[Components][OptionsResolver] Fix&improve replaceDefaults() description

Author: bicpi

components/options_resolver.rst

@@ -177,21 +177,12 @@ override this default. You don't need to configure ``username`` as an optional
 option. The ``OptionsResolver`` already knows that options with a default
 value are optional.
 
-The OptionsResolver component also has an
-:method:`Symfony\\Component\\OptionsResolver\\OptionsResolver::replaceDefaults`
-method. This can be used to override the previous default value. The closure
-that is passed has 2 parameters:
-
-* ``$options`` (an :class:`Symfony\\Component\\OptionsResolver\\Options`
-  instance), with all the default options
-* ``$value``, the previous set default value
-
 Default Values that depend on another Option
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Suppose you add a ``port`` option to the ``Mailer`` class, whose default
 value you guess based on the host. You can do that easily by using a
-Closure as the default value::
+closure as the default value::
 
     use Symfony\Component\OptionsResolver\Options;
     use Symfony\Component\OptionsResolver\OptionsResolverInterface;
@@ -214,9 +205,83 @@ Closure as the default value::
 
 .. caution::
 
-    The first argument of the Closure must be typehinted as ``Options``,
+    The first argument of the closure must be typehinted as ``Options``,
     otherwise it is considered as the value.
 
+Overwriting Default Values
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A previously set default value can be overwritten by invoking
+:method:`Symfony\\Component\\OptionsResolver\\OptionsResolver::setDefaults`
+again. When using a closure as the new value it is passed 2 arguments:
+
+    * ``$options``: an :class:`Symfony\\Component\\OptionsResolver\\Options`
+      instance with all the other default options
+    * ``$previousValue``: the previous set default value
+
+.. code-block:: php
+
+    use Symfony\Component\OptionsResolver\Options;
+    use Symfony\Component\OptionsResolver\OptionsResolverInterface;
+
+    // ...
+    protected function setDefaultOptions(OptionsResolverInterface $resolver)
+    {
+        // ...
+        $resolver->setDefaults(array(
+            'encryption' => 'ssl',
+            'host' => 'localhost',
+        ));
+
+        // ...
+        $resolver->setDefaults(array(
+            'encryption' => 'tls', // simple overwrite
+            'host' => function (Options $options, $previousValue) {
+                return 'localhost' == $previousValue ? '127.0.0.1' : $previousValue;
+            },
+        ));
+    }
+
+.. tip::
+
+    If the previous default value is calculated by an expensive closure and
+    you don't need access to it, you can use the
+    :method:`Symfony\\Component\\OptionsResolver\\OptionsResolver::replaceDefaults`
+    method instead. It acts like ``setDefaults`` but simply erases the
+    previous value to improve performance. This means that the previous
+    default value is not available when overwriting with another closure::
+
+        use Symfony\Component\OptionsResolver\Options;
+        use Symfony\Component\OptionsResolver\OptionsResolverInterface;
+
+        // ...
+        protected function setDefaultOptions(OptionsResolverInterface $resolver)
+        {
+            // ...
+            $resolver->setDefaults(array(
+                'encryption' => 'ssl',
+                'heavy' => function (Options $options) {
+                    // Some heavy calculations to create the $result
+
+                    return $result;
+                },
+            ));
+
+            $resolver->replaceDefaults(array(
+                'encryption' => 'tls', // simple overwrite
+                'heavy' => function (Options $options) {
+                    // $previousValue not available
+                    // ...
+
+                    return $someOtherResult;
+                },
+            ));
+        }
+
+.. note::
+
+    Existing option keys that you do not mention when overwriting are preserved.
+
 Configure allowed Values
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -270,7 +335,7 @@ Normalize the Options
 
 Some values need to be normalized before you can use them. For instance,
 pretend that the ``host`` should always start with ``http://``. To do that,
-you can write normalizers. These Closures will be executed after all options
+you can write normalizers. These closures will be executed after all options
 are passed and should return the normalized value. You can configure these
 normalizers by calling
 :method:`Symfony\\Components\\OptionsResolver\\OptionsResolver::setNormalizers`::