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/EchoStore.ts

@@ -0,0 +1,72 @@
+/*
+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 { EventEmitter } from "events";
+import { CachedEcho } from "./CachedEcho";
+import { Room } from "matrix-js-sdk/src/models/room";
+import { RoomCachedEcho } from "./RoomCachedEcho";
+import { RoomEchoContext } from "./RoomEchoContext";
+import { AsyncStoreWithClient } from "../AsyncStoreWithClient";
+import defaultDispatcher from "../../dispatcher/dispatcher";
+import { ActionPayload } from "../../dispatcher/payloads";
+
+type ContextKey = string;
+
+const roomContextKey = (room: Room): ContextKey => `room-${room.roomId}`;
+
+export class EchoStore extends AsyncStoreWithClient<any> {
+    private static _instance: EchoStore;
+
+    private caches = new Map<ContextKey, CachedEcho<any, any, any>>();
+
+    constructor() {
+        super(defaultDispatcher);
+    }
+
+    public static get instance(): EchoStore {
+        if (!EchoStore._instance) {
+            EchoStore._instance = new EchoStore();
+        }
+        return EchoStore._instance;
+    }
+
+    public getOrCreateEchoForRoom(room: Room): RoomCachedEcho {
+        if (this.caches.has(roomContextKey(room))) {
+            return this.caches.get(roomContextKey(room)) as RoomCachedEcho;
+        }
+        const echo = new RoomCachedEcho(new RoomEchoContext(room));
+        echo.setClient(this.matrixClient);
+        this.caches.set(roomContextKey(room), echo);
+        return echo;
+    }
+
+    protected async onReady(): Promise<any> {
+        for (const echo of this.caches.values()) {
+            echo.setClient(this.matrixClient);
+        }
+    }
+
+    protected async onNotReady(): Promise<any> {
+        for (const echo of this.caches.values()) {
+            echo.setClient(null);
+        }
+    }
+
+    protected async onAction(payload: ActionPayload): Promise<any> {
+        // We have nothing to actually listen for
+        return Promise.resolve();
+    }
+}