initialize_service_on_app_start_example.ts

// best practice arch
// app
//  L core
//      L core.module.ts
//      L core.providers.ts
//      L core.model.ts

// core.module.ts

import { ModuleWithProviders, NgModule, Optional, SkipSelf } from '@angular/core';
import { CoreConfigurationContract, CORE_CONFIGURATION } from '@core/core.model';
import { APP_INITIALIZER_PROVIDERS } from '@core/core.providers';

const MODULES = [];

const INNER_COMPONENTS = [];

const OUTER_COMPONENTS = [];

@NgModule({
  imports: [...MODULES],
  declarations: [...INNER_COMPONENTS, ...OUTER_COMPONENTS],
  exports: [...OUTER_COMPONENTS]
})
export class CoreModule {
  constructor(@Optional() @SkipSelf() parentModule: CoreModule) {
    if (!!parentModule) {
      console.error('"CoreModule" already loaded. Import it _ONLY_ on main "AppRootModule".');
    }
  }

  static forRoot(coreConfigurationFactory: () => CoreConfigurationContract): ModuleWithProviders {
    return {
      ngModule: CoreModule,
      providers: [
        { provide: CORE_CONFIGURATION, useFactory: coreConfigurationFactory },
        ...APP_INITIALIZER_PROVIDERS
      ]
    };
  }
}



// core.providers.ts

import { APP_INITIALIZER, Provider } from '@angular/core';

export const load<Name>ServiceFnFactory = (<name>Service: <Name>Service): (() => Promise<any>) => () => {
  <name>Service.init();
  return Promise.resolve();
  /*
    or
    <name>Service.init().pipe().subscribe(() => Promise.resolve())
  */
};

export const APP_INITIALIZER_PROVIDERS: Provider[] = [
  {
    provide: APP_INITIALIZER,
    useFactory: load<Name>ServiceFnFactory,
    deps: [<Name>Service],
    multi: true
  }
];

// core.model.ts

export interface CoreConfigurationContract {}

export const CORE_CONFIGURATION = new InjectionToken<CoreConfigurationContract>('<DOMAIN>_CORE_CONFIGURATION_TOKEN');