Add test for snapshot class

test/viewmodels/base/Snapshot-test.ts

@@ -0,0 +1,41 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import { Snapshot } from "../../../src/viewmodels/base/Snapshot";
+
+interface TestSnapshot {
+    key1: string;
+    key2: number;
+    key3: boolean;
+}
+
+describe("Snapshot", () => {
+    it("should accept an initial value", () => {
+        const snapshot = new Snapshot<TestSnapshot>({ key1: "foo", key2: 5, key3: false }, jest.fn());
+        expect(snapshot.current).toStrictEqual({ key1: "foo", key2: 5, key3: false });
+    });
+
+    it("should call emit callback when state changes", () => {
+        const emit = jest.fn();
+        const snapshot = new Snapshot<TestSnapshot>({ key1: "foo", key2: 5, key3: false }, emit);
+        snapshot.merge({ key3: true });
+        expect(emit).toHaveBeenCalledTimes(1);
+    });
+
+    it("should swap out entire snapshot on set call", () => {
+        const snapshot = new Snapshot<TestSnapshot>({ key1: "foo", key2: 5, key3: false }, jest.fn());
+        const newValue = { key1: "bar", key2: 8, key3: true };
+        snapshot.set(newValue);
+        expect(snapshot.current).toStrictEqual(newValue);
+    });
+
+    it("should merge partial snapshot on merge call", () => {
+        const snapshot = new Snapshot<TestSnapshot>({ key1: "foo", key2: 5, key3: false }, jest.fn());
+        snapshot.merge({ key2: 10 });
+        expect(snapshot.current).toStrictEqual({ key1: "foo", key2: 10, key3: false });
+    });
+});