farmOS/modules/farm/farm_quantity/farm_quantity.module

<?php
/**
 * @file
 * Farm quantity module.
 */

// Include Features code.
include_once 'farm_quantity.features.inc';

/**
 * Implements hook_farm_info().
 */
function farm_quantity_farm_info() {
  return array(
    'system_of_measurement' => farm_quantity_system_of_measurement(),
  );
}

/**
 * Implements hook_farm_ui_entities().
 */
function farm_quantity_farm_ui_entities() {
  return array(
    'taxonomy_term' => array(
      'farm_quantity_units' => array(
        'label' => t('Quantity unit'),
        'label_plural' => t('Quantity units'),
      ),
    ),
  );
}

/**
 * Implements hook_restws_field_collection_info().
 */
function farm_quantity_restws_field_collection_info() {
  return array(
    'field_farm_quantity' => array(
      'alias' => 'quantity',
      'label' => t('Quantity'),
      'multiple' => TRUE,
      'fields' => array(
        'measure' => array(
          'field_name' => 'field_farm_quantity_measure',
          'field_label' => t('Measure'),
          'field_type' => 'text',
          'field_value' => 'value',
        ),
        'value' => array(
          'field_name' => 'field_farm_quantity_value',
          'field_label' => t('Value'),
          'field_type' => 'decimal',
          'field_value' => 'decimal',
        ),
        'unit' => array(
          'field_name' => 'field_farm_quantity_units',
          'field_label' => t('Unit'),
          'field_type' => 'taxonomy_term',
          'field_value' => 'tid',
        ),
        'label' => array(
          'field_name' => 'field_farm_quantity_label',
          'field_label' => t('Label'),
          'field_type' => 'text',
          'field_value' => 'value',
        ),
      ),
    ),
  );
}

/**
 * Implements hook_permission().
 */
function farm_quantity_permission() {
  $perms = array(
    'administer farm_quantity module' => array(
      'title' => t('Administer farm quantity module'),
    ),
  );
  return $perms;
}

/**
 * Implements hook_farm_access_perms().
 */
function farm_quantity_farm_access_perms($role) {
  $perms = array();

  // Load the list of farm roles.
  $roles = farm_access_roles();

  // If this role has 'config' access, grant access to quantity configuration.
  if (!empty($roles[$role]['access']['config'])) {
    $perms[] = 'administer farm_quantity module';
  }

  return $perms;
}

/**
 * Implements hook_menu().
 */
function farm_quantity_menu() {

  // Quantity configuration form.
  $items['admin/config/farm/quantity'] = array(
    'title' => 'Quantity',
    'description' => 'Quantity configuration settings.',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('farm_quantity_settings_form'),
    'access arguments' => array('administer farm_quantity module'),
  );

  return $items;
}

/**
 * Quantity settings form.
 */
function farm_quantity_settings_form($form, &$form_state) {

  // Metric or US/Imperial.
  $form['farm_quantity_unit_system'] = array(
    '#type' => 'radios',
    '#title' => t('System of measurement'),
    '#description' => t('Select the system of measurement you would like to use in farmOS. Changing this setting after data has been entered is not recommended.'),
    '#options' => array(
      'metric' => t('Metric'),
      'us' => t('US/Imperial'),
    ),
    '#default_value' => farm_quantity_system_of_measurement(),
  );

  // Return it as a system settings form.
  return system_settings_form($form);
}

/**
 * Helper function for loading the default system of measurement.
 *
 * @return string
 *   Returns either 'metric' or 'us' (defaults to 'metric').
 */
function farm_quantity_system_of_measurement() {
  return variable_get('farm_quantity_unit_system', 'metric');
}

