This PR was merged into the 2.2 branch.

Discussion
----------

[Form][2.2] Fixed Form::submit() to react to dynamic form modifications

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

ref #3767, #3768, #4548, #8748
cc @Burgov

This PR ensures that fields that are added during the submission process of a form are submitted as well. For example:

```php
$builder = $this->createFormBuilder()
    ->add('country', 'choice', ...)
    ->getForm();

$builder->get('country')->addEventListener(FormEvents::POST_SUBMIT, function (FormEvent $event) {
    $form = $event->getForm()->getParent();
    $country = $event->getForm()->getData();

    $form->add('province', 'choice', /* ... something with $country ... */);
});
```

Currently, the field "province" will not be submitted, because the submission loop uses `foreach`, which ignores changes in the underyling array.

Additionally, this PR ensures that `setData()` is called on *every* element of a form (except those inheriting their parent data) when `setData()` is called on the root element (i.e., during initialization). Currently, when some of the intermediate nodes (e.g. embedded forms) are submitted with an empty value, `setData()` won't be called on the nodes below (i.e. the fields of the embedded form) until `get*Data()` is called on them. If `getData()` is *not* called before `createView()`, any effects of `*_DATA` event listeners attached to those fields will not be visible in the view.

Commits
-------

cd27e1f [Form] Extracted ReferencingArrayIterator out of VirtualFormAwareIterator
ccaaedf [Form] PropertyPathMapper::mapDataToForms() *always* calls setData() on every child to ensure that all *_DATA events were fired when the initialization phase is over (except for virtual forms)
19b483f [Form] Removed superfluous reset() call
00bc270 [Form] Fixed: submit() reacts to dynamic modifications of the form children

This PR was merged into the 2.3 branch.

Discussion
----------

[Form][2.3] Fixed Form::submit() and Form::setData() to react to dynamic form modifications

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

ref #3767, #3768, #4548, #8748
cc @Burgov

This PR ensures that fields that are added during the submission process of a form are submitted as well. For example:

```php
$builder = $this->createFormBuilder()
    ->add('country', 'choice', ...)
    ->getForm();

$builder->get('country')->addEventListener(FormEvents::POST_SUBMIT, function (FormEvent $event) {
    $form = $event->getForm()->getParent();
    $country = $event->getForm()->getData();

    $form->add('province', 'choice', /* ... something with $country ... */);
});
```

Currently, the field "province" will not be submitted, because the submission loop uses `foreach`, which ignores changes in the underyling array.

Contrary to #8827 (for 2.2), this PR also allows dynamic modifications during `setData()`:

```php
$builder = $this->createFormBuilder()
    ->add('country', 'choice', ...)
    ->getForm();

$builder->get('country')->addEventListener(FormEvents::POST_SET_DATA, function (FormEvent $event) {
    $form = $event->getForm()->getParent();
    $country = $event->getData();

    $form->add('province', 'choice', /* ... something with $country ... */);
});
```

For 2.2, this fix is not possible, because it would require changing the signature `DataMapperInterface::mapDataToForms($data, array $forms)` to `DataMapperInterface::mapDataToForms($data, array &$forms)`, which would be a BC break. For 2.3, this change is not necessary (instead of an array we now pass an iterator, which is passed by reference anyway).

Additionally, this PR ensures that `setData()` is called on *every* element of a form (except those inheriting their parent data) when `setData()` is called on the root element (i.e., during initialization). Currently, when some of the intermediate nodes (e.g. embedded forms) are submitted with an empty value, `setData()` won't be called on the nodes below (i.e. the fields of the embedded form) until `get*Data()` is called on them. If `getData()` is *not* called before `createView()`, any effects of `*_DATA` event listeners attached to those fields will not be visible in the view.

Commits
-------

7a34d96 Merge branch 'form-submit-2.2' into form-submit-2.3
cd27e1f [Form] Extracted ReferencingArrayIterator out of VirtualFormAwareIterator
3cb8a80 [Form] Added a test that ensures that setData() reacts to dynamic modifications of a form's children
07d14e5 [Form] Removed exception in Button::setData(): setData() is now always called for all elements in the form tree during the initialization of the tree
b9a3770 [Form] Removed call to deprecated method
878e27c Merge branch 'form-submit-2.2' into form-submit-2.3
ccaaedf [Form] PropertyPathMapper::mapDataToForms() *always* calls setData() on every child to ensure that all *_DATA events were fired when the initialization phase is over (except for virtual forms)
19b483f [Form] Removed superfluous reset() call
5d60a4f Merge branch 'form-submit-2.2' into form-submit-2.3
00bc270 [Form] Fixed: submit() reacts to dynamic modifications of the form children

symfony/symfony#8827