Replace many usages of useObservableEagerState with useBehavior

This hook is simpler in its implementation (therefore hopefully more correct & performant) and enforces a type-level distinction between raw Observables and Behaviors.

src/button/ReactionToggleButton.tsx

@@ -24,8 +24,6 @@ import {
 import { useTranslation } from "react-i18next";
 import { logger } from "matrix-js-sdk/lib/logger";
 import classNames from "classnames";
-import { useObservableState } from "observable-hooks";
-import { map } from "rxjs";
 
 import { useReactionsSender } from "../reactions/useReactionsSender";
 import styles from "./ReactionToggleButton.module.css";
@@ -36,6 +34,7 @@ import {
 } from "../reactions";
 import { Modal } from "../Modal";
 import { type CallViewModel } from "../state/CallViewModel";
+import { useBehavior } from "../useBehavior";
 
 interface InnerButtonProps extends ComponentPropsWithoutRef<"button"> {
   raised: boolean;
@@ -180,12 +179,8 @@ export function ReactionToggleButton({
   const [showReactionsMenu, setShowReactionsMenu] = useState(false);
   const [errorText, setErrorText] = useState<string>();
 
-  const isHandRaised = useObservableState(
-    vm.handsRaised$.pipe(map((v) => !!v[identifier])),
-  );
-  const canReact = useObservableState(
-    vm.reactions$.pipe(map((v) => !v[identifier])),
-  );
+  const isHandRaised = !!useBehavior(vm.handsRaised$)[identifier];
+  const canReact = !useBehavior(vm.reactions$)[identifier];
 
   useEffect(() => {
     // Clear whenever the reactions menu state changes.

src/grid/Grid.tsx

@@ -24,16 +24,16 @@ import {
   createContext,
   memo,
   use,
+  useCallback,
   useEffect,
   useMemo,
   useRef,
   useState,
+  useSyncExternalStore,
 } from "react";
 import useMeasure from "react-use-measure";
 import classNames from "classnames";
 import { logger } from "matrix-js-sdk/lib/logger";
-import { useObservableEagerState } from "observable-hooks";
-import { fromEvent, map, startWith } from "rxjs";
 
 import styles from "./Grid.module.css";
 import { useMergedRefs } from "../useMergedRefs";
@@ -155,11 +155,6 @@ export function useVisibleTiles(callback: VisibleTilesCallback): void {
   );
 }
 
-const windowHeightObservable$ = fromEvent(window, "resize").pipe(
-  startWith(null),
-  map(() => window.innerHeight),
-);
-
 export interface LayoutProps<LayoutModel, TileModel, R extends HTMLElement> {
   ref?: Ref<R>;
   model: LayoutModel;
@@ -261,7 +256,13 @@ export function Grid<
   const [gridRoot, gridRef2] = useState<HTMLElement | null>(null);
   const gridRef = useMergedRefs<HTMLElement>(gridRef1, gridRef2);
 
-  const windowHeight = useObservableEagerState(windowHeightObservable$);
+  const windowHeight = useSyncExternalStore(
+    useCallback((onChange) => {
+      window.addEventListener("resize", onChange);
+      return (): void => window.removeEventListener("resize", onChange);
+    }, []),
+    useCallback(() => window.innerHeight, []),
+  );
   const [layoutRoot, setLayoutRoot] = useState<HTMLElement | null>(null);
   const [generation, setGeneration] = useState<number | null>(null);
   const [visibleTilesCallback, setVisibleTilesCallback] =

src/grid/OneOnOneLayout.tsx

@@ -13,6 +13,7 @@ import { type OneOnOneLayout as OneOnOneLayoutModel } from "../state/CallViewMod
 import { type CallLayout, arrangeTiles } from "./CallLayout";
 import styles from "./OneOnOneLayout.module.css";
 import { type DragCallback, useUpdateLayout } from "./Grid";
+import { useBehavior } from "../useBehavior";
 
 /**
  * An implementation of the "one-on-one" layout, in which the remote participant
@@ -32,7 +33,7 @@ export const makeOneOnOneLayout: CallLayout<OneOnOneLayoutModel> = ({
   scrolling: function OneOnOneLayoutScrolling({ ref, model, Slot }): ReactNode {
     useUpdateLayout();
     const { width, height } = useObservableEagerState(minBounds$);
-    const pipAlignmentValue = useObservableEagerState(pipAlignment$);
+    const pipAlignmentValue = useBehavior(pipAlignment$);
     const { tileWidth, tileHeight } = useMemo(
       () => arrangeTiles(width, height, 1),
       [width, height],

src/grid/SpotlightExpandedLayout.tsx

@@ -6,12 +6,12 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { type ReactNode, useCallback } from "react";
-import { useObservableEagerState } from "observable-hooks";
 
 import { type SpotlightExpandedLayout as SpotlightExpandedLayoutModel } from "../state/CallViewModel";
 import { type CallLayout } from "./CallLayout";
 import { type DragCallback, useUpdateLayout } from "./Grid";
 import styles from "./SpotlightExpandedLayout.module.css";
+import { useBehavior } from "../useBehavior";
 
 /**
  * An implementation of the "expanded spotlight" layout, in which the spotlight
@@ -46,7 +46,7 @@ export const makeSpotlightExpandedLayout: CallLayout<
     Slot,
   }): ReactNode {
     useUpdateLayout();
-    const pipAlignmentValue = useObservableEagerState(pipAlignment$);
+    const pipAlignmentValue = useBehavior(pipAlignment$);
 
     const onDragPip: DragCallback = useCallback(
       ({ xRatio, yRatio }) =>

src/grid/SpotlightPortraitLayout.tsx

@@ -13,6 +13,7 @@ import { type CallLayout, arrangeTiles } from "./CallLayout";
 import { type SpotlightPortraitLayout as SpotlightPortraitLayoutModel } from "../state/CallViewModel";
 import styles from "./SpotlightPortraitLayout.module.css";
 import { useUpdateLayout, useVisibleTiles } from "./Grid";
+import { useBehavior } from "../useBehavior";
 
