Angular pass service with dependencies to module using forRoot

Question

I have a service for authentication based on JWT. To reuse this service in all my projects i created a library which should be shipped with npm.

For this service to work i need some API-Calls. In every project the API could look completely different so i don't want to provide this functionality inside my library instead inject another service which handles my API-Calls.

My idea was to create a module which contains my service and provide an interface to describe the service for API-Calls and inject it forRoot. The Problem is that my api service has some dependencies like HttpClient and i cannot simple instantiate it in my app.module.

My library looks like:

auth.module.ts

import { NgModule, ModuleWithProviders, InjectionToken } from '@angular/core';
import { AuthService } from '../services/auth.service';
import { AuthAPI } from '../models/authAPI';
import { AuthapiConfigService } from '../services/authapi-config.service';


@NgModule()
export class AuthModule {

  static forRoot(apiService: AuthAPI): ModuleWithProviders {
    return {
      ngModule: AuthModule,
      providers: [
        AuthService,
        {
          provide: AuthapiConfigService,
          useValue: apiService
        }
      ]
    };
  }
}

auth-api.interface.ts

import { Observable } from 'rxjs';

export interface AuthAPI {
  reqLogin(): Observable<{ access_token: string; }>;
  reqRegister(): Observable<{ access_token: string; }>;
}

auth-api-config.service.ts

import { InjectionToken } from '@angular/core';
import { AuthAPI } from '../models/authAPI';
/**
 * This is not a real service, but it looks like it from the outside.
 * It's just an InjectionTToken used to import the config object, provided from the outside
 */
export const AuthapiConfigService = new InjectionToken<AuthAPI>('API-Service');

auth.service.ts

 constructor(@Inject(AuthapiConfigService) private apiService) {}

How i am trying to implement it:

auth-rest-service.ts

import { Injectable } from '@angular/core';
import { AuthAPI } from 'projects/library-project/src/lib/auth/models/authAPI';
import { Observable } from 'rxjs';
import { HttpClient } from '@angular/common/http';

@Injectable({
  providedIn: 'root'
})
export class AuthRestService implements AuthAPI  {

  constructor(private http: HttpClient) {}

  reqLogin(): Observable<{ access_token: string; }> {
    return this.http.post<{access_token: string}>(`/login`, 'test');
  }

  reqRegister(): Observable<{ access_token: string; }> {
    return this.http.post<{access_token: string}>(`/login`, 'test');
  }

}

app.module.ts

import { AuthRestService } from './components/auth-service/auth-rest.service';


@NgModule({
  declarations: [
   ...
  ],
  imports: [
    ...
    AuthModule.forRoot(AuthRestService),
    ...
  ],
  providers: [AuthModule],
  bootstrap: [AppComponent]
})
export class AppModule { }

I can't create an instance of AuthRestService because of the dependencies this service has (HttpClient). Is there any method to tell angular to provide me this service.

Answer 1

This is possible with usage of angular's Injector.

import { Injector, ModuleWithProviders, NgModule, Optional, Provider, SkipSelf } from '@angular/core';
import { isFunction } from 'lodash';

export function resolveService(cfg: SharedConfig, inj: Injector): IncompleteService {
  const provider = cfg?.service;
  // if service is an angular provider, use Injector, otherwise return service instance as simple value
  const service = isFunction(service) ? inj.get(provider) : provider;
  return service;
}

/**
 * Service to be implemented from outside the module.
 */
@Injectable()
export abstract class IncompleteService {
  abstract strategyMethod();
}

// Optional: A config object is optional of course, but usually it fits the needs.
export interface SharedConfig {
  service: IncompleteService | Type<IncompleteService> | InjectionToken<IncompleteService>;
  // other config properties...
}

/*
 * Optional: If a Config interface is used, one might resolve the config itself 
 * using other dependencies (e.g. load JSON via HTTPClient). Hence an InjectionToken 
 * is necessary.
 */
export const SHARED_CONFIG = new InjectionToken<SharedConfig>('shared-config');

// Optional: If SharedConfig is resolved with dependencies, it must be provided itself.  
export type ModuleConfigProvider = ValueProvider | ClassProvider | ExistingProvider | FactoryProvider;

/**
 * One can provide the config as is, i.e. "{ service: MyService }" or resolved by 
 * injection, i.e.
 * { provide: SHARED_CONFIG: useFactory: myConfigFactory, deps: [DependentService1, DependentService2] }
 */
@NgModule({
  declarations: [],
  imports: []
})
export class SharedModule {
  static forRoot(config: SharedConfig | ModuleConfigProvider): ModuleWithProviders<SharedModule> {
    // dynamic (config is Provider) or simple (config is SharedConfig)
    return {
      ngModule: SharedModule,
      providers: [
        (config as ModuleConfigProvider).provide ? (config as Provider) : { provide: SHARED_CONFIG, useValue: config },
        { provide: IncompleteService, useFactory: resolveService, deps: [SHARED_CONFIG, Injector] },
        // ... provide additional things
      ],
    };
}


/**
 * In general not really useful, because usually an instance of IncompleteService
 * need other dependencies itself. Hence you cannot provide this instance without
 * creating it properly. But for the sake of completeness, it should work as well.
 */
@NgModule({
  declarations: [],
  imports: []
})
export class MostSimpleSharedModule {
  static forRoot(service: IncompleteService): ModuleWithProviders<SharedModule> {
    // dynamic (config is Provider) or simple (config is SharedConfig)
    return {
      ngModule: SharedModule,
      providers: [
        { provide: IncompleteService, useValue: service },
        // ... provide additional things
      ],
    };
}

EDIT

If you really need an interface iso. an (injectable) abstract class IncompleteService, you just need to define another InjectionToken<IncompleteServiceInterface> and provide this token explicitly.