use DrupalCoreFormConfigFormBase;

class DemoForm extends ConfigFormBase {

// ...

public function buildForm(array $form, array &$form_state) {

  $config = $this->config('demo.settings');

  $form['email'] = array(
    '#type' => 'email',
    '#title' => $this->t('Your email address.'),
    '#default_value' => $config->get('demo.email_address')
  );

  return $form;
}

// ...

}

# Test API endpoints

test_api.get:
  path: 'my-api/get.json'
  defaults: { _controller: '\Drupal\test_api\Controller\TestAPIController::get_example' }
  methods:  [GET]
  requirements:
    _access: 'TRUE'

test_api.put:
  path: 'my-api/put.json'
  defaults: { _controller: '\Drupal\test_api\Controller\TestAPIController::put_example' }
  methods:  [PUT]
  requirements:
    _access: 'TRUE'

test_api.post:
  path: 'my-api/post.json'
  defaults: { _controller: '\Drupal\test_api\Controller\TestAPIController::post_example' }
  methods:  [POST]
  requirements:
    _access: 'TRUE'

test_api.delete:
  path: 'my-api/delete.json'
  defaults: { _controller: '\Drupal\test_api\Controller\TestAPIController::delete_example' }
  methods:  [DELETE]
  requirements:
    _access: 'TRUE'

<?php
/**
 * @file
 * Contains \Drupal\test_api\Controller\TestAPIController.
 */
namespace Drupal\test_api\Controller;
use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\JsonResponse;
/**
 * Controller routines for test_api routes.
 */
class TestAPIController extends ControllerBase {
  /**
   * Callback for `my-api/get.json` API method.
   */
  public function get_example( Request $request ) {
    $response['data'] = 'Some test data to return';
    $response['method'] = 'GET';
    return new JsonResponse( $response );
  }
  /**
   * Callback for `my-api/put.json` API method.
   */
  public function put_example( Request $request ) {
    $response['data'] = 'Some test data to return';
    $response['method'] = 'PUT';
    return new JsonResponse( $response );
  }
  /**
   * Callback for `my-api/post.json` API method.
   */
  public function post_example( Request $request ) {
    // This condition checks the `Content-type` and makes sure to 
    // decode JSON string from the request body into array.
    if ( 0 === strpos( $request->headers->get( 'Content-Type' ), 'application/json' ) ) {
      $data = json_decode( $request->getContent(), TRUE );
      $request->request->replace( is_array( $data ) ? $data : [] );
    }
    $response['data'] = 'Some test data to return';
    $response['method'] = 'POST';
    return new JsonResponse( $response );
  }
  /**
   * Callback for `my-api/delete.json` API method.
   */
  public function delete_example( Request $request ) {
    $response['data'] = 'Some test data to return';
    $response['method'] = 'DELETE';
    return new JsonResponse( $response );
  }
}

{#graphql
query {
  admin:userById(id: "1") {
    uid
    name
  }
  user:currentUserContext {
    uid
  }
}
#}

{% extends '@bartik/block.html.twig' %}
{% block content %}
  {% embed '@graphql_twig/query.html.twig' %}
    {% block content %}
      {% set admin = graphql.admin %}
      {% set user = graphql.user %}
      <div{{ content_attributes.addClass('content') }}>
        {{ content }} and
          {% if user.uid == admin.uid %}
            you, {{ admin.name }}.
          {% else %}
            you, appreciated anonymous visitor.
          {% endif %}
      </div>
    {% endblock %}
  {% endembed %}
{% endblock %}