Add local echo capabilities for rooms

The structure here might need some documentation and work, but overall the idea is that all calls pass through a CachedEcho instance, which are self-updating.

src/stores/local-echo/RoomCachedEcho.ts

@@ -0,0 +1,77 @@
+/*
+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 { CachedEcho, implicitlyReverted, PROPERTY_UPDATED } from "./CachedEcho";
+import { getRoomNotifsState, setRoomNotifsState } from "../../RoomNotifs";
+import { RoomEchoContext } from "./RoomEchoContext";
+import { _t } from "../../languageHandler";
+import { Volume } from "../../RoomNotifsTypes";
+import { MatrixEvent } from "matrix-js-sdk/src/models/event";
+
+export type CachedRoomValues = Volume;
+
+export enum CachedRoomKey {
+    NotificationVolume,
+}
+
+export class RoomCachedEcho extends CachedEcho<RoomEchoContext, CachedRoomKey, CachedRoomValues> {
+    private properties = new Map<CachedRoomKey, CachedRoomValues>();
+
+    public constructor(context: RoomEchoContext) {
+        super(context, (k) => this.properties.get(k));
+    }
+
+    protected onClientChanged(oldClient, newClient) {
+        this.properties.clear();
+        if (oldClient) {
+            oldClient.removeListener("accountData", this.onAccountData);
+        }
+        if (newClient) {
+            // Register the listeners first
+            newClient.on("accountData", this.onAccountData);
+
+            // Then populate the properties map
+            this.updateNotificationVolume();
+        }
+    }
+
+    private onAccountData = (event: MatrixEvent) => {
+        if (event.getType() === "m.push_rules") {
+            const currentVolume = this.properties.get(CachedRoomKey.NotificationVolume) as Volume;
+            const newVolume = getRoomNotifsState(this.context.room.roomId) as Volume;
+            if (currentVolume !== newVolume) {
+                this.updateNotificationVolume();
+            }
+        }
+    };
+
+    private updateNotificationVolume() {
+        this.properties.set(CachedRoomKey.NotificationVolume, getRoomNotifsState(this.context.room.roomId));
+        this.emit(PROPERTY_UPDATED, CachedRoomKey.NotificationVolume);
+    }
+
+    // ---- helpers below here ----
+
+    public get notificationVolume(): Volume {
+        return this.getValue(CachedRoomKey.NotificationVolume);
+    }
+
+    public set notificationVolume(v: Volume) {
+        this.setValue(_t("Change notification settings"), CachedRoomKey.NotificationVolume, v, async () => {
+            setRoomNotifsState(this.context.room.roomId, v);
+        }, implicitlyReverted);
+    }
+}