feat: add error message parameter to onError callback in subscription

packages/ts/frontend/src/Connect.ts

@@ -42,7 +42,7 @@ export interface Subscription<T> {
   onComplete(callback: () => void): Subscription<T>;
 
   /** Called when an exception occured in the subscription. */
-  onError(callback: () => void): Subscription<T>;
+  onError(callback: (message: string) => void): Subscription<T>;
 
   /** Called when a new value is available. */
   onNext(callback: (value: T) => void): Subscription<T>;

packages/ts/frontend/src/FluxConnection.ts

@@ -80,7 +80,7 @@ export class FluxConnection extends EventTarget {
   readonly #endpointInfos = new Map<string, EndpointInfo>();
   #nextId = 0;
   readonly #onCompleteCallbacks = new Map<string, () => void>();
-  readonly #onErrorCallbacks = new Map<string, () => void>();
+  readonly #onErrorCallbacks = new Map<string, (message: string) => void>();
   readonly #onNextCallbacks = new Map<string, (value: any) => void>();
   readonly #onStateChangeCallbacks = new Map<string, (event: FluxSubscriptionStateChangeEvent) => void>();
   readonly #statusOfSubscriptions = new Map<string, FluxSubscriptionState>();
@@ -154,7 +154,7 @@ export class FluxConnection extends EventTarget {
         this.#onCompleteCallbacks.set(id, callback);
         return hillaSubscription;
       },
-      onError: (callback: () => void): Subscription<any> => {
+      onError: (callback: (message: string) => void): Subscription<any> => {
         this.#onErrorCallbacks.set(id, callback);
         return hillaSubscription;
       },
@@ -285,7 +285,7 @@ export class FluxConnection extends EventTarget {
       } else {
         const callback = this.#onErrorCallbacks.get(id);
         if (callback) {
-          callback();
+          callback(message.message);
         }
         this.#removeSubscription(id);
         if (!callback) {

packages/ts/frontend/test/FluxConnection.test.ts

@@ -125,6 +125,7 @@ describe('@vaadin/hilla-frontend', () => {
       const msg: ClientErrorMessage = { '@type': 'error', id: '0', message: 'it failed' };
       emitMessage(msg);
       expect(onError).to.have.been.calledOnce;
+      expect(onError).to.have.been.calledWith('it failed');
     });
 
     it('should not deliver messages after completing', () => {