feat(appsync): expose the AppSyncDomain of the custom domain of an Ap…

…pSync api (#21554)

When configuring a custom domain on an AppSync api you couldn't easily get the AppSyncDomainName attribute of the associated AWS::AppSync::DomainName. This fix will expose that attribute so that the provided example doesn't throw errors.

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-appsync/README.md

@@ -330,7 +330,7 @@ const zone = route53.HostedZone.fromHostedZoneAttributes(this, `HostedZone`, {
 new route53.CnameRecord(this, `CnameApiRecord`, {
   recordName: 'api',
   zone,
-  domainName: myDomainName,
+  domainName: api.appSyncDomainName,
 });
 ```
 

packages/@aws-cdk/aws-appsync/lib/graphqlapi.ts

@@ -478,6 +478,7 @@ export class GraphqlApi extends GraphqlApiBase {
   private schemaResource: CfnGraphQLSchema;
   private api: CfnGraphQLApi;
   private apiKeyResource?: CfnApiKey;
+  private domainNameResource?: CfnDomainName;
 
   constructor(scope: Construct, id: string, props: GraphqlApiProps) {
     super(scope, id);
@@ -510,18 +511,17 @@ export class GraphqlApi extends GraphqlApiBase {
     this.schemaResource = this.schema.bind(this);
 
     if (props.domainName) {
-      const domainName = new CfnDomainName(this, 'DomainName', {
+      this.domainNameResource = new CfnDomainName(this, 'DomainName', {
         domainName: props.domainName.domainName,
         certificateArn: props.domainName.certificate.certificateArn,
         description: `domain for ${this.name} at ${this.graphqlUrl}`,
       });
-
       const domainNameAssociation = new CfnDomainNameApiAssociation(this, 'DomainAssociation', {
         domainName: props.domainName.domainName,
         apiId: this.apiId,
       });
 
-      domainNameAssociation.addDependsOn(domainName);
+      domainNameAssociation.addDependsOn(this.domainNameResource);
     }
 
     if (modes.some((mode) => mode.authorizationType === AuthorizationType.API_KEY)) {
@@ -774,4 +774,15 @@ export class GraphqlApi extends GraphqlApiBase {
   public addSubscription(fieldName: string, field: ResolvableField): ObjectType {
     return this.schema.addSubscription(fieldName, field);
   }
+
+
+  /**
+   * The AppSyncDomainName of the associated custom domain
+   */
+  public get appSyncDomainName(): string {
+    if (!this.domainNameResource) {
+      throw new Error('Cannot retrieve the appSyncDomainName without a domainName configuration');
+    }
+    return this.domainNameResource.attrAppSyncDomainName;
+  }
 }

packages/@aws-cdk/aws-appsync/test/appsync-domain.test.ts

@@ -40,4 +40,30 @@ describe('Tests of AppSync Domain Name', () => {
       },
     );
   });
+
+  test('appSyncDomainName exposes the domain of the associated AWS::AppSync::DomainName', () => {
+    const api = new appsync.GraphqlApi(stack, 'baseApi', {
+      name: 'api',
+      schema: appsync.Schema.fromAsset(
+        path.join(__dirname, 'appsync.test.graphql'),
+      ),
+      domainName: {
+        certificate,
+        domainName: 'aws.amazon.com',
+      },
+    });
+
+    expect(stack.resolve(api.appSyncDomainName)).toEqual({ 'Fn::GetAtt': ['baseApiDomainName52E3D63D', 'AppSyncDomainName'] });
+  });
+
+  test('appSyncDomainName should throw an error when no custom domain has been configured', () => {
+    const api = new appsync.GraphqlApi(stack, 'baseApi', {
+      name: 'api',
+      schema: appsync.Schema.fromAsset(
+        path.join(__dirname, 'appsync.test.graphql'),
+      ),
+    });
+
+    expect(() => api.appSyncDomainName).toThrow('Cannot retrieve the appSyncDomainName without a domainName configuration');
+  });
 });