How to unit-test an Angular v19 `resource` and trigger the evaluation of input-signal changes?

I have a Service which contains a resource (for a list of country-objects). Unit-testing the usual way seems not to work, even with fakeAsync and tick. Changes on the Subject which feeds the request-input do not trigger the loader.

export class CountryService {
private http = inject(HttpClient);
private translocoService = inject(TranslocoService);
private language = toSignal(this.translocoService.langChanges$);

public readonly countryListResource = resource({
request: this.language,
loader: (params) => {
const language = params.request;
return lastValueFrom(this.fetchCountries(language));
},
}).asReadonly();

public constructor() {
effect(() => console.log("CountryService: language changed to", this.language()));
this.translocoService.langChanges$.subscribe((lang) =>
console.log("translocoService.langChanges$ changed: " + lang),
);
}

private fetchCountries(language: string): Observable<Country[]> {
return this.http
.get<Country[]>(`assets/countries/countries-${language}.json`);
}
}


This is the unit-test:

describe("CountryService", () => {
let service: CountryService;
let httpMock: HttpTestingController;
let languageSubject = new Subject<string>();

beforeEach(() => {
TestBed.configureTestingModule({
providers: [
provideHttpClient(),
provideHttpClientTesting(),
{ provide: TranslocoService, useValue: { langChanges$: languageSubject } },
],
});

service = TestBed.inject(CountryService);
httpMock = TestBed.inject(HttpTestingController);
});

it("should retrieve a list of countries from assets", fakeAsync(() => {
const expectedEmpty = service.countryListResource.value();
expect(expectedEmpty).toBeUndefined();

// act
languageSubject.next("en");
tick();

// assert [1]
const req = httpMock.expectOne("assets/countries/countries-en.json");

// act
req.flush(mockCountries); // Simulates backend response
tick();

// assert [2]
expect(service.countryListResource.value()).toEqual(mockCountries);
}));
});


The test fails with (see assert [1]):

Error: Expected one matching request for criteria "Match URL: assets/countries/countries-en.json", found none.
​

In the output on the console, the subscription of the langChanges$ outputs, but the effect doesn't output anything.

Is there a way to trigger the evaluation of the resource request Signal?

BTW the service works without problems in production, only the unit-test causes me headaches.

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