@Injectable()
export class SharedService {
  private _subject = new Subject<any>();

  constructor() { }

  shared_new_event(event) {
    this._subject.next(event);
  }

  get emit_shared_event() {
    return this._subject.asObservable();
  }
}

constructor( private shared_service: SharedService) { }

ngAfterViewInit() {
    this.shared_service.emit_shared_event.forEach(e => this.getDonationAmount(e));
  }

onclick_donate_amount($event) {
    this.shared_service.shared_new_event($event);
  }

describe('Shared Service', () => {
    let service: SharedService;
    let component: DonationComponent;

    beforeEach(() => {
        service = new SharedService();
    });

    it('should emit a new event ', () => {
        spyOn(service, 'shared_new_event').and.returnValue(Observable.of(40));
    });

    component.ngAfterViewInit();

    expect(component.onclick_donate_amount).toBe(40);
});

Cannot read property 'ngAfterViewInit' of undefined