Dependency Injection

Angular has built-in support for dependency injection.

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class HeroService {
  constructor() {}
}

Dependency Providers

• providedIn
  • root — application level.
  • any — provide a unique instance in each lazy loaded module.
  • platform — provide a single instance for all applications on the page.

Injection Tokens

Angular uses Injection Tokens to identify dependencies.

export const HERO_SERVICE_TOKEN = new InjectionToken<CoursesService>(
  'HERO_SERVICE_TOKEN'
);

Providers

• TypeProvider
• ValueProvider
• ClassProvider
• ConstructorProvider
• ExistingProvider
• FactoryProvider

Factory Provider

• useFactory — invoked to create the instance with resolved values of deps.
• deps — list of tokes the factory depended on creating the instance.

@NgModule({
  imports: [],
  declarations: [],
  providers: [
    {
      provide: HERO_SERVICE_TOKEN,
      useFactory: heroServiceProviderFactory,
      deps: [HttpClient],
    },
  ],
})
export class HeroModule {}