如何在 angular 8 中使用注入器注入使用注入令牌的服务

Question

I'm developing a package that is used by two projects in my company. In project A and only there we got a message service that is provided in core.module like this:

        {
          provide: 'MESSAGE_SERVICE',
          useClass: MessageService
        }

In my component, I need to inject this service. I'm doing it using injector . However the method get that accepts a string as a parameter is deprecated so I cannot use it, although it works. When I try to use a new API with InjectionToken I always get undefined. Here's my code:

  protected messageService: any;

  constructor(protected injector: Injector) {
    const messageServiceInjectionToken = new InjectionToken('MESSAGE_SERVICE');
     // i get null
    this.messageService = this.injector.get<any>(messageServiceInjectionToken, null, InjectFlags.Optional);
    // It works but method is deprecated
    this.messageService = this.injector.get('MESSAGE_SERVICE', null);
    // It works but I cannot use MessageService class directly as it is not present in application B
    this.messageService = this.injector.get(MessageService);
  }

What is the proper way to get my service without using direct imports of said class? I thought of creating a mock class in project B so I would be able to use MessageService class. However, I'd like to know if there's a better way.

EDIT

The shared service is just a base class to other implementations, it is provided using a factory so there's not dependency injection in it. This is how it is provided:

export function parentServiceFactory(platformService: PlatformService, injector: Injector) {
  if (platformService.isIOS) {
    return new IosChildService(injector);
  }

  if (platformService.isAndroid) {
    return new AndroidChildService(injector);
  }

  return new DesktopChildService(injector);
}

@NgModule({
providers: [
    {
      provide: ParentService,
      useFactory: parentServiceFactory,
      deps: [PlatformService, Injector]
    }
]
})
export class SharedModule {}

Answer 1

The purpose of that injection token is to be able to not use the injector.

Instead =, use this syntax

constructor(@Inject('MESSAGE_SERVICE')protected service: YourInterface)

Although if you make a service, I don't see the point of using an injection token.

Answer 2

You can create an angular library and then use it in any number of projects

Here's how to create an angular2 library https://makina-corpus.com/blog/metier/2017/how-to-create-an-angular-library

with angular-cli https://medium.com/@nikolasleblanc/building-an-angular-4-component-library-with-the-angular-cli-and-ng-packagr-53b2ade0701e