WidgetBase::formMultipleElements

protected WidgetBase::formMultipleElements(FieldItemListInterface $items, array &$form, FormStateInterface $form_state)

protected function formMultipleElements(FieldItemListInterface $items, array &$form, FormStateInterface $form_state) {
  $field_name = $this->fieldDefinition->getName();
  $cardinality = $this->fieldDefinition->getFieldStorageDefinition()->getCardinality();
  $parents = $form['#parents'];

  // Determine the number of widgets to display.
  switch ($cardinality) {
    case FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED:
      $field_state = static::getWidgetState($parents, $field_name, $form_state);
      $max = $field_state['items_count'];
      $is_multiple = TRUE;
      break;

    default:
      $max = $cardinality - 1;
      $is_multiple = ($cardinality > 1);
      break;
  }

  $title = $this->fieldDefinition->getLabel();
  $description = FieldFilteredMarkup::create(\Drupal::token()->replace($this->fieldDefinition->getDescription()));

  $elements = array();

  for ($delta = 0; $delta <= $max; $delta++) {
    // Add a new empty item if it doesn't exist yet at this delta.
    if (!isset($items[$delta])) {
      $items->appendItem();
    }

    // For multiple fields, title and description are handled by the wrapping
    // table.
    if ($is_multiple) {
      $element = [
        '#title' => $this->t('@title (value @number)', ['@title' => $title, '@number' => $delta + 1]),
        '#title_display' => 'invisible',
        '#description' => '',
      ];
    }
    else {
      $element = [
        '#title' => $title,
        '#title_display' => 'before',
        '#description' => $description,
      ];
    }

    $element = $this->formSingleElement($items, $delta, $element, $form, $form_state);

    if ($element) {
      // Input field for the delta (drag-n-drop reordering).
      if ($is_multiple) {
        // We name the element '_weight' to avoid clashing with elements
        // defined by widget.
        $element['_weight'] = array(
          '#type' => 'weight',
          '#title' => $this->t('Weight for row @number', array('@number' => $delta + 1)),
          '#title_display' => 'invisible',
          // Note: this 'delta' is the FAPI #type 'weight' element's property.
          '#delta' => $max,
          '#default_value' => $items[$delta]->_weight ? : $delta,
          '#weight' => 100,
        );
      }

      $elements[$delta] = $element;
    }
  }

  if ($elements) {
    $elements += array(
      '#theme' => 'field_multiple_value_form',
      '#field_name' => $field_name,
      '#cardinality' => $cardinality,
      '#cardinality_multiple' => $this->fieldDefinition->getFieldStorageDefinition()->isMultiple(),
      '#required' => $this->fieldDefinition->isRequired(),
      '#title' => $title,
      '#description' => $description,
      '#max_delta' => $max,
    );

    // Add 'add more' button, if not working with a programmed form.
    if ($cardinality == FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED && !$form_state->isProgrammed()) {
      $id_prefix = implode('-', array_merge($parents, array($field_name)));
      $wrapper_id = Html::getUniqueId($id_prefix . '-add-more-wrapper');
      $elements['#prefix'] = '<div id="' . $wrapper_id . '">';
      $elements['#suffix'] = '</div>';

      $elements['add_more'] = array(
        '#type' => 'submit',
        '#name' => strtr($id_prefix, '-', '_') . '_add_more',
        '#value' => t('Add another item'),
        '#attributes' => array('class' => array('field-add-more-submit')),
        '#limit_validation_errors' => array(array_merge($parents, array($field_name))),
        '#submit' => array(array(get_class($this), 'addMoreSubmit')),
        '#ajax' => array(
          'callback' => array(get_class($this), 'addMoreAjax'),
          'wrapper' => $wrapper_id,
          'effect' => 'fade',
        ),
      );
    }
  }

  return $elements;
}