fix(apigatewayv2): unable to retrieve domain url for default stage

packages/@aws-cdk/aws-apigatewayv2/lib/http/stage.ts

@@ -18,6 +18,11 @@ export interface IHttpStage extends IStage {
    */
   readonly api: IHttpApi;
 
+  /**
+   * The custom domain URL to this stage
+   */
+  readonly domainUrl: string;
+
   /**
    * Metric for the number of client-side errors captured in a given period.
    *
@@ -96,6 +101,7 @@ export interface HttpStageAttributes extends StageAttributes {
 }
 
 abstract class HttpStageBase extends StageBase implements IHttpStage {
+  public abstract readonly domainUrl: string;
   public abstract readonly api: IHttpApi;
 
   public metricClientError(props?: MetricOptions): Metric {
@@ -140,6 +146,10 @@ export class HttpStage extends HttpStageBase {
       get url(): string {
         throw new Error('url is not available for imported stages.');
       }
+
+      get domainUrl(): string {
+        throw new Error('domainUrl is not available for imported stages.');
+      }
     }
     return new Import(scope, id);
   }
@@ -177,9 +187,6 @@ export class HttpStage extends HttpStageBase {
     return `https://${this.api.apiId}.execute-api.${s.region}.${s.urlSuffix}/${urlPath}`;
   }
 
-  /**
-   * The custom domain URL to this stage
-   */
   public get domainUrl(): string {
     if (!this._apiMapping) {
       throw new Error('domainUrl is not available when no API mapping is associated with the Stage');

packages/@aws-cdk/aws-apigatewayv2/test/http/api.test.ts

@@ -1,9 +1,10 @@
 import { Match, Template } from '@aws-cdk/assertions';
+import { Certificate } from '@aws-cdk/aws-certificatemanager';
 import { Metric } from '@aws-cdk/aws-cloudwatch';
 import * as ec2 from '@aws-cdk/aws-ec2';
 import { Duration, Stack } from '@aws-cdk/core';
 import {
-  CorsHttpMethod,
+  CorsHttpMethod, DomainName,
   HttpApi, HttpAuthorizer, HttpIntegrationType, HttpMethod, HttpRouteAuthorizerBindOptions, HttpRouteAuthorizerConfig,
   HttpRouteIntegrationBindOptions, HttpRouteIntegrationConfig, IHttpRouteAuthorizer, IHttpRouteIntegration, HttpNoneAuthorizer, PayloadFormatVersion,
 } from '../../lib';
@@ -374,6 +375,27 @@ describe('HttpApi', () => {
     expect(() => api.apiEndpoint).toThrow(/apiEndpoint is not configured/);
   });
 
+  test('domainUrl can be retrieved for default stage', () => {
+    const stack = new Stack();
+    const dn = new DomainName(stack, 'DN', {
+      domainName: 'example.com',
+      certificate: Certificate.fromCertificateArn(stack, 'cert', 'arn:aws:acm:us-east-1:111111111111:certificate'),
+    });
+
+    const api = new HttpApi(stack, 'Api', {
+      createDefaultStage: true,
+      defaultDomainMapping: {
+        domainName: dn,
+      },
+    });
+
+    expect(stack.resolve(api.defaultStage?.domainUrl)).toEqual({
+      'Fn::Join': ['', [
+        'https://', { Ref: 'DNFDC76583' }, '/',
+      ]],
+    });
+  });
+
 
   describe('default authorization settings', () => {
     test('can add default authorizer', () => {