Initial import

src/client/sse.ts

@@ -0,0 +1,104 @@
+import { validateMessage } from "../shared/message.js";
+import { Transport } from "../shared/transport.js";
+import { JSONRPCMessage } from "../types/index.js";
+
+/**
+ * Client transport for SSE: this will connect to a server using Server-Sent Events for receiving
+ * messages and make separate POST requests for sending messages.
+ *
+ * This uses the EventSource API in browsers. You can install the `eventsource` package for Node.js.
+ */
+export class SSEClientTransport implements Transport {
+  private _eventSource?: EventSource;
+  private _endpoint?: URL;
+  private _abortController?: AbortController;
+
+  onclose?: () => void;
+  onerror?: (error: Error) => void;
+  onmessage?: (message: JSONRPCMessage) => void;
+
+  connect(url: URL): Promise<void> {
+    return new Promise((resolve, reject) => {
+      this._eventSource = new EventSource(url.href);
+      this._abortController = new AbortController();
+
+      this._eventSource.onerror = (event) => {
+        const error = new Error(`SSE error: ${JSON.stringify(event)}`);
+        reject(error);
+        this.onerror?.(error);
+      };
+
+      this._eventSource.onopen = () => {
+        // The connection is open, but we need to wait for the endpoint to be received.
+      };
+
+      this._eventSource.addEventListener("endpoint", (event: Event) => {
+        const messageEvent = event as MessageEvent;
+
+        try {
+          this._endpoint = new URL(messageEvent.data, url);
+          if (this._endpoint.origin !== url.origin) {
+            throw new Error(
+              `Endpoint origin does not match connection origin: ${this._endpoint.origin}`,
+            );
+          }
+        } catch (error) {
+          reject(error);
+          this.onerror?.(error as Error);
+
+          void this.close();
+          return;
+        }
+
+        resolve();
+      });
+
+      this._eventSource.onmessage = (event: Event) => {
+        const messageEvent = event as MessageEvent;
+        let message: JSONRPCMessage;
+        try {
+          message = JSON.parse(messageEvent.data);
+          validateMessage(message);
+        } catch (error) {
+          this.onerror?.(error as Error);
+          return;
+        }
+
+        this.onmessage?.(message);
+      };
+    });
+  }
+
+  async close(): Promise<void> {
+    this._abortController?.abort();
+    this._eventSource?.close();
+    this.onclose?.();
+  }
+
+  async send(message: JSONRPCMessage): Promise<void> {
+    if (!this._endpoint) {
+      throw new Error("Not connected");
+    }
+
+    try {
+      const response = await fetch(this._endpoint, {
+        method: "POST",
+        headers: {
+          "Content-Type": "application/json",
+        },
+        body: JSON.stringify(message),
+        signal: this._abortController?.signal,
+      });
+
+      if (!response.ok) {
+        const text = await response.text().catch(() => null);
+        throw new Error(
+          `Error POSTing to endpoint (HTTP ${response.status}): ${text}`,
+        );
+      }
+    } catch (error) {
+      this.onerror?.(error as Error);
+      throw error;
+    }
+  }
+}