93219: Support Router in InitService

For Router to work properly, APP_BASE_HREF must be resolved _before_ the APP_INITIALIZER factory is called (otherwise Angular will attempt to initialize APP_BASE_HREF too soon)
To fix this we add a pre-initialization hook to APP_CONFIG so BrowserInitService can resolve it before APP_INITIALIZER

src/app/init.service.ts

@@ -6,30 +6,18 @@
  * http://www.dspace.org/license/
  */
 import { Store } from '@ngrx/store';
-import { AppState } from './app.reducer';
 import { CheckAuthenticationTokenAction } from './core/auth/auth.actions';
 import { CorrelationIdService } from './correlation-id/correlation-id.service';
 import { DSpaceTransferState } from '../modules/transfer-state/dspace-transfer-state.service';
+import { APP_INITIALIZER, Provider, Type } from '@angular/core';
+import { TransferState } from '@angular/platform-browser';
+import { APP_CONFIG } from '../config/app-config.interface';
+import { environment } from '../environments/environment';
+import { AppState } from './app.reducer';
 
 /**
  * Performs the initialization of the app.
- *
- * Should have distinct extensions for server & browser for the specifics.
- * Should be provided in AppModule as follows
- * ```
- * {
- *   provide: APP_INITIALIZER
- *   useFactory: (initService: InitService) => initService.init(),
- *   deps: [ InitService ],
- *   multi: true,
- * }
- * ```
- *
- * In order to be injected in the common APP_INITIALIZER,
- * concrete subclasses should be provided in their respective app modules as
- * ```
- * { provide: InitService, useClass: SpecificInitService }
- * ```
+ * Should be extended to implement server- & browser-specific functionality.
  */
 export abstract class InitService {
   protected constructor(
@@ -40,14 +28,63 @@ export abstract class InitService {
   }
 
   /**
-   * Main initialization method, to be used as the APP_INITIALIZER factory.
-   *
-   * Note that the body of this method and the callback it returns are called
-   * at different times.
-   * This is important to take into account when other providers depend on the
-   * initialization logic (e.g. APP_BASE_HREF)
+   * The initialization providers to use in `*AppModule`
+   * - this concrete {@link InitService}
+   * - {@link APP_CONFIG} with optional pre-initialization hook
+   * - {@link APP_INITIALIZER}
+   * <br>
+   * Should only be called on concrete subclasses of InitService for the initialization hooks to work
    */
-  abstract init(): () => Promise<boolean>;
+  public static providers(): Provider[] {
+    if (!InitService.isPrototypeOf(this)) {
+      throw new Error(
+        'Initalization providers should only be generated from concrete subclasses of InitService'
+      );
+    }
+    return [
+      {
+        provide: InitService,
+        useClass: this as unknown as Type<InitService>,
+      },
+      {
+        provide: APP_CONFIG,
+        useFactory: (transferState: TransferState) => {
+          this.resolveAppConfig(transferState);
+          return environment;
+        },
+        deps: [ TransferState ]
+      },
+      {
+        provide: APP_INITIALIZER,
+        useFactory: (initService: InitService) => initService.init(),
+        deps: [ InitService ],
+        multi: true,
+      },
+    ];
+  }
+
+  /**
+   * Optional pre-initialization method to ensure that {@link APP_CONFIG} is fully resolved before {@link init} is called.
+   *
+   * For example, Router depends on APP_BASE_HREF, which in turn depends on APP_CONFIG.
+   * In production mode, APP_CONFIG is resolved from the TransferState when the app is initialized.
+   * If we want to use Router within APP_INITIALIZER, we have to make sure APP_BASE_HREF is resolved beforehand.
+   * In this case that means that we must transfer the configuration from the SSR state during pre-initialization.
+   * @protected
+   */
+  protected static resolveAppConfig(
+    transferState: TransferState
+  ): void {
+    // overriden in subclasses if applicable
+  }
+
+  /**
+   * Main initialization method.
+   * @protected
+   */
+  protected abstract init(): () => Promise<boolean>;
+
+  // Common initialization steps
 
   protected checkAuthenticationToken(): void {
     this.store.dispatch(new CheckAuthenticationTokenAction());