Cascade Dropdown in Angular Reactive Form

I leave you HERE your fork working as you need.

All you have todo do is subscribe to the changes of the 'Country' field with valueChanges, something like what I do here with the formFieldCountryValueChanges$ Observable:

private formFieldCountryValueChanges$: Observable<any>;

ngOnInit() {
    this.myForm = this.formBuilder.group({
      Country: ['', [Validators.required]],
      State: ['', [Validators.required]],
    });

    this.countries = this.selectService.getCountries();

    this.formFieldCountryValueChanges$ = this.myForm
      .get('Country')
      .valueChanges.pipe(
        tap((_arrayStates) => {
          this.myForm.patchValue({ State: '' });
          return _arrayStates;
        })
      );
    this.formFieldCountryValueChanges$.subscribe();
  }