Merge pull request #1304 from nestjs/feat/readiness-watcher

feat: add event emitter readiness watcher class #1063

lib/event-emitter-readiness.watcher.ts

@@ -0,0 +1,19 @@
+import { Injectable } from '@nestjs/common';
+import { promiseWithResolvers } from './utils/promise-with-resolvers';
+
+@Injectable()
+export class EventEmitterReadinessWatcher {
+  private readonly readyPromise = promiseWithResolvers();
+
+  waitUntilReady() {
+    return this.readyPromise.promise;
+  }
+
+  setReady() {
+    this.readyPromise.resolve();
+  }
+
+  setErrored(error: Error) {
+    this.readyPromise.reject(error);
+  }
+}

lib/event-emitter.module.ts

@@ -1,6 +1,7 @@
 import { DynamicModule, Module } from '@nestjs/common';
 import { DiscoveryModule } from '@nestjs/core';
 import { EventEmitter2 } from 'eventemitter2';
+import { EventEmitterReadinessWatcher } from './event-emitter-readiness.watcher';
 import { EventSubscribersLoader } from './event-subscribers.loader';
 import { EventsMetadataAccessor } from './events-metadata.accessor';
 import { EventEmitterModuleOptions } from './interfaces';
@@ -15,12 +16,13 @@ export class EventEmitterModule {
       providers: [
         EventSubscribersLoader,
         EventsMetadataAccessor,
+        EventEmitterReadinessWatcher,
         {
           provide: EventEmitter2,
           useValue: new EventEmitter2(options),
         },
       ],
-      exports: [EventEmitter2],
+      exports: [EventEmitter2, EventEmitterReadinessWatcher],
     };
   }
 }

lib/event-subscribers.loader.ts

@@ -17,6 +17,7 @@ import {
 } from '@nestjs/core/injector/instance-wrapper';
 import { Module } from '@nestjs/core/injector/module';
 import { EventEmitter2 } from 'eventemitter2';
+import { EventEmitterReadinessWatcher } from './event-emitter-readiness.watcher';
 import { EventsMetadataAccessor } from './events-metadata.accessor';
 import { OnEventOptions } from './interfaces';
 
@@ -33,10 +34,16 @@ export class EventSubscribersLoader
     private readonly metadataAccessor: EventsMetadataAccessor,
     private readonly metadataScanner: MetadataScanner,
     private readonly moduleRef: ModuleRef,
+    private readonly eventEmitterReadinessWatcher: EventEmitterReadinessWatcher,
   ) {}
 
   onApplicationBootstrap() {
-    this.loadEventListeners();
+    try {
+      this.loadEventListeners();
+      this.eventEmitterReadinessWatcher.setReady();
+    } catch (e) {
+      this.eventEmitterReadinessWatcher.setErrored(e as Error);
+    }
   }
 
   onApplicationShutdown() {
@@ -95,7 +102,12 @@ export class EventSubscribersLoader
         listenerMethod(
           event,
           (...args: unknown[]) =>
-            this.wrapFunctionInTryCatchBlocks(instance, methodKey, args, options),
+            this.wrapFunctionInTryCatchBlocks(
+              instance,
+              methodKey,
+              args,
+              options,
+            ),
           options,
         );
       }

lib/index.ts

@@ -1,4 +1,5 @@
 export { EventEmitter2 } from 'eventemitter2';
+export { EVENT_PAYLOAD } from './constants';
 export * from './decorators';
+export * from './event-emitter-readiness.watcher';
 export * from './event-emitter.module';
-export { EVENT_PAYLOAD } from './constants';

lib/utils/promise-with-resolvers.ts

@@ -0,0 +1,15 @@
+/**
+ * A polyfill for the Promise.withResolvers method that is not available in older versions of Node.js
+ * @returns A promise and its resolve and reject functions
+ */
+export function promiseWithResolvers() {
+  let resolve: () => void;
+  let reject: (reason?: any) => void;
+  const promise = new Promise<void>((res, rej) => {
+    resolve = res;
+    reject = rej;
+  });
+
+  // @ts-expect-error "resolve" and "reject" are assigned in the promise constructor
+  return { promise, resolve, reject };
+}

tests/e2e/module-e2e.spec.ts

@@ -1,6 +1,7 @@
 import { INestApplication } from '@nestjs/common';
 import { Test } from '@nestjs/testing';
 import { EventEmitter2 } from 'eventemitter2';
+import { EventEmitterReadinessWatcher } from '../../lib';
 import { AppModule } from '../src/app.module';
 import {
   TEST_EVENT_MULTIPLE_PAYLOAD,
@@ -157,7 +158,9 @@ describe('EventEmitterModule - e2e', () => {
     await app.init();
 
     const eventEmitter = app.get(EventEmitter2);
-    const result = eventEmitter.emit('error-handling-suppressed.request-scoped');
+    const result = eventEmitter.emit(
+      'error-handling-suppressed.request-scoped',
+    );
 
     expect(result).toBeTruthy();
   });
@@ -166,14 +169,27 @@ describe('EventEmitterModule - e2e', () => {
     await app.init();
 
     const eventEmitter = app.get(EventEmitter2);
-    expect(eventEmitter.emitAsync('error-throwing.provider')).rejects.toThrow("This is a test error");
+    expect(eventEmitter.emitAsync('error-throwing.provider')).rejects.toThrow(
+      'This is a test error',
+    );
   });
 
   it('should throw when an unexpected error occurs from request scoped and suppressErrors is false', async () => {
     await app.init();
 
     const eventEmitter = app.get(EventEmitter2);
-    expect(eventEmitter.emitAsync('error-throwing.request-scoped')).rejects.toThrow("This is a test error");
+    expect(
+      eventEmitter.emitAsync('error-throwing.request-scoped'),
+    ).rejects.toThrow('This is a test error');
+  });
+
+  it('should be able to wait until the event emitter is ready', async () => {
+    const eventsConsumerRef = app.get(EventsControllerConsumer);
+    await app.init();
+
+    const eventEmitterWatcher = app.get(EventEmitterReadinessWatcher);
+    await expect(eventEmitterWatcher.waitUntilReady()).resolves.toBeUndefined();
+    expect(eventsConsumerRef.eventPayload).toEqual(TEST_EVENT_PAYLOAD);
   });
 
   afterEach(async () => {