 interface GridCSSProperties extends CSSProperties {
   "--grid-gap": string;
@@ -65,8 +66,7 @@ export const makeSpotlightPortraitLayout: CallLayout<
       width,
       model.grid.length,
     );
-    const withIndicators =
-      useObservableEagerState(model.spotlight.media$).length > 1;
+    const withIndicators = useBehavior(model.spotlight.media$).length > 1;
 
     return (
       <div

src/reactions/useReactionsSender.tsx

@@ -16,12 +16,12 @@ import {
 } from "react";
 import { type MatrixRTCSession } from "matrix-js-sdk/lib/matrixrtc";
 import { logger } from "matrix-js-sdk/lib/logger";
-import { useObservableEagerState } from "observable-hooks";
 
 import { useMatrixRTCSessionMemberships } from "../useMatrixRTCSessionMemberships";
 import { useClientState } from "../ClientContext";
 import { ElementCallReactionEventType, type ReactionOption } from ".";
 import { type CallViewModel } from "../state/CallViewModel";
+import { useBehavior } from "../useBehavior";
 
 interface ReactionsSenderContextType {
   supportsReactions: boolean;
@@ -70,7 +70,7 @@ export const ReactionsSenderProvider = ({
     [memberships, myUserId, myDeviceId],
   );
 
-  const reactions = useObservableEagerState(vm.reactions$);
+  const reactions = useBehavior(vm.reactions$);
   const myReaction = useMemo(
     () =>
       myMembershipIdentifier !== undefined
@@ -79,7 +79,7 @@ export const ReactionsSenderProvider = ({
     [myMembershipIdentifier, reactions],
   );
 
-  const handsRaised = useObservableEagerState(vm.handsRaised$);
+  const handsRaised = useBehavior(vm.handsRaised$);
   const myRaisedHand = useMemo(
     () =>
       myMembershipIdentifier !== undefined

src/room/GroupCallView.tsx

@@ -24,7 +24,6 @@ import {
   type MatrixRTCSession,
 } from "matrix-js-sdk/lib/matrixrtc";
 import { useNavigate } from "react-router-dom";
-import { useObservableEagerState } from "observable-hooks";
 
 import type { IWidgetApiRequest } from "matrix-widget-api";
 import {
@@ -72,6 +71,7 @@ import {
 import { useTypedEventEmitter } from "../useEvents";
 import { muteAllAudio$ } from "../state/MuteAllAudioModel.ts";
 import { useAppBarTitle } from "../AppBar.tsx";
+import { useBehavior } from "../useBehavior.ts";
 
 declare global {
   interface Window {
@@ -110,7 +110,7 @@ export const GroupCallView: FC<Props> = ({
   );
   const memberships = useMatrixRTCSessionMemberships(rtcSession);
 
-  const muteAllAudio = useObservableEagerState(muteAllAudio$);
+  const muteAllAudio = useBehavior(muteAllAudio$);
   const leaveSoundContext = useLatest(
     useAudioContext({
       sounds: callEventAudioSounds,

src/room/InCallView.tsx

@@ -25,7 +25,7 @@ import useMeasure from "react-use-measure";
 import { type MatrixRTCSession } from "matrix-js-sdk/lib/matrixrtc";
 import classNames from "classnames";
 import { BehaviorSubject, map } from "rxjs";
-import { useObservable, useObservableEagerState } from "observable-hooks";
+import { useObservable } from "observable-hooks";
 import { logger } from "matrix-js-sdk/lib/logger";
 import { RoomAndToDeviceEvents } from "matrix-js-sdk/lib/matrixrtc/RoomAndToDeviceKeyTransport";
 import {
@@ -112,6 +112,7 @@ import { useMatrixRTCSessionMemberships } from "../useMatrixRTCSessionMembership
 import { useMediaDevices } from "../MediaDevicesContext.ts";
 import { EarpieceOverlay } from "./EarpieceOverlay.tsx";
 import { useAppBarHidden, useAppBarSecondaryButton } from "../AppBar.tsx";
+import { useBehavior } from "../useBehavior.ts";
 
 const canScreenshare = "getDisplayMedia" in (navigator.mediaDevices ?? {});
 
@@ -251,7 +252,7 @@ export const InCallView: FC<InCallViewProps> = ({
     room: livekitRoom,
   });
 
-  const muteAllAudio = useObservableEagerState(muteAllAudio$);
+  const muteAllAudio = useBehavior(muteAllAudio$);
 
   // This seems like it might be enough logic to use move it into the call view model?
   const [didFallbackToRoomKey, setDidFallbackToRoomKey] = useState(false);
@@ -302,15 +303,15 @@ export const InCallView: FC<InCallViewProps> = ({
     () => void toggleRaisedHand(),
   );
 
-  const windowMode = useObservableEagerState(vm.windowMode$);
-  const layout = useObservableEagerState(vm.layout$);
-  const tileStoreGeneration = useObservableEagerState(vm.tileStoreGeneration$);
+  const windowMode = useBehavior(vm.windowMode$);
+  const layout = useBehavior(vm.layout$);
+  const tileStoreGeneration = useBehavior(vm.tileStoreGeneration$);
   const [debugTileLayout] = useSetting(debugTileLayoutSetting);
-  const gridMode = useObservableEagerState(vm.gridMode$);
-  const showHeader = useObservableEagerState(vm.showHeader$);
-  const showFooter = useObservableEagerState(vm.showFooter$);
-  const earpieceMode = useObservableEagerState(vm.earpieceMode$);
-  const audioOutputSwitcher = useObservableEagerState(vm.audioOutputSwitcher$);
+  const gridMode = useBehavior(vm.gridMode$);
+  const showHeader = useBehavior(vm.showHeader$);
+  const showFooter = useBehavior(vm.showFooter$);
+  const earpieceMode = useBehavior(vm.earpieceMode$);
+  const audioOutputSwitcher = useBehavior(vm.audioOutputSwitcher$);
   const switchCamera = useSwitchCamera(vm.localVideo$);
 
