Change the root node name of a custom REST resource's XML encoding

1. Why do you want to change the "root element"?
2. JSON-encoded responses do not have a root, so I'm not sure what you mean.
3. So, assuming you're referring to XML, I'd like to warn you that XML support may be dropped, because it's completely broken: #2800873: Add XML GET REST test coverage, work around XML encoder quirks
4. Why is this tagged with services? Are you using https://www.drupal.org/project/services perhaps?

1. Ok.
2. Ok.
3. You did not respond to this.
4. Ok. Removing the tag.

Can you explain why you're using XML? Like I said, chances are we'll drop XML support. You're the very first user I encounter who actually is using the xml format.

Now that I have the necessary background info, I can look into this and formulate a proper answer. In the mean time, please answer the above question.

Thanks, that's a very good reason!

Using a different root node for the xml format is equivalent with using a custom format. So, you must specify your own format. Let's assume it's called foo_xml, and the root node should be foo.

Then you'll want to do something like this, in a custom module:

use \Drupal\serialization\Encoder\XmlEncoder;

class FooXmlEncoder extends XmlEncoder {

  /**
   * {@inheritdoc}
   */
  static protected $format = ['foo_xml'];

  /**
   * {@inheritdoc}
   */
  public function getBaseEncoder() {
    $base_encoder = parent::getBaseEncoder();
    $base_encoder->setRootNodeName('foo');
    return $base_encoder;
  }
}

And this in its *.services.yml file:

  serializer.encoder.foo_xml:
    class: Drupal\your_custom_module\Encoder\XmlEncoder
    tags:
      - { name: encoder, format: foo_xml }

Then you can interact with your custom REST resource via http://example.com/your/custom/rest/resource?_format=foo_xml… and the root node will no longer be <response>, but <foo>.

Great! :)

fwiw, I had this need as well, I just added an annotation on my resource and the used that in the RequestHandler which I overtook. (the things you have todo migrating from D6 services to D8 isn't always pleasant, but oh well)

This approach will be more reliable. Formats will continue to work the same way, they're an API. A particular controller is not an API; it may change between releases (and in the case of RequestHandler, it already has, and certainly will again in the future), which means you'll need to chase changes.

For the sake of reference and assisting anyone viewing this page, XmlEncoder includes a method allowing you to change the XML root node name:

$encoder = new XmlEncoder();
$encoder->setRootNodeName('Request');
$RequestDetail = $encoder->encode([
  'Name' => 'value',
  'OtherName' => 'value'
], 'xml');

gives you:

<?xml version="1.0"?>
<Request>
  <Name>value</Name>
  <OtherName>value</OtherName>
</Request>

Worked perfectly...
But how do I add context to my new format, just like "xml_format_output" or "xml_encoding"?

@guilmour-asc this might help: https://stackoverflow.com/questions/47523962/how-set-encoding-utf-8-and-...