GNU Emacs Lisp Reference Manual

29.22: Change Hooks

These hook variables let you arrange to take notice of all changes in all buffers (or in a particular buffer, if you make them buffer-local). See also Special Properties, for how to detect changes to specific parts of the text.

The functions you use in these hooks should save and restore the match data if they do anything that uses regular expressions; otherwise, they will interfere in bizarre ways with the editing operations that call them.

Variable: before-change-functions

This variable holds a list of a functions to call before any buffer modification. Each function gets two arguments, the beginning and end of the region that is about to change, represented as integers. The buffer that is about to change is always the current buffer.

Variable: after-change-functions

This variable holds a list of a functions to call after any buffer modification. Each function receives three arguments: the beginning and end of the region just changed, and the length of the text that existed before the change. (To get the current length, subtract the region beginning from the region end.) All three arguments are integers. The buffer that's about to change is always the current buffer.

Variable: before-change-function

This obsolete variable holds one function to call before any buffer modification (or nil for no function). It is called just like the functions in before-change-functions.

Variable: after-change-function

This obsolete variable holds one function to call after any buffer modification (or nil for no function). It is called just like the functions in after-change-functions.

The four variables above are temporarily bound to nil during the time that any of these functions is running. This means that if one of these functions changes the buffer, that change won't run these functions. If you do want a hook function to make changes that run these functions, make it bind these variables back to their usual values.

One inconvenient result of this protective feature is that you cannot have a function in after-change-functions or before-change-functions which changes the value of that variable. But that's not a real limitation. If you want those functions to change the list of functions to run, simply add one fixed function to the hook, and code that function to look in another variable for other functions to call. Here is an example:

(setq my-own-after-change-functions nil)
(defun indirect-after-change-function (beg end len)
  (let ((list my-own-after-change-functions))
    (while list
      (funcall (car list) beg end len)
      (setq list (cdr list)))))
(add-hooks 'after-change-functions
           'indirect-after-change-function)

Variable: first-change-hook

This variable is a normal hook that is run whenever a buffer is changed that was previously in the unmodified state.