Here's the example from the examples module found here
use Drupal\Core\Form\FormStateInterface;
/**
* Implements the ajax demo form controller.
*
* This example demonstrates using ajax callbacks to add people's names to a
* list of picnic attendees.
*
* @see \Drupal\Core\Form\FormBase
* @see \Drupal\Core\Form\ConfigFormBase
*/
class AjaxAddMore extends DemoBase {
/**
* Form with 'add more' and 'remove' buttons.
*
* This example shows a button to "add more" - add another textfield, and
* the corresponding "remove" button.
*/
public function buildForm(array $form, FormStateInterface $form_state) {
$form['description'] = [
'#type' => 'item',
'#markup' => $this->t('This example shows an add-more and a remove-last button.'),
];
// Gather the number of names in the form already.
$num_names = $form_state->get('num_names');
// We have to ensure that there is at least one name field.
if ($num_names === NULL) {
$name_field = $form_state->set('num_names', 1);
$num_names = 1;
}
$form['#tree'] = TRUE;
$form['names_fieldset'] = [
'#type' => 'fieldset',
'#title' => $this->t('People coming to picnic'),
'#prefix' => '<div id="names-fieldset-wrapper">',
'#suffix' => '</div>',
];
for ($i = 0; $i < $num_names; $i++) {
$form['names_fieldset']['name'][$i] = [
'#type' => 'textfield',
'#title' => $this->t('Name'),
];
}
$form['names_fieldset']['actions'] = [
'#type' => 'actions',
];
$form['names_fieldset']['actions']['add_name'] = [
'#type' => 'submit',
'#value' => $this->t('Add one more'),
'#submit' => ['::addOne'],
'#ajax' => [
'callback' => '::addmoreCallback',
'wrapper' => 'names-fieldset-wrapper',
],
];
// If there is more than one name, add the remove button.
if ($num_names > 1) {
$form['names_fieldset']['actions']['remove_name'] = [
'#type' => 'submit',
'#value' => $this->t('Remove one'),
'#submit' => ['::removeCallback'],
'#ajax' => [
'callback' => '::addmoreCallback',
'wrapper' => 'names-fieldset-wrapper',
],
];
}
$form['actions']['submit'] = [
'#type' => 'submit',
'#value' => $this->t('Submit'),
];
return $form;
}
/**
* {@inheritdoc}
*/
public function getFormId() {
return 'form_api_example_ajax_addmore';
}
/**
* Callback for both ajax-enabled buttons.
*
* Selects and returns the fieldset with the names in it.
*/
public function addmoreCallback(array &$form, FormStateInterface $form_state) {
return $form['names_fieldset'];
}
/**
* Submit handler for the "add-one-more" button.
*
* Increments the max counter and causes a rebuild.
*/
public function addOne(array &$form, FormStateInterface $form_state) {
$name_field = $form_state->get('num_names');
$add_button = $name_field + 1;
$form_state->set('num_names', $add_button);
// Since our buildForm() method relies on the value of 'num_names' to
// generate 'name' form elements, we have to tell the form to rebuild. If we
// don't do this, the form builder will not call buildForm().
$form_state->setRebuild();
}
/**
* Submit handler for the "remove one" button.
*
* Decrements the max counter and causes a form rebuild.
*/
public function removeCallback(array &$form, FormStateInterface $form_state) {
$name_field = $form_state->get('num_names');
if ($name_field > 1) {
$remove_button = $name_field - 1;
$form_state->set('num_names', $remove_button);
}
// Since our buildForm() method relies on the value of 'num_names' to
// generate 'name' form elements, we have to tell the form to rebuild. If we
// don't do this, the form builder will not call buildForm().
$form_state->setRebuild();
}
/**
* Final submit handler.
*
* Reports what values were finally set.
*/
public function submitForm(array &$form, FormStateInterface $form_state) {
$values = $form_state->getValue(['names_fieldset', 'name']);
$output = $this->t('These people are coming to the picnic: @names', [
'@names' => implode(', ', $values),
]
);
$this->messenger()->addMessage($output);
}
}
