silverstripe-widgets/code/controller/WidgetContentControllerExtension.php

<?php
/**
 * Add this to ContentController to enable widgets
 *
 * @package widgets
 */
class WidgetContentControllerExtension extends Extension
{
	/**
	 *
	 * @var array
	 */
	private static $allowed_actions = array(
		'handleWidget'
	);

	/**
	 * Handles widgets attached to a page through one or more {@link WidgetArea}
	 * elements.
	 *
	 * Iterated through each $has_one relation with a {@link WidgetArea} and
	 * looks for connected widgets by their database identifier.
	 *
	 * Assumes URLs in the following format: <URLSegment>/widget/<Widget-ID>.
	 *
	 * @return RequestHandler
	 */
	public function handleWidget()
		{$SQL_id = $this->owner->getRequest()->param('ID');
		if(!$SQL_id) {return false;
		}
		/** @var SiteTree $widgetOwner */
		$widgetOwner = $this->owner->data();
		while($widgetOwner->InheritSideBar && $widgetOwner->Parent()->exists()){
			$widgetOwner = $widgetOwner->Parent();
		}

		// find WidgetArea relations
		$widgetAreaRelations = array();
		$hasOnes = $widgetOwner->hasOne();

		if(!$hasOnes) {
			return false;
		}

        foreach ($hasOnes as $hasOneName => $hasOneClass) {
            if ($hasOneClass == 'WidgetArea' || is_subclass_of($hasOneClass, 'WidgetArea')) {
                $widgetAreaRelations[] = $hasOneName;
            }
        }

        // find widget
        $widget = null;

        foreach ($widgetAreaRelations as $widgetAreaRelation) {
            if ($widget) {
                break;
            }

			$widget = $widgetOwner->$widgetAreaRelation()->Widgets()
				->filter('ID', $SQL_id)
			->First();
		}

        if (!$widget) {
            user_error('No widget found', E_USER_ERROR);
        }

        return $widget->getController();
    }
}