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();
        $config = $this->config('book.settings');
        $form['book_allowed_types'] = [
            '#type' => 'checkboxes',
            '#title' => $this->t('Content types allowed in book outlines'),
            '#default_value' => $config->get('allowed_types'),
            '#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'),
            '#default_value' => $config->get('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);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $allowed_types = array_filter($form_state->getValue('book_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);
        $this->config('book.settings')
            ->set('allowed_types', $allowed_types)
            ->set('child_type', $form_state->getValue('book_child_type'))
            ->save();
        parent::submitForm($form, $form_state);
    }

}