[Form] let `TextType` implements `DataTransformerInterface`

closes #5906.

The submitted data should always be
transformed back to the model as a string
as NULL in this case could stand for "unset
this value" whereas a string property of a
class could rely on the string type.

Furthermore, this prevents potential issues
with PHP 7 which allows type hinting of
strings in functions.

UPGRADE-3.1.md

@@ -18,6 +18,8 @@ Form
  * Support for data objects that implements both `Traversable` and `ArrayAccess`
    in `ResizeFormListener::preSubmit` method has been deprecated and will be
    removed in Symfony 4.0.
+ * `TextType` now implements `DataTransformerInterface` and will always return
+   an empty string when `empty_data` option is explicitly assigned to it.
 
 FrameworkBundle
 ---------------

src/Symfony/Component/Form/CHANGELOG.md

@@ -7,6 +7,7 @@ CHANGELOG
  * deprecated the "choices_as_values" option of ChoiceType
  * deprecated support for data objects that implements both `Traversable` and
    `ArrayAccess` in `ResizeFormListener::preSubmit` method
+ * implemented `DataTransformerInterface` in `TextType`
 
 3.0.0
 -----

src/Symfony/Component/Form/Extension/Core/Type/TextType.php

@@ -12,10 +12,24 @@
 namespace Symfony\Component\Form\Extension\Core\Type;
 
 use Symfony\Component\Form\AbstractType;
+use Symfony\Component\Form\DataTransformerInterface;
+use Symfony\Component\Form\FormBuilderInterface;
 use Symfony\Component\OptionsResolver\OptionsResolver;
 
-class TextType extends AbstractType
+class TextType extends AbstractType implements DataTransformerInterface
 {
+    public function buildForm(FormBuilderInterface $builder, array $options)
+    {
+        // When empty_data is explicitly set to an empty string,
+        // a string should always be returned when NULL is submitted
+        // This gives more control and thus helps preventing some issues
+        // with PHP 7 which allows type hinting strings in functions
+        // See https://github.com/symfony/symfony/issues/5906#issuecomment-203189375
+        if ('' === $options['empty_data']) {
+            $builder->addViewTransformer($this);
+        }
+    }
+
     /**
      * {@inheritdoc}
      */
@@ -33,4 +47,22 @@ class TextType extends AbstractType
     {
         return 'text';
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function transform($data)
+    {
+        // Model data should not be transformed
+        return $data;
+    }
+
+    /**
+     * {@inheritdoc}
+     *.
+     */
+    public function reverseTransform($data)
+    {
+        return null === $data ? '' : $data;
+    }
 }

src/Symfony/Component/Form/Tests/Extension/Core/Type/TextTypeTest.php

@@ -0,0 +1,37 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Form\Tests\Extension\Core\Type;
+
+use Symfony\Component\Form\Test\TypeTestCase as TestCase;
+
+class TextTypeTest extends TestCase
+{
+    public function testSubmitNullReturnsNull()
+    {
+        $form = $this->factory->create('Symfony\Component\Form\Extension\Core\Type\TextType', 'name');
+
+        $form->submit(null);
+
+        $this->assertNull($form->getData());
+    }
+
+    public function testSubmitNullReturnsEmptyStringWithEmptyDataAsString()
+    {
+        $form = $this->factory->create('Symfony\Component\Form\Extension\Core\Type\TextType', 'name', array(
+            'empty_data' => '',
+        ));
+
+        $form->submit(null);
+
+        $this->assertSame('', $form->getData());
+    }
+}