class BookSettingsForm

class BookSettingsForm extends ConfigFormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'book_admin_settings';
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getEditableConfigNames() {
        return [
            'book.settings',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $types = node_type_get_names();
        $form['book_allowed_types'] = [
            '#type' => 'checkboxes',
            '#title' => $this->t('Content types allowed in book outlines'),
            '#config_target' => new ConfigTarget('book.settings', 'allowed_types', toConfig: static::class . '::filterAndSortAllowedTypes'),
            '#options' => $types,
            '#description' => $this->t('Users with the %outline-perm permission can add all content types.', [
                '%outline-perm' => $this->t('Administer book outlines'),
            ]),
            '#required' => TRUE,
        ];
        $form['book_child_type'] = [
            '#type' => 'radios',
            '#title' => $this->t('Content type for the <em>Add child page</em> link'),
            '#config_target' => 'book.settings:child_type',
            '#options' => $types,
            '#required' => TRUE,
        ];
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        $child_type = $form_state->getValue('book_child_type');
        if ($form_state->isValueEmpty([
            'book_allowed_types',
            $child_type,
        ])) {
            $form_state->setErrorByName('book_child_type', $this->t('The content type for the %add-child link must be one of those selected as an allowed book outline type.', [
                '%add-child' => $this->t('Add child page'),
            ]));
        }
        parent::validateForm($form, $form_state);
    }
    
    /**
     * Transformation callback for the book_allowed_types config value.
     *
     * @param array $allowed_types
     *   The config value to transform.
     *
     * @return array
     *   The transformed value.
     */
    public static function filterAndSortAllowedTypes(array $allowed_types) : array {
        $allowed_types = array_filter($allowed_types);
        // We need to save the allowed types in an array ordered by machine_name so
        // that we can save them in the correct order if node type changes.
        // @see book_node_type_update().
        sort($allowed_types);
        return $allowed_types;
    }

}