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How to Add hook_form_alter() in Drupal 8
One of the most powerful features of Drupal is its hook system. It allows developers to extend and alter existing behavior without hacking core code. Among these, `hook_form_alter()` and its variants are widely used to modify form behavior, add or remove elements, apply custom validation, and more.
Today, we will walk through the order of form alteration hooks, practical examples of how to use them, and debugging tips to make the form customizations effective and efficient.
An Overview:
Understanding the Order of Form Alter Hooks
When a form is created in Drupal, various modules and themes can implement hooks to alter it. These hooks are called in a specific order:
- hook_form_alter()
- hook_form_BASE_FORM_ID_alter()
- hook_form_FORM_ID_alter()
These hooks are called within each module, and for each module, the more general hooks are invoked first, followed by the more specific ones. For example:
- hook_form_alter() is called for all forms.
- hook_form_BASE_FORM_ID_alter() targets forms that share a base form ID.
- hook_form_FORM_ID_alter() is used to target a specific form.
The complete calling order is:
- All hook implementations from Module A
- Then from Module B
- Followed by any base theme(s)
- And finally, the current theme
Furthermore, module execution order is based on system weight and module name.
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A Common Use Case: Altering the Node Form
A frequent scenario is adding or modifying fields on the node form. We can access the node object like this:
$form['#node'];To alter the node form in the custom module, use:
use Drupal\Core\Form\FormStateInterface;
/
* Implements hook_form_alter().
*/
function yourmodule_form_alter(&$form, FormStateInterface $form_state, $form_id) {
if ($form_id == 'node_article_edit_form' || $form_id == 'node_article_form') {
// Modify the node article form here.
}
}
Also, always clear the cache after writing hook functions to make Drupal recognize them.
Use the `drush cr` command (if you’re using Drush 8+) or go to Manage > Configuration > Performance > Clear All Caches.
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Debugging Tips: Is Your Hook Being Called?
To verify if the hook is working, we can use the Devel module along with Kint, a debugging tool that prints structured variable output.
Install and enable both modules, then in the hook, use:
function yourmodule_form_alter(&$form, &$form_state, $form_id) {
kint($form); // This prints all form variables.
}
To target a specific form like `node-question-form`:
function yourmodule_form_node_question_form_alter(&$form, &$form_state, $form_id) {
kint($form); // Debug the specific form.
}
Alternatively, we can use `echo` or `print_r()` and check the browser’s source code using `Ctrl+F` to locate the debug text.
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Practical Example: Forcing Sort Order in an Exposed Views Form
Here’s how to control the sort direction of a Views exposed filter based on the user’s selection:
function MYTHEME_form_alter(&$form, &$form_state, $form_id) {
if ($form_id !== 'views_exposed_form' || $form['#id'] !== 'views-exposed-form-search-custom-page-1') {
return;
}
$user_input = $form_state->getUserInput();
if (empty($user_input['sort_by'])) {
return;
}
if ($user_input['sort_by'] == 'relevance') {
$user_input['sort_order'] = 'ASC';
} elseif ($user_input['sort_by'] == 'created') {
$user_input['sort_order'] = 'DESC';
}
$form_state->setUserInput($user_input);
}
Replace `”views-exposed-form-search-custom-page-1″` with the actual form ID, and ensure that `”relevance”` and `”created”` match the sort field machine names.
Altering Forms in Custom Themes
If we are working within a custom theme, we can use the following format in your `THEMENAME.theme` file:
function bootstrap_ga_form_user_login_form_alter(&$form, &$form_state, $form_id) {
$form['#validate'][] = '_bootstrap_ga_form_user_login_form_validate';
}
In this case, `bootstrap_ga` is the name of the custom theme. This example adds custom validation to the user login form.
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Conclusion
Modifying forms in Drupal using hook functions is a core part of custom module and theme development.
In short, our Support Engineers demonstrated how to add hook_form_alter() in Drupal 8.
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