Extents are available XEmacs and forks from it. They act simlar to text properties. Unlike text properties, which are attached only to characters in buffers or strings, extents can also be attached to windows and frames.

Both GnuEmacs overlays and text properties correspond to extents in XEmacs. If you are using overlays in your code, you can immediately port to XEmacs with the ‘fsf-compat’ package available by putting (require 'overlay) at the start of your file, though the XEmacs developers encourage you to write an implementation in terms of extents.

Example Code

Here is some example code to write your own compatiblity layer. In newer XEmacsen, it might be possible to use overlays. This will work for Emacs, and XEmacs has a compatibility layer (see above) that translates overlays into extents. I know it is weird, and it may also not work for all properties, since they often have different names and different semantics.

See also http://proofgeneral.inf.ed.ac.uk/components for Emacs/XEmacs compatibility using a common abstraction of extents and overlays, dubbed spans.

Example from ansi-color.el

(See AnsiColor for more.)

(defun ansi-color-make-extent (from to &optional object)
  "Make an extent for the range [FROM, TO) in OBJECT.

OBJECT defaults to the current buffer.  XEmacs uses `make-extent', Emacs
uses `make-overlay'.  XEmacs can use a buffer or a string for OBJECT,
Emacs requires OBJECT to be a buffer."
  (if (functionp 'make-extent)
      (make-extent from to object)
    ;; In Emacs, the overlay might end at the process-mark in comint
    ;; buffers.  In that case, new text will be inserted before the
    ;; process-mark, ie. inside the overlay (using insert-before-marks).
    ;; In order to avoid this, we use the `insert-behind-hooks' overlay
    ;; property to make sure it works.
    (let ((overlay (make-overlay from to object)))
      (overlay-put overlay 'modification-hooks '(ansi-color-freeze-overlay))

(defun ansi-color-freeze-overlay (overlay is-after begin end &optional len)
  "Prevent OVERLAY from being extended.
This function can be used for the `modification-hooks' overlay
  ;; if stuff was inserted at the end of the overlay
  (when (and is-after
	     (= 0 len)
	     (= end (overlay-end overlay)))
    ;; reset the end of the overlay
    (move-overlay overlay (overlay-start overlay) begin)))

Example from wiki.el

(See WikiMode for more.)

(defun wiki-make-extent (from to map with-glyph)
  "Make an extent for the range [FROM, TO) in the current buffer.
MAP is the local keymap to use, if any.
WITH-GLYPH non-nil will add a question-mark after the extent.
XEmacs uses `make-extent', Emacs uses `make-overlay'."
  ;; I don't use (fboundp 'make-extent) because of (require 'lucid)
  (if (featurep 'xemacs)
      ;; Extents for XEmacs
      (let ((extent (make-extent from to)))
	(set-extent-property extent 'face 'info-xref)
	(set-extent-property extent 'mouse-face 'highlight)
	(when map
	  (set-extent-property extent 'keymap map))
	(set-extent-property extent 'evaporate t)
	(set-extent-property extent 'wikiname t)
	(when with-glyph
	  (set-extent-property extent 'end-glyph (make-glyph '("?"))))
    ;; Overlays for Emacs
    (let ((overlay (make-overlay from to)))
      (overlay-put overlay 'face 'info-xref)
      (overlay-put overlay 'mouse-face 'highlight)
      (when map
	(overlay-put overlay 'local-map map))
      (overlay-put overlay 'evaporate t)
      (overlay-put overlay 'wikiname t)
      (when with-glyph
	(overlay-put overlay 'after-string "?"))

(defun wiki-delete-extents (&optional start end)
  "Delete all extents/overlays created by `wiki-make-extent'.
If optional arguments START and END are given, only the overlays in that
region will be deleted.  XEmacs uses extents, Emacs uses overlays."
  (if (featurep 'xemacs)
      (let ((extents (extent-list nil start end))
	(while extents
	  (setq extent (car extents)
		extents (cdr extents))
	  (when (extent-property extent 'wikiname)
	    (delete-extent extent))))
    (let ((overlays (overlays-in (or start (point-min))
				 (or end (point-max))))
      (while overlays
	(setq overlay (car overlays)
	      overlays (cdr overlays))
	(when (overlay-get overlay 'wikiname)
	  (delete-overlay overlay))))))

CategoryCode ComparativeEmacsology