Added PubSub.waitFor (#13)

src/main/PubSub.ts

@@ -1,3 +1,5 @@
+import { AbortablePromise } from './AbortablePromise';
+
 /**
  * Publish–subscribe pattern implementation that guarantees the delivery of published messages even if any of listeners
  * would throw an error.
@@ -32,6 +34,41 @@ export class PubSub<T = void> {
     }
   }
 
+  /**
+   * Waits for a message that satisfies the given predicate to be published and resolves with that message.
+   *
+   * @template R A subtype of T that the predicate function identifies as satisfying the condition.
+   * @param predicate A function that takes the message as a parameter and returns true if the message satisfies the condition, otherwise false.
+   * @returns An {@link AbortablePromise} that resolves with the published message that satisfies the predicate.
+   */
+  waitFor<R extends T>(predicate: (message: T) => message is R): AbortablePromise<R>;
+
+  /**
+   * Waits for a message that satisfies the given predicate to be published and resolves with that message.
+   *
+   * @param predicate A function that takes the message as a parameter and returns true if the message satisfies the condition, otherwise false.
+   * @returns An {@link AbortablePromise} that resolves with the published message that satisfies the predicate.
+   */
+  waitFor(predicate: (message: T) => boolean): AbortablePromise<T>;
+
+  /**
+   * Waits for a message that satisfies the given predicate to be published and resolves with that message.
+   *
+   * @param predicate A function that takes the message as a parameter and returns true if the message satisfies the condition, otherwise false.
+   * @returns An {@link AbortablePromise} that resolves with the published message that satisfies the predicate.
+   */
+  waitFor(predicate: (message: T) => boolean) {
+    return new AbortablePromise((resolve, _reject, signal) => {
+      const unsubscribe = this.subscribe(message => {
+        if (predicate(message)) {
+          resolve(message);
+          unsubscribe();
+        }
+      });
+      signal.addEventListener('abort', unsubscribe);
+    });
+  }
+
   /**
    * Adds a listener that would receive all messages published via {@link publish}.
    *

src/test/PubSub.test.ts

@@ -54,4 +54,29 @@ describe('PubSub', () => {
     expect(listenerMock1).toHaveBeenCalledTimes(0);
     expect(listenerMock2).toHaveBeenCalledTimes(1);
   });
+
+  test('waits for a specific message and resolves with that message', async () => {
+    const pubSub = new PubSub<number>();
+    const promise = pubSub.waitFor((message): message is 42 => message === 42);
+
+    pubSub.publish(10);
+    pubSub.publish(20);
+    pubSub.publish(42);
+    pubSub.publish(30);
+
+    await expect(promise).resolves.toBe(42);
+  });
+
+  test('aborts the waiter of the specific message', () => {
+    const predicateMock = jest.fn();
+    const pubSub = new PubSub<number>();
+    const promise = pubSub.waitFor(message => message === 42);
+
+    promise.then(predicateMock, () => {});
+    promise.abort();
+
+    pubSub.publish(42);
+
+    expect(predicateMock).not.toHaveBeenCalled();
+  });
 });