angular-form-directive-validator.ts

@Directive({
  selector: '[vali][ngModel]',
  providers: [
    {
      provide: NG_VALIDATORS,
      useExisting: ValiDirective,
      multi: true
    }
  ]

})
export class ValiDirective implements Validator {

  validate(formControl: AbstractControl): ValidationErrors {
    if(formControl.value === '3'){
        return {vali: { error: 'vali'}}
    }
    return null;
  };
}