In the latest release of Angular 6, a service is registered in a module using the providedIn property in the service metadata:

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

However the documentation still also refers to registering the service in the module providers array in the module metadata just like we did in Angular 5:

@NgModule({
  providers: [HeroService],
})
export class AppModule {}

So,

Which method should be used to make the injector aware of the service that it should inject?
Will the module providers array method be deprecated?

Pardeep Jain

Basically you can use either, But as per new CLI provideIn will be automatically added while creating service

providedIn

There is now a new, recommended, way to register a provider, directly inside the @Injectable() decorator, using the new providedIn attribute. It accepts 'root' as a value or any module of your application. When you use 'root', your injectable will be registered as a singleton in the application, and you don’t need to add it to the providers of the root module. Similarly, if you use providedIn: UsersModule, the injectable is registered as a provider of the UsersModule without adding it to the providers of the module.

This new way has been introduced to have a better tree-shaking in the application. Currently a service added to the providers of a module will end up in the final bundle, even if it is not used in the application, which is a bit sad.

For more information please refer here

来源：https://stackoverflow.com/questions/50208642/angular-6-dependency-injection

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