From f5b4254add2c308b5cc540cc7bbc18eb92cdc368 Mon Sep 17 00:00:00 2001
From: Marcelo Paternostro <marcelo.paternostro@datadoghq.com>
Date: Sun, 13 Apr 2025 19:50:24 -0400
Subject: [PATCH] event notifier

---
 src/shared/eventNotifier.test.ts | 145 ++++++++++++++++++++++++++++++
 src/shared/eventNotifier.ts      | 149 +++++++++++++++++++++++++++++++
 2 files changed, 294 insertions(+)
 create mode 100644 src/shared/eventNotifier.test.ts
 create mode 100644 src/shared/eventNotifier.ts

diff --git a/src/shared/eventNotifier.test.ts b/src/shared/eventNotifier.test.ts
new file mode 100644
index 00000000..2923f7a0
--- /dev/null
+++ b/src/shared/eventNotifier.test.ts
@@ -0,0 +1,145 @@
+import { createEventNotifier } from "./eventNotifier.js";
+
+describe("EventNotifier", () => {
+  let notifier: ReturnType<typeof createEventNotifier<string>>;
+
+  beforeEach(() => {
+    notifier = createEventNotifier<string>();
+  });
+
+  test("should notify listeners in registration order", () => {
+    const events: string[] = [];
+    notifier.onEvent((event) => events.push(`first: ${event}`));
+    notifier.onEvent((event) => events.push(`second: ${event}`));
+    notifier.onEvent((event) => events.push(`third: ${event}`));
+
+    notifier.notify("test event");
+
+    expect(events).toEqual([
+      "first: test event",
+      "second: test event",
+      "third: test event",
+    ]);
+  });
+
+  test("should not notify unsubscribed listeners", () => {
+    const events: string[] = [];
+    const subscription = notifier.onEvent((event) => events.push(event));
+
+    notifier.notify("first event");
+    subscription.close();
+    notifier.notify("second event");
+
+    expect(events).toEqual(["first event"]);
+  });
+
+  test("should handle function events", () => {
+    const events: string[] = [];
+    notifier.onEvent((event) => events.push(event));
+
+    notifier.notify(() => "dynamic event");
+
+    expect(events).toEqual(["dynamic event"]);
+  });
+
+  test("should handle errors through error handler", () => {
+    const errors: Error[] = [];
+    notifier.onError((error) => errors.push(error));
+
+    notifier.onEvent(() => {
+      throw new Error("test error");
+    });
+
+    notifier.notify("test event");
+
+    expect(errors).toHaveLength(1);
+    expect(errors[0]).toBeInstanceOf(Error);
+    expect(errors[0].message).toBe("test error");
+  });
+
+  test("should not notify after close", () => {
+    const events: string[] = [];
+    notifier.onEvent((event) => events.push(event));
+
+    notifier.notify("first event");
+    notifier.close();
+    notifier.notify("second event");
+
+    expect(events).toEqual(["first event"]);
+  });
+
+  test("should handle multiple subscriptions and unsubscriptions", () => {
+    const events: string[] = [];
+    const subscription1 = notifier.onEvent((event) =>
+      events.push(`1: ${event}`)
+    );
+    const subscription2 = notifier.onEvent((event) =>
+      events.push(`2: ${event}`)
+    );
+
+    notifier.notify("first event");
+    subscription1.close();
+    notifier.notify("second event");
+    subscription2.close();
+    notifier.notify("third event");
+
+    expect(events).toEqual([
+      "1: first event",
+      "2: first event",
+      "2: second event",
+    ]);
+  });
+
+  test("should handle error handler after close", () => {
+    const errors: Error[] = [];
+    notifier.onError((error) => errors.push(error));
+    notifier.close();
+
+    notifier.onEvent(() => {
+      throw new Error("test error");
+    });
+
+    notifier.notify("test event");
+
+    expect(errors).toHaveLength(0);
+  });
+
+  test("should clear error handler on close", () => {
+    const errors: Error[] = [];
+    notifier.onError((error) => errors.push(error));
+
+    // Close should clear the error handler
+    notifier.close();
+
+    // Setting up a new listener after close
+    notifier.onEvent(() => {
+      throw new Error("test error");
+    });
+
+    // This should not trigger the error handler since the notifier is closed
+    notifier.notify("test event");
+
+    expect(errors).toHaveLength(0);
+  });
+
+  test("should use the last set error handler", () => {
+    const errors1: Error[] = [];
+    const errors2: Error[] = [];
+
+    // First error handler
+    notifier.onError((error) => errors1.push(error));
+
+    // Second error handler should replace the first one
+    notifier.onError((error) => errors2.push(error));
+
+    notifier.onEvent(() => {
+      throw new Error("test error");
+    });
+
+    notifier.notify("test event");
+
+    expect(errors1).toHaveLength(0);
+    expect(errors2).toHaveLength(1);
+    expect(errors2[0].message).toBe("test error");
+  });
+});
diff --git a/src/shared/eventNotifier.ts b/src/shared/eventNotifier.ts
new file mode 100644
index 00000000..349a9446
--- /dev/null
+++ b/src/shared/eventNotifier.ts
@@ -0,0 +1,149 @@
+/**
+ * Provides a simple, type-safe event notification implementation. This module allows components to implement the
+ * observer pattern with minimal boilerplate and proper type checking.
