gtk:file-filter-add-custom (filter needed func)

filter -- a gtk:file-filter object
needed -- a gtk:file-filter-flags value for the flags indicating the information that the custom filter function needs
func -- a gtk:file-filter-func callback function, if the function returns true, then the file will be displayed

Adds rule to a filter that allows files based on a custom callback function. The bitfield needed which is passed in provides information about what sorts of information that the filter function needs. This allows GTK to avoid retrieving expensive information when it is not needed by the filter.

(defun custom-file-filter (filter-info)
  ;; Select files with upcase characters in the display name
  (let ((display-name (gtk:file-filter-info-display-name filter-info)))
    (string= display-name
             (string-upcase display-name))))
...
(let ((filter-custom (gtk:file-filter-new)))
  ;; Add a custom file filter
  (setf (gtk:file-filter-name filter-custom) "Custom Filter")
  (gtk:file-filter-add-custom filter-custom
                              :display-name
                              #'custom-file-filter)
  (gtk:file-chooser-add-filter chooser filter-custom)
  ... )