Intro#

If you want to create only one instance of any service for a whole application, you want to create a Singleton.

From wikipedia:

In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one “single” instance.

Why you may want it?

The most common reason is to share some valuable state between all parts of your application.

Take a look at simple application configuration service:

@Injectable()
export class SettingsService {
  private settings = new Map();

  public get(key: string): any {
    return this.settings.get(key);
  }

  public set(key: string, value: any): any {
    return this.settings.set(key, value);
  }
}

and it’s module:

@NgModule({
  imports: [BrowserModule],
  declarations: [ApplicationComponent],
  bootstrap: [ApplicationComponent],
  providers: [SettingsService]
})
export class AppModule {}

Basically I want to use the same settings for a whole application:

@Component({
  selector: 'app',
  template: ''
})
class ApplicationComponent {
  constructor(private settings: SettingsService) {
    settings.set('FEATURE', true);
  }
}

And then use it in some component:

this.isFeatureAvailable = settings.get('FEATURE');
...
<div *ngIf="isFeatureAvailable"><super-feature></super-feature><div>

#angular #dependency-injection