CopyFieldValueTestForm.php

<?php

namespace Drupal\system_test\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Provides a form to test Drupal.behaviors.copyFieldValue.
 */
class CopyFieldValueTestForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'copy_field_value_test_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['#attached']['library'][] = 'system/drupal.system';
        $form['#attached']['drupalSettings']['copyFieldValue']['edit-source-field'] = [
            'edit-target-field',
        ];
        $form['source_field'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Source Field'),
            '#default_value' => '',
            '#description' => $this->t('Source input field to provide text value.'),
            '#required' => TRUE,
        ];
        $form['target_field'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Target Field'),
            '#default_value' => '',
            '#description' => $this->t('Target input field to get value from source field.'),
            '#required' => TRUE,
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // We are only testing the JavaScript part of form. We are not submitting
        // form.
    }

}