Implement MessagePreviewViewModel (#29514)

* Implement message preview vm

* Write tests

src/components/viewmodels/roomlist/MessagePreviewViewModel.tsx

@@ -0,0 +1,57 @@
+/*
+ * 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,
+    };
+}