framework/base/StaticInstanceTrait.php
public static function instance($refresh = false)
    {
        $className = get_called_class();
        if ($refresh || !isset(self::$_instances[$className])) {
            self::$_instances[$className] = Yii::createObject($className);
        }
        return self::$_instances[$className];
    }
public static function instance($refresh = false)
    {
        $className = get_called_class();
        if ($refresh || !isset(self::$_instances[$className])) {
            self::$_instances[$className] = Yii::createObject($className);
        }
        return self::$_instances[$className];
    }
Returns static class instance, which can be used to obtain meta information.
@param bool $refresh whether to re-create static instance even, if it is already cached.
@return static class instance.