validator.directive.ts

@Directive({
  selector: '[myValidator]',
  providers: [{
    provide: NG_VALIDATORS,
    useExisting: MyValidatorDirective,
    multi: true,
  }]
})
class MyValidatorDirective implements Validator {
  validate(control: AbstractControl): ValidationErrors | null {
    return { 'myValidator': true };
  }
}