Add SubscriptionListener to streamline nullability handling

2 files changed, 33 insertions, 16 deletions

diff --git a/gui/packages/desktop/src/renderer/app.js b/gui/packages/desktop/src/renderer/app.js
index 6d68cf4e1b..66154bcc9d 100644
--- a/gui/packages/desktop/src/renderer/app.js
+++ b/gui/packages/desktop/src/renderer/app.js
@@ -14,7 +14,7 @@ import { createMemoryHistory } from 'history';
 
 import makeRoutes from './routes';
 import ReconnectionBackoff from './lib/reconnection-backoff';
-import { DaemonRpc, ConnectionObserver } from './lib/daemon-rpc';
+import { DaemonRpc, ConnectionObserver, SubscriptionListener } from './lib/daemon-rpc';
 import NotificationController from './lib/notification-controller';
 import setShutdownHandler from './lib/shutdown-handler';
 import { NoAccountError } from './errors';
@@ -499,18 +499,19 @@ export default class AppRenderer {
     this._connectedToDaemon = false;
   }
 
-  async _subscribeStateListener() {
-    await this._daemonRpc.subscribeStateListener((newState, error) => {
-      if (error) {
-        log.error(`Failed to deserialize the new state: ${error.message}`);
-      }
-
-      if (newState) {
+  _subscribeStateListener(): Promise<void> {
+    const listener = new SubscriptionListener(
+      (newState: TunnelStateTransition) => {
         log.debug(`Got state update: '${JSON.stringify(newState)}'`);
 
         this._onChangeTunnelState(newState);
-      }
-    });
+      },
+      (error: Error) => {
+        log.error(`Failed to deserialize the new state: ${error.message}`);
+      },
+    );
+
+    return this._daemonRpc.subscribeStateListener(listener);
   }
 
   _fetchInitialState() {
diff --git a/gui/packages/desktop/src/renderer/lib/daemon-rpc.js b/gui/packages/desktop/src/renderer/lib/daemon-rpc.js
index ca45c74af2..198a3e764d 100644
--- a/gui/packages/desktop/src/renderer/lib/daemon-rpc.js
+++ b/gui/packages/desktop/src/renderer/lib/daemon-rpc.js
@@ -278,6 +278,24 @@ export class ConnectionObserver {
   };
 }
 
+export class SubscriptionListener<T> {
+  _eventHandler: (payload: T) => void;
+  _errorHandler: (error: Error) => void;
+
+  constructor(eventHandler: (payload: T) => void, errorHandler: (error: Error) => void) {
+    this._eventHandler = eventHandler;
+    this._errorHandler = errorHandler;
+  }
+
+  _onEvent(payload: T) {
+    this._eventHandler(payload);
+  }
+
+  _onError(error: Error) {
+    this._errorHandler(error);
+  }
+}
+
 export interface DaemonRpcProtocol {
   connect({ path: string }): void;
   disconnect(): void;
@@ -297,7 +315,7 @@ export interface DaemonRpcProtocol {
   disconnectTunnel(): Promise<void>;
   getLocation(): Promise<Location>;
   getState(): Promise<TunnelStateTransition>;
-  subscribeStateListener((state: ?TunnelStateTransition, error: ?Error) => void): Promise<void>;
+  subscribeStateListener(listener: SubscriptionListener<TunnelStateTransition>): Promise<void>;
   addConnectionObserver(observer: ConnectionObserver): void;
   removeConnectionObserver(observer: ConnectionObserver): void;
   getAccountHistory(): Promise<Array<AccountToken>>;
@@ -484,15 +502,13 @@ export class DaemonRpc implements DaemonRpcProtocol {
     }
   }
 
-  subscribeStateListener(
-    listener: (state: ?TunnelStateTransition, error: ?Error) => void,
-  ): Promise<void> {
+  subscribeStateListener(listener: SubscriptionListener<TunnelStateTransition>): Promise<void> {
     return this._transport.subscribe('new_state', (payload) => {
       try {
         const newState = validate(TunnelStateTransitionSchema, payload);
-        listener(newState, null);
+        listener._onEvent(newState);
       } catch (error) {
-        listener(null, new ResponseParseError('Invalid payload from new_state', error));
+        listener._onError(new ResponseParseError('Invalid payload from new_state', error));
       }
     });
   }