original_string: public function header(Closure $callback = null)
      {
          if (\is_null($callback)) {
              return $this->header;
          }
  
          $this->header = $callback;
  
          return null;
      }
code: public function header(Closure $callback = null)
      {
          if (\is_null($callback)) {
              return $this->header;
          }
  
          $this->header = $callback;
  
          return null;
      }
code_tokens: public function header ( Closure $ callback = null ) { if ( \ is_null ( $ callback ) ) { return $ this -> header ; } $ this -> header = $ callback ; return null ; }
docstring: Add or append grid header attributes.
  
  @param  \Closure|null  $callback
  
  @return \Closure|array|null
docstring_tokens: Add or append grid header attributes .