How can i modify Magento 2 response to include additional info?

Question

When I try to access API when i'm not authorized I want to customize response to return link where I can get token. What class should I override or change? I have no idea where to start with this and sorry for this long post but i can't post shorter. :/ I did not tried anything because I don't know how. Just let me post question please

Answer 1

Ok, i added this:

if ($this->getStatusCode() == 401)
        $this->setHeader('WWW-Authenticate', 'basic');

In \Magento\Framework\Webapi\Rest\Response.php class at the end of _renderMessages() and that works fine but it is not practice to directly change core functionality. So i made modul Rewriter. Made etc/di.xml that looks like:

 <?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Framework\Webapi\Rest\Response" type="Sdu\Rewriter\Rest\Response" />
</config>

And made new Response.php class in this new module:

<?php
namespace Sdu\Rewriter\Rest;


class Response extends \Magento\Framework\Webapi\Rest\Response
{
   public function _renderMessages()
   {
       $responseHttpCode = null;
       /** @var \Exception $exception */
       foreach ($this->getException() as $exception) {
           $maskedException = $this->_errorProcessor->maskException($exception);
           $messageData = [
               'message' => $maskedException->getMessage(),
           ];
           if ($maskedException->getErrors()) {
               $messageData['errors'] = [];
               foreach ($maskedException->getErrors() as $errorMessage) {
                   $errorData['message'] = $errorMessage->getRawMessage();
                   $errorData['parameters'] = $errorMessage->getParameters();
                   $messageData['errors'][] = $errorData;
               }
           }
           if ($maskedException->getCode()) {
               $messageData['code'] = $maskedException->getCode();
           }
           if ($maskedException->getDetails()) {
               $messageData['parameters'] = $maskedException->getDetails();
           }
           if ($this->_appState->getMode() == \Magento\Framework\App\State::MODE_DEVELOPER) {
               $messageData['trace'] = $exception instanceof \Magento\Framework\Webapi\Exception
                   ? $exception->getStackTrace()
                   : $exception->getTraceAsString();
           }
           $responseHttpCode = $maskedException->getHttpCode();
       }
       // set HTTP code of the last error, Content-Type, and all rendered error messages to body
       $this->setHttpResponseCode($responseHttpCode);
       $this->setMimeType($this->_renderer->getMimeType());
       if ($this->getStatusCode() == 401)
           $this->setHeader('WWW-Authenticate', 'basic');
       $this->setBody($this->_renderer->render($messageData));
       return $this;
   }
}

You can see added to lines at the end of method.