   // Ideally we could detect taps by listening for click events and checking
@@ -527,16 +528,12 @@ export const InCallView: FC<InCallViewProps> = ({
         targetHeight,
         model,
       }: TileProps<TileViewModel, HTMLDivElement>): ReactNode {
-        const spotlightExpanded = useObservableEagerState(
-          vm.spotlightExpanded$,
-        );
-        const onToggleExpanded = useObservableEagerState(
-          vm.toggleSpotlightExpanded$,
-        );
-        const showSpeakingIndicatorsValue = useObservableEagerState(
+        const spotlightExpanded = useBehavior(vm.spotlightExpanded$);
+        const onToggleExpanded = useBehavior(vm.toggleSpotlightExpanded$);
+        const showSpeakingIndicatorsValue = useBehavior(
           vm.showSpeakingIndicators$,
         );
-        const showSpotlightIndicatorsValue = useObservableEagerState(
+        const showSpotlightIndicatorsValue = useBehavior(
           vm.showSpotlightIndicators$,
         );
 

src/room/ReactionsOverlay.tsx

@@ -6,16 +6,16 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { type ReactNode } from "react";
-import { useObservableState } from "observable-hooks";
 
 import styles from "./ReactionsOverlay.module.css";
 import { type CallViewModel } from "../state/CallViewModel";
+import { useBehavior } from "../useBehavior";
 
 export function ReactionsOverlay({ vm }: { vm: CallViewModel }): ReactNode {
-  const reactionsIcons = useObservableState(vm.visibleReactions$);
+  const reactionsIcons = useBehavior(vm.visibleReactions$);
   return (
     <div className={styles.container}>
-      {reactionsIcons?.map(({ sender, emoji, startX }) => (
+      {reactionsIcons.map(({ sender, emoji, startX }) => (
         <span
           // Reactions effects are considered presentation elements. The reaction
           // is also present on the sender's tile, which assistive technology can

src/settings/SettingsModal.tsx

@@ -10,7 +10,6 @@ import { useTranslation } from "react-i18next";
 import { type MatrixClient } from "matrix-js-sdk";
 import { Button, Root as Form, Separator } from "@vector-im/compound-web";
 import { type Room as LivekitRoom } from "livekit-client";
-import { useObservableEagerState } from "observable-hooks";
 
 import { Modal } from "../Modal";
 import styles from "./SettingsModal.module.css";
@@ -34,6 +33,7 @@ import { DeveloperSettingsTab } from "./DeveloperSettingsTab";
 import { FieldRow, InputField } from "../input/Input";
 import { useSubmitRageshake } from "./submit-rageshake";
 import { useUrlParams } from "../UrlParams";
+import { useBehavior } from "../useBehavior";
 
 type SettingsTab =
   | "audio"
@@ -112,7 +112,7 @@ export const SettingsModal: FC<Props> = ({
   // rather than the input section.
   const { controlledAudioDevices } = useUrlParams();
   // If we are on iOS we will show a button to open the native audio device picker.
-  const iosDeviceMenu = useObservableEagerState(iosDeviceMenu$);
+  const iosDeviceMenu = useBehavior(iosDeviceMenu$);
 
   const audioTab: Tab<SettingsTab> = {
     key: "audio",

src/settings/settings.ts

@@ -6,10 +6,11 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { logger } from "matrix-js-sdk/lib/logger";
-import { BehaviorSubject, type Observable } from "rxjs";
-import { useObservableEagerState } from "observable-hooks";
+import { BehaviorSubject } from "rxjs";
 
 import { PosthogAnalytics } from "../analytics/PosthogAnalytics";
+import { type Behavior } from "../state/Behavior";
+import { useBehavior } from "../useBehavior";
 
 export class Setting<T> {
   public constructor(
@@ -38,7 +39,7 @@ export class Setting<T> {
   private readonly key: string;
 
   private readonly _value$: BehaviorSubject<T>;
-  public readonly value$: Observable<T>;
+  public readonly value$: Behavior<T>;
 
   public readonly setValue = (value: T): void => {
     this._value$.next(value);
@@ -53,7 +54,7 @@ export class Setting<T> {
  * React hook that returns a settings's current value and a setter.
  */
 export function useSetting<T>(setting: Setting<T>): [T, (value: T) => void] {
-  return [useObservableEagerState(setting.value$), setting.setValue];
+  return [useBehavior(setting.value$), setting.setValue];
 }
 
 // null = undecided

src/state/Behavior.ts

@@ -5,7 +5,7 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
-import { BehaviorSubject, Observable } from "rxjs";
+import { BehaviorSubject, distinctUntilChanged, Observable } from "rxjs";
 
 import { type ObservableScope } from "./ObservableScope";
 
@@ -45,7 +45,7 @@ Observable.prototype.behavior = function <T>(
 ): Behavior<T> {
   const subject$ = new BehaviorSubject<T | typeof nothing>(nothing);
   // Push values from the Observable into the BehaviorSubject
-  this.pipe(scope.bind()).subscribe(subject$);
+  this.pipe(scope.bind(), distinctUntilChanged()).subscribe(subject$);
   if (subject$.value === nothing)
     throw new Error("Behavior failed to synchronously emit an initial value");
   return subject$ as Behavior<T>;

src/state/CallViewModel.test.ts

@@ -75,6 +75,7 @@ import {
 import { ObservableScope } from "./ObservableScope";
 import { MediaDevices } from "./MediaDevices";
 import { getValue } from "../utils/observable";
+import { constant } from "./Behavior";
 
 const getUrlParams = vi.hoisted(() => vi.fn(() => ({})));
 vi.mock("../UrlParams", () => ({ getUrlParams }));
@@ -157,9 +158,10 @@ function summarizeLayout$(l$: Observable<Layout>): Observable<LayoutSummary> {
         case "grid":
           return combineLatest(
             [
-              l.spotlight?.media$ ?? of(undefined),
+              l.spotlight?.media$ ?? constant(undefined),
               ...l.grid.map((vm) => vm.media$),
             ],
+            // eslint-disable-next-line rxjs/finnish -- false positive
             (spotlight, ...grid) => ({
               type: l.type,
               spotlight: spotlight?.map((vm) => vm.id),
@@ -178,7 +180,8 @@ function summarizeLayout$(l$: Observable<Layout>): Observable<LayoutSummary> {
           );
         case "spotlight-expanded":
           return combineLatest(
-            [l.spotlight.media$, l.pip?.media$ ?? of(undefined)],
+            [l.spotlight.media$, l.pip?.media$ ?? constant(undefined)],
+            // eslint-disable-next-line rxjs/finnish -- false positive
             (spotlight, pip) => ({
               type: l.type,
               spotlight: spotlight.map((vm) => vm.id),

src/state/CallViewModel.ts

@@ -339,7 +339,7 @@ class ScreenShare {
     participant: LocalParticipant | RemoteParticipant,
     encryptionSystem: EncryptionSystem,
     liveKitRoom: LivekitRoom,
-    displayname$: Observable<string>,
+    displayName$: Observable<string>,
   ) {
     this.participant$ = new BehaviorSubject(participant);
 
