Async config file (#682)

* initial

* only donwload config once

* formatting

* update sample config

* sentry

* refactor load state

* fix build yaml

* Upper case enums

* change how defaults work. review fixes

* abstract initialization

* copyright

* gitignore styleing

* refactor initialization

* use dafualt as fallback

* internalInstance rename

* review

* remove acidentally added posthog file

* DSN rename

* update Copyright

* remove olm from the initializer

Co-authored-by: Timo K <timok@element.io>

src/config/Config.ts

@@ -0,0 +1,64 @@
+/*
+Copyright 2021-2022 New Vector Ltd
+
+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 { DEFAULT_CONFIG, IConfigOptions } from "./ConfigOptions";
+
+export class Config {
+  private static internalInstance: Config;
+  public static get instance(): Config {
+    if (!this.internalInstance)
+      throw new Error("Config instance read before config got initialized");
+    return this.internalInstance;
+  }
+  public static init(): Promise<void> {
+    if (Config?.internalInstance?.initPromise) {
+      return Config.internalInstance.initPromise;
+    }
+    Config.internalInstance = new Config();
+    Config.internalInstance.initPromise = new Promise<void>((resolve) => {
+      downloadConfig("../config.json").then((config) => {
+        Config.internalInstance.config = { ...DEFAULT_CONFIG, ...config };
+        resolve();
+      });
+    });
+    return Config.internalInstance.initPromise;
+  }
+
+  public config: IConfigOptions;
+  private initPromise: Promise<void>;
+}
+
+async function downloadConfig(
+  configJsonFilename: string
+): Promise<IConfigOptions> {
+  const url = new URL(configJsonFilename, window.location.href);
+  url.searchParams.set("cachebuster", Date.now().toString());
+  const res = await fetch(url, {
+    cache: "no-cache",
+    method: "GET",
+  });
+
+  if (res.status === 404 || res.status === 0) {
+    // Lack of a config isn't an error, we should just use the defaults.
+    // Also treat a blank config as no config, assuming the status code is 0, because we don't get 404s from file:
+    // URIs so this is the only way we can not fail if the file doesn't exist when loading from a file:// URI.
+    return {} as IConfigOptions;
+  }
+
+  if (res.ok) {
+    return res.json();
+  }
+}