How to write unit test for a service having breakpoint observer?

Question

I have a service having a breakpoint observer. But how we can trigger window resize event for breakpoint observer to cover all branch and lines.

export class BreakPointService {
isSmallDevice ;

  constructor(private breakpointObserver: BreakpointObserver) {
    this.breakpointObserver.observe([`(max-width: 899px)`,
                                    `(min-width: 900px)`])
      .subscribe(result => {

        if (result.matches) {
          if (result.breakpoints[`(max-width: 899px)`]) {
            this.isSmallDevice = true;
          }
          if (result.breakpoints[`(min-width: 900px)`]) {
            this.isSmallDevice = false;
          }
        }
      });
  }
}

Answer 1

I have had to create a test in a presentational component recently, where the responsive is used in a logical way. For this I have created methods outside the constructor or ngOnInit, which I test independently. And regarding the module I have injected it in this way TestBed.inject(BreakpointObserver); This was my scheme:

fdescribe('yourComponent', () => {
  let component: yourComponent;
  let fixture: ComponentFixture<yourComponent>;

  beforeEach(async () => {

    await TestBed.configureTestingModule({
      schemas: [
        CUSTOM_ELEMENTS_SCHEMA, NO_ERRORS_SCHEMA
      ],
      declarations: [ yourComponent ],
      providers: [],
      imports: [ yourModule],
    })
    .compileComponents();
    TestBed.inject(BreakpointObserver); // <------ inject  
  });

  beforeEach(() => {
    fixture = TestBed.createComponent(yourComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('Can load instance', () => {
    expect(component).toBeTruthy();
  });

  it('Test for the method how do somthing', () => {
    // <---------- your test
  });
});