@@ -349,7 +349,7 @@ class ScreenShare {
       this.participant$.asObservable(),
       encryptionSystem,
       liveKitRoom,
-      displayname$.behavior(this.scope),
+      displayName$.behavior(this.scope),
       participant.isLocal,
     );
   }
@@ -1271,14 +1271,14 @@ export class CallViewModel extends ViewModel {
   /**
    * Whether audio is currently being output through the earpiece.
    */
-  public readonly earpieceMode$: Observable<boolean> = combineLatest(
+  public readonly earpieceMode$: Behavior<boolean> = combineLatest(
     [
       this.mediaDevices.audioOutput.available$,
       this.mediaDevices.audioOutput.selected$,
     ],
     (available, selected) =>
       selected !== undefined && available.get(selected.id)?.type === "earpiece",
-  ).pipe(this.scope.state());
+  ).behavior(this.scope);
 
   /**
    * Callback to toggle between the earpiece and the loudspeaker.
@@ -1286,7 +1286,7 @@ export class CallViewModel extends ViewModel {
    * This will be `null` in case the target does not exist in the list
    * of available audio outputs.
    */
-  public readonly audioOutputSwitcher$: Observable<{
+  public readonly audioOutputSwitcher$: Behavior<{
     targetOutput: "earpiece" | "speaker";
     switch: () => void;
   } | null> = combineLatest(
@@ -1298,7 +1298,7 @@ export class CallViewModel extends ViewModel {
       const selectionType = selected && available.get(selected.id)?.type;
 
       // If we are in any output mode other than spaeker switch to speaker.
-      const newSelectionType =
+      const newSelectionType: "earpiece" | "speaker" =
         selectionType === "speaker" ? "earpiece" : "speaker";
       const newSelection = [...available].find(
         ([, d]) => d.type === newSelectionType,
@@ -1311,7 +1311,7 @@ export class CallViewModel extends ViewModel {
         switch: () => this.mediaDevices.audioOutput.select(id),
       };
     },
-  );
+  ).behavior(this.scope);
 
   public readonly reactions$ = this.reactionsSubject$
     .pipe(

src/state/MediaDevices.ts

@@ -10,7 +10,6 @@ import {
   filter,
   map,
   merge,
-  of,
   pairwise,
   startWith,
   Subject,
@@ -34,6 +33,7 @@ import {
 import { getUrlParams } from "../UrlParams";
 import { platform } from "../Platform";
 import { switchWhen } from "../utils/observable";
+import { type Behavior, constant } from "./Behavior";
 
 // This hardcoded id is used in EX ios! It can only be changed in coordination with
 // the ios swift team.
@@ -74,11 +74,11 @@ export interface MediaDevice<Label, Selected> {
   /**
    * A map from available device IDs to labels.
    */
-  available$: Observable<Map<string, Label>>;
+  available$: Behavior<Map<string, Label>>;
   /**
    * The selected device.
    */
-  selected$: Observable<Selected | undefined>;
+  selected$: Behavior<Selected | undefined>;
   /**
    * Selects a new device.
    */
@@ -94,36 +94,37 @@ export interface MediaDevice<Label, Selected> {
  *   `availableOutputDevices$.includes((d)=>d.forEarpiece)`
  */
 export const iosDeviceMenu$ =
-  platform === "ios" ? of(true) : alwaysShowIphoneEarpieceSetting.value$;
+  platform === "ios" ? constant(true) : alwaysShowIphoneEarpieceSetting.value$;
 
 function availableRawDevices$(
   kind: MediaDeviceKind,
-  usingNames$: Observable<boolean>,
+  usingNames$: Behavior<boolean>,
   scope: ObservableScope,
-): Observable<MediaDeviceInfo[]> {
+): Behavior<MediaDeviceInfo[]> {
   const logError = (e: Error): void =>
     logger.error("Error creating MediaDeviceObserver", e);
   const devices$ = createMediaDeviceObserver(kind, logError, false);
   const devicesWithNames$ = createMediaDeviceObserver(kind, logError, true);
 
-  return usingNames$.pipe(
-    switchMap((withNames) =>
-      withNames
-        ? // It might be that there is already a media stream running somewhere,
-          // and so we can do without requesting a second one. Only switch to the
-          // device observer that explicitly requests the names if we see that
-          // names are in fact missing from the initial device enumeration.
-          devices$.pipe(
-            switchWhen(
-              (devices, i) => i === 0 && devices.every((d) => !d.label),
-              devicesWithNames$,
-            ),
-          )
-        : devices$,
-    ),
-    startWith([]),
-    scope.state(),
-  );
+  return usingNames$
+    .pipe(
+      switchMap((withNames) =>
+        withNames
+          ? // It might be that there is already a media stream running somewhere,
+            // and so we can do without requesting a second one. Only switch to the
+            // device observer that explicitly requests the names if we see that
+            // names are in fact missing from the initial device enumeration.
+            devices$.pipe(
+              switchWhen(
+                (devices, i) => i === 0 && devices.every((d) => !d.label),
+                devicesWithNames$,
+              ),
+            )
+          : devices$,
+      ),
+      startWith([]),
+    )
+    .behavior(scope);
 }
 
 function buildDeviceMap(
@@ -161,42 +162,44 @@ function selectDevice$<Label>(
 }
 
 class AudioInput implements MediaDevice<DeviceLabel, SelectedAudioInputDevice> {
-  private readonly availableRaw$: Observable<MediaDeviceInfo[]> =
+  private readonly availableRaw$: Behavior<MediaDeviceInfo[]> =
     availableRawDevices$("audioinput", this.usingNames$, this.scope);
 
