class ForumSettingsForm

class ForumSettingsForm extends ConfigFormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'forum_admin_settings';
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getEditableConfigNames() {
        return [
            'forum.settings',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $config = $this->config('forum.settings');
        $options = [
            5,
            10,
            15,
            20,
            25,
            30,
            35,
            40,
            50,
            60,
            80,
            100,
            150,
            200,
            250,
            300,
            350,
            400,
            500,
        ];
        $form['forum_hot_topic'] = [
            '#type' => 'select',
            '#title' => $this->t('Hot topic threshold'),
            '#default_value' => $config->get('topics.hot_threshold'),
            '#options' => array_combine($options, $options),
            '#description' => $this->t('The number of replies a topic must have to be considered "hot".'),
        ];
        $options = [
            10,
            25,
            50,
            75,
            100,
        ];
        $form['forum_per_page'] = [
            '#type' => 'select',
            '#title' => $this->t('Topics per page'),
            '#default_value' => $config->get('topics.page_limit'),
            '#options' => array_combine($options, $options),
            '#description' => $this->t('Default number of forum topics displayed per page.'),
        ];
        $order = [
            1 => $this->t('Date - newest first'),
            2 => $this->t('Date - oldest first'),
            3 => $this->t('Posts - most active first'),
            4 => $this->t('Posts - least active first'),
        ];
        $form['forum_order'] = [
            '#type' => 'radios',
            '#title' => $this->t('Default order'),
            '#default_value' => $config->get('topics.order'),
            '#options' => $order,
            '#description' => $this->t('Default display order for topics.'),
        ];
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->config('forum.settings')
            ->set('topics.hot_threshold', $form_state->getValue('forum_hot_topic'))
            ->set('topics.page_limit', $form_state->getValue('forum_per_page'))
            ->set('topics.order', $form_state->getValue('forum_order'))
            ->save();
        parent::submitForm($form, $form_state);
    }

}