how to extend service with dependencies in angular 2

Question

I have a parent service which has some dependencies like

@Injectable()
export class ParentService{
  constructor(private http:Http, private customService:CustomService){}
}

and I want to extend the service

@Injectable()
export class ChildService extends ParentService{
  constructor (){
    super(??) <= typescript now asking to enter two parameters according to ParentServie's constructor
  }
}

Edit-----------------

@Injectable()
export class ParentService{
  constructor(private http:Http, private customService:CustomService){}
  get(){this.http.get(...)}
}

@Injectable()
export class ChildService extends ParentService{
  constructor (private http:Http, private customService:CustomService){
    super(http, customService)
  }
}

Then I can use in components?

export class Cmp {
  constructor(private childService:ChildService){
    this.childService.get()
  }
}

Answer 1

The parameters of the super class need to be repeated and passed to the super call:

@Injectable()
export class ChildService extends ParentService{
  constructor (http:Http, customService:CustomService){
    super(http, customService);
  }
}

There are some "hacks" to work around like Inheritance and dependency injection

Answer 2

Simply make base service... non-@Injectable()! For example, you need to supply a method to retrieve user profile in LoginService. This method will be different for different instances of LoginService, and parent class must not know anything about where from this method came: it can be lambda, it can be exported function, it can be a method of another service. To achieve this, you can declare parent service which Parent service:

// Don't annotate it with @Injectable() ! It's a simple class.
export abstract class BaseLoginService {
  constructor(
    // This won't be injected automatically, 
    // child class must call super() and provide value for this argument.
    // Note: it's private, can't be accessed outside of BaseLoginService class
    private http: HttpClient,
    // Method used to retrieve user profile
    private profileGetter: () => Observable<UserDetails>, 
    private router: Router
  ) {
    this.profileGetter().subscribe(); // Do something with this method
  }

Then extend it in child class:

// Child class must be annotated with @Injectable(),
// if you don't need to extend it further
@Injectable() 
export class LoginService extends BaseLoginService {

  constructor(
    // Note: no public/private/protected here - "http"
    // will be just an argument
    http: HttpClient, 
    private profileService: ProfileService, // This is the private field of LoginService class
    router: Router,
    // Some service used by LoginService class, parent class BaseLoginService
    // doesn't need to know about SomeOtherService
    private someOtherService: SomeOtherService
  ) {
    super(
      http,
      // Need lambda here to capture profileService instance. If
      // profileService.getUserDetailsMethod doesn't use instance
      // fields or methods, then we can simply write 
      // "profileService.getUserDetailsMethod" (without quotes, obviously).
      () => profileService.getUserDetailsMethod(), 
      router
    );
    this.someOtherService.doSomething(); // Custom initialization code
  }

Note: in providers section of module specify LoginService instead of parent BaseLoginService:

providers: [
  LoginService,

and use it in component classes:

export class LoginComponent implements OnInit {
  constructor(
    private loginService: LoginService
  ) {
  }

If you need to use parent service (e.g. in shared components which only need functionality from parent service class), then provide BaseLoginService this way:

providers: [
  {provide: BaseLoginService, useExisting: LoginService}, // For those components which need functionality from base class only
  LoginService, // Still need this one for those components which need functionality from child class

Answer 3

If you make the DI services in the abstract class protected and omit the constructor in the subclass, then the services are available in the subclass.