Add lifecycle customisation point after logout

This will help specific deployments that need to do something custom here such
as redirect the user or call some API after Element has logged out and cleared
storage.

src/Lifecycle.ts

@@ -49,6 +49,7 @@ import {SSO_HOMESERVER_URL_KEY, SSO_ID_SERVER_URL_KEY} from "./BasePlatform";
 import ThreepidInviteStore from "./stores/ThreepidInviteStore";
 import CountlyAnalytics from "./CountlyAnalytics";
 import CallHandler from './CallHandler';
+import LifecycleCustomisations from "./customisations/Lifecycle";
 
 const HOMESERVER_URL_KEY = "mx_hs_url";
 const ID_SERVER_URL_KEY = "mx_is_url";
@@ -716,6 +717,7 @@ export async function onLoggedOut(): Promise<void> {
     dis.dispatch({action: 'on_logged_out'}, true);
     stopMatrixClient();
     await clearStorage({deleteEverything: true});
+    LifecycleCustomisations.onLoggedOutAndStorageCleared?.();
 }
 
 /**

src/customisations/Lifecycle.ts

@@ -0,0 +1,30 @@
+/*
+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.
+*/
+
+function onLoggedOutAndStorageCleared(): void {
+    // E.g. redirect user or call other APIs after logout
+}
+
+// This interface summarises all available customisation points and also marks
+// them all as optional. This allows customisers to only define and export the
+// customisations they need while still maintaining type safety.
+export interface ILifecycleCustomisations {
+    onLoggedOutAndStorageCleared?: typeof onLoggedOutAndStorageCleared;
+}
+
+// A real customisation module will define and export one or more of the
+// customisation points that make up `ILifecycleCustomisations`.
+export default {} as ILifecycleCustomisations;