class SequenceDataDefinition

class SequenceDataDefinition extends ListDataDefinition {
    
    /**
     * Gets the description of how the sequence should be sorted.
     *
     * Only the top level of the array should be sorted. Top-level keys should be
     * discarded when using 'value' sorting. If the sequence is an associative
     * array 'key' sorting is recommended, if not 'value' sorting is recommended.
     *
     * @return string|null
     *   May be 'key' (to sort by key), 'value' (to sort by value, discarding
     *   keys), or NULL (if the schema does not describe how the sequence should
     *   be sorted).
     */
    public function getOrderBy() {
        return isset($this->definition['orderby']) ? $this->definition['orderby'] : NULL;
    }

}