Drupal provides a powerful framework for creating custom elements for use in forms. One example of a custom element is the Link field. Suppose you want to change the default label on a Link field to read "Link text." How do you alter it?
In his blogpost Custom Drupal Elements for Forms, Silvio J. Gutierrez provides a great explanation how to define a custom form element. Altering a custom form element follows in a similar manner.
To alter a custom form element:
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Register a process callback
-
Create a process callback.
What’s a callback?
In Drupal a callback is a function that allows you to define how some type of processing happens. You pass this function name as a parameter to a Drupal API function and this function is then called at the appropriate time.
How do I register my callback?
When creating a custom form element you would use the #process attribute in hook_element_info() to register a callback. Similarly, when altering a custom form element you use the #process attribute in hook_element_info_alter() to register your callback. In this example, I want to make changes to Link fields. My code is as follows:
/**
* Implements hook_element_info_alter().
*/
function my_module_element_info_alter(&$type) {
if (isset($type['link_field'])) {
$type['link_field']['#process'][] = 'my_module_link_field_process';
}
}
How do I create my process callback?
The signature for a callback function for altering a custom form element is the same as if you were creating the element. See hook_element_info() for more information. In this case want to change the label for the Link field title. My code is as follows:
/**
* Process Callback Function
*
* Callback to process the My Module link field.
*/
function my_module_link_field_process($element, $form_state, $complete_form) {
if (isset($element['#field_name'])) {
switch ($element['#field_name']) {
case 'field_s_my_link':
$element['title']['#title'] = t('Link text');
break;
}
}
return $element;
}
Using these steps I was able to add help text and alter all sorts of custom form elements. Next time you have a custom element that needs even more customizing, give this a try, and let me know how it goes!
Acknowledgements
My thanks to my colleague Greg Garvey who pointed me to this solution!
Other resources:
See this discussion from the Link issue queue.
