Implementing my own incremental search

Question

What would be the best way to implement an Emacs Lisp command that pops up the minibuffer, lets the user edit a search string there, and runs code to update the search results after each keystroke (or other command) that changes the minibuffer contents? To be specific, I'm asking about how to run a Lisp function in response to text changes; running a Lisp function in response to each keystroke may be close enough.

I'm talking about something that resembles the well-known incremental search (I-search) commands built into Emacs. isearch.el shows that I-search itself sets up isearch-mode-map such that each ASCII/multibyte character is manually mapped onto the isearch-printing-char command. (It loops over all the numerical character codepoints and assigns the same command to each codepoint).

If possible, it would probably be easier to have normal minibuffer editing, and simply have a hook function that gets called after every keystroke (or other command) that changes the minibuffer contents.

Answer 1

The minibuffer is a normal buffer, so you can use post-command-hook or after-change-functions (or both) to react to edits. You can use minibuffer-with-setup-hook to set them up:

(minibuffer-with-setup-hook
    (lambda ()
      (add-hook 'post-command-hook #'my-minibuf-after-cmd nil 'local)
      (add-hook 'after-change-functions #'my-minibuf-after-edit nil 'local))
  (read-string ...))

Using both can be useful: it's often a good idea to limit the amount of code run in after-change-functions either for performance reasons (a single command (such as filling a paragraph) can sometimes cause thousands of individual changes) or to avoid undesired interactions (after-change-functions is run right when the change takes place, before the command is finished, so it might run in an unusual context). In constrast post-command-hook is only run once per user interaction and "at top-level", so it is customary to limit after-change-functions to record the change in some global variable, and then use that var in post-command-hook.