How to implement a custom field formatter

field_formatter.module

<?php
/**
* @file
* Sets up a field formatter, which can run custom theming or whatever else for a field.
*
* Once you have implemented the formatter, choose it on the 'display options' for a node.
*/


/**
 * Implements hook_field_formatter_info().
 *
 * Provides a custom formatter for node_reference fields.
 */
function superpoll_field_formatter_info() {
  return array(
    'superpoll_option' => array(
      'label' => t('Option with title, thumbnail, description and vote button.'),
      'field types' => array('node_reference'),
    )
  );
}

/**
 * Implements hook_field_formatter_prepare_view().
 */
function superpoll_field_formatter_prepare_view($entity_type, $entities, $field, $instances, $langcode, &$items, $displays) {
  node_reference_field_formatter_prepare_view($entity_type, $entities, $field, $instances, $langcode, &$items, $displays);
}

/**
 * Implements hook_field_formatter_view().
 */
function superpoll_field_formatter_view($entity_type, $entity, $field, $instance, $langcode, $items, $display) {
  // Type is always superpoll_option - we don't expose any other formatters, so
  // we don't need to check $display['type'].
  $result = array();
  foreach ($items as $delta => $item) {
    if ($item['access']) {
      $result[$delta] = array(
        '#theme' => 'superpoll_option',
        '#node' => $item['node'],
        '#parent_nid' => $entity->hostEntityId(),
        // Doesn't seem like this uses any extra memory really.
        '#parent_node' => node_load($entity->hostEntityId()),
      );
    }
  }
  return $result;
}