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Some things I'd like to show in the mode-line are quite expensive to compute, so I'd rather use an idle timer.

Is there a general way to do this? or does this need to be added individually? (using a method similar to which-function-mode, for example).

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  • Here is what I did a few years ago as a workaround and have been happily using it ever since: Use a pre-command-hook and a post-command-hook to record (for purposes of comparison) four (4) values each command loop: this-command; last-command; the currently (selected-window); and, the previously (selected-window). Set up a set of rules regarding which commands have a high rate of repetition; e.g., undo / redo; left-char / right-char; scroll-up / scroll-down; etc... [To be continued in the following comment hereinbelow.] – lawlist Nov 15 '20 at 16:27
  • Continuation: If this-command is eq to last-command and the current command is a memq of the list of the aforementioned high rate of repetition commands, then use a pre-recorded value for the expensive command. When the aforementioned situation is not happening, anything that is expensive gets calculated and stored to a variable using format-mode-line to convert it to a string, which can contain colors as well as other properties for mouse-over help, etc. When the aforementioned condition exists, the stored values are used for the expensive components of the mode-line. – lawlist Nov 15 '20 at 16:37
  • Continuation: Here is the function I use for the post-command-hook: (defun ml-post-command-hook-fn () (setq ml-selected-window--post-command-hook (selected-window))) Here is the function I use for the pre-command-hook: (defun ml-pre-command-hook-fn () (setq ml-selected-window--pre-command-hook (selected-window)) (setq ml-this-command this-command) (setq ml-last-command last-command)). These values can also be used to differentiate the mode-line in the selected-window versus a non-selected window, so the colors and so forth for the components can be different; e.g., dim / bold. – lawlist Nov 15 '20 at 16:43
  • Continuation: If changing colors in the mode-line for the mode-line face and the mode-line-inactive face and the minibuffer-prompt face to better distinguish between a selected window versus a non-selected window -- in conjunction with the method described in the previous comments -- be sure to use the face-remapping-alist instead of expensive functions such as set-face-attribute and the like. Use methods such as setq-default and setq-local ... The manual suggests using face-remap-add-relative instead of directly controlling the face-remapping-alist, but I am adventurous. – lawlist Nov 15 '20 at 16:53
  • Continuation: Be aware that this-command and last-command have a short shelf-life, which is why it must be recorded and that recorded value is used when the mode-line is being computed / updated; i.e., when comparing ml-this-command to ml-last-command. When the mode-line is computed / updated the (selected-window) also changes, which why it is necessary to use the recorded values instead for comparison purposes; i.e., ml-selected-window--post-command-hook versus ml-selected-window--pre-command-hook (both of which are custom global variables). – lawlist Nov 15 '20 at 17:09
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This can be done using idle timers, although this can get a little involved if you have multiple idle timers and want to prevent them setting each other off.

I've wrapped this up into a small package: mode-line-idle, a small dedicated package which does just this, without any configuration or minor-modes.

Calls to the main function can be done in :eval blocks in the mode-line-format variable.

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