UPGRADE FROM 2.2 to 2.3

Form

Although this was not officially supported nor documented, it was possible to set the option "validation_groups" to false, resulting in the group "Default" being validated. Now, if you set "validation_groups" to false, the validation of a form will be skipped (except for a few integrity checks on the form).

If you want to validate a form in group "Default", you should either explicitly set "validation_groups" to "Default" or alternatively set it to null.

Before:
```
// equivalent notations for validating in group "Default"
"validation_groups" => null
"validation_groups" => "Default"
"validation_groups" => false

// notation for skipping validation
"validation_groups" => array()
```
After:
```
// equivalent notations for validating in group "Default"
"validation_groups" => null
"validation_groups" => "Default"

// equivalent notations for skipping validation
"validation_groups" => false
"validation_groups" => array()
```

The array type hint from DataMapperInterface was removed. You should adapt implementations of that interface accordingly.

Before:

use Symfony\Component\Form\DataMapperInterface;

class MyDataMapper
{
    public function mapFormsToData(array $forms, $data)
    {
        // ...
    }

    public function mapDataToForms($data, array $forms)
    {
        // ...
    }
}

After:

use Symfony\Component\Form\DataMapperInterface;

class MyDataMapper
{
    public function mapFormsToData($forms, $data)
    {
        // ...
    }

    public function mapDataToForms($data, $forms)
    {
        // ...
    }
}

Instead of an array, the methods here are now passed a RecursiveIteratorIterator containing an InheritDataAwareIterator by default, so you don't need to handle forms inheriting their parent data (former "virtual forms") in the data mapper anymore.

Before:

use Symfony\Component\Form\Util\VirtualFormAwareIterator;

public function mapFormsToData(array $forms, $data)
{
    $iterator = new \RecursiveIteratorIterator(
        new VirtualFormAwareIterator($forms)
    );

    foreach ($iterator as $form) {
        // ...
    }
}

After:

public function mapFormsToData($forms, $data)
{
    foreach ($forms as $form) {
        // ...
    }
}

The *_SET_DATA events are now guaranteed to be fired after the children were added by the FormBuilder (unless setData() is called manually). Before, the *_SET_DATA events were sometimes thrown before adding child forms, which made it impossible to remove child forms dynamically.

A consequence of this change is that you need to set the "auto_initialize" option to false for FormInterface instances that you pass to FormInterface::add():

Before:
```
$form = $factory->create('form');
$form->add($factory->createNamed('field', 'text'));
```
This code will now throw an exception with the following message:

Automatic initialization is only supported on root forms. You should set the "auto_initialize" option to false on the field "field".

Consequently, you need to set the "auto_initialize" option:

After (Alternative 1):
```
$form = $factory->create('form');
$form->add($factory->createNamed('field', 'text', array(), array(
    'auto_initialize' => false,
)));
```
The problem also disappears if you work with FormBuilder instances instead of Form instances:

After (Alternative 2):
```
$builder = $factory->createBuilder('form');
$builder->add($factory->createBuilder('field', 'text'));
$form = $builder->getForm();
```
The best solution is in most cases to let add() handle the field creation:

After (Alternative 3):
```
$form = $factory->create('form');
$form->add('field', 'text');
```
After (Alternative 4):
```
$builder = $factory->createBuilder('form');
$builder->add('field', 'text');
$form = $builder->getForm();
```
Previously, when the "data" option of a field was set to null and the containing form was mapped to an object, the field would receive the data from the object as default value. This functionality was unintended and fixed to use null as default value in Symfony 2.3.

In cases where you made use of the previous behavior, you should now remove the "data" option altogether.

Before:
```
$builder->add('field', 'text', array(
   'data' => $defaultData ?: null,
));
```
After:
```
$options = array();
if ($defaultData) {
    $options['data'] = $defaultData;
}
$builder->add('field', 'text', $options);
```

PropertyAccess

PropertyAccessor was changed to continue its search for a property or method even if a non-public match was found. This means that the property "author" in the following class will now correctly be found:

class Article
{
    public $author;

    private function getAuthor()
    {
        // ...
    }
}

Although this is uncommon, similar cases exist in practice.

Instead of the PropertyAccessDeniedException that was thrown here, the more generic NoSuchPropertyException is thrown now if no public property nor method are found by the PropertyAccessor. PropertyAccessDeniedException was removed completely.

Before:

use Symfony\Component\PropertyAccess\Exception\PropertyAccessDeniedException;
use Symfony\Component\PropertyAccess\Exception\NoSuchPropertyException;

try {
    $value = $accessor->getValue($article, 'author');
} catch (PropertyAccessDeniedException $e) {
    // Method/property was found but not public
} catch (NoSuchPropertyException $e) {
    // Method/property was not found
}

After:

use Symfony\Component\PropertyAccess\Exception\NoSuchPropertyException;

try {
    $value = $accessor->getValue($article, 'author');
} catch (NoSuchPropertyException $e) {
    // Method/property was not found or not public
}

DomCrawler

Crawler::each() and Crawler::reduce() now return Crawler instances instead of DomElement instances:

Before:

$data = $crawler->each(function ($node, $i) {
    return $node->nodeValue;
});

After:

$data = $crawler->each(function ($crawler, $i) {
    return $crawler->text();
});

Console

New verbosity levels have been added, therefore if you used to do check the output verbosity level directly for VERBOSITY_VERBOSE you probably want to update it to a greater than comparison:

Before:
```
if (OutputInterface::VERBOSITY_VERBOSE === $output->getVerbosity()) { ... }
```
After:
```
if (OutputInterface::VERBOSITY_VERBOSE <= $output->getVerbosity()) { ... }
```

BrowserKit

If you are receiving responses with non-3xx Status Code and Location header please be aware that you won't be able to use auto-redirects on these kind of responses.

If you are correctly passing 3xx Status Code with Location header, you don't have to worry about the change.

If you were using responses with Location header and non-3xx Status Code, you have to update your code to manually create another request to URL grabbed from the Location header.