class OEmbedForm

class OEmbedForm extends AddFormBase {

  /**
   * The oEmbed URL resolver service.
   *
   * @var \Drupal\media\OEmbed\UrlResolverInterface
   */
  protected $urlResolver;

  /**
   * The oEmbed resource fetcher service.
   *
   * @var \Drupal\media\OEmbed\ResourceFetcherInterface
   */
  protected $resourceFetcher;

  /**
   * Constructs a new OEmbedForm.
   *
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager.
   * @param \Drupal\media_library\MediaLibraryUiBuilder $library_ui_builder
   *   The media library UI builder.
   * @param \Drupal\media\OEmbed\UrlResolverInterface $url_resolver
   *   The oEmbed URL resolver service.
   * @param \Drupal\media\OEmbed\ResourceFetcherInterface $resource_fetcher
   *   The oEmbed resource fetcher service.
   * @param \Drupal\media_library\OpenerResolverInterface $opener_resolver
   *   The opener resolver.
   */
  public function __construct(EntityTypeManagerInterface $entity_type_manager, MediaLibraryUiBuilder $library_ui_builder, UrlResolverInterface $url_resolver, ResourceFetcherInterface $resource_fetcher, OpenerResolverInterface $opener_resolver = NULL) {
    parent::__construct($entity_type_manager, $library_ui_builder, $opener_resolver);
    $this->urlResolver = $url_resolver;
    $this->resourceFetcher = $resource_fetcher;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static($container
      ->get('entity_type.manager'), $container
      ->get('media_library.ui_builder'), $container
      ->get('media.oembed.url_resolver'), $container
      ->get('media.oembed.resource_fetcher'), $container
      ->get('media_library.opener_resolver'));
  }

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return $this
      ->getBaseFormId() . '_oembed';
  }

  /**
   * {@inheritdoc}
   */
  protected function getMediaType(FormStateInterface $form_state) {
    if ($this->mediaType) {
      return $this->mediaType;
    }
    $media_type = parent::getMediaType($form_state);
    if (!$media_type
      ->getSource() instanceof OEmbedInterface) {
      throw new \InvalidArgumentException('Can only add media types which use an oEmbed source plugin.');
    }
    return $media_type;
  }

  /**
   * {@inheritdoc}
   */
  protected function buildInputElement(array $form, FormStateInterface $form_state) {
    $media_type = $this
      ->getMediaType($form_state);
    $providers = $media_type
      ->getSource()
      ->getProviders();

    // Add a container to group the input elements for styling purposes.
    $form['container'] = [
      '#type' => 'container',
    ];
    $form['container']['url'] = [
      '#type' => 'url',
      '#title' => $this
        ->t('Add @type via URL', [
        '@type' => $this
          ->getMediaType($form_state)
          ->label(),
      ]),
      '#description' => $this
        ->t('Allowed providers: @providers.', [
        '@providers' => implode(', ', $providers),
      ]),
      '#required' => TRUE,
      '#attributes' => [
        'placeholder' => 'https://',
      ],
    ];
    $form['container']['submit'] = [
      '#type' => 'submit',
      '#value' => $this
        ->t('Add'),
      '#button_type' => 'primary',
      '#validate' => [
        '::validateUrl',
      ],
      '#submit' => [
        '::addButtonSubmit',
      ],
      // @todo Move validation in https://www.drupal.org/node/2988215
      '#ajax' => [
        'callback' => '::updateFormCallback',
        'wrapper' => 'media-library-wrapper',
        // Add a fixed URL to post the form since AJAX forms are automatically
        // posted to <current> instead of $form['#action'].
        // @todo Remove when https://www.drupal.org/project/drupal/issues/2504115
        //   is fixed.
        'url' => Url::fromRoute('media_library.ui'),
        'options' => [
          'query' => $this
            ->getMediaLibraryState($form_state)
            ->all() + [
            FormBuilderInterface::AJAX_FORM_REQUEST => TRUE,
          ],
        ],
      ],
    ];
    return $form;
  }

  /**
   * Validates the oEmbed URL.
   *
   * @param array $form
   *   The complete form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current form state.
   */
  public function validateUrl(array &$form, FormStateInterface $form_state) {
    $url = $form_state
      ->getValue('url');
    if ($url) {
      try {
        $resource_url = $this->urlResolver
          ->getResourceUrl($url);
        $this->resourceFetcher
          ->fetchResource($resource_url);
      } catch (ResourceException $e) {
        $form_state
          ->setErrorByName('url', $e
          ->getMessage());
      }
    }
  }

  /**
   * Submit handler for the add button.
   *
   * @param array $form
   *   The form render array.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The form state.
   */
  public function addButtonSubmit(array $form, FormStateInterface $form_state) {
    $this
      ->processInputValues([
      $form_state
        ->getValue('url'),
    ], $form, $form_state);
  }

}