Allow to implement sorting in a Magento frontend Block easily.

SortableTrait.php

<?php

trait Foo_Catalog_Block_List_SortableTrait
{
    /**
     * @var string
     */
    protected $_orderVarName = 'order';

    /**
     * @var string
     */
    protected $_directionVarName = 'dir';

    /**
     * @var string[]
     */
    protected $_availableOrder = [];

    /**
     * @var string|null
     */
    protected $_orderField = null;

    /**
     * @var string
     */
    protected $_direction = 'asc';

    /**
     * @var bool
     */
    protected $_paramsMemorizeAllowed = true;

    /**
     * @return Mage_Catalog_Model_Session
     */
    abstract protected function getSessionModel();

    /**
     * @return string
     */
    abstract protected function getDefaultOrderField();

    /**
     * @return mixed
     */
    abstract protected function getAttributeUsedForSortByArray();

    /**
     * @param string $key
     * @return array
     */
    abstract protected function _getData($key);

    /**
     * @return Mage_Core_Controller_Request_Http
     */
    abstract public function getRequest();

    /**
     * @param string $key
     * @param mixed $value
     * @return $this
     */
    abstract public function setData($key, $value);

    /**
     * @param string $route
     * @param array $params
     * @return mixed
     */
    abstract public function getUrl($route = '', $params = []); // signature of \Mage_Core_Block_Abstract::getUrl()

    /**
     *
     */
    protected function _construct()
    {
        $this->_orderField  = $this->getDefaultOrderField();
        $this->_availableOrder = $this->getAttributeUsedForSortByArray();
        parent::_construct();
    }

    /**
     * @return $this
     */
    public function disableParamsMemorizing()
    {
        $this->_paramsMemorizeAllowed = false;
        return $this;
    }

    /**
     * @param string $param
     * @param string $value
     * @return $this
     */
    protected function _memorizeParam($param, $value)
    {
        $session = $this->getSessionModel();
        if ($this->_paramsMemorizeAllowed && !$session->getParamsMemorizeDisabled()) {
            $session->setData($param, $value);
        }
        return $this;
    }

    /**
     * @param $collection
     * @return $this
     */
    public function setCollection(Mage_Catalog_Model_Resource_Product_Collection $collection)
    {
        if ($this->getCurrentOrder()) {
            $collection->setOrder($this->getCurrentOrder(), $this->getCurrentDirection());
        }
        parent::setCollection($collection);
        return $this;
    }

    /**
     * @return string
     */
    public function getOrderVarName()
    {
        return $this->_orderVarName;
    }

    /**
     * @return string
     */
    public function getDirectionVarName()
    {
        return $this->_directionVarName;
    }

    /**
     * @return null
     */
    public function getCurrentOrder()
    {
        $order = $this->_getData('_current_grid_order');
        if ($order) {
            return $order;
        }

        $orders = $this->getAvailableOrders();
        $defaultOrder = $this->_orderField;
        if (!isset($orders[$defaultOrder])) {
            $keys = array_keys($orders);
            $defaultOrder = $keys[0];
        }

        $order = $this->getRequest()->getParam($this->getOrderVarName());
        if ($order && isset($orders[$order])) {
            if ($order == $defaultOrder) {
                $this->getSessionModel()->unsSortOrder();
            } else {
                $this->_memorizeParam('sort_order', $order);
            }
        } else {
            $order = $this->getSessionModel()->getSortOrder();
        }
        // validate session value
        if (!$order || !isset($orders[$order])) {
            $order = $defaultOrder;
        }
        $this->setData('_current_grid_order', $order);
        return $order;
    }

    /**
     * @return string
     */
    public function getCurrentDirection()
    {
        $dir = $this->_getData('_current_grid_direction');
        if ($dir) {
            return $dir;
        }

        $directions = array('asc', 'desc');
        $dir = strtolower($this->getRequest()->getParam($this->getDirectionVarName()));
        if ($dir && in_array($dir, $directions)) {
            if ($dir == $this->_direction) {
                $this->getSessionModel()->unsSortDirection();
            } else {
                $this->_memorizeParam('sort_direction', $dir);
            }
        } else {
            $dir = $this->getSessionModel()->getSortDirection();
        }
        // validate direction
        if (!$dir || !in_array($dir, $directions)) {
            $dir = $this->_direction;
        }
        $this->setData('_current_grid_direction', $dir);
        return $dir;
    }

    /**
     * @param $field
     * @return $this
     */
    public function setDefaultOrder($field)
    {
        if (isset($this->_availableOrder[$field])) {
            $this->_orderField = $field;
        }
        return $this;
    }

    /**
     * @param $dir
     * @return $this
     */
    public function setDefaultDirection($dir)
    {
        if (in_array(strtolower($dir), ['asc', 'desc'])) {
            $this->_direction = strtolower($dir);
        }
        return $this;
    }

    /**
     * @return array
     */
    public function getAvailableOrders()
    {
        return $this->_availableOrder;
    }

    /**
     * @param $orders
     * @return $this
     */
    public function setAvailableOrders($orders)
    {
        $this->_availableOrder = $orders;
        return $this;
    }

    /**
     * @param $order
     * @param $value
     * @return $this
     */
    public function addOrderToAvailableOrders($order, $value)
    {
        $this->_availableOrder[$order] = $value;
        return $this;
    }

    /**
     * @param $order
     * @return $this
     */
    public function removeOrderFromAvailableOrders($order)
    {
        if (isset($this->_availableOrder[$order])) {
            unset($this->_availableOrder[$order]);
        }
        return $this;
    }

    /**
     * @param $order
     * @return bool
     */
    public function isOrderCurrent($order)
    {
        return ($order === $this->getCurrentOrder());
    }

    /**
     * @param $order
     * @return mixed
     */
    public function getSortUrl($order)
    {
        $sortDirection = $this->_direction;
        if ($this->isOrderCurrent($order)) {
            $sortDirection = $this->oppositeDirectionOf($this->getCurrentDirection());
        }
        return $this->getOrderUrl($order, $sortDirection);
    }

    /**
     * @param $order
     * @return string
     */
    private function oppositeDirectionOf($order)
    {
        return ($order === 'asc') ? 'desc' : 'asc';
    }

    /**
     * @param $order
     * @param $direction
     * @return mixed
     */
    public function getOrderUrl($order, $direction)
    {
        if ($order === null) {
            $order = $this->getCurrentOrder() ? $this->getCurrentOrder() : $this->_availableOrder[0];
        }
        return $this->getPagerUrl(array(
            $this->getOrderVarName()=>$order,
            $this->getDirectionVarName()=>$direction,
        ));
    }

    /**
     * @param array $params
     * @return mixed
     */
    public function getPagerUrl($params = [])
    {
        $urlParams = [];
        $urlParams['_current']  = true;
        $urlParams['_escape']   = true;
        $urlParams['_use_rewrite']   = true;
        $urlParams['_query']    = $params;
        return $this->getUrl('*/*/*', $urlParams);
    }
}