-  public readonly available$ = this.availableRaw$.pipe(
-    map(buildDeviceMap),
-    this.scope.state(),
-  );
+  public readonly available$ = this.availableRaw$
+    .pipe(map(buildDeviceMap))
+    .behavior(this.scope);
 
   public readonly selected$ = selectDevice$(
     this.available$,
     audioInputSetting.value$,
-  ).pipe(
-    map((id) =>
-      id === undefined
-        ? undefined
-        : {
-            id,
-            // We can identify when the hardware device has changed by watching for
-            // changes in the group ID
-            hardwareDeviceChange$: this.availableRaw$.pipe(
-              map((devices) => devices.find((d) => d.deviceId === id)?.groupId),
-              pairwise(),
-              filter(([before, after]) => before !== after),
-              map(() => undefined),
-            ),
-          },
-    ),
-    this.scope.state(),
-  );
+  )
+    .pipe(
+      map((id) =>
+        id === undefined
+          ? undefined
+          : {
+              id,
+              // We can identify when the hardware device has changed by watching for
+              // changes in the group ID
+              hardwareDeviceChange$: this.availableRaw$.pipe(
+                map(
+                  (devices) => devices.find((d) => d.deviceId === id)?.groupId,
+                ),
+                pairwise(),
+                filter(([before, after]) => before !== after),
+                map(() => undefined),
+              ),
+            },
+      ),
+    )
+    .behavior(this.scope);
 
   public select(id: string): void {
     audioInputSetting.setValue(id);
   }
 
   public constructor(
-    private readonly usingNames$: Observable<boolean>,
+    private readonly usingNames$: Behavior<boolean>,
     private readonly scope: ObservableScope,
   ) {
     this.available$.subscribe((available) => {
@@ -212,47 +215,48 @@ class AudioOutput
     "audiooutput",
     this.usingNames$,
     this.scope,
-  ).pipe(
-    map((availableRaw) => {
-      const available: Map<string, AudioOutputDeviceLabel> =
-        buildDeviceMap(availableRaw);
-      // Create a virtual default audio output for browsers that don't have one.
-      // Its device ID must be the empty string because that's what setSinkId
-      // recognizes.
-      if (available.size && !available.has("") && !available.has("default"))
-        available.set("", {
-          type: "default",
-          name: availableRaw[0]?.label || null,
-        });
-      // Note: creating virtual default input devices would be another problem
-      // entirely, because requesting a media stream from deviceId "" won't
-      // automatically track the default device.
-      return available;
-    }),
-    this.scope.state(),
-  );
+  )
+    .pipe(
+      map((availableRaw) => {
+        const available: Map<string, AudioOutputDeviceLabel> =
+          buildDeviceMap(availableRaw);
+        // Create a virtual default audio output for browsers that don't have one.
+        // Its device ID must be the empty string because that's what setSinkId
+        // recognizes.
+        if (available.size && !available.has("") && !available.has("default"))
+          available.set("", {
+            type: "default",
+            name: availableRaw[0]?.label || null,
+          });
+        // Note: creating virtual default input devices would be another problem
+        // entirely, because requesting a media stream from deviceId "" won't
+        // automatically track the default device.
+        return available;
+      }),
+    )
+    .behavior(this.scope);
 
   public readonly selected$ = selectDevice$(
     this.available$,
     audioOutputSetting.value$,
-  ).pipe(
-    map((id) =>
-      id === undefined
-        ? undefined
-        : {
-            id,
-            virtualEarpiece: false,
-          },
-    ),
-    this.scope.state(),
-  );
-
+  )
+    .pipe(
+      map((id) =>
+        id === undefined
+          ? undefined
+          : {
+              id,
+              virtualEarpiece: false,
+            },
+      ),
+    )
+    .behavior(this.scope);
   public select(id: string): void {
     audioOutputSetting.setValue(id);
   }
 
   public constructor(
-    private readonly usingNames$: Observable<boolean>,
+    private readonly usingNames$: Behavior<boolean>,
     private readonly scope: ObservableScope,
   ) {
     this.available$.subscribe((available) => {
@@ -287,7 +291,7 @@ class ControlledAudioOutput
 
       return available;
     },
-  ).pipe(this.scope.state());
+  ).behavior(this.scope);
 
   private readonly deviceSelection$ = new Subject<string>();
 
@@ -309,7 +313,7 @@ class ControlledAudioOutput
         ? undefined
         : { id, virtualEarpiece: id === EARPIECE_CONFIG_ID };
     },
-  ).pipe(this.scope.state());
+  ).behavior(this.scope);
 
   public constructor(private readonly scope: ObservableScope) {
     this.selected$.subscribe((device) => {
@@ -335,22 +339,21 @@ class VideoInput implements MediaDevice<DeviceLabel, SelectedDevice> {
     "videoinput",
     this.usingNames$,
     this.scope,
-  ).pipe(map(buildDeviceMap));
-
+  )
+    .pipe(map(buildDeviceMap))
+    .behavior(this.scope);
   public readonly selected$ = selectDevice$(
     this.available$,
     videoInputSetting.value$,
-  ).pipe(
-    map((id) => (id === undefined ? undefined : { id })),
-    this.scope.state(),
-  );
-
+  )
+    .pipe(map((id) => (id === undefined ? undefined : { id })))
+    .behavior(this.scope);
   public select(id: string): void {
     videoInputSetting.setValue(id);
   }
 
   public constructor(
-    private readonly usingNames$: Observable<boolean>,
+    private readonly usingNames$: Behavior<boolean>,
     private readonly scope: ObservableScope,
   ) {
     // This also has the purpose of subscribing to the available devices
@@ -378,12 +381,12 @@ export class MediaDevices {
   // you to do to receive device names in lieu of a more explicit permissions
   // API. This flag never resets to false, because once permissions are granted
   // the first time, the user won't be prompted again until reload of the page.
-  private readonly usingNames$ = this.deviceNamesRequest$.pipe(
-    map(() => true),
-    startWith(false),
-    this.scope.state(),
-  );
-
+  private readonly usingNames$ = this.deviceNamesRequest$
+    .pipe(
+      map(() => true),
+      startWith(false),
+    )
+    .behavior(this.scope);
   public readonly audioInput: MediaDevice<
     DeviceLabel,
     SelectedAudioInputDevice

