Change Emacs window appearance when it loses focus

When I program I use two screens with Emacs on both with two buffers split in each window totaling 4 open source files on screen at any one time.

I switch between buffers with C-x b and between Windows with Alt-TAB. I change the appearance of buffers when I switch between them by defining different faces for mode-line and mode-line-inactive. But how do I inactivate a buffer when I switch from the Emacs window completely to another Emacs window via Alt-TAB?

It's probably also relevant that I'm using Emacs 23.2.1 on Ubuntu 1开发者_运维问答1.04 with Gnome 2.32.1.

PS: The question How to automatically save files on lose focus in Emacs is after a different goal but with the same original event of "window losing focus".

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It may depend on your window manager and how it manages multiple windows, or frames, in emacs parlance. The code below works like a champ in fvwm but not always in gnome.

I map a keystroke, C-o, to go between frames, this helps when you want to go to the other frame but an alt-tab would take you through a number of superfluous apps on the way.

If you're running a single instance of emacs with two frames you could use something like the following:

(defun pgr-previous-frame ()
  "go to the previous frame"
  (interactive)
  (pgr-switch-frame (previous-frame)))

(defun pgr-next-frame ()
  "go to the next frame"
  (interactive)
  (pgr-switch-frame (next-frame)))

(defun pgr-switch-frame (frame)
  "go to the specified frame and raise it"
  ;; reset the frame properties here
  (select-frame frame)     
  (raise-frame frame)
  ;;change the display in some manner here
  )

You could also try adding some advice to raise-frame and lower-frame haven't tried it but it's worth a try.

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I really liked @logoscia answer, which allowed me to do this more generic version. It uses the mode-line-inactive face when no focus.

(add-hook 'focus-out-hook
      (lambda ()
        (copy-face 'mode-line '--mode-line-backup)
        (copy-face 'mode-line-inactive 'mode-line)))
(add-hook 'focus-in-hook
      (lambda ()
        (copy-face '--mode-line-backup 'mode-line)))

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In Emacs 24.4 and later, you can use focus-in-hook and focus-out-hook. This piece of code seems to work, such that the active window of an inactive frame has the same colour as an inactive window:

(defvar my-mode-line-active-background "gray75")
(defvar my-mode-line-inactive-background "gray40")

(defun my-unhighlight-mode-line ()
  (set-face-attribute 'mode-line nil
                      :background my-mode-line-inactive-background))

(add-hook 'focus-out-hook 'my-unhighlight-mode-line)

(defun my-highlight-mode-line ()
  (set-face-attribute 'mode-line nil
                      :background my-mode-line-active-background))

(add-hook 'focus-in-hook 'my-highlight-mode-line)

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I don't know if it can be done only with Emacs, but a possible alternative is running wmctrl in a shell script which perodically checks which window has the focus and if there is a change then it lets Emacs know via emacsclient which can send lisp code to a running Emacs for evaluation:

-e'--eval' Tell Emacs to evaluate some Emacs Lisp code, instead of visiting some files. When this option is given, the arguments to `emacsclient' are interpreted as a list of expressions to evaluate, not as a list of files to visit.