feat(core): expose listening stream from http adapter host

packages/core/helpers/http-adapter-host.ts

@@ -1,3 +1,4 @@
+import { Observable, Subject } from 'rxjs';
 import { AbstractHttpAdapter } from '../adapters/http-adapter';
 
 /**
@@ -16,6 +17,8 @@ export class HttpAdapterHost<
   T extends AbstractHttpAdapter = AbstractHttpAdapter,
 > {
   private _httpAdapter?: T;
+  private _listen$ = new Subject<void>();
+  private isListening = false;
 
   /**
    * Accessor for the underlying `HttpAdapter`
@@ -35,4 +38,31 @@ export class HttpAdapterHost<
   get httpAdapter(): T {
     return this._httpAdapter;
   }
+
+  /**
+   * Observable that allows to subscribe to the `listen` event.
+   * This event is emitted when the HTTP application is listening for incoming requests.
+   */
+  get listen$(): Observable<void> {
+    return this._listen$.asObservable();
+  }
+
+  /**
+   * Sets the listening state of the application.
+   */
+  set listening(listening: boolean) {
+    this.isListening = listening;
+
+    if (listening) {
+      this._listen$.next();
+      this._listen$.complete();
+    }
+  }
+
+  /**
+   * Returns a boolean indicating whether the application is listening for incoming requests.
+   */
+  get listening(): boolean {
+    return this.isListening;
+  }
 }

packages/core/nest-application.ts

@@ -294,8 +294,12 @@ export class NestApplication
   public async listen(port: number | string, hostname: string): Promise<any>;
   public async listen(port: number | string, ...args: any[]): Promise<any> {
     this.assertNotInPreviewMode('listen');
-    !this.isInitialized && (await this.init());
 
+    if (!this.isInitialized) {
+      await this.init();
+    }
+
+    const httpAdapterHost = this.container.getHttpAdapterHostRef();
     return new Promise((resolve, reject) => {
       const errorHandler = (e: any) => {
         this.logger.error(e?.toString?.());
@@ -323,6 +327,8 @@ export class NestApplication
           if (address) {
             this.httpServer.removeListener('error', errorHandler);
             this.isListening = true;
+
+            httpAdapterHost.listening = true;
             resolve(this.httpServer);
           }
           if (isCallbackInOriginalArgs) {

packages/core/test/helpers/application-ref-host.spec.ts

@@ -2,11 +2,27 @@ import { expect } from 'chai';
 import { HttpAdapterHost } from '../../helpers/http-adapter-host';
 
 describe('HttpAdapterHost', () => {
-  const applicationRefHost = new HttpAdapterHost();
+  let applicationRefHost: HttpAdapterHost;
+  beforeEach(() => {
+    applicationRefHost = new HttpAdapterHost();
+  });
+
   it('should wrap application reference', () => {
     const ref = {};
     applicationRefHost.httpAdapter = ref as any;
 
     expect(applicationRefHost.httpAdapter).to.be.eql(ref);
   });
+
+  it('should emit listen event when listening is set to true', done => {
+    applicationRefHost.listen$.subscribe(() => {
+      expect(applicationRefHost.listening).to.be.true;
+      done();
+    });
+    applicationRefHost.listening = true;
+  });
+
+  it('listening should return false if the application isnt listening yet', () => {
+    expect(applicationRefHost.listening).to.be.false;
+  });
 });