class DateFormatFormBase

abstract class DateFormatFormBase extends EntityForm {
    
    /**
     * The date formatter service.
     *
     * @var \Drupal\Core\Datetime\DateFormatterInterface
     */
    protected $dateFormatter;
    
    /**
     * The date format storage.
     *
     * @var \Drupal\Core\Config\Entity\ConfigEntityStorageInterface
     */
    protected $dateFormatStorage;
    
    /**
     * Constructs a new date format form.
     *
     * @param \Drupal\Core\Datetime\DateFormatterInterface $date_formatter
     *   The date service.
     * @param \Drupal\Core\Config\Entity\ConfigEntityStorageInterface $date_format_storage
     *   The date format storage.
     */
    public function __construct(DateFormatterInterface $date_formatter, ConfigEntityStorageInterface $date_format_storage) {
        $this->dateFormatter = $date_formatter;
        $this->dateFormatStorage = $date_format_storage;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('date.formatter'), $container->get('entity_type.manager')
            ->getStorage('date_format'));
    }
    
    /**
     * Checks for an existing date format.
     *
     * @param string|int $entity_id
     *   The entity ID.
     * @param array $element
     *   The form element.
     *
     * @return bool
     *   TRUE if this format already exists, FALSE otherwise.
     */
    public function exists($entity_id, array $element) {
        return (bool) $this->dateFormatStorage
            ->getQuery()
            ->condition('id', $element['#field_prefix'] . $entity_id)
            ->execute();
    }
    
    /**
     * {@inheritdoc}
     */
    public function form(array $form, FormStateInterface $form_state) {
        $form['label'] = [
            '#type' => 'textfield',
            '#title' => 'Name',
            '#maxlength' => 100,
            '#description' => $this->t('Name of the date format'),
            '#default_value' => $this->entity
                ->label(),
        ];
        $form['id'] = [
            '#type' => 'machine_name',
            '#description' => $this->t('A unique machine-readable name. Can only contain lowercase letters, numbers, and underscores.'),
            '#disabled' => !$this->entity
                ->isNew(),
            '#default_value' => $this->entity
                ->id(),
            '#machine_name' => [
                'exists' => [
                    $this,
                    'exists',
                ],
                'replace_pattern' => '([^a-z0-9_]+)|(^custom$)',
                'error' => $this->t('The machine-readable name must be unique, and can only contain lowercase letters, numbers, and underscores. Additionally, it can not be the reserved word "custom".'),
            ],
        ];
        $form['date_format_pattern'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Format string'),
            '#maxlength' => 100,
            '#description' => $this->t('A user-defined date format. See the <a href="https://www.php.net/manual/datetime.format.php#refsect1-datetime.format-parameters">PHP manual</a> for available options.'),
            '#required' => TRUE,
            '#attributes' => [
                'data-drupal-date-formatter' => 'source',
            ],
            '#field_suffix' => ' <small class="js-hide" data-drupal-date-formatter="preview">' . $this->t('Displayed as %date_format', [
                '%date_format' => '',
            ]) . '</small>',
        ];
        $form['langcode'] = [
            '#type' => 'language_select',
            '#title' => $this->t('Language'),
            '#languages' => LanguageInterface::STATE_ALL,
            '#default_value' => $this->entity
                ->language()
                ->getId(),
        ];
        $form['#attached']['drupalSettings']['dateFormats'] = $this->dateFormatter
            ->getSampleDateFormats();
        $form['#attached']['library'][] = 'system/drupal.system.date';
        return parent::form($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        parent::validateForm($form, $form_state);
        // The machine name field should already check to see if the requested
        // machine name is available.
        $pattern = trim($form_state->getValue('date_format_pattern'));
        foreach ($this->dateFormatStorage
            ->loadMultiple() as $format) {
            if ($format->getPattern() == $pattern && $format->id() == $this->entity
                ->id()) {
                $this->messenger()
                    ->addStatus($this->t('The existing format/name combination has not been altered.'));
                continue;
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $form_state->setValue('pattern', trim($form_state->getValue('date_format_pattern')));
        parent::submitForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function save(array $form, FormStateInterface $form_state) {
        $status = $this->entity
            ->save();
        if ($status == SAVED_UPDATED) {
            $this->messenger()
                ->addStatus($this->t('Custom date format updated.'));
        }
        else {
            $this->messenger()
                ->addStatus($this->t('Custom date format added.'));
        }
        $form_state->setRedirectUrl($this->entity
            ->toUrl('collection'));
    }

}