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/utils/Whenable.ts

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+/*
+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 { IDestroyable } from "./IDestroyable";
+import { arrayFastClone } from "./arrays";
+
+export type WhenFn<T> = (w: Whenable<T>) => void;
+
+/**
+ * Whenables are a cheap way to have Observable patterns mixed with typical
+ * usage of Promises, without having to tear down listeners or calls. Whenables
+ * are intended to be used when a condition will be met multiple times and
+ * the consumer needs to know *when* that happens.
+ */
+export abstract class Whenable<T> implements IDestroyable {
+    private listeners: {condition: T | null, fn: WhenFn<T>}[] = [];
+
+    /**
+     * Sets up a call to `fn` *when* the `condition` is met.
+     * @param condition The condition to match.
+     * @param fn The function to call.
+     * @returns This.
+     */
+    public when(condition: T, fn: WhenFn<T>): Whenable<T> {
+        this.listeners.push({condition, fn});
+        return this;
+    }
+
+    /**
+     * Sets up a fall to `fn` *when* any of the `conditions` are met.
+     * @param conditions The conditions to match.
+     * @param fn The function to call.
+     * @returns This.
+     */
+    public whenAnyOf(conditions: T[], fn: WhenFn<T>): Whenable<T> {
+        for (const condition of conditions) {
+            this.when(condition, fn);
+        }
+        return this;
+    }
+
+    /**
+     * Sets up a call to `fn` *when* any condition is met.
+     * @param fn The function to call.
+     * @returns This.
+     */
+    public whenAnything(fn: WhenFn<T>): Whenable<T> {
+        this.listeners.push({condition: null, fn});
+        return this;
+    }
+
+    /**
+     * Notifies all the whenables of a given condition.
+     * @param condition The new condition that has been met.
+     */
+    protected notifyCondition(condition: T) {
+        const listeners = arrayFastClone(this.listeners); // clone just in case the handler modifies us
+        for (const listener of listeners) {
+            if (listener.condition === null || listener.condition === condition) {
+                try {
+                    listener.fn(this);
+                } catch (e) {
+                    console.error(`Error calling whenable listener for ${condition}:`, e);
+                }
+            }
+        }
+    }
+
+    public destroy() {
+        this.listeners = [];
+    }
+}