hook_form_FORM_ID_alter

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

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/developing/api/8/cache.

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.

Form alter hooks are called in the following order: hook_form_alter(), hook_form_BASE_FORM_ID_alter(), hook_form_FORM_ID_alter(). See hook_form_alter() for more details.

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

Form generation

Describes how to generate and manipulate forms and process form submissions.

Hooks

Define functions that alter the behavior of Drupal core.

File

core/lib/Drupal/Core/Form/form.api.php, line 245

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'] = array(
    '#type' => 'checkbox',
    '#title' => t("I agree with the website's terms and conditions."),
    '#required' => TRUE,
  );
}