symfony FormType：EventListener中的setData

Question

I've written a symfony FormType-Class which has a POST_SUBMIT EventListener, in which I wanted to set a default value for an existing field dependant on another, filled form element.

In other words: dependant on a selected, existing user (entity select) I want to set the default data of another entity-select field (country and state) to the correspondant value.

The code looks like this:

$formModifier = function (FormInterface $form, Person $person = null) 
{
    if ($person) {
        $form->get('country')->setData(
            $person->getCountry()
        );
    } 
};

By selecting the user the form is submitted but not successfully validated, so the Country should be selected. The Person-Object exists, the value is correct and the statement is reached.

But the value isn't selected. Isn't this possible by using a POST_SUBMIT-FormEvent?

Additional info: symfony 2.7, PHP 5.6, implementation done like the second example under http://symfony.com/doc/2.7/cookbook/form/dynamic_form_modification.html#dynamic-generation-for-submitted-forms

Thanks in advance!

Martin

Edit: The formModifier is called like this:

$builder->get('field')->addEventListener(
    FormEvents::POST_SUBMIT,
    function (FormEvent $event) use ($formModifier) {
        $person = $event->getForm()->getData();
        $formModifier($event->getForm()->getParent(), $person)
    }
};