Implement MessagePreviewViewModel (#29514)

* Implement message preview vm

* Write tests

test/unit-tests/components/viewmodels/roomlist/MessagePreviewViewModel-test.tsx

@@ -0,0 +1,58 @@
+/*
+ * 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 { renderHook, waitFor } from "jest-matrix-react";
+import { type Room } from "matrix-js-sdk/src/matrix";
+
+import { createTestClient, mkStubRoom } from "../../../../test-utils";
+import { type MessagePreview, MessagePreviewStore } from "../../../../../src/stores/room-list/MessagePreviewStore";
+import { useMessagePreviewViewModel } from "../../../../../src/components/viewmodels/roomlist/MessagePreviewViewModel";
+
+describe("MessagePreviewViewModel", () => {
+    let room: Room;
+
+    beforeEach(() => {
+        const matrixClient = createTestClient();
+        room = mkStubRoom("roomId", "roomName", matrixClient);
+    });
+
+    it("should do an initial fetch of the message preview", async () => {
+        // Mock the store to return some text.
+        jest.spyOn(MessagePreviewStore.instance, "getPreviewForRoom").mockImplementation(async (room) => {
+            return { text: "Hello world!" } as MessagePreview;
+        });
+
+        const { result: vm } = renderHook(() => useMessagePreviewViewModel(room));
+
+        // Eventually, vm.message should have the text from the store.
+        await waitFor(() => {
+            expect(vm.current.message).toEqual("Hello world!");
+        });
+    });
+
+    it("should fetch message preview again on update from store", async () => {
+        // Mock the store to return the text in variable message.
+        let message = "Hello World!";
+        jest.spyOn(MessagePreviewStore.instance, "getPreviewForRoom").mockImplementation(async (room) => {
+            return { text: message } as MessagePreview;
+        });
+        jest.spyOn(MessagePreviewStore, "getPreviewChangedEventName").mockImplementation((room) => {
+            return "UPDATE";
+        });
+
+        const { result: vm } = renderHook(() => useMessagePreviewViewModel(room));
+
+        // Let's assume the message changed.
+        message = "New message!";
+        MessagePreviewStore.instance.emit("UPDATE");
+
+        /// vm.message should be the updated message.
+        await waitFor(() => {
+            expect(vm.current.message).toEqual(message);
+        });
+    });
+});