Defining Custom Field Types in Drupal

gistfile1.php

<?php

/**
 * Implements hook_field_schema().
 */
function license_plate_field_schema($field) {
  return array(
    'columns' => array(
      'plate_number' => array(
        'type' => 'varchar',
        'length' => 8,
        'not null' => TRUE,
      ),
      'state' => array(
        'type' => 'varchar',
        'length' => 2,
        'not null' => FALSE,
      ),
    ),
  );
}

/**
 * Implements hook_field_info().
 */
function license_plate_field_info() {
  return array(
    'license_plate' => array(
      'label' => t('License Plate'),
      'description' => t('This field stores a license plate number in the database.'),
      'default_widget' => 'license_plate_field',
      'default_formatter' => 'license_plate_default',
    ),
  );
}

/**
 * Implements hook_field_widget_info().
 */
function license_plate_field_widget_info() {
  return array(
    'license_plate_field' => array(
      'label' => t('Text field'),
      'field types' => array('license_plate'),
    ),
  );
}

/**
 * Implements hook_field_settings_form().
 */
function license_plate_field_settings_form($field, $instance, $has_data) {
  $settings = $field['settings'];
  // Add your global settings fields here
  $form = array();
  return $form;
}

/**
 * Implements hook_field_instance_settings_form().
 */
function license_plate_field_instance_settings_form($field, $instance) {
  $settings = $instance['settings'];
  // Add your instance settings fields here.
  $form = array();
  return $form;
}

/**
 * Implements hook_field_widget_form().
 */
function license_plate_field_widget_form(&$form, &$form_state, $field, $instance, $langcode, $items, $delta, $element) {
  switch ($instance['widget']['type']) {
    case 'license_plate_field' :
      $element['license_plate'] = array(
        '#type' => 'fieldset',
        '#title' => $element['#title'],
        '#tree' => TRUE,
      );
      $element['license_plate']['plate_number'] = array(
        '#type' => 'textfield',
        '#title' => t('Plate number'),
        '#default_value' => isset($items[$delta]['plate_nuber']) ? $items[$delta]['plate_nuber'] : '',
        '#required' => $element['#required'],
        '#size' => 8,
        '#attributes' => array('maxlength' => 8),
      );
      $element['license_plate']['state'] = array(
        '#type' => 'select',
        '#title' => t('State'),
        '#default_value' => isset($items[$delta]['state']) ? $items[$delta]['state'] : '',
        '#options' => array(
          'AL' => 'Alabama',
          'AK' => 'Alaska',
          'AZ' => 'Arizona',
        ),
        '#required' => $element['#required'],
      );
      break;
  }
  return $element;
}

/**
 * Implements hook_field_presave().
 */
function license_plate_field_presave($entity_type, $entity, $field, $instance, $langcode, &$items) {
  foreach ($items as $delta => $item) {
    if (isset($item['license_plate']['plate_number'])) {
      $items[$delta]['plate_number'] = $item['license_plate']['plate_number'];
      $items[$delta]['state'] = $item['license_plate']['state'];
    }
  }
}

/**
 * Implements hook_field_is_empty().
 */
function license_plate_field_is_empty($item, $field) {
  if (empty($item['license_plate']['plate_number'])) {
    return TRUE;
  }
  return FALSE;
}

/**
 * Implements hook_field_validate().
 */
function license_plate_field_validate($entity_type, $entity, $field, $instance, $langcode, $items, &$errors) {
  // Loop through field items in the case of multiple values.
  foreach ($items as $delta => $item) {
    if (isset($item['plate_number']) && $item['plate_number'] != '') {
      if (!valid_license_plate($item['plate_number'])) {
        $errors[$field['field_name']][$langcode][$delta][] = array(
          'message' => t('Invalid license plate number.'),
        );
      }
    }
  }
}

/**
 * Implements hook_field_formatter_info().
 */
function license_plate_field_formatter_info() {
  return array(
    'license_plate_default' => array(
      'label' => t('Default'),
      'field types' => array('license_plate'),
    ),
  );
}

/**
 * Implements hook_field_formatter_view().
 */
function license_plate_field_formatter_view($entity_type, $entity, $field, $instance, $langcode, $items, $display) {
  $element = array();
  switch ($display['type']) {
    case 'license_plate_default' :
      foreach ($items as $delta => $item) {
        if (isset($item['plate_number'])) {
          $element[$delta]['#markup'] = $item['state'] . ' ' . $item['plate_number'];
        }
      }
      break;
  }
  return $element;
}