public function on($columnNames)
    {
        if (!is_array($columnNames)) {
            $columnNames = [(string)$columnNames];
        }

        call_user_func($this->callback, $columnNames);
    }
public function on($columnNames)
    {
        if (!is_array($columnNames)) {
            $columnNames = [(string)$columnNames];
        }

        call_user_func($this->callback, $columnNames);
    }
public function on ( $ columnNames ) { if ( ! is_array ( $ columnNames ) ) { $ columnNames = [ ( string ) $ columnNames ] ; } call_user_func ( $ this -> callback , $ columnNames ) ; }
Defines the index on the supplied column names.

@param string|string[] $columnNames

@return void
Defines the index on the supplied column names .