Implement MessagePreviewViewModel (#29514)

* Implement message preview vm

* Write tests

src/components/viewmodels/roomlist/MessagePreviewViewModel.tsx

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+/*
+ * 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 { useCallback, useEffect, useState } from "react";
+
+import type { Room } from "matrix-js-sdk/src/matrix";
+import { type MessagePreview, MessagePreviewStore } from "../../../stores/room-list/MessagePreviewStore";
+import { useEventEmitter } from "../../../hooks/useEventEmitter";
+
+interface MessagePreviewViewState {
+    /**
+     * A string representation of the message preview if available.
+     */
+    message?: string;
+}
+
+/**
+ * View model for rendering a message preview for a given room list item.
+ * @param room The room for which we're rendering the message preview.
+ * @see {@link MessagePreviewViewState} for what this view model returns.
+ */
+export function useMessagePreviewViewModel(room: Room): MessagePreviewViewState {
+    const [messagePreview, setMessagePreview] = useState<MessagePreview | null>(null);
+
+    const updatePreview = useCallback(async (): Promise<void> => {
+        /**
+         * The second argument to getPreviewForRoom is a tag id which doesn't really make
+         * much sense within the context of the new room list. We can pass an empty string
+         * to match all tags for now but we should remember to actually change the implementation
+         * in the store once we remove the legacy room list.
+         */
+        const newPreview = await MessagePreviewStore.instance.getPreviewForRoom(room, "");
+        setMessagePreview(newPreview);
+    }, [room]);
+
+    /**
+     * Update when the message preview has changed for this room.
+     */
+    useEventEmitter(MessagePreviewStore.instance, MessagePreviewStore.getPreviewChangedEventName(room), () => {
+        updatePreview();
+    });
+
+    /**
+     * Do an initial fetch of the message preview.
+     */
+    useEffect(() => {
+        updatePreview();
+    }, [updatePreview]);
+
+    return {
+        message: messagePreview?.text,
+    };
+}

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

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+/*
+ * 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);
+        });
+    });
+});