fix: RN event source does not capture headers in a timely manner

Author: joker23

packages/sdk/react-native/__tests__/fromExternal/react-native-sse/EventSource.test.ts

@@ -167,4 +167,60 @@ describe('EventSource', () => {
       }),
     );
   });
+
+  test('calls onopen with parsed response headers during streaming (onprogress, pre-DONE)', () => {
+    const onopen = jest.fn();
+    eventSource.onopen = onopen;
+
+    mockXhr.getAllResponseHeaders = jest.fn(
+      () => 'X-Ld-Fd-Fallback: true\r\nX-Ld-Fd-Fallback-Ttl: 60\r\nContent-Type: text/event-stream',
+    );
+    mockXhr.responseText = '';
+
+    jest.runAllTimers();
+
+    // Simulate a chunk arriving while the connection is still LOADING (readyState 3).
+    // This is the real runtime path for a live stream: DONE (readyState 4) only
+    // fires once the connection closes, so gating 'open' on DONE would mean the
+    // event, and the fallback headers it carries, never fires during normal streaming.
+    mockXhr.readyState = 3;
+    mockXhr.status = 200;
+    mockXhr.onprogress();
+
+    expect(onopen).toHaveBeenCalledTimes(1);
+    expect(onopen).toHaveBeenCalledWith(
+      expect.objectContaining({
+        type: 'open',
+        headers: expect.objectContaining({
+          'x-ld-fd-fallback': 'true',
+          'x-ld-fd-fallback-ttl': '60',
+        }),
+      }),
+    );
+  });
+
+  test('dispatches open exactly once when onprogress precedes an onreadystatechange DONE', () => {
+    const onopen = jest.fn();
+    eventSource.onopen = onopen;
+
+    mockXhr.getAllResponseHeaders = jest.fn(
+      () => 'X-Ld-Fd-Fallback: true\r\nX-Ld-Fd-Fallback-Ttl: 60\r\nContent-Type: text/event-stream',
+    );
+    mockXhr.responseText = '';
+
+    jest.runAllTimers();
+
+    // onprogress observes the connection first (LOADING) and transitions to OPEN.
+    mockXhr.readyState = 3;
+    mockXhr.status = 200;
+    mockXhr.onprogress();
+
+    // A later onreadystatechange at DONE must not re-dispatch open: the status is
+    // already OPEN (no longer CONNECTING), so its open-dispatch guard is a no-op.
+    mockXhr.readyState = 4;
+    mockXhr.status = 200;
+    mockXhr.onreadystatechange();
+
+    expect(onopen).toHaveBeenCalledTimes(1);
+  });
 });

packages/sdk/react-native/src/fromExternal/react-native-sse/EventSource.ts

@@ -7,10 +7,17 @@
  * 4. replaced all for of loops with foreach
  *
  * Additional changes:
- * 1. separated event handling to use onprogress for data changes
- *    and onreadystatechange for status changes. This is to address
- *    an issue with Vega OS where they do not fire a readyStatechange
- *    event when the response is received.
+ * 1. separated event handling to use onprogress for data changes and
+ *    onreadystatechange for status changes, since some platforms (e.g. Vega OS)
+ *    never fire a readystatechange event while the response is still streaming in.
+ * 2. onprogress now also performs the CONNECTING -> OPEN transition and dispatches
+ *    open, guarded by the same this._status === CONNECTING check onreadystatechange
+ *    already used. Previously open only fired from onreadystatechange at
+ *    readyState DONE, which for a long-lived stream only happens at connection
+ *    close, so the parsed response headers carried on open were never seen while
+ *    the stream was actually running. onprogress fires as soon as the response
+ *    starts loading, so headers are available as soon as the connection opens
+ *    instead of only at teardown.
  */
 import type { EventSourceEvent, EventSourceListener, EventSourceOptions, EventType } from './types';
 
@@ -158,6 +165,16 @@ export default class EventSource<E extends string = never> {
         );
 
         if (this._xhr.status >= 200 && this._xhr.status < 400) {
+          if (this._status === this.CONNECTING) {
+            this._retryCount = 0;
+            this._status = this.OPEN;
+            this.dispatch('open', {
+              type: 'open',
+              headers: this._parseResponseHeaders(this._xhr.getAllResponseHeaders()),
+            });
+            this._logger?.debug('[EventSource][onprogress][OPEN] Connection opened.');
+          }
+
           this._handleEvent(this._xhr.responseText || '');
         } else {
           this._status = this.ERROR;