Allow to implement sorting in a Magento frontend Block easily.

SortableTrait.php

<?php
/**
 * Code extracted/inspired from the \Mage_Catalog_Block_Product_List_Toolbar class
 * for being more easily reused
 */
trait My_Namespace_Block_List_SortableTrait
{

    protected $_orderVarName        = 'order';

    protected $_directionVarName    = 'dir';

    protected $_availableOrder      = array();

    protected $_orderField          = null;

    protected $_direction           = 'asc';

    protected $_paramsMemorizeAllowed = true;

    abstract protected function getSessionModel();
    abstract protected function getDefaultOrderField();
    abstract protected function getAttributeUsedForSortByArray();
    abstract public function getUrl($route = '', $params = array()); // signature of \Mage_Core_Block_Abstract::getUrl()

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

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

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

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

    public function getOrderVarName()
    {
        return $this->_orderVarName;
    }

    public function getDirectionVarName()
    {
        return $this->_directionVarName;
    }

    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;
    }

    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;
    }

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

    public function setDefaultDirection($dir)
    {
        if (in_array(strtolower($dir), array('asc', 'desc'))) {
            $this->_direction = strtolower($dir);
        }
        return $this;
    }

    public function getAvailableOrders()
    {
        return $this->_availableOrder;
    }

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

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

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

    public function isOrderCurrent($order)
    {
        return ($order === $this->getCurrentOrder());
    }

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

    private function oppositeDirectionOf($order)
    {
        return ($order === 'asc') ? 'desc' : 'asc';
    }

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

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

}