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While configuring powerline, I use my own format for the mode-line. I now want to apply a different face to the battery load string, depending on the battery status (dis/charging). However, when I compute the charging status within my mode-line-format construct, Emacs gets really slow and I can't mark region with shift + arrow anymore.

You can see the mode-line definition in my configuration that I keep on Bitbucket: init-mode-line.el:102.

The idea behind it was to have a boolean describing the battery status, and use it in the manner of active, that I already use.

Here is the output of profile-report:

Profile report

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    Did you try profiling battery-status-function? gnu.org/software/emacs/manual/html_node/elisp/Profiling.html (here's the reference for how to do it, in case you need it). – wvxvw Jun 30 '15 at 14:01
  • This is a very good advice, I did not know that thank you :). Not sure how it can help me though. I have attached a screenshot of the profile-report output. I guess each redraw of the mode-line opens the battery file and read from it, which seems to be resource demanding. – Mathieu Marques Jun 30 '15 at 14:15
  • I don't know what battery-pmset does, but I would try memoisation, i.e. I'd try to store the old value somewhere and only poll the device once every couple of seconds or so. – wvxvw Jun 30 '15 at 14:42
  • Ended up using an advice, idea taken from smart-mode-line.el, but I'm depending on battery-update-interval. – Mathieu Marques Jun 30 '15 at 17:05
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Your code calls battery-status-function for every mode line update. This function inherently slow, because it has to invoke external programs to obtain battery information. Since Emacs is single-threaded, each invocation freezes Emacs until the function returns. As the mode line is updated frequently, by almost every command, and often multiple times per command, these small delays amount to a very noticeable lag in all operations of Emacs, which renders Emacs almost entirely unusable.

I had the very same issue when I tried to customize the battery status appearance in my mode line, and wrote fancy-battery.el to address it. This package queries and caches the battery status independently of mode line updates, and thus reduces the frequency of invocations of battery-status-function, removing the lag entirely. It's written for the standard mode line, but it should work well with powerline, too.

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Shamelessly taken from smart-mode-line.el:1592. The defadvice will colorize the battery load string before every call to battery-update.

(defadvice battery-update (before my/pl-colorize-battery-advice activate)
  "Colorize the battery load string depending on its status (dis/charging)."

  (if (string-equal "AC" (cdr (assoc 76 (funcall battery-status-function))))
      (copy-face 'my/pl-battery-charging-face 'my/pl-battery-face)
    (copy-face 'my/pl-battery-discharging-face 'my/pl-battery-face)))

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