feat(core): support APP_INITIALIZER work with observable

add support to observable with APP_INITIALIZER

close angular#15088

Co-authored-by: Andrew Kushnir <43554145+AndrewKushnir@users.noreply.github.com>

packages/core/src/application_init.ts

@@ -7,7 +7,7 @@
  */
 
 import {Inject, Injectable, InjectionToken, Optional} from './di';
-import {isPromise} from './util/lang';
+import {isObservable, isPromise} from './util/lang';
 import {noop} from './util/noop';
 
 
@@ -16,8 +16,8 @@ import {noop} from './util/noop';
  * one or more initialization functions.
  *
  * The provided functions are injected at application startup and executed during
- * app initialization. If any of these functions returns a Promise, initialization
- * does not complete until the Promise is resolved.
+ * app initialization. If any of these functions returns a Promise or a Observable, initialization
+ * does not complete until the Promise is resolved or the Observable is completed.
  *
  * You can, for example, create a factory function that loads language data
  * or an external configuration, and provide that function to the `APP_INITIALIZER` token.
@@ -68,6 +68,11 @@ export class ApplicationInitStatus {
         const initResult = this.appInits[i]();
         if (isPromise(initResult)) {
           asyncInitPromises.push(initResult);
+        } else if (isObservable(initResult)) {
+          // Note: `.toPromise()` will be deprecated in RxJS v7 and removed completely in later
+          // versions. Once RxJS is updated in the repo to v7, this code should be refactored and
+          // the `lastValueFrom` function should be used instead.
+          asyncInitPromises.push(initResult.toPromise());
         }
       }
     }

packages/core/test/application_init_spec.ts

@@ -7,6 +7,7 @@
  */
 import {Injector} from '@angular/core';
 import {APP_INITIALIZER, ApplicationInitStatus} from '@angular/core/src/application_init';
+import {Observable, Subscriber} from 'rxjs';
 
 import {inject, TestBed, waitForAsync} from '../testing';
 
@@ -28,7 +29,7 @@ import {inject, TestBed, waitForAsync} from '../testing';
          })));
     });
 
-    describe('with async initializers', () => {
+    describe('with async promise initializers', () => {
       let resolve: (result: any) => void;
       let promise: Promise<any>;
       let completerResolver = false;
@@ -57,7 +58,7 @@ import {inject, TestBed, waitForAsync} from '../testing';
         });
       });
 
-      it('should update the status once all async initializers are done',
+      it('should update the status once all async promise initializers are done',
          waitForAsync(inject([ApplicationInitStatus], (status: ApplicationInitStatus) => {
            (status as any).runInitializers();
 
@@ -73,5 +74,58 @@ import {inject, TestBed, waitForAsync} from '../testing';
            });
          })));
     });
+
+    describe('with app initializers represented using observables', () => {
+      let subscriber: Subscriber<any>;
+      let observable: Observable<any>;
+      let completerResolver = false;
+      beforeEach(() => {
+        observable = new Observable((res) => {
+          subscriber = res;
+        });
+        TestBed.configureTestingModule({
+          providers: [
+            {provide: APP_INITIALIZER, multi: true, useValue: () => observable},
+          ]
+        });
+      });
+
+      it('should update the status once all async observable initializers are done',
+         waitForAsync(inject([ApplicationInitStatus], (status: ApplicationInitStatus) => {
+           // Accessing internal `runInitializers` function of the `ApplicationInitStatus` class
+           // instance for testing purposes to invoke initializer functions.
+           (status as any).runInitializers();
+
+           setTimeout(() => {
+             completerResolver = true;
+             subscriber.next();
+             subscriber.complete();
+           });
+
+           expect(status.done).toBe(false);
+           status.donePromise.then(() => {
+             expect(status.done).toBe(true);
+             expect(completerResolver).toBe(true);
+           });
+         })));
+
+      it('should update the status once async observable initializers has error',
+         waitForAsync(inject([ApplicationInitStatus], (status: ApplicationInitStatus) => {
+           // Accessing internal `runInitializers` function of the `ApplicationInitStatus` class
+           // instance for testing purposes to invoke initializer functions.
+           (status as any).runInitializers();
+
+           setTimeout(() => {
+             completerResolver = true;
+             subscriber.error();
+           });
+
+           expect(status.done).toBe(false);
+           status.donePromise.catch(() => {
+             expect(status.done).toBe(false);
+             expect(completerResolver).toBe(true);
+           });
+         })));
+    });
   });
 }