calculated_field.module

<?php

/** 
  Process each form to add the JavaScript code and submission function to handle submission
  In case of field forms, attach a JavaScript settings block to each field
 */
function calculated_field_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'field_ui_field_edit_form') {

    $function = 'calculated_field_' . $form['#field']['field_name'] . '_javascript';
    if (function_exists($function)) {
      $form['field']['settings']['javascript'] = array(
        '#markup' => t('This javascript setting is being populated with function <strong>!field()</strong>', array('!field' => $function)),
      );
    } else {
      $form['field']['settings']['javascript'] = array(
       '#type' => 'textarea',
       '#title' => t('Javascript settings'),
       '#default_value' => isset($form['#field']['settings']['javascript']) ? $form['#field']['settings']['javascript'] : '',
       '#description' => t('Alternatively, implement <strong>!function()</strong>', array('!function' => $function)),
     );
    }
  }

  $form['#attached']['js'][] = drupal_get_path('module', 'calculated_field') . '/js/calculated_field.js';
  $form['#attributes']['onsubmit'] = "jQuery('select.disabled-by-calculated-field').removeAttr('disabled');";
  $form['#submit'][] = 'calculated_field_process_submission';
}

function calculated_field_field_widget_form_alter(&$element, &$form_state, $context) {
  $js = NULL;

  if (!isset($context['field']['field_name'])) {
    return;
  }

  $function = 'calculated_field_' . $context['field']['field_name'] . '_javascript';
  if (function_exists($function)) {
    $js = $function();
  } else {
      if (isset($context['field']['settings']['javascript'])) {
          $js = $context['field']['settings']['javascript'];
      }
  }

  // Only if we attached JavaScript
  if ($js) {
      $item = array(
        '#type' => 'checkbox',
        '#title' => t('Calculate field'),
        '#attributes' => array(
          'id' => $context['field']['field_name'] . '_locked',
          'name' => $context['field']['field_name'] . '_locked',
        ),
      );

      // Edited: Disable locking (not used)
      //if (!isset($context['field']['settings']['locked']) || $context['field']['settings']['locked']) {
        $item['#attributes']['checked'] = 'checked';
      //}

      // Some items are nested
      $form_element = &$element;
      if (isset($element['value'])) {
        $form_element = &$element['value'];
      }

      $form_element['#suffix'] = '<div class="element-invisible">' . render($item) . '</div>';

      // In case of specific fields we need to do some extra processing
      $form_element['#attributes']['readonly'] = 'readonly';
      switch ($form_element['#type']) {
        case 'date_combo':
          $element['#after_build'][] = 'calculated_field_date_combo_after_build';
          break;
        case 'select':
          $form_element['#attributes']['class'][] = 'disabled-by-calculated-field';
          $form_element['#attributes']['disabled'] = 'disabled';
          break;
      }

      // Inject JavaScript code in settings for this field
      drupal_add_js(array(
        'calculated_field' => array(
          $context['field']['field_name'] => $js,
        ),
      ), 'setting');
  }
}

// The actual HTML input elements are nested
function calculated_field_date_combo_after_build($element, &$form_state) {
  $element['value']['#attributes']['readonly'] = 'readonly';
  $element['value']['date']['#attributes']['readonly'] = 'readonly';
  return $element;
}

/**
  Save the settings in the field settings
 */
function calculated_field_process_submission($form, &$form_state) {
  foreach (array_keys($form_state['input']) as $field_name) {
    if ($field = field_read_field($field_name)) {
      if (isset($field['settings']['javascript'])) {
        $locked = FALSE;
        if (!empty($form_state['input'][$field_name . '_locked']))
          $locked = TRUE;

        if (!isset($field['settings']['locked']) || $field['settings']['locked'] != $locked) {
            $new_field = array('field_name' => $field_name);
            $new_field['settings']['locked'] = $locked;
            field_update_field($new_field);
        }
      }
    }
  }
}