Sprout Form + Solspace Calendar

SfscPlugin.php

<? namespace Craft;

class SfscPlugin extends BasePlugin
{
  public function getName()
  {
    return Craft::t('Event Attendance');
  }

  public function getVersion()
  {
    return '1.0';
  }

  public function getDeveloper()
  {
    return 'oof. Studio';
  }

  public function getDeveloperUrl()
  {
    return 'http://oof.studio/';
  }

  public function hasCpSection()
  {
    return false;
  }

  # Hook to events, etc. emitted by other plugins
  public function init()
  {
    craft()->on('sproutForms.onBeforeSaveEntry', function (Event $e) {
      // Store the submission data temporarily:
      $submission = $e->params['entry'];

      // Fetch your Calendar Event by getting its ID off the $event:
      $event = craft()->calendar_events->getEventById(craft()->request->getParam('yoda'));

      // Get current quantity and subtract the specified `quantity`.
      $currentQty = $event->getContent()->stockQty;
      $newQty = $currentQty - $submission->quantity;

      // Update the stock, and save.
      $event->getContent()->setAttribute('stockQty', $newQty);
      craft()->calendar_events->saveEvent($event);
    });
  }
}