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/EchoContext.ts

@@ -0,0 +1,68 @@
+/*
+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 { EchoTransaction, RunFn, TransactionStatus } from "./EchoTransaction";
+import { arrayFastClone } from "../../utils/arrays";
+import { IDestroyable } from "../../utils/IDestroyable";
+import { Whenable } from "../../utils/Whenable";
+
+export enum ContextTransactionState {
+    NotStarted,
+    PendingErrors,
+    AllSuccessful
+}
+
+export abstract class EchoContext extends Whenable<ContextTransactionState> implements IDestroyable {
+    private _transactions: EchoTransaction[] = [];
+    public readonly startTime: Date = new Date();
+
+    public get transactions(): EchoTransaction[] {
+        return arrayFastClone(this._transactions);
+    }
+
+    public beginTransaction(auditName: string, runFn: RunFn): EchoTransaction {
+        const txn = new EchoTransaction(auditName, runFn);
+        this._transactions.push(txn);
+        txn.whenAnything(this.checkTransactions);
+
+        // We have no intent to call the transaction again if it succeeds (in fact, it'll
+        // be really angry at us if we do), so call that the end of the road for the events.
+        txn.when(TransactionStatus.DoneSuccess, () => txn.destroy());
+
+        return txn;
+    }
+
+    private checkTransactions = () => {
+        let status = ContextTransactionState.AllSuccessful;
+        for (const txn of this.transactions) {
+            if (txn.status === TransactionStatus.DoneError) {
+                status = ContextTransactionState.PendingErrors;
+                break;
+            } else if (txn.status === TransactionStatus.Pending) {
+                status = ContextTransactionState.NotStarted;
+                // no break as we might hit something which broke
+            }
+        }
+        this.notifyCondition(status);
+    };
+
+    public destroy() {
+        for (const txn of this.transactions) {
+            txn.destroy();
+        }
+        super.destroy();
+    }
+}