet however

src/state/SessionBehaviors.ts

@@ -0,0 +1,72 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import {
+  type CallMembership,
+  isLivekitTransport,
+  type LivekitTransport,
+  type MatrixRTCSession,
+  MatrixRTCSessionEvent,
+} from "matrix-js-sdk/lib/matrixrtc";
+import { fromEvent, map } from "rxjs";
+
+import { type ObservableScope } from "./ObservableScope";
+import { type Behavior } from "./Behavior";
+
+export const sessionBehaviors$ = (
+  scope: ObservableScope,
+  matrixRTCSession: MatrixRTCSession,
+): {
+  memberships$: Behavior<CallMembership[]>;
+  membershipsWithTransport$: Behavior<
+    { membership: CallMembership; transport?: LivekitTransport }[]
+  >;
+  transports$: Behavior<LivekitTransport[]>;
+} => {
+  const memberships$ = scope.behavior(
+    fromEvent(
+      matrixRTCSession,
+      MatrixRTCSessionEvent.MembershipsChanged,
+      (_, memberships: CallMembership[]) => memberships,
+    ),
+  );
+  /**
+   * Lists the transports used by ourselves, plus all other MatrixRTC session
+   * members. For completeness this also lists the preferred transport and
+   * whether we are in multi-SFU mode or sticky events mode (because
+   * advertisedTransport$ wants to read them at the same time, and bundling data
+   * together when it might change together is what you have to do in RxJS to
+   * avoid reading inconsistent state or observing too many changes.)
+   */
+  const membershipsWithTransport$: Behavior<
+    { membership: CallMembership; transport?: LivekitTransport }[]
+  > = scope.behavior(
+    memberships$.pipe(
+      map((memberships) => {
+        return memberships.map((membership) => {
+          const oldestMembership = memberships[0] ?? membership;
+          const transport = membership.getTransport(oldestMembership);
+          return {
+            membership,
+            transport: isLivekitTransport(transport) ? transport : undefined,
+          };
+        });
+      }),
+    ),
+  );
+
+  const transports$ = scope.behavior(
+    membershipsWithTransport$.pipe(
+      map((mts) => mts.flatMap(({ transport: t }) => (t ? [t] : []))),
+    ),
+  );
+  return {
+    memberships$,
+    membershipsWithTransport$,
+    transports$,
+  };
+};