How to mock @Select in ngxs when using a mock store

Question

I am using ngxs for state handling in angular, and I am trying to test our components as units, so preferably only with mock stores, states etc.

What we have in our component is something like:

export class SelectPlatformComponent {

  @Select(PlatformListState) platformList$: Observable<PlatformListStateModel>;

  constructor(private store: Store, private fb: FormBuilder) {
    this.createForm();
    this.selectPlatform();
  }

  createForm() {
    this.selectPlatformForm = this.fb.group({
      platform: null,
    });
  }

  selectPlatform() {
    const platformControl = this.selectPlatformForm.get('platform');
    platformControl.valueChanges.forEach(
      (value: Platform) => {
        console.log("select platform " + value);
        this.store.dispatch(new PlatformSelected(value));
      }
    );
  }

}

And our fixture setup looks like this, so we can check calls on the store:

describe('SelectPlatformComponent', () => {
  let component: SelectPlatformComponent;
  let fixture: ComponentFixture<SelectPlatformComponent>;
  let store: Store;

  beforeEach(async(() => {
    const storeSpy = jasmine.createSpyObj('Store', ['dispatch']);
    TestBed.configureTestingModule({
      imports: [ReactiveFormsModule],
      declarations: [SelectPlatformComponent],
      providers: [{provide: Store, useValue: storeSpy}]

    })
      .compileComponents();
    store = TestBed.get(Store);
  }));

But when we run this, we get the following error:

Error: SelectFactory not connected to store!
    at SelectPlatformComponent.createSelect (webpack:///./node_modules/@ngxs/store/fesm5/ngxs-store.js?:1123:23)
    at SelectPlatformComponent.get [as platformList$] (webpack:///./node_modules/@ngxs/store/fesm5/ngxs-store.js?:1150:89)
    at Object.eval [as updateDirectives] (ng:///DynamicTestModule/SelectPlatformComponent.ngfactory.js:78:87)
    at Object.debugUpdateDirectives [as updateDirectives] (webpack:///./node_modules/@angular/core/fesm5/core.js?:11028:21)
    at checkAndUpdateView (webpack:///./node_modules/@angular/core/fesm5/core.js?:10425:14)
    at callViewAction (webpack:///./node_modules/@angular/core/fesm5/core.js?:10666:21)
    at execComponentViewsAction (webpack:///./node_modules/@angular/core/fesm5/core.js?:10608:13)
    at checkAndUpdateView (webpack:///./node_modules/@angular/core/fesm5/core.js?:10431:5)
    at callWithDebugContext (webpack:///./node_modules/@angular/core/fesm5/core.js?:11318:25)
    at Object.debugCheckAndUpdateView [as checkAndUpdateView] (webpack:///./node_modules/@angular/core/fesm5/core.js?:10996:12)

I could enable the entire ngxs module for this, but then I would need to create services mocks to inject into state objects, which I do not like because I am then not testing the component in isolation anymore. I tried to create a mock SelectFactory, but it seems it is not exported from the module.

Is there a way to mock the SelectFactory, or inject some mocks into the platformList$ directly? Other suggestions?

Answer 1

I stumbled upon the same problem and I found it's not possible with Angular's DI Mechanism alone. Though, it is possible by getting the actual instance created and then mock it like this:

beforeEach(async(() => {
   TestBed.configureTestingModule({
      declarations: [MyComponent]
      imports: [NgxsModule.forRoot([])] // import real module without state
   });

   const store:Store = TestBed.get(Store);
   spyOn(store, 'select').and.returnValue(of(null)); // be sure to mock the implementation here
   spyOn(store, 'selectSnapshot').and.returnValue(null); // same here
}));

If you are using memoized selectors (e.g. @Select(MyState.selector)) inside your component, be sure to ALWAYS mock the store's select function. If you don't, NGXS will try to instantiate the class MyState regardless of it being not provided to the NgxsModule.forRoot([]). This is not a problem in a lot of cases but as soon as you have dependencies inside the constructor of MyState (Angular DI dependencies) you would also need to provide those to the providers array.