chore: import upstream snapshot with attribution

apps/webapp/app/hooks/useEventSource.tsx

@@ -0,0 +1,43 @@
+import { useEffect, useState } from "react";
+
+type EventSourceOptions = {
+  init?: EventSourceInit;
+  event?: string;
+  disabled?: boolean;
+};
+
+/**
+ * Subscribe to an event source and return the latest event.
+ * @param url The URL of the event source to connect to
+ * @param options The options to pass to the EventSource constructor
+ * @returns The last event received from the server
+ */
+export function useEventSource(
+  url: string | URL,
+  { event = "message", init, disabled }: EventSourceOptions = {}
+) {
+  const [data, setData] = useState<string | null>(null);
+
+  useEffect(() => {
+    if (disabled) {
+      return;
+    }
+
+    // reset data if dependencies change
+    setData(null);
+
+    const eventSource = new EventSource(url, init);
+    eventSource.addEventListener(event ?? "message", handler);
+
+    function handler(event: MessageEvent) {
+      setData(event.data || "UNKNOWN_EVENT_DATA");
+    }
+
+    return () => {
+      eventSource.removeEventListener(event ?? "message", handler);
+      eventSource.close();
+    };
+  }, [url, event, init, disabled]);
+
+  return data;
+}