Using hook_form_alter() to modify formsfor Drupal 8 , 9 , 10 , and 11

In Drupal, the Form API provides a powerful way to modify existing forms through the use of the hook_form_alter() function. This functionality is crucial when you need to make changes to forms provided by other modules or the core system, allowing for extensive customization without altering the original module's code.

What is hook_form_alter()?

The hook_form_alter() function is a hook provided by Drupal that allows developers to alter the structure of any form before it is rendered. By implementing this hook, you can add, remove, or change form elements based on your custom requirements.

Why Use hook_form_alter()?

• Flexibility: Customize forms provided by Drupal core or contributed modules to better fit your application's needs.
• Non-Invasive: Apply changes without directly modifying the original module code, reducing the risk during updates.
• Comprehensive Control: Allow for the addition of new validation, submission handlers, and conditionally required fields.

Basic Usage of hook_form_alter()

Here's a basic example of how to use hook_form_alter() to modify a form. Assume you want to add a new field to the user registration form:

    /**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(array &$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
    // Target the user registration form.
    if ($form_id === 'user_register_form') {
        // Add a new field for phone number.
        $form['phone_number'] = [
            '#type' => 'textfield',
            '#title' => t('Phone Number'),
            '#required' => TRUE,
            '#description' => t('Enter your phone number.'),
        ];
    }
}

Explanation of the Code

In this example:

• The hook_form_alter() function is placed within a custom module named mymodule.
• The function targets the user registration form by checking if $form_id equals 'user_register_form'.
• If the form matches, a new text field for phone_number is added to the registration form.
• The added field is set as required, ensuring users provide a phone number upon registration.

Advanced Usage

The hook_form_alter() function can also be used in more advanced scenarios, such as:

• Changing form element attributes or default values.
• Modifying form submission handlers to customize how data is processed.
• Altering form validation logic to include new or changed validation rules.

Conclusion

Using hook_form_alter(), you can effectively tailor Drupal forms to meet specific application needs. This provides extensive flexibility for developers to implement custom logic or UI/UX enhancements without compromising the integrity of the core or contributed modules. By making strategic use of this hook, you create a robust platform tailored to your unique requirements.

What's Next?

In the next topic, we'll explore hook_form_FORM_ID_alter() for targeted form modifications, offering you even greater precision in form customization. Stay tuned to learn how to apply changes to specific forms with exacting focus, optimizing your development process.