src/state/MediaViewModel.ts

@@ -258,7 +258,7 @@ abstract class BaseMediaViewModel extends ViewModel {
     audioSource: AudioSource,
     videoSource: VideoSource,
     livekitRoom: LivekitRoom,
-    public readonly displayname$: Observable<string>,
+    public readonly displayName$: Behavior<string>,
   ) {
     super();
     const audio$ = observeTrackReference$(participant$, audioSource).behavior(
@@ -390,7 +390,7 @@ abstract class BaseUserMediaViewModel extends BaseMediaViewModel {
     participant$: Observable<LocalParticipant | RemoteParticipant | undefined>,
     encryptionSystem: EncryptionSystem,
     livekitRoom: LivekitRoom,
-    displayname$: Observable<string>,
+    displayName$: Behavior<string>,
     public readonly handRaised$: Behavior<Date | null>,
     public readonly reaction$: Behavior<ReactionOption | null>,
   ) {
@@ -402,7 +402,7 @@ abstract class BaseUserMediaViewModel extends BaseMediaViewModel {
       Track.Source.Microphone,
       Track.Source.Camera,
       livekitRoom,
-      displayname$,
+      displayName$,
     );
 
     const media$ = participant$
@@ -469,7 +469,7 @@ export class LocalUserMediaViewModel extends BaseUserMediaViewModel {
     participant$: Observable<LocalParticipant | undefined>,
     encryptionSystem: EncryptionSystem,
     livekitRoom: LivekitRoom,
-    displayname$: Observable<string>,
+    displayName$: Behavior<string>,
     handRaised$: Behavior<Date | null>,
     reaction$: Behavior<ReactionOption | null>,
   ) {
@@ -479,7 +479,7 @@ export class LocalUserMediaViewModel extends BaseUserMediaViewModel {
       participant$,
       encryptionSystem,
       livekitRoom,
-      displayname$,
+      displayName$,
       handRaised$,
       reaction$,
     );
@@ -562,7 +562,7 @@ export class RemoteUserMediaViewModel extends BaseUserMediaViewModel {
     participant$: Observable<RemoteParticipant | undefined>,
     encryptionSystem: EncryptionSystem,
     livekitRoom: LivekitRoom,
-    displayname$: Observable<string>,
+    displayname$: Behavior<string>,
     handRaised$: Behavior<Date | null>,
     reaction$: Behavior<ReactionOption | null>,
   ) {
@@ -627,7 +627,7 @@ export class ScreenShareViewModel extends BaseMediaViewModel {
     participant$: Observable<LocalParticipant | RemoteParticipant>,
     encryptionSystem: EncryptionSystem,
     livekitRoom: LivekitRoom,
-    displayname$: Observable<string>,
+    displayname$: Behavior<string>,
     public readonly local: boolean,
   ) {
     super(

src/state/MuteAllAudioModel.ts

@@ -9,6 +9,8 @@ import { combineLatest, startWith } from "rxjs";
 
 import { setAudioEnabled$ } from "../controls";
 import { muteAllAudio as muteAllAudioSetting } from "../settings/settings";
+import { globalScope } from "./ObservableScope";
+import "../state/Behavior"; // Patches in the Observable.behavior method
 
 /**
  * This can transition into sth more complete: `GroupCallViewModel.ts`
@@ -16,4 +18,4 @@ import { muteAllAudio as muteAllAudioSetting } from "../settings/settings";
 export const muteAllAudio$ = combineLatest(
   [setAudioEnabled$.pipe(startWith(true)), muteAllAudioSetting.value$],
   (outputEnabled, settingsMute) => !outputEnabled || settingsMute,
-);
+).behavior(globalScope);

src/state/ObservableScope.ts

@@ -33,3 +33,8 @@ export class ObservableScope {
     this.ended$.complete();
   }
 }
+
+/**
+ * The global scope, a scope which never ends.
+ */
+export const globalScope = new ObservableScope();

src/state/TileViewModel.ts

@@ -5,10 +5,9 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
-import { type Observable } from "rxjs";
-
 import { ViewModel } from "./ViewModel";
 import { type MediaViewModel, type UserMediaViewModel } from "./MediaViewModel";
+import { type Behavior } from "./Behavior";
 
 let nextId = 0;
 function createId(): string {
@@ -18,15 +17,15 @@ function createId(): string {
 export class GridTileViewModel extends ViewModel {
   public readonly id = createId();
 
-  public constructor(public readonly media$: Observable<UserMediaViewModel>) {
+  public constructor(public readonly media$: Behavior<UserMediaViewModel>) {
     super();
   }
 }
 
 export class SpotlightTileViewModel extends ViewModel {
   public constructor(
-    public readonly media$: Observable<MediaViewModel[]>,
-    public readonly maximised$: Observable<boolean>,
+    public readonly media$: Behavior<MediaViewModel[]>,
+    public readonly maximised$: Behavior<boolean>,
   ) {
     super();
   }

src/tile/GridTile.test.tsx

@@ -9,7 +9,6 @@ import { type RemoteTrackPublication } from "livekit-client";
 import { test, expect } from "vitest";
 import { render, screen } from "@testing-library/react";
 import { axe } from "vitest-axe";
-import { of } from "rxjs";
 import { type MatrixRTCSession } from "matrix-js-sdk/lib/matrixrtc";
 
 import { GridTile } from "./GridTile";
@@ -17,6 +16,7 @@ import { mockRtcMembership, withRemoteMedia } from "../utils/test";
 import { GridTileViewModel } from "../state/TileViewModel";
 import { ReactionsSenderProvider } from "../reactions/useReactionsSender";
 import type { CallViewModel } from "../state/CallViewModel";
+import { constant } from "../state/Behavior";
 
 global.IntersectionObserver = class MockIntersectionObserver {
   public observe(): void {}
@@ -53,13 +53,13 @@ test("GridTile is accessible", async () => {
         memberships: [],
       } as unknown as MatrixRTCSession;
       const cVm = {
-        reactions$: of({}),
-        handsRaised$: of({}),
+        reactions$: constant({}),
+        handsRaised$: constant({}),
       } as Partial<CallViewModel> as CallViewModel;
       const { container } = render(
         <ReactionsSenderProvider vm={cVm} rtcSession={fakeRtcSession}>
           <GridTile
-            vm={new GridTileViewModel(of(vm))}
+            vm={new GridTileViewModel(constant(vm))}
             onOpenProfile={() => {}}
             targetWidth={300}
             targetHeight={200}

