32.25 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 32.19.4 Properties with Special Meanings, 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 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 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. All three arguments are integers. The buffer that's about to change is always the current buffer.

The length of the old text is the difference between the buffer positions before and after that text as it was before the change. As for the changed text, its length is simply the difference between the first two arguments.

Macro: combine-after-change-calls body...

The macro executes body normally, but arranges to call the after-change functions just once for a series of several changes--if that seems safe.

If a program makes several text changes in the same area of the buffer, using the macro combine-after-change-calls around that part of the program can make it run considerably faster when after-change hooks are in use. When the after-change hooks are ultimately called, the arguments specify a portion of the buffer including all of the changes made within the combine-after-change-calls body.

Warning: You must not alter the values of after-change-functions within the body of a combine-after-change-calls form.

Note: If the changes you combine occur in widely scattered parts of the buffer, this will still work, but it is not advisable, because it may lead to inefficient behavior for some change hook functions.

The two 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.

Variable: inhibit-modification-hooks

If this variable is non-nil, all of the change hooks are disabled; none of them run. This affects all the hook variables described above in this section, as well as the hooks attached to certain special text properties (see section 32.19.4 Properties with Special Meanings) and overlay properties (see section 38.9.1 Overlay Properties).

This variable is available starting in Emacs 21.