feature #12314 [Form] Add "prototype_data" option to collection type (kgilden)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #12314).

Discussion
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[Form] Add "prototype_data" option to collection type

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #5095
| License       | MIT
| Doc PR        | symfony/symfony-docs#4367

Makes it possible to have default data for collection prototypes.

Commits
-------

80b0a80 [Form] Add "prototype_data" option to collection type

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

@@ -29,7 +29,9 @@ class CollectionType extends AbstractType
         if ($options['allow_add'] && $options['prototype']) {
             $prototype = $builder->create($options['prototype_name'], $options['type'], array_replace(array(
                 'label' => $options['prototype_name'].'label__',
-            ), $options['options']));
+            ), $options['options'], array(
+                'data' => $options['prototype_data'],
+            )));
             $builder->setAttribute('prototype', $prototype->getForm());
         }
 
@@ -84,6 +86,7 @@ class CollectionType extends AbstractType
             'allow_add' => false,
             'allow_delete' => false,
             'prototype' => true,
+            'prototype_data' => null,
             'prototype_name' => '__name__',
             'type' => 'text',
             'options' => array(),

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

@@ -274,4 +274,19 @@ class CollectionTypeTest extends \Symfony\Component\Form\Test\TypeTestCase
 
         $this->assertSame('__test__label__', $form->createView()->vars['prototype']->vars['label']);
     }
+
+    public function testPrototypeData()
+    {
+        $form = $this->factory->create('collection', array(), array(
+            'type' => 'text',
+            'allow_add' => true,
+            'prototype' => true,
+            'prototype_data' => 'foo',
+            'options' => array(
+                'data' => 'bar',
+            ),
+        ));
+
+        $this->assertSame('foo', $form->createView()->vars['prototype']->vars['value']);
+    }
 }