Magically named cancel functions in Views UI forms have been deprecated

Previously forms that call \Drupal\views_ui\ViewsUI::getStandardButtons() could have a custom cancel submit handler automatically applied if it was named $form_id . '_cancel'. If this function did not exist the cancel button would use ViewUI::standardCancel() instead.

These magically named cancel functions have been deprecated. Specify a submit handler in a class method instead.

Before:

<?php

function my_form_id_cancel($form, FormStateInterface $form_state) {
  // Custom handler.
}

class MyForm extends ViewsFormBase {

  public function buildForm(array $form, FormStateInterface $form_state) {
    $view->getStandardButtons($form, $form_state, 'my_form_id');
  }

}
?>

After:

<?php
class MyForm extends ViewsFormBase {

  public function buildForm(array $form, FormStateInterface $form_state) {
    $view->getStandardButtons($form, $form_state, 'my_form_id');
    $form['actions']['cancel']['#submit'] = [[$this, 'cancelSubmit']],
  }

  public function cancelSubmit($form, FormStateInterface $form_state) {
    // Custom handler.
  }

}
?>