class ConfigEditor

class ConfigEditor extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'devel_config_system_edit_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, $config_name = '') {
        $config = $this->config($config_name);
        if ($config === FALSE || $config->isNew()) {
            drupal_set_message(t('Config @name does not exist in the system.', array(
                '@name' => $config_name,
            )), 'error');
            return;
        }
        $data = $config->getOriginal();
        if (empty($data)) {
            drupal_set_message(t('Config @name exists but has no data.', array(
                '@name' => $config_name,
            )), 'warning');
            return;
        }
        try {
            $output = Yaml::encode($data);
        } catch (InvalidDataTypeException $e) {
            drupal_set_message(t('Invalid data detected for @name : %error', array(
                '@name' => $config_name,
                '%error' => $e->getMessage(),
            )), 'error');
            return;
        }
        $form['current'] = array(
            '#type' => 'details',
            '#title' => $this->t('Current value for %variable', array(
                '%variable' => $config_name,
            )),
            '#attributes' => array(
                'class' => array(
                    'container-inline',
                ),
            ),
        );
        $form['current']['value'] = array(
            '#type' => 'item',
            '#markup' => dpr($output, TRUE),
        );
        $form['name'] = array(
            '#type' => 'value',
            '#value' => $config_name,
        );
        $form['new'] = array(
            '#type' => 'textarea',
            '#title' => $this->t('New value'),
            '#default_value' => $output,
            '#rows' => 24,
            '#required' => TRUE,
        );
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Save'),
        ];
        $form['actions']['cancel'] = array(
            '#type' => 'link',
            '#title' => $this->t('Cancel'),
            '#url' => $this->buildCancelLinkUrl(),
        );
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        $value = $form_state->getValue('new');
        // try to parse the new provided value
        try {
            $parsed_value = Yaml::decode($value);
            // Config::setData needs array for the new configuration and
            // a simple string is valid YAML for any reason.
            if (is_array($parsed_value)) {
                $form_state->setValue('parsed_value', $parsed_value);
            }
            else {
                $form_state->setErrorByName('new', $this->t('Invalid input'));
            }
        } catch (InvalidDataTypeException $e) {
            $form_state->setErrorByName('new', $this->t('Invalid input: %error', array(
                '%error' => $e->getMessage(),
            )));
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $values = $form_state->getValues();
        try {
            $this->configFactory()
                ->getEditable($values['name'])
                ->setData($values['parsed_value'])
                ->save();
            drupal_set_message($this->t('Configuration variable %variable was successfully saved.', array(
                '%variable' => $values['name'],
            )));
            $this->logger('devel')
                ->info('Configuration variable %variable was successfully saved.', array(
                '%variable' => $values['name'],
            ));
            $form_state->setRedirectUrl(Url::fromRoute('devel.configs_list'));
        } catch (\Exception $e) {
            drupal_set_message($e->getMessage(), 'error');
            $this->logger('devel')
                ->error('Error saving configuration variable %variable : %error.', array(
                '%variable' => $values['name'],
                '%error' => $e->getMessage(),
            ));
        }
    }
    
    /**
     * Builds the cancel link url for the form.
     *
     * @return Url
     *   Cancel url
     */
    private function buildCancelLinkUrl() {
        $query = $this->getRequest()->query;
        if ($query->has('destination')) {
            $options = UrlHelper::parse($query->get('destination'));
            $url = Url::fromUri('internal:/' . $options['path'], $options);
        }
        else {
            $url = Url::fromRoute('devel.configs_list');
        }
        return $url;
    }

}