_config.php

<?php

SiteTree::add_extension('ViewableData','ExtsViewableDataDecorator');

ExtsViewableDataDecorator.php

<?php

class ExtsViewableDataDecorator extends Extension {

	/**
	* Adds first/last/start/end class to a list item.
	*
	* Sometimes you need to:
	* Lay out a load of thumbnails in a grid.
	* Style only the first or last elements in a vertical/horizontal list of items
	*
	* You can do this using nth-child pseudoselectors, but it is dangerous.
	*
	* Instead, on any list of elements:
	*
	* - add a _first_ class to the very first one
	* - add a _last_ class to the very last one
	* - add a _start_ class to the first item on a row (e.g. every 4th item, starting with the first one)
	* - add an _end_ class to the last item on a row (e.g. every 4th item, starting with the fourth one)
	*
	* @param $rowLength length of each row
	* @return string list class
	*/
	public function ListClass( $rowLength=2 ) {

		$classes = array();

		// first
		if ($this->owner->iteratorPos == 0) {
			$classes[] = 'first';
		}

		// last
		if($this->owner->iteratorPos == $this->owner->iteratorTotalItems - 1) {
			$classes[] = 'last';
		}

		// start
		if ($this->owner->iteratorPos % $rowLength == 0) {
			$classes[] = 'start';
		}

		// end
		if ($this->owner->iteratorPos % $rowLength == $rowLength - 1) {
			$classes[] = 'end';
		}

		$html = join(' ', $classes);
		return $html;
	}

}