foo.php

<?php

/**
 * @file
 * Contains \Drupal\sdag_media\Plugin\Field\FieldWidget\EntityReferenceDialogWidget.
 */

namespace Drupal\sdag_media\Plugin\Field\FieldWidget;

use Drupal\Component\Utility\NestedArray;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Url;

/**
 * Plugin implementation of the 'sdag_media_dialog' widget.
 *
 * @FieldWidget(
 *   id = "sdag_media_dialog",
 *   label = @Translation("Media resource selector"),
 *   field_types = {
 *     "entity_reference",
 *     "entity_reference_metadata"
 *   },
 *   settings = {
 *     "progress_indicator" = "throbber"
 *   }
 * )
 */
class EntityReferenceDialogWidget extends WidgetBase {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $field_name = $this->fieldDefinition->getName();
    $parents = $form['#parents'];
    $element['#field_name'] = $field_name;

    // Load the items for form rebuilds from the field state as they might not be
    // in $form_state['values'] because of validation limitations.
    $field_state = static::getWidgetState($parents, $field_name, $form_state);
    if (isset($field_state['items'])) {
      $items->setValue($field_state['items']);
    }

    $uuid = \Drupal::service('uuid')->generate();

    $element['#type'] = 'details';
    $element['#tree'] = TRUE;
    $element['#collapsible'] = TRUE;
    $element['#collapsed'] = FALSE;

    $element['#attributes']['id'] = 'field-widget-' . $uuid;
    $element['#attributes']['data-resource-widget'] = $uuid;
    /*
    $element['#attached']['css'][] = array(
      'type' => 'file',
      'data' => drupal_get_path('module', 'sdag_media') . '/css/sdag_media.widget.css',
    );
    */

    // $element['#type'] = 'container';
    // $element['#attributes']['class'][] = 'form-item';
    $element['#attributes']['class'][] = 'clearfix';

    $element['resource'] = array(
      '#type' => 'hidden',
      '#value' => $items->offsetGet($delta)->target_id,
      '#attributes' => array(
        'data-resource-id' => 1,
      ),
    );

    $element['item'] = array(
      '#type' => 'container',
      '#attributes' => array(
        'class' => array(
          'form-item',
          'clearfix',
        ),
      ),
    );

    if ($items->offsetGet($delta)->entity) {
      $preview = entity_view($items->offsetGet($delta)->entity, 'preview');
      $element['item']['preview']['#type'] = 'container';
      $element['item']['preview']['#attributes']['class'][] = 'widget-preview';
      $element['item']['preview']['entity'] = array(
        '#markup' => render($preview),
      );
    }

    $element['item']['operations']['#type'] = 'container';
    $element['item']['operations']['#attributes']['class'][] = 'widget-operations';

    $element['item']['operations']['search'] = array(
      '#type' => 'link',
      '#title' => t('Select'),
      '#value' => t('Select'),
      '#url' => Url::fromRoute('view.sdag_media_resource_dialog.entity_reference_dialog_widget', array('arg_0' => $uuid)),
      '#options' => array(
        'query' => array(
          'resource-widget' => $uuid,
        ),
      ),
      '#attributes' => array(
        'class' => array('button', 'use-ajax'),
        'data-accepts' => 'application/vnd.drupal-modal',
        'data-dialog-options' => json_encode(array(
          'height' => '500',
          'width' => '90%',
          'title' => 'Select a image',
        )),
      ),
      '#limit_validation_errors' => array(array_merge($parents, array($field_name))),
    );

    $element['item']['operations']['remove'] = array(
      '#type' => 'submit',
      '#name' => 'field-widget--' . $field_name . '--remove',
      '#value' => t('Remove'),
      '#ajax' => array(
        'callback' => '\Drupal\sdag_media\Plugin\Field\FieldWidget\EntityReferenceDialogWidget::updateWidget',
        'wrapper' => 'field-widget-' . $uuid,
      ),
      '#attributes' => array(
        'data-resource-remove' => 1,
      ),
      '#submit' => array('\Drupal\sdag_media\Plugin\Field\FieldWidget\EntityReferenceDialogWidget::operationRemoveSubmit'),
      '#limit_validation_errors' => array(array_merge($parents, array($field_name))),
    );

    $element['item']['operations']['update'] = array(
      '#type' => 'submit',
      '#name' => 'field-widget--' . $field_name . '--update',
      '#value' => t('Update Preview'),
      '#ajax' => array(
        'callback' => '\Drupal\sdag_media\Plugin\Field\FieldWidget\EntityReferenceDialogWidget::updateWidget',
        'wrapper' => 'field-widget-' . $uuid,
      ),
      '#attributes' => array(
        'data-resource-update' => 1,
        'class' => array('js-hide'),
      ),
      '#submit' => array('\Drupal\sdag_media\Plugin\Field\FieldWidget\EntityReferenceDialogWidget::operationUpdatePreview'),
      '#limit_validation_errors' => array(array_merge($parents, array($field_name))),
    );

    return $element;
  }

  public static function operationRemoveSubmit($form, FormStateInterface $form_state) {
    $trigger_element = $form_state->getTriggeringElement();
    $parents = $trigger_element['#array_parents'];
    $parents = array_slice($parents, 0, -3);
    $element = NestedArray::getValue($form, $parents);
    $field_name = $element['#field_name'];

    $parents = $element['#field_parents'];
    $field_state = static::getWidgetState($parents, $field_name, $form_state);
    $field_state['items'] = array();
    $field_state['items_count'] = count($field_state['items']);
    static::setWidgetState($parents, $field_name, $form_state, $field_state);
    $form_state->setRebuild(TRUE);
  }

  public static function operationUpdatePreview($form, FormStateInterface $form_state) {
    // Determine whether it was the upload or the remove button that was clicked,
    // and set $element to the managed_file element that contains that button.
    $parents = $form_state->getTriggeringElement()['#array_parents'];
    $parents = array_slice($parents, 0, -3);
    $element = NestedArray::getValue($form, $parents);
    $field_name = $element['#field_name'];

    $parents = $element['#field_parents'];
    $field_state = static::getWidgetState($parents, $field_name, $form_state);
    $field_state['items'] = array();

    $form_state_parents = array_merge($parents, array($field_name));
    $values = NestedArray::getValue($form_state->getUserInput(), $form_state_parents);

    foreach ($values as $delta => $item) {
      $field_state['items'][$delta] = array(
        'target_id' => $item['resource'],
        'resource' => $item['resource'],
      );
    }

    $field_state['items_count'] = count($field_state['items']);
    static::setWidgetState($parents, $field_name, $form_state, $field_state);
    $form_state->setRebuild();
  }

  public static function updateWidget($form, FormStateInterface $form_state) {
    $parents = $form_state->getTriggeringElement()['#array_parents'];
    $parents = array_slice($parents, 0, -3);
    $element = NestedArray::getValue($form, $parents);
    return $element;
  }

  /**
   * {@inheritdoc}
   */
  public function massageFormValues(array $values, array $form, FormStateInterface $form_state) {
    foreach ($values as &$value) {
      $value['target_id'] = $value['resource'];
    };
    return $values;
  }

}