sdk implementation

sdk/src/utils/LazyBehavior.test.ts

@@ -0,0 +1,52 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { describe, expect, it, vi } from "vitest";
+import { Observable, Subject } from "rxjs";
+
+import { LazyBehavior } from "./LazyBehavior";
+
+describe("LazyBehavior", () => {
+  it("runs its source only while subscribed", () => {
+    const teardown = vi.fn();
+    let starts = 0;
+    const values$ = new Subject<number>();
+    const source$ = new Observable<number>((subscriber) => {
+      starts++;
+      const subscription = values$.subscribe(subscriber);
+      return (): void => {
+        teardown();
+        subscription.unsubscribe();
+      };
+    });
+    const lazy$ = new LazyBehavior(source$, 0);
+    expect(starts).toBe(0);
+    expect(lazy$.value).toBe(0);
+
+    const seenByFirst: number[] = [];
+    const first = lazy$.subscribe((v) => seenByFirst.push(v));
+    expect(starts).toBe(1);
+    values$.next(1);
+    expect(seenByFirst).toEqual([0, 1]);
+    expect(lazy$.value).toBe(1);
+
+    const seenBySecond: number[] = [];
+    const second = lazy$.subscribe((v) => seenBySecond.push(v));
+    expect(starts).toBe(1);
+    expect(seenBySecond).toEqual([1]);
+
+    first.unsubscribe();
+    expect(teardown).not.toHaveBeenCalled();
+    second.unsubscribe();
+    expect(teardown).toHaveBeenCalledTimes(1);
+
+    values$.next(2);
+    expect(lazy$.value).toBe(1);
+    lazy$.subscribe().unsubscribe();
+    expect(starts).toBe(2);
+  });
+});