`oneOf` validator for polymorphic data

[maxime1992]

When dealing with polymorphic data and using the remap functions it'd be nice to have a oneOf validator. It'd take as argument keys of the formGroup so it'd also be type safe.

Example here https://github.com/cloudnc/ngx-sub-form/blob/6ecdb2017e/src/app/main/listing/listing-form/vehicle-listing/vehicle-product.component.ts#L24-L25

And it could look like the following:

public getFormGroupControlOptions(): FormGroupOptions<YourPolymorphicForm> {
  return {
    validators: [NgxSubFormValidators.OneOf('prop1', 'prop2', 'prop3')],
  };
}

[andreElrico]

Im currently developing a feature to deal with polymorphic data in a much simpler fashion.
For me its feels a bit overkill to make 3 subforms when you have for e.g. 3 animals.

                        animal
              /           |           \
              dogForm  mouseForm  catForm

It would make sense to have 4 forms when the subforms are "large and complex". I will let you know on my progress.
The basic idea is to have only ONE full-form and then use ng-switch like you do and disable all not used formControls.

[maxime1992]

The validator wouldn't be aware whether you implement the 3 sub forms in the same component or if you break them down. The validator would be here only to guarantee that the form would be in valid state if exactly 1 value of the 3 possible ones is defined.

[maxime1992]

Validators are not really supposed to be part of ngx-sub-form. I mixed things up here because we do need this in our own app so I'm just going to close this one