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/EchoTransaction.ts

@@ -0,0 +1,65 @@
+/*
+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 { Whenable } from "../../utils/Whenable";
+
+export type RunFn = () => Promise<void>;
+
+export enum TransactionStatus {
+    Pending,
+    DoneSuccess,
+    DoneError,
+}
+
+export class EchoTransaction extends Whenable<TransactionStatus> {
+    private _status = TransactionStatus.Pending;
+    private didFail = false;
+
+    public constructor(
+        public readonly auditName,
+        public runFn: RunFn,
+    ) {
+        super();
+    }
+
+    public get didPreviouslyFail(): boolean {
+        return this.didFail;
+    }
+
+    public get status(): TransactionStatus {
+        return this._status;
+    }
+
+    public run() {
+        if (this.status === TransactionStatus.DoneSuccess) {
+            throw new Error("Cannot re-run a successful echo transaction");
+        }
+        this.setStatus(TransactionStatus.Pending);
+        this.runFn()
+            .then(() => this.setStatus(TransactionStatus.DoneSuccess))
+            .catch(() => this.setStatus(TransactionStatus.DoneError));
+    }
+
+    private setStatus(status: TransactionStatus) {
+        this._status = status;
+        if (status === TransactionStatus.DoneError) {
+            this.didFail = true;
+        } else if (status === TransactionStatus.DoneSuccess) {
+            this.didFail = false;
+        }
+        this.notifyCondition(status);
+    }
+}