Simple POC for moving download button to action bar

src/utils/LazyValue.ts

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+/*
+Copyright 2021 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.
+*/
+
+/**
+ * Utility class for lazily getting a variable.
+ */
+export class LazyValue<T> {
+    private val: T;
+    private prom: Promise<T>;
+    private done = false;
+
+    public constructor(private getFn: () => Promise<T>) {
+    }
+
+    /**
+     * Whether or not a cached value is present.
+     */
+    public get present(): boolean {
+        // we use a tracking variable just in case the final value is falsey
+        return this.done;
+    }
+
+    /**
+     * Gets the value without invoking a get. May be undefined until the
+     * value is fetched properly.
+     */
+    public get cachedValue(): T {
+        return this.val;
+    }
+
+    /**
+     * Gets a promise which resolves to the value, eventually.
+     */
+    public get value(): Promise<T> {
+        if (this.prom) return this.prom;
+        this.prom = this.getFn();
+
+        // Fork the promise chain to avoid accidentally making it return undefined always.
+        this.prom.then(v => {
+            this.val = v;
+            this.done = true;
+        });
+
+        return this.prom;
+    }
+}