/**
 * Define information about available quantity measures.
 *
 * @return array
 *   Returns an array of measure information.
 */
function farm_quantity_measures() {
  return array(
    'count' => array(
      'label' => t('Count'),
    ),
    'length' => array(
      'label' => t('Length/depth'),
    ),
    'weight' => array(
      'label' => t('Weight'),
    ),
    'area' => array(
      'label' => t('Area'),
    ),
    'volume' => array(
      'label' => t('Volume'),
    ),
    'time' => array(
      'label' => t('Time'),
    ),
    'temperature' => array(
      'label' => t('Temperature'),
    ),
    'pressure' => array(
      'label' => t('Pressure'),
    ),
    'water_content' => array(
      'label' => t('Water content'),
    ),
    'value' => array(
      'label' => t('Value'),
    ),
    'rate' => array(
      'label' => t('Rate'),
    ),
    'rating' => array(
      'label' => t('Rating'),
    ),
    'ratio' => array(
      'label' => t('Ratio'),
    ),
    'probability' => array(
      'label' => t('Probability'),
    ),
  );
}

/**
 * Define available options for the Measure field.
 */
function farm_quantity_measure_options() {

  // Start an empty options array.
  $options = array();

  // Load information about measures.
  $measures = farm_quantity_measures();

  // Iterate through the measures and build a list of options.
  foreach ($measures as $measure => $data) {
    $options[$measure] = $data['label'];
  }

  // Return the array of options.
  return $options;
}

/**
 * Return an array of standard unit names for given measures.
 *
 * @todo
 *   This is a first step towards more complete unit management in farmOS. It
 *   is provided as a central helper function for other modules to use as a
 *   placeholder until then.
 *
 * @param string $measure
 *   An optional measure to filter by.
 *   See available options in farm_quantity_measures() above.
 *
 * @return array
 *   Returns an array of standard unit strings for a given measure. Or an array
 *   of all unit strings, organized into sub-arrays keyed by measure.
 */
function farm_quantity_units($measure = '') {

  // Load the available measures.
  $measures = farm_quantity_measures();

  // Build an empty array of quantity unit sub-arrays, keyed by measure.
  $units = array();
  foreach ($measures as $name => $info) {
    $units[$name] = array();
  }

  // Look up the system of measurement.
  $system = farm_quantity_system_of_measurement();

  // Depending on the system of measurement, add unit options to measures.
  if ($system == 'metric') {

    // Length units.
    $units['length'][] = t('kilometers');
    $units['length'][] = t('meters');
    $units['length'][] = t('centimeters');

    // Weight units.
    $units['weight'][] = t('kilograms');
    $units['weight'][] = t('grams');

    // Area units.
    $units['area'][] = t('hectares');
    $units['area'][] = t('square meters');

    // Volume units.
    $units['volume'][] = t('kiloliters');
    $units['volume'][] = t('liters');
    $units['volume'][] = t('milliliters');

    // Temperature units.
    $units['temperature'][] = 'C';
  }
  elseif ($system == 'us') {

    // Length units.
    $units['length'][] = t('miles');
    $units['length'][] = t('feet');
    $units['length'][] = t('inches');

    // Weight units.
    $units['weight'][] = t('tons');
    $units['weight'][] = t('lbs');
    $units['weight'][] = t('ounces');

    // Area units.
    $units['area'][] = t('acres');
    $units['area'][] = t('square feet');

    // Volume units.
    $units['volume'][] = t('cubic yards');
    $units['volume'][] = t('cubic feet');
    $units['volume'][] = t('gallons');
    $units['volume'][] = t('quarts');
    $units['volume'][] = t('cups');

    // Temperature units.
    $units['temperature'][] = t('F');
  }

  // Time units are independent of system.
  $units['time'][] = t('years');
  $units['time'][] = t('months');
  $units['time'][] = t('weeks');
  $units['time'][] = t('days');
  $units['time'][] = t('hours');
  $units['time'][] = t('minutes');
  $units['time'][] = t('seconds');

  // If a measure is specified, only return units for that measure.
  if (!empty($measure)) {
    if (!empty($units[$measure])) {
      return $units[$measure];
    }
    else {
      return array();
    }
  }

  // Return all quantity units.
  return $units;
}

/**
 * Implements hook_preprocess_field().
 */
function farm_quantity_preprocess_field(&$vars) {

  // Add quantity CSS when a Quantity field is displayed.
  if (!empty($vars['element']['#field_name']) && $vars['element']['#field_name'] == 'field_farm_quantity') {
    drupal_add_css(drupal_get_path('module', 'farm_quantity') . '/farm_quantity.css');
  }
}