Angular 6 service with interface

Question

I am building an application with Angular (6.0.7) and I am trying to create a service with the new:

@Injectable({
  providedIn: 'root'
})

But how can I type an injection with an Interface?

---

The problem

I have 2 services, Authentication.service and SessionStorage.service. I want to inject the sessionstorage into the authentication service. That can be done via:

constructor(private sessionStorage: SessionStorage) {
}

No problem there. But for Object Orientated purposes I want to have an interface above this service (so that I can implement both localstorage service as sessionstorage service). Thus it is only logical that I want to type the injected class with the interface, but this cannot be done the same way Angular 5 and lower does it.

So how can I type the injection into this global service with my interface?

---

I've tried

The Angular service typings describe an InjectableProvider, but this does not match any of the parameters of the siblings of InjectableProvider, so this gives a compiler (and tslint) error.

@Injectable({
  providedIn: 'root'
}, {provide: IStorageService, useClass: SessionStorage})

Answer 1

This can be done with InjectionToken, which is a replacement for the obsolete OpaqueToken

export const AuthenticationProvider = new InjectionToken(
  "AuthenticationProvider",
  { providedIn: "root", factory: () => new CognitoAuthenticationProvider() }
);

...

@Injectable()
export class CognitoAuthenticationProvider implements IAuthenticationProvider {

...

@Injectable({
  providedIn: "root"
})
export class AuthenticationService {
  constructor(
    @Inject(AuthenticationProvider)
    private authenticationProvider: IAuthenticationProvider,
    private http: HttpClient
  ) {}

Answer 2

I used something like the following to solve this

app.module.ts

providers: [
  { provide: AlmostInterface, useClass: environment.concrete }
  ...
]

AlmostInterface.ts

export abstract class AlmostInterface {
   abstract myMethod();
}

MyConcrete.ts

export class MyConcrete implements AlmostInterface {
   myMethod() { ... }; // implementation
}

export class MyConcreteAlternative implements AlmostInterface {
   myMethod() { ... }; // implementation
}

environment.ts

export const environment = {
  production: false,
  concrete: MyConcreteAlternative
};

environment.prod.ts

export const environment = {
  production: true,
  concrete: MyConcrete
};

Answer 3

I think you can't use typescript interfaces for dependency injection as typescript interfaces don't exist at runtime (only for typesafety at compile time).
I would suggest using an abstract class for it.

EDIT: It seems you can use useClass in the first parameter of @Injectable, not as a second like your example. Combining that with @k0zakinio's answer results in:

@Injectable({
  providedIn: 'root',
  useClass: environment.concrete,
  deps: []
})
export abstract class SessionStorage { }

It also seems you need to declare your dependencies via deps or inject, checkout this github issue. I hope this time my answer is of more help.