class StatisticsSettingsForm

class StatisticsSettingsForm extends ConfigFormBase {
    
    /**
     * The module handler.
     *
     * @var \Drupal\Core\Extension\ModuleHandlerInterface
     */
    protected $moduleHandler;
    
    /**
     * Constructs a \Drupal\statistics\StatisticsSettingsForm object.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The factory for configuration objects.
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler.
     */
    public function __construct(ConfigFactoryInterface $config_factory, ModuleHandlerInterface $module_handler) {
        parent::__construct($config_factory);
        $this->moduleHandler = $module_handler;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('config.factory'), $container->get('module_handler'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'statistics_settings_form';
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getEditableConfigNames() {
        return [
            'statistics.settings',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $config = $this->config('statistics.settings');
        // Content counter settings.
        $form['content'] = [
            '#type' => 'details',
            '#title' => t('Content viewing counter settings'),
            '#open' => TRUE,
        ];
        $form['content']['statistics_count_content_views'] = [
            '#type' => 'checkbox',
            '#title' => t('Count content views'),
            '#default_value' => $config->get('count_content_views'),
            '#description' => t('Increment a counter each time content is viewed.'),
        ];
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->config('statistics.settings')
            ->set('count_content_views', $form_state->getValue('statistics_count_content_views'))
            ->save();
        // The popular statistics block is dependent on these settings, so clear the
        // block plugin definitions cache.
        if ($this->moduleHandler
            ->moduleExists('block')) {
            \Drupal::service('plugin.manager.block')->clearCachedDefinitions();
        }
        parent::submitForm($form, $form_state);
    }

}