new cookbook article on using empty data for form classes

cookbook/form/index.rst

@@ -11,3 +11,4 @@ Form
     create_custom_field_type
     create_form_type_extension
     use_virtuals_forms
+    use_empty_data

cookbook/form/use_empty_data.rst

@@ -0,0 +1,47 @@
+.. index::
+   single: Form; Empty data
+
+How to configure Empty Data for a Form Class
+============================================
+
+The ``empty_data`` option allows you to specify an empty data set for your
+form class. This empty data set would be used if you bind your form, but
+haven't yet called ``setData()``.
+
+By default, ``empty_data`` is set to ``null``. Or, if you have specified
+a ``data_class`` option for your form class, it will default to a new instance
+of that class. That instance will be created by calling the constructor
+with no arguments.
+
+If you want to override this default behavior, there are two ways to do this.
+
+Option 1: Instantiate a new Class
+---------------------------------
+
+One reason you might use this option is if you want to use a constructor
+that takes arguments. Remember, the default ``data_class`` option calls
+that constructor with no arguments::
+
+    public function getDefaultOptions()
+    {
+        return array(
+            'empty_data' => new User($this->someDependency),
+        );
+    }
+
+Option 2: Provide a Closure
+---------------------------
+
+Using a closure is the preferred method, since it will only create the object
+if it is needed.
+
+The closure must accept a ``FormInterface`` instance as the first argument::
+
+    public function getDefaultOptions()
+    {
+        return array(
+            'empty_data' => function (FormInterface $form) {
+                return new User($form->get('username')->getData());
+            },
+        );
+    }

cookbook/map.rst.inc

@@ -80,6 +80,7 @@
   * :doc:`/cookbook/form/create_custom_field_type`
   * :doc:`/cookbook/form/create_form_type_extension`
   * :doc:`/cookbook/form/use_virtuals_forms`
+  * :doc:`/cookbook/form/use_empty_data`
   * (validation) :doc:`/cookbook/validation/custom_constraint`
   * (doctrine) :doc:`/cookbook/doctrine/file_uploads`
 

reference/forms/types/options/empty_data.rst.inc

@@ -20,3 +20,8 @@ value is selected, you can do it like this:
         'empty_value' => 'Choose your gender',
         'empty_data'  => null
     ));
+
+.. note::
+
+    If you want to set the ``empty_data`` option for your entire form class,
+    see the cookbook article :doc:`/cookbook/form/use_empty_data`