WidgetBase::addMoreSubmit

public static WidgetBase::addMoreSubmit(array $form, FormStateInterface $form_state)

public static function addMoreSubmit(array $form, FormStateInterface $form_state) {
  $button = $form_state->getTriggeringElement();

  // Go one level up in the form, to the widgets container.
  $element = NestedArray::getValue($form, array_slice($button['#array_parents'], 0, -1));
  $field_name = $element['#field_name'];
  $parents = $element['#field_parents'];

  // Increment the items count.
  $field_state = static::getWidgetState($parents, $field_name, $form_state);
  $field_state['items_count']++;
  static::setWidgetState($parents, $field_name, $form_state, $field_state);

  $form_state->setRebuild();
}