src/tile/GridTile.tsx

@@ -35,7 +35,7 @@ import {
   ToggleMenuItem,
   Menu,
 } from "@vector-im/compound-web";
-import { useObservableEagerState, useObservableState } from "observable-hooks";
+import { useObservableEagerState } from "observable-hooks";
 
 import styles from "./GridTile.module.css";
 import {
@@ -49,6 +49,7 @@ import { useLatest } from "../useLatest";
 import { type GridTileViewModel } from "../state/TileViewModel";
 import { useMergedRefs } from "../useMergedRefs";
 import { useReactionsSender } from "../reactions/useReactionsSender";
+import { useBehavior } from "../useBehavior";
 
 interface TileProps {
   ref?: Ref<HTMLDivElement>;
@@ -81,19 +82,19 @@ const UserMediaTile: FC<UserMediaTileProps> = ({
 }) => {
   const { toggleRaisedHand } = useReactionsSender();
   const { t } = useTranslation();
-  const video = useObservableEagerState(vm.video$);
-  const unencryptedWarning = useObservableEagerState(vm.unencryptedWarning$);
-  const encryptionStatus = useObservableEagerState(vm.encryptionStatus$);
+  const video = useBehavior(vm.video$);
+  const unencryptedWarning = useBehavior(vm.unencryptedWarning$);
+  const encryptionStatus = useBehavior(vm.encryptionStatus$);
   const audioStreamStats = useObservableEagerState<
     RTCInboundRtpStreamStats | RTCOutboundRtpStreamStats | undefined
   >(vm.audioStreamStats$);
   const videoStreamStats = useObservableEagerState<
     RTCInboundRtpStreamStats | RTCOutboundRtpStreamStats | undefined
   >(vm.videoStreamStats$);
-  const audioEnabled = useObservableEagerState(vm.audioEnabled$);
-  const videoEnabled = useObservableEagerState(vm.videoEnabled$);
-  const speaking = useObservableEagerState(vm.speaking$);
-  const cropVideo = useObservableEagerState(vm.cropVideo$);
+  const audioEnabled = useBehavior(vm.audioEnabled$);
+  const videoEnabled = useBehavior(vm.videoEnabled$);
+  const speaking = useBehavior(vm.speaking$);
+  const cropVideo = useBehavior(vm.cropVideo$);
   const onSelectFitContain = useCallback(
     (e: Event) => {
       e.preventDefault();
@@ -101,8 +102,8 @@ const UserMediaTile: FC<UserMediaTileProps> = ({
     },
     [vm],
   );
-  const handRaised = useObservableState(vm.handRaised$);
-  const reaction = useObservableState(vm.reaction$);
+  const handRaised = useBehavior(vm.handRaised$);
+  const reaction = useBehavior(vm.reaction$);
 
   const AudioIcon = locallyMuted
     ? VolumeOffSolidIcon
@@ -205,8 +206,8 @@ const LocalUserMediaTile: FC<LocalUserMediaTileProps> = ({
   ...props
 }) => {
   const { t } = useTranslation();
-  const mirror = useObservableEagerState(vm.mirror$);
-  const alwaysShow = useObservableEagerState(vm.alwaysShow$);
+  const mirror = useBehavior(vm.mirror$);
+  const alwaysShow = useBehavior(vm.alwaysShow$);
   const latestAlwaysShow = useLatest(alwaysShow);
   const onSelectAlwaysShow = useCallback(
     (e: Event) => {
@@ -256,8 +257,8 @@ const RemoteUserMediaTile: FC<RemoteUserMediaTileProps> = ({
   ...props
 }) => {
   const { t } = useTranslation();
-  const locallyMuted = useObservableEagerState(vm.locallyMuted$);
-  const localVolume = useObservableEagerState(vm.localVolume$);
+  const locallyMuted = useBehavior(vm.locallyMuted$);
+  const localVolume = useBehavior(vm.localVolume$);
   const onSelectMute = useCallback(
     (e: Event) => {
       e.preventDefault();
@@ -328,8 +329,8 @@ export const GridTile: FC<GridTileProps> = ({
 }) => {
   const ourRef = useRef<HTMLDivElement | null>(null);
   const ref = useMergedRefs(ourRef, theirRef);
-  const media = useObservableEagerState(vm.media$);
-  const displayName = useObservableEagerState(media.displayname$);
+  const media = useBehavior(vm.media$);
+  const displayName = useBehavior(media.displayName$);
 
   if (media instanceof LocalUserMediaViewModel) {
     return (

src/tile/SpotlightTile.test.tsx

@@ -9,7 +9,6 @@ import { test, expect, vi } from "vitest";
 import { isInaccessible, render, screen } from "@testing-library/react";
 import { axe } from "vitest-axe";
 import userEvent from "@testing-library/user-event";
-import { of } from "rxjs";
 
 import { SpotlightTile } from "./SpotlightTile";
 import {
@@ -18,6 +17,7 @@ import {
   withRemoteMedia,
 } from "../utils/test";
 import { SpotlightTileViewModel } from "../state/TileViewModel";
+import { constant } from "../state/Behavior";
 
 global.IntersectionObserver = class MockIntersectionObserver {
   public observe(): void {}
@@ -44,7 +44,12 @@ test("SpotlightTile is accessible", async () => {
           const toggleExpanded = vi.fn();
           const { container } = render(
             <SpotlightTile
-              vm={new SpotlightTileViewModel(of([vm1, vm2]), of(false))}
+              vm={
+                new SpotlightTileViewModel(
+                  constant([vm1, vm2]),
+                  constant(false),
+                )
+              }
               targetWidth={300}
               targetHeight={200}
               expanded={false}

