Not able to flush multiple times to test subscriptions

[fdub]

I'm not able to test a component which uses a subscription to receive updates because it's not possible to call flush(...) multiple times on a TestOperation. The contained observer is completed after calling flush for the first time.

Here's an example of a failing test showing this behavior.

    it('should receive updates', (done) => {
        TestBed.configureTestingModule({
            imports: [ApolloTestingModule],
        });
        const apollo = TestBed.inject(Apollo);
        const controller = TestBed.inject(ApolloTestingController);
        
        const query = gql`
            subscription dates {
                date
            }`;
        const data1 = {date: '1970-01-01'};
        const data2 = {date: '1970-01-02'};

        let resultData: any;
        apollo.subscribe<any>({query: query}).subscribe({
            next: (result) => {
                resultData = result.data;
            },
            error: (e) => {
                done.fail(e);
            },
        });

        const op = controller.expectOne(query);
        op.flush({data: data1});
        op.flush({data: data2});

        expect(resultData).toEqual(data2);
        done();
    });

An additional argument keepOpen: bool (with default false) or an additional function like flushAndKeepOpen(...), omitting the call to complete(), could solve this.

[MikaStark]

In addition, I want to propose to add this possibility to Queries too.
Sometimes you may need to test multiple behaviors dependening on results (nullability, list emptiness, greater/lower, contains, etc.).

[rmckeel]

I fully agree, yes, would love to see this. Our use case is mocking a GraphQL query that has the pollInterval option set. Without being able to flush twice and complete (I'll skip the obvious potty joke here ;) it is hard to test that our software operates properly when the operation is polled again and either changes or stays the same.

[Maximaximum]

Hm, strange that this has not been implemented yet. It seems to be such a core thing

[Killerbear]

same problem here. we are using a subscription on valueChanges to update our template accordingly if something was added or removed in the data.
But without the ability to flush multiple times, we can't test this.

[Frozen-byte]

Is there any workaround for this other than completely mocking the GraphQL Service?
When inspect the actual sourcecode:

apollo-angular/packages/apollo-angular/testing/src/operation.ts

Line 23 in 007ee40

this.observer.complete();

The complete statement may be prevented on a subscription document.

My Shoot at this issue: #2316 feel free to test, comment and review.