public function header(Closure $callback = null)
    {
        if (\is_null($callback)) {
            return $this->header;
        }

        $this->header = $callback;

        return null;
    }
public function header(Closure $callback = null)
    {
        if (\is_null($callback)) {
            return $this->header;
        }

        $this->header = $callback;

        return null;
    }
public function header ( Closure $ callback = null ) { if ( \ is_null ( $ callback ) ) { return $ this -> header ; } $ this -> header = $ callback ; return null ; }
Add or append grid header attributes.

@param  \Closure|null  $callback

@return \Closure|array|null
Add or append grid header attributes .