class TaxonomyAutocompleteWidget

class TaxonomyAutocompleteWidget extends WidgetBase {

  /**
   * Implements Drupal\field\Plugin\Type\Widget\WidgetInterface::settingsForm().
   */
  public function settingsForm(array $form, array &$form_state) {
    $element['placeholder'] = array(
      '#type' => 'textfield',
      '#title' => t('Placeholder'),
      '#default_value' => $this
        ->getSetting('placeholder'),
      '#description' => t('Text that will be shown inside the field until a value is entered. This hint is usually a sample value or a brief description of the expected format.'),
    );
    return $element;
  }

  /**
   * Implements Drupal\field\Plugin\Type\Widget\WidgetInterface::formElement().
   */
  public function formElement(array $items, $delta, array $element, $langcode, array &$form, array &$form_state) {
    $field = $this->field;
    $tags = array();
    foreach ($items as $item) {
      $tags[$item['tid']] = isset($item['taxonomy_term']) ? $item['taxonomy_term'] : taxonomy_term_load($item['tid']);
    }
    $element += array(
      '#type' => 'textfield',
      '#default_value' => taxonomy_implode_tags($tags),
      '#autocomplete_path' => $this
        ->getSetting('autocomplete_path') . '/' . $field['field_name'],
      '#size' => $this
        ->getSetting('size'),
      '#placeholder' => $this
        ->getSetting('placeholder'),
      '#maxlength' => 1024,
      '#element_validate' => array(
        'taxonomy_autocomplete_validate',
      ),
    );
    return $element;
  }

  /**
   * Implements Drupal\field\Plugin\Type\Widget\WidgetInterface::massageFormValues()
   */
  public function massageFormValues(array $values, array $form, array &$form_state) {

    // Autocomplete widgets do not send their tids in the form, so we must detect
    // them here and process them independently.
    $items = array();
    $field = $this->field;

    // Collect candidate vocabularies.
    foreach ($field['settings']['allowed_values'] as $tree) {
      if ($vocabulary = entity_load('taxonomy_vocabulary', $tree['vocabulary'])) {
        $vocabularies[$vocabulary
          ->id()] = $vocabulary;
      }
    }

    // Translate term names into actual terms.
    foreach ($values as $value) {

      // See if the term exists in the chosen vocabulary and return the tid;
      // otherwise, create a new term.
      if ($possibilities = entity_load_multiple_by_properties('taxonomy_term', array(
        'name' => trim($value),
        'vid' => array_keys($vocabularies),
      ))) {
        $term = array_pop($possibilities);
        $item = array(
          'tid' => $term
            ->id(),
        );
      }
      else {
        $vocabulary = reset($vocabularies);
        $term = entity_create('taxonomy_term', array(
          'vid' => $vocabulary
            ->id(),
          'name' => $value,
        ));
        $item = array(
          'tid' => 0,
          'entity' => $term,
        );
      }
      $items[] = $item;
    }
    return $items;
  }

}