<?php
/**
 * CExpressionDependency class file.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @link http://www.yiiframework.com/
 * @copyright 2008-2013 Yii Software LLC
 * @license http://www.yiiframework.com/license/
 */

/**
 * CExpressionDependency represents a dependency based on the result of a PHP expression.
 *
 * CExpressionDependency performs dependency checking based on the
 * result of a PHP {@link expression}.
 * The dependency is reported as unchanged if and only if the result is
 * the same as the one evaluated when storing the data to cache.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @package system.caching.dependencies
 * @since 1.0
 */
class CExpressionDependency extends CCacheDependency
{
	/**
	 * @var string the PHP expression whose result is used to determine the dependency.
	 * The expression can also be a valid serializable PHP callback.
	 * It will be passed with a parameter which is the dependency object itself.
	 *
	 * The PHP expression will be evaluated using {@link evaluateExpression}.
	 *
	 * A PHP expression can be any PHP code that has a value. To learn more about what an expression is,
	 * please refer to the {@link http://www.php.net/manual/en/language.expressions.php php manual}.
	 */
	public $expression;

	/**
	 * Constructor.
	 * @param string $expression the PHP expression whose result is used to determine the dependency.
	 */
	public function __construct($expression='true')
	{
		$this->expression=$expression;
	}

	/**
	 * Generates the data needed to determine if dependency has been changed.
	 * This method returns the result of the PHP expression.
	 * @return mixed the data needed to determine if dependency has been changed.
	 */
	protected function generateDependentData()
	{
		return $this->evaluateExpression($this->expression);
	}
}