Update the session in-place to avoid a screen/app reload

src/Lifecycle.ts

@@ -606,6 +606,34 @@ export async function hydrateSession(credentials: IMatrixClientCreds): Promise<M
     return doSetLoggedIn(credentials, overwrite);
 }
 
+/**
+ * Similar to hydrateSession(), this will update the credentials used by the current
+ * session in-place. Services will not be restarted, and storage will not be deleted.
+ * @param {IMatrixClientCreds} credentials The credentials to use
+ * @returns {Promise} promise which resolves to the new MatrixClient once it has been started
+ */
+export async function hydrateSessionInPlace(credentials: IMatrixClientCreds): Promise<MatrixClient> {
+    const oldUserId = MatrixClientPeg.get().getUserId();
+    const oldDeviceId = MatrixClientPeg.get().getDeviceId();
+    if (credentials.userId !== oldUserId || credentials.deviceId !== oldDeviceId) {
+        throw new Error("Attempted to hydrate in-place with a different session");
+    }
+
+    const cli = MatrixClientPeg.get();
+    if (!cli) {
+        throw new Error("Attempted to hydrate a non-existent MatrixClient");
+    }
+
+    logger.info("Lifecycle#hydrateInPlace: Persisting credentials and updating access token");
+    await persistCredentials(credentials);
+    MatrixClientPeg.updateUsingCreds(credentials);
+
+    // reset the token timers
+    TokenLifecycle.instance.startTimers(credentials);
+
+    return cli;
+}
+
 /**
  * fires on_logging_in, optionally clears localstorage, persists new credentials
  * to localstorage, starts the new client.

src/MatrixClientPeg.ts

@@ -101,6 +101,14 @@ export interface IMatrixClientPeg {
      * @param {IMatrixClientCreds} creds The new credentials to use.
      */
     replaceUsingCreds(creds: IMatrixClientCreds): void;
+
+    /**
+     * Similar to replaceUsingCreds(), but without the replacement operation.
+     * Credentials that can be updated in-place will be updated. All others
+     * will be ignored.
+     * @param {IMatrixClientCreds} creds The new credentials to use.
+     */
+    updateUsingCreds(creds: IMatrixClientCreds): void;
 }
 
 /**
@@ -166,6 +174,11 @@ class MatrixClientPegClass implements IMatrixClientPeg {
         this.createClient(creds);
     }
 
+    public updateUsingCreds(creds: IMatrixClientCreds): void {
+        this.currentClientCreds = creds;
+        this.matrixClient.setAccessToken(creds.accessToken);
+    }
+
     public async assign(): Promise<any> {
         for (const dbType of ['indexeddb', 'memory']) {
             try {

src/TokenLifecycle.ts

@@ -18,7 +18,7 @@ import { logger } from "matrix-js-sdk/src/logger";
 import { MatrixClient } from "matrix-js-sdk/src";
 
 import { IMatrixClientCreds, MatrixClientPeg } from "./MatrixClientPeg";
-import { hydrateSession } from "./Lifecycle";
+import { hydrateSessionInPlace } from "./Lifecycle";
 
 export interface IRenewedMatrixClientCreds extends Pick<IMatrixClientCreds,
     "accessToken" | "accessTokenExpiryTs" | "accessTokenRefreshToken"> {}
@@ -144,11 +144,11 @@ export class TokenLifecycle {
                 this.startTimers(credentials);
             } else {
                 logger.info("TokenLifecycle#expireExchange: Updating client credentials using rehydration");
-                await hydrateSession({
+                await hydrateSessionInPlace({
                     ...credentials,
                     ...result, // override from credentials
                 });
-                // hydrateSession will ultimately call back to startTimers() for us, so no need to do it here.
+                // hydrateSessionInPlace will ultimately call back to startTimers() for us, so no need to do it here.
             }
         } catch (e) {
             logger.error("TokenLifecycle#expireExchange: Error getting new credentials. Rescheduling.", e);