A way to keep original values around when a Symfony form is submitted with an empty value. Useful for password fields especially

composer.json

{
    "name": "pmg/keep-value-listener",
    "description": "Example code for a blog post",
    "license": "MIT",
    "require": {
        "symfony/form": "~3.0"
    },
    "require-dev": {
        "phpunit/phpunit": "~5.0"
    },
    "autoload": {
        "files": [
            "KeepValueListener.php"
        ]
    }
}

KeepValueListener.php

<?php
/**
 * Copyright (c) 2016 PMG <https://www.pmg.com>
 * 
 * License: MIT
 */

use Symfony\Component\PropertyAccess\PropertyAccess;
use Symfony\Component\PropertyAccess\PropertyAccessorInterface;
use Symfony\Component\Form\FormEvents;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Use inside your form types.
 * 
 *    public function buildForm(FormBuilderInterface $build, array $options)
 *    {
 *      $build->add('password', PasswordType::class);
 *      $build->add('remove_password', CheckboxType::class, [
 *        'mapped' => false,
 *      ]);
 *      $build->addEventSubscriber(new KeepValueListener('password', 'remove_password'));
 *    }
 */
final class KeepValueListener implements EventSubscriberInterface
{
    /**
     * The name of a field whose value should be kept if an 
     * empty value is submitted.
     *
     * @var string
     */
    private $keepField;

    /**
     * The boolean (checkbox) field that tells the listener to clear
     * the field.
     *
     * @var string
     */
    private $clearField;

    /**
     * @var PropertyAccessorInterface
     */
    private $accessor;

    public function __construct($keepField, $clearField=null, PropertyAccessorInterface $accessor=null)
    {
        $this->keepField = $keepField;
        $this->clearField = $clearField;
        $this->accessor = $accessor ?: PropertyAccess::createPropertyAccessor();
    }

    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents()
    {
        return [
            FormEvents::PRE_SUBMIT => 'onPreSubmit',
        ];
    }

    public function onPreSubmit(FormEvent $event)
    {
        $field = $event->getForm()->get($this->keepField);
        $submitData = $event->getData();

        $erase = false;
        if ($this->clearField && isset($submitData[$this->clearField])) {
            $erase = self::asBool($submitData[$this->clearField]);
        }

        if ($erase) {
            $submitData[$this->keepField] = $field->getConfig()->getEmptyData();
            $event->setData($submitData);
            return;
        }

        if (empty($submitData[$this->keepField])) {
            $submitData[$this->keepField] = $this->accessor->getValue(
                $event->getForm()->getData(),
                $field->getPropertyPath()
            );
            $event->setData($submitData);
        }
    }

    private static function asBool($value)
    {
        return filter_var($value, FILTER_VALIDATE_BOOLEAN);
    }
}

KeepValueListenerTest.php

<?php
/**
 * Copyright (c) 2016 PMG <https://www.pmg.com>
 * 
 * License: MIT
 */

use Symfony\Component\Form\Forms;
use Symfony\Component\Form\Extension\Core\Type as CoreTypes;

class KeepValueListenerTest extends \PHPUnit_Framework_TestCase
{
    private $formFactory;

    public function testFormSubmissionWithClearFieldSetRemovesTheValue()
    {
        $form = $this->createForm([]);

        $form->submit([
            'password' => 'test',
            'clear_password' => '1',
        ]);

        $data = $form->getData();

        $this->assertNull($data['password']);
    }

    public function testEmptyFieldValueInSubmissionKeepsTheOriginalValueAround()
    {
        $form = $this->createForm([
            'password' => 'original',
        ]);

        $form->submit([
            'password' => '',
        ]);

        $data = $form->getData();

        $this->assertEquals('original', $data['password']);
    }

    public function testNonEmptyValueInSubmissionReplacesTheExistingValue()
    {
        $form = $this->createForm([
            'password' => 'original',
        ]);

        $form->submit([
            'password' => 'changed',
        ]);

        $data = $form->getData();

        $this->assertEquals('changed', $data['password']);
    }

    protected function setUp()
    {
        $this->formFactory = Forms::createFormFactory();
    }

    private function createForm($data)
    {
        return $this->formFactory->createBuilder(CoreTypes\FormType::class, $data)
            ->add('password', CoreTypes\PasswordType::class, [
                'empty_data' => null,
            ])
            ->add('clear_password', CoreTypes\CheckboxType::class)
            ->addEventSubscriber(new KeepValueListener('password', 'clear_password'))
            ->getForm();
    }
}