Custom field widget form - Multiple values

Question

I've create a custom field form widget for an entity reference field. When only one value is allowed it shows a (filtered) select list, but I want to allow user to assign multiple reference values to that field.

In my use case I want it to be shown as checkboxes when multiple values are allowed, but by default it shows my checkboxes in a 'add-another' fieldset.

How can this be accomplished?

Below some code, $type is now a field setting and is manually set.

// Construct element
$widget = $element;
$widget['#delta'] = $delta;
$type = $settings['option_limit_type'];

... extra logic ...

switch ($type) {
      case 'select':
        $widget += array(
          '#type' => 'select',
          '#prefix' => '<div id="' . $settings[$element['#field_name']]['ajax_wrapper'] . '">',
          '#suffix' => '</div>',
          '#options' => $options,
          '#delta' => $delta,
          '#element_validate' => array('_entityreference_hierachy_option_limit_validate'),
          '#default_value' =>  $saved_target_id,
        );
        break;
      case 'checkboxes':
        $widget += array(
          '#type' => 'checkboxes',
          '#prefix' => '<div id="' . $settings[$element['#field_name']]['ajax_wrapper'] . '">',
          '#suffix' => '</div>',
          '#options' => $options,
          '#delta' => $delta,
          '#element_validate' => array('_entityreference_hierachy_option_limit_validate'),
          '#default_value' =>  $saved_target_id,
        );
        break;
    }
return $widget;

Answer 1

You can use drupal form api attributes like

---

 case 'select':
        $widget += array(
          '#type' => 'select',
          '#prefix' => '<div id="' . $settings[$element['#field_name']]['ajax_wrapper'] . '">',
          '#suffix' => '</div>',
          '#options' => $options,
          '#delta' => $delta,
          '#element_validate' => array('_entityreference_hierachy_option_limit_validate'),
          '#default_value' =>  $saved_target_id,
          '#multiple' => $multiple,
          '#size' => $multiple ? min(12,count($options)) : 0, 
        );
        break;

---

https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7#multiple

Answer 2

In hook_field_widget_info() use 'FIELD_BEHAVIOR_CUSTOM' like this:

 'field_my_checkboxes' => array(
  'label' => t('Let the user select from a set of checkboxes.'),
  'field types' => array('field_myfield'),
  'behaviors' => array(
    'multiple values' => FIELD_BEHAVIOR_CUSTOM,
  ),

In this case 'Add another' link and fieldset won't be shown but you need to handle user's input manually. Example: https://www.drupal.org/node/1464770