testing - How override Provider in Angular 5 for only one test?

Question

In one of my unit test files, I have to mock several times the same service with different mocks.

import { MyService } from '../services/myservice.service';
import { MockMyService1 } from '../mocks/mockmyservice1';
import { MockMyService2 } from '../mocks/mockmyservice2';
describe('MyComponent', () => {

    beforeEach(async(() => {
        TestBed.configureTestingModule({
        declarations: [
            MyComponent
        ],
        providers: [
            { provide: MyService, useClass: MockMyService1 }
        ]
        })
        .compileComponents();
    }));

    beforeEach(() => {
        fixture = TestBed.createComponent(MapComponent);
        mapComponent = fixture.componentInstance;
        fixture.detectChanges();
    });

    describe('MyFirstTest', () => {
        it('should test with my first mock', () => {
            /**
             * Test with my first mock
             */
        });
    });

    describe('MySecondTest', () => {
        // Here I would like to change { provide: MyService, useClass: MockMyService1 } to { provide: MyService, useClass: MockMyService2 }

        it('should test with my second mock', () => {
            /**
             * Test with my second mock
             */
        });
    });
});

I see that the function overrideProvider exists, but I did not manage to use it in my test. When I use it in a "it", the provider doesn't change. I didn't manage to find an example where this function is called. Could you explain me how to use it properly? Or have you an other method to do that?

Answer 1

As of angular 6 I noticed that overrideProvider works with the useValue property. So in order to make it work try something like:

class MockRequestService1 {
  ...
}

class MockRequestService2 {
  ...
}

then write you TestBed like:

// example with injected service
TestBed.configureTestingModule({
  // Provide the service-under-test
  providers: [
    SomeService, {
      provide: SomeInjectedService, useValue: {}
    }
  ]
});

And whenever you want to override the provider just use:

TestBed.overrideProvider(SomeInjectedService, {useValue: new MockRequestService1()});
// Inject both the service-to-test and its (spy) dependency
someService = TestBed.get(SomeService);
someInjectedService = TestBed.get(SomeInjectedService);

Either in a beforeEach() function or place it in an it() function.