+ */
+
+/**
+ * A type-safe event notifier that manages event listeners and notifications.
+ *
+ * @template T The type of events this notifier will handle
+ * @template E The type of error that can be handled (defaults to Error)
+ *
+ *   EventNotifier provides:
+ *
+ *   - Type-safe event subscriptions via `onEvent`
+ *   - Synchronized event notifications via `notify`
+ *   - Automatic cleanup of resources via `close`
+ *   - Status tracking via `active` property
+ *   - Error handling via optional error callback
+ */
+export type EventNotifier<T, E = Error> = {
+  /**
+   * Registers a listener function to be called when events are notified. Listeners are notified in the order they
+   * were registered.
+   *
+   * @example
+   *
+   * ```ts
+   * const notifier = createEventNotifier<string>();
+   * const subscription = notifier.onEvent((message) => {
+   *   console.log(`Received message: ${message}`);
+   * });
+   *
+   * // Later, to stop listening:
+   * subscription.close();
+   * ```
+   *
+   * @param listener A function that will be called with the notified event
+   * @returns A SyncCloseable that, when closed, will unregister the listener
+   */
+  onEvent: (listener: (event: T) => unknown) => { close: () => void };
+
+  /**
+   * Notifies all registered listeners with the provided event.
+   *
+   * This method:
+   *
+   * - Calls all registered listeners with the event in their registration order
+   * - Ignores errors thrown by listeners (they won't affect other listeners)
+   * - Ignores returned promises (results are not awaited)
+   * - Does nothing if there are no listeners
+   * - If the event is a function, it will be called if there are listeners and its return value will be
+   *   used as the event.
+   *
+   * @example
+   *
+   * ```ts
+   * const notifier = createEventNotifier<{ type: string; data: unknown }>();
+   * notifier.onEvent((event) => {
+   *   console.log(`Received ${event.type} with data:`, event.data);
+   * });
+   *
+   * notifier.notify({ type: 'update', data: { id: 123, status: 'complete' } });
+   * ```
+   *
+   * @param event The event to send to all listeners or a function that returns such event.
+   */
+  notify: (event: T | (() => T)) => void;
+
+  /**
+   * Sets an error handler for the notifier. This handler will be called when a listener throws an error.
+   *
+   * @param handler A function that will be called with any errors thrown by listeners
+   */
+  onError: (handler: (error: E) => void) => void;
+
+  /**
+   * Closes the notifier and removes all listeners.
+   *
+   * @warning Failing to call close() on subscriptions or the notifier itself may lead to memory leaks.
+   */
+  close: () => void;
+};
+
+/**
+ * Creates a type-safe event notifier.
+ *
+ * @example
+ *
+ * ```ts
+ * // Simple string event notifier
+ * const stringNotifier = createEventNotifier<string>();
+ *
+ * // Complex object event notifier
+ * interface UserEvent {
+ *   type: 'created' | 'updated' | 'deleted';
+ *   userId: number;
+ *   data?: Record<string, unknown>;
+ * }
+ * const userNotifier = createEventNotifier<UserEvent>();
+ * ```
+ *
+ * @template T The type of events this notifier will handle
+ * @template E The type of error that can be handled (defaults to Error)
+ * @returns A new EventNotifier instance
+ */
+export const createEventNotifier = <T, E = Error>(): EventNotifier<T, E> => {
+  const listeners = new Set<(event: T) => unknown>();
+  let errorHandler: ((error: E) => void) | undefined;
+
+  return {
+    close: () => {
+      listeners.clear();
+      errorHandler = undefined;
+    },
+
+    onEvent: (listener) => {
+      listeners.add(listener);
+      return {
+        close: () => {
+          listeners.delete(listener);
+        },
+      };
+    },
+
+    notify: (event: T | (() => T)) => {
+      if (!listeners.size) {
+        return;
+      }
+
+      if (typeof event === "function") {
+        event = (event as () => T)();
+      }
+
+      for (const listener of listeners) {
+        try {
+          void listener(event);
+        } catch (error) {
+          if (errorHandler) {
+            errorHandler(error as E);
+          }
+        }
+      }
+    },
+
+    onError: (handler) => {
+      errorHandler = handler;
+    },
+  };
+};