application_init.ts

/**
 * @license
 * Copyright Google LLC All Rights Reserved.
 *
 * Use of this source code is governed by an MIT-style license that can be
 * found in the LICENSE file at https://angular.io/license
 */

import {Observable} from 'rxjs';

import {inject, Injectable, InjectionToken} from './di';
import {RuntimeError, RuntimeErrorCode} from './errors';
import {isPromise, isSubscribable} from './util/lang';

/**
 * A [DI token](guide/glossary#di-token "DI token definition") that you can use to provide
 * 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 or an 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.
 * The function is executed during the application bootstrap process,
 * and the needed data is available on startup.
 *
 * @see {@link ApplicationInitStatus}
 *
 * @usageNotes
 *
 * The following example illustrates how to configure a multi-provider using `APP_INITIALIZER` token
 * and a function returning a promise.
 *
 * ```
 *  function initializeApp(): Promise<any> {
 *    return new Promise((resolve, reject) => {
 *      // Do some asynchronous stuff
 *      resolve();
 *    });
 *  }
 *
 *  @NgModule({
 *   imports: [BrowserModule],
 *   declarations: [AppComponent],
 *   bootstrap: [AppComponent],
 *   providers: [{
 *     provide: APP_INITIALIZER,
 *     useFactory: () => initializeApp,
 *     multi: true
 *    }]
 *   })
 *  export class AppModule {}
 * ```
 *
 * It's also possible to configure a multi-provider using `APP_INITIALIZER` token and a function
 * returning an observable, see an example below. Note: the `HttpClient` in this example is used for
 * demo purposes to illustrate how the factory function can work with other providers available
 * through DI.
 *
 * ```
 *  function initializeAppFactory(httpClient: HttpClient): () => Observable<any> {
 *   return () => httpClient.get("https://someUrl.com/api/user")
 *     .pipe(
 *        tap(user => { ... })
 *     );
 *  }
 *
 *  @NgModule({
 *    imports: [BrowserModule, HttpClientModule],
 *    declarations: [AppComponent],
 *    bootstrap: [AppComponent],
 *    providers: [{
 *      provide: APP_INITIALIZER,
 *      useFactory: initializeAppFactory,
 *      deps: [HttpClient],
 *      multi: true
 *    }]
 *  })
 *  export class AppModule {}
 * ```
 *
 * @publicApi
 */
export const APP_INITIALIZER =
    new InjectionToken<ReadonlyArray<() => Observable<unknown>| Promise<unknown>| void>>(
        'Application Initializer');

/**
 * A class that reflects the state of running {@link APP_INITIALIZER} functions.
 *
 * @publicApi
 */
@Injectable({providedIn: 'root'})
export class ApplicationInitStatus {
  // Using non null assertion, these fields are defined below
  // within the `new Promise` callback (synchronously).
  private resolve!: (...args: any[]) => void;
  private reject!: (...args: any[]) => void;

  private initialized = false;
  public readonly done = false;
  public readonly donePromise: Promise<any> = new Promise((res, rej) => {
    this.resolve = res;
    this.reject = rej;
  });

  private readonly appInits = inject(APP_INITIALIZER, {optional: true}) ?? [];

  constructor() {
    if ((typeof ngDevMode === 'undefined' || ngDevMode) && !Array.isArray(this.appInits)) {
      throw new RuntimeError(
          RuntimeErrorCode.INVALID_MULTI_PROVIDER,
          'Unexpected type of the `APP_INITIALIZER` token value ' +
              `(expected an array, but got ${typeof this.appInits}). ` +
              'Please check that the `APP_INITIALIZER` token is configured as a ' +
              '`multi: true` provider.');
    }
  }

  /** @internal */
  runInitializers() {
    if (this.initialized) {
      return;
    }

    const asyncInitPromises = [];
    for (const appInits of this.appInits) {
      const initResult = appInits();
      if (isPromise(initResult)) {
        asyncInitPromises.push(initResult);
      } else if (isSubscribable(initResult)) {
        const observableAsPromise = new Promise<void>((resolve, reject) => {
          initResult.subscribe({complete: resolve, error: reject});
        });
        asyncInitPromises.push(observableAsPromise);
      }
    }

    const complete = () => {
      // @ts-expect-error overwriting a readonly
      this.done = true;
      this.resolve();
    };

    Promise.all(asyncInitPromises)
        .then(() => {
          complete();
        })
        .catch(e => {
          this.reject(e);
        });

    if (asyncInitPromises.length === 0) {
      complete();
    }
    this.initialized = true;
  }
}