Throttle RoomState.members handler to improve performance

Lazy Loading emits a RoomState.members for each member which in the case of a room like
Matrix HQ means it happens over 8000 times in a very short span of time causing the UI
to lock up.

src/hooks/useRoomMembers.ts

@@ -0,0 +1,31 @@
+/*
+Copyright 2020 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 {useState} from "react";
+import {Room} from "matrix-js-sdk/src/models/room";
+import {RoomMember} from "matrix-js-sdk/src/models/room-member";
+
+import {useEventEmitter} from "./useEventEmitter";
+import {throttle} from "lodash";
+
+// Hook to simplify listening to Matrix Room members
+export const useRoomMembers = (room: Room, throttleWait = 250) => {
+    const [members, setMembers] = useState<RoomMember[]>(room.getJoinedMembers());
+    useEventEmitter(room.currentState, "RoomState.members", throttle(() => {
+        setMembers(room.getJoinedMembers());
+    }, throttleWait, {leading: true, trailing: true}));
+    return members;
+};