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->has_one();

		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(
				sprintf('"Widget"."ID" = %d', $SQL_id)
			)->First();
		}

		if(!$widget) {
			user_error('No widget found', E_USER_ERROR);
		}
		
		return $widget->getController();
	}
}