src/tile/SpotlightTile.tsx

@@ -23,7 +23,7 @@ import {
 } from "@vector-im/compound-design-tokens/assets/web/icons";
 import { animated } from "@react-spring/web";
 import { type Observable, map } from "rxjs";
-import { useObservableEagerState, useObservableRef } from "observable-hooks";
+import { useObservableRef } from "observable-hooks";
 import { useTranslation } from "react-i18next";
 import classNames from "classnames";
 import { type TrackReferenceOrPlaceholder } from "@livekit/components-core";
@@ -43,6 +43,7 @@ import { useMergedRefs } from "../useMergedRefs";
 import { useReactiveState } from "../useReactiveState";
 import { useLatest } from "../useLatest";
 import { type SpotlightTileViewModel } from "../state/TileViewModel";
+import { useBehavior } from "../useBehavior";
 
 interface SpotlightItemBaseProps {
   ref?: Ref<HTMLDivElement>;
@@ -73,7 +74,7 @@ const SpotlightLocalUserMediaItem: FC<SpotlightLocalUserMediaItemProps> = ({
   vm,
   ...props
 }) => {
-  const mirror = useObservableEagerState(vm.mirror$);
+  const mirror = useBehavior(vm.mirror$);
   return <MediaView mirror={mirror} {...props} />;
 };
 
@@ -87,8 +88,8 @@ const SpotlightUserMediaItem: FC<SpotlightUserMediaItemProps> = ({
   vm,
   ...props
 }) => {
-  const videoEnabled = useObservableEagerState(vm.videoEnabled$);
-  const cropVideo = useObservableEagerState(vm.cropVideo$);
+  const videoEnabled = useBehavior(vm.videoEnabled$);
+  const cropVideo = useBehavior(vm.cropVideo$);
 
   const baseProps: SpotlightUserMediaItemBaseProps &
     RefAttributes<HTMLDivElement> = {
@@ -130,10 +131,10 @@ const SpotlightItem: FC<SpotlightItemProps> = ({
 }) => {
   const ourRef = useRef<HTMLDivElement | null>(null);
   const ref = useMergedRefs(ourRef, theirRef);
-  const displayName = useObservableEagerState(vm.displayname$);
-  const video = useObservableEagerState(vm.video$);
-  const unencryptedWarning = useObservableEagerState(vm.unencryptedWarning$);
-  const encryptionStatus = useObservableEagerState(vm.encryptionStatus$);
+  const displayName = useBehavior(vm.displayName$);
+  const video = useBehavior(vm.video$);
+  const unencryptedWarning = useBehavior(vm.unencryptedWarning$);
+  const encryptionStatus = useBehavior(vm.encryptionStatus$);
 
   // Hook this item up to the intersection observer
   useEffect(() => {
@@ -200,8 +201,8 @@ export const SpotlightTile: FC<Props> = ({
   const { t } = useTranslation();
   const [ourRef, root$] = useObservableRef<HTMLDivElement | null>(null);
   const ref = useMergedRefs(ourRef, theirRef);
-  const maximised = useObservableEagerState(vm.maximised$);
-  const media = useObservableEagerState(vm.media$);
+  const maximised = useBehavior(vm.maximised$);
+  const media = useBehavior(vm.media$);
   const [visibleId, setVisibleId] = useState<string | undefined>(media[0]?.id);
   const latestMedia = useLatest(media);
   const latestVisibleId = useLatest(visibleId);

src/useAudioContext.test.tsx

@@ -10,12 +10,12 @@ import { type FC } from "react";
 import { render } from "@testing-library/react";
 import userEvent, { type UserEvent } from "@testing-library/user-event";
 import { BrowserRouter } from "react-router-dom";
-import { of } from "rxjs";
 
 import { MediaDevicesContext } from "./MediaDevicesContext";
 import { useAudioContext } from "./useAudioContext";
 import { soundEffectVolume as soundEffectVolumeSetting } from "./settings/settings";
 import { mockMediaDevices } from "./utils/test";
+import { constant } from "./state/Behavior";
 
 const staticSounds = Promise.resolve({
   aSound: new ArrayBuffer(0),
@@ -128,8 +128,8 @@ test("will use the correct device", () => {
     <MediaDevicesContext
       value={mockMediaDevices({
         audioOutput: {
-          available$: of(new Map<never, never>()),
-          selected$: of({ id: "chosen-device", virtualEarpiece: false }),
+          available$: constant(new Map<never, never>()),
+          selected$: constant({ id: "chosen-device", virtualEarpiece: false }),
           select: () => {},
         },
       })}
@@ -161,8 +161,8 @@ test("will use the pan if earpiece is selected", async () => {
     <MediaDevicesContext
       value={mockMediaDevices({
         audioOutput: {
-          available$: of(new Map<never, never>()),
-          selected$: of({ id: "chosen-device", virtualEarpiece: true }),
+          available$: constant(new Map<never, never>()),
+          selected$: constant({ id: "chosen-device", virtualEarpiece: true }),
           select: () => {},
         },
       })}

src/useBehavior.ts

@@ -0,0 +1,25 @@
+/*
+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 { useCallback, useSyncExternalStore } from "react";
+
+import { type Behavior } from "./state/Behavior";
+
+/**
+ * React hook which reactively reads the value of a behavior.
+ */
+export function useBehavior<T>(behavior: Behavior<T>): T {
+  const subscribe = useCallback(
+    (onChange: () => void) => {
+      const s = behavior.subscribe(onChange);
+      return (): void => s.unsubscribe();
+    },
+    [behavior],
+  );
+  const getValue = useCallback(() => behavior.value, [behavior]);
+  return useSyncExternalStore(subscribe, getValue);
+}

src/utils/test.ts

@@ -217,7 +217,7 @@ export async function withLocalMedia(
       kind: E2eeType.PER_PARTICIPANT,
     },
     mockLivekitRoom({ localParticipant }),
-    of(roomMember.rawDisplayName ?? "nodisplayname"),
+    constant(roomMember.rawDisplayName ?? "nodisplayname"),
     constant(null),
     constant(null),
   );
@@ -256,7 +256,7 @@ export async function withRemoteMedia(
       kind: E2eeType.PER_PARTICIPANT,
     },
     mockLivekitRoom({}, { remoteParticipants$: of([remoteParticipant]) }),
-    of(roomMember.rawDisplayName ?? "nodisplayname"),
+    constant(roomMember.rawDisplayName ?? "nodisplayname"),
     constant(null),
     constant(null),
   );