function hook_form_FORM_ID_alter

Provide a form-specific alteration instead of the global hook_form_alter().

Implementations are responsible for adding cache contexts/tags/max-age as needed. See https://www.drupal.org/docs/8/api/cache-api/cache-api.

Modules can implement hook_form_FORM_ID_alter() to modify a specific form, rather than implementing hook_form_alter() and checking the form ID, or using long switch statements to alter multiple forms.

The call order is as follows: all existing form alter functions are called for module A, then all for module B, etc., followed by all for any base theme(s), and finally for the theme itself. The module order is determined by system weight, then by module name.

Within each module, form alter hooks are called in the following order: first, hook_form_alter(); second, hook_form_BASE_FORM_ID_alter(); third, hook_form_FORM_ID_alter(). So, for each module, the more general hooks are called first followed by the more specific.

Parameters

$form: Nested array of form elements that comprise the form.

$form_state: The current state of the form. The arguments that \Drupal::formBuilder()->getForm() was originally called with are available in the array $form_state->getBuildInfo()['args'].

$form_id: String representing the name of the form itself. Typically this is the name of the function that generated the form.

See also

hook_form_alter()

hook_form_BASE_FORM_ID_alter()

\Drupal\Core\Form\FormBuilderInterface::prepareForm()

Related topics

File

core/lib/Drupal/Core/Form/form.api.php, line 254

Callbacks and hooks related to form system.

Code

function hook_form_FORM_ID_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {

  // Modification for the form with the given form ID goes here. For example, if
  // FORM_ID is "user_register_form" this code would run only on the user
  // registration form.
  // Add a checkbox to registration form about agreeing to terms of use.
  $form['terms_of_use'] = [
    '#type' => 'checkbox',
    '#title' => t("I agree with the website's terms and conditions."),
    '#required' => TRUE,
  ];
}