Add dialog to navigate long room topics (#8517)

src/hooks/room/useTopic.ts

@@ -0,0 +1,40 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { useEffect, useState } from "react";
+import { EventType } from "matrix-js-sdk/src/@types/event";
+import { MatrixEvent } from "matrix-js-sdk/src/models/event";
+import { Room } from "matrix-js-sdk/src/models/room";
+import { RoomStateEvent } from "matrix-js-sdk/src/models/room-state";
+
+import { useTypedEventEmitter } from "../useEventEmitter";
+
+export const getTopic = (room: Room) => {
+    return room?.currentState?.getStateEvents(EventType.RoomTopic, "")?.getContent()?.topic;
+};
+
+export function useTopic(room: Room): string {
+    const [topic, setTopic] = useState(getTopic(room));
+    useTypedEventEmitter(room.currentState, RoomStateEvent.Events, (ev: MatrixEvent) => {
+        if (ev.getType() !== EventType.RoomTopic) return;
+        setTopic(getTopic(room));
+    });
+    useEffect(() => {
+        setTopic(getTopic(room));
+    }, [room]);
+
+    return topic;
+}