How and when to use createForm vs createFormBuilder - forms

Question

I am working with Symfony 3.4 and wonder how and when to use createForm vs createFormBuilder
In my case I need to create two different forms for the same data class, e.g. an user object User. The forms only differ in some additional fields.
For example one registration form for normal users with additional fields to accept the legal terms, subscribe to the newsletter, etc. And one registration form for admin users with some other additional fields. In both cases the same User class is used and the additional fields are not mapped:
public function regUserAction(Request $request) {
// Create and show user reg form
}
public function regAdminAction(Request $request) {
// Create and show admin reg form
}
Since the User form is needed/used in two different places it would be good idea to create a UserFormType and create the form using createForm
public function regUserAction(Request $request) {
$form = $this->createForm(UserFormType::class, $user, array());
}
public function regAdminAction(Request $request) {
$form = $this->createForm(UserFormType::class, $user, array());
}
class UserFormType extends AbstractType {
// setup of form fields, FormEvents handlers, etc.
}
Pro: The setup of the form has to implemented only once in UserFormType
Con: It is not possible to add custom fields to the created form, in regUserAction and regAdminAction, is it?
Since the User form needs to be customized it would be a good idea to use createFormBuilder instead:
public function regUserAction(Request $request) {
$formBuilder = $this->createFormBuilder($user, array())
->add(... common form setup ...)
->add(... custom user fields ...)
}
public function regAdminAction(Request $request) {
$formBuilder = $this->createFormBuilder($user, array())
->add(... common form setup ...)
->add(... custom admin fields ...)
}
Pro: Possible to add custom fields to the form
Con: Common setup of the form has to be implemented again for each form.
Is there any way to mix these two approaches? To implement the common form setup only once AND to add custom fields per form?

Answer 1

You can modify the form in the FormType using FormEvents like described here in the Symfony Docs. There is an example modifying a form based on user data.

Answer 2

Maybe you can implement the code in a service and return the $formBuilder object to customise it in your actions. Or you can implement a sort of factory pattern and build your $formBuilder based on the params you will pass to it.