class SettingsForm

class SettingsForm extends ConfigFormBase {
    
    /**
     * The module handler service.
     *
     * @var \Drupal\Core\Extension\ModuleHandlerInterface
     */
    protected $moduleHandler;
    
    /**
     * Constructs a \Drupal\system\ConfigFormBase object.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The factory for configuration objects.
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler service.
     */
    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 buildForm(array $form, FormStateInterface $form_state) {
        $config = $this->config('subpathauto.settings');
        $form['depth'] = [
            '#type' => 'select',
            '#title' => $this->t('Maximum depth of sub-paths to alias'),
            '#options' => array_merge([
                0 => $this->t('Disabled'),
            ], range(1, MenuTreeStorage::MAX_DEPTH - 1)),
            '#default_value' => $config->get('depth'),
            '#description' => $this->t('Increasing this value may decrease performance.'),
        ];
        $is_redirect_installed = $this->moduleHandler
            ->moduleExists('redirect');
        $form['redirect_support'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Support for redirects'),
            '#default_value' => $is_redirect_installed ? $config->get('redirect_support') : FALSE,
            '#disabled' => !$is_redirect_installed,
            '#description' => $is_redirect_installed ? $this->t('If checked, redirects will be taken into account when resolving sub aliases.') : $this->t('If checked, redirects will be taken into account when resolving sub aliases. The redirect module should be installed for this setting.'),
        ];
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $values = $form_state->getValues();
        $this->config('subpathauto.settings')
            ->set('depth', $values['depth'])
            ->set('redirect_support', $values['redirect_support'])
            ->save();
        parent::submitForm($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'subpathauto_settings_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function getEditableConfigNames() {
        return [
            'subpathauto.settings',
        ];
    }

}