Create a Behavior type

.eslintrc.cjs

@@ -44,7 +44,7 @@ module.exports = {
     ],
     // To encourage good usage of RxJS:
     "rxjs/no-exposed-subjects": "error",
-    "rxjs/finnish": "error",
+    "rxjs/finnish": ["error", { names: { "^this$": false } }],
   },
   settings: {
     react: {

src/state/Behavior.ts

@@ -0,0 +1,52 @@
+/*
+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 { BehaviorSubject, Observable } from "rxjs";
+
+import { type ObservableScope } from "./ObservableScope";
+
+/**
+ * A stateful, read-only reactive value. As an Observable, it is "hot" and
+ * always replays the current value upon subscription.
+ *
+ * A Behavior is to BehaviorSubject what Observable is to Subject; it does not
+ * provide a way to imperatively set new values. For more info on the
+ * distinction between Behaviors and Observables, see
+ * https://monoid.dk/post/behaviors-and-streams-why-both/.
+ */
+export type Behavior<T> = Omit<BehaviorSubject<T>, "next" | "observers">;
+
+/**
+ * Creates a Behavior which never changes in value.
+ */
+export function constant<T>(value: T): Behavior<T> {
+  return new BehaviorSubject(value);
+}
+
+declare module "rxjs" {
+  interface Observable<T> {
+    /**
+     * Converts this Observable into a Behavior. This requires the Observable to
+     * synchronously emit an initial value.
+     */
+    behavior(scope: ObservableScope): Behavior<T>;
+  }
+}
+
+const nothing = Symbol("nothing");
+
+Observable.prototype.behavior = function <T>(
+  this: Observable<T>,
+  scope: ObservableScope,
+): Behavior<T> {
+  const subject$ = new BehaviorSubject<T | typeof nothing>(nothing);
+  // Push values from the Observable into the BehaviorSubject
+  this.pipe(scope.bind()).subscribe(subject$);
+  if (subject$.value === nothing)
+    throw new Error("Behavior failed to synchronously emit an initial value");
+  return subject$ as Behavior<T>;
+};