Re: efficiently viewing Unix timestamps as dates

[Stefan Monnier]

> mouse to look at the date as a tooltip.  I didn't like how it looked
> with the `display' property: you would hit backspace from a string of 8
> digits to 7, and the display property would remain the previously
> computed date.

You have to explicit remove the `display' property before refreshing
with font-lock.  My sample code did not bother to do it because it was
just a proof of concept.

A naive way to do that is to add `display' to
font-lock-extra-managed-props, tho this will also remove `display'
properties added by other packages for other purposes.
So a better way is to add not just a `display' property but also another
property that is specific to your package, so that you can recognize
which `display' properties are yours, and then in font-lock-keywords you
can add a pseudo-keyword which will look for those display properties
and remove them.

Something like (untested):

 (defun foo-remove-display (limit)
   (let ((beg (point)))
     (while (< beg limit)
       (let ((next (next-single-property-change beg 'display nil end))
             (prop (get-text-property beg 'display)))
         (if (and prop (get-text-property beg 'foo-owned))
             (put-text-property beg next 'display nil))
         (setq beg next))))
   nil)

 (add-hook 'foo-mode-hook
           (lambda ()
             (font-lock-add-keywords
              nil
              '((foo-remove-display)
                ("[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]"
                 (0 `(face foo-owned t nil display
                           ,(format-time-string "%F %T"
                                                (seconds-to-time
                                                 (car
                                                  (read-from-string
                                                   (concat
                                                    (match-string 0)
                                                    ".0"))))))))))))

-- Stefan