Re: Callbacks from modules

[Eli Zaretskii <eliz@gnu.org>]

> From: Ivan Andrus <darthandrus@gmail.com>
> Date: Wed, 25 Nov 2015 23:08:48 -0700
> 
> I’ve started writing an Emacs module to access NSSpeechSynthesizer on OS X (and 
> maybe GNUStep though I haven’t tested it there).  I’ve been using the mod-test 
> module as an example and I have it working reasonably well except that I don’t 
> have any idea how to run a callback.
> 
> When the synthesizer is done speaking it sends an Objective-C message 
> speechSynthesizer:didFinishSpeaking.  From there I would like to run an elisp 
> hook, say `ns-speech-finished-speaking-hook`.  How can I do this?  Do I have to 
> squirrel away a pointer to an emacs_env somehow?  I tried naively storing env 
> from a previous call but, not surprisingly, the pointer is invalid when I try 
> to use it.

How would you do that in Emacs's core code?  A module is just a
(limited) extension of the Emacs core, so when you ask such questions,
you should first think how Emacs core does that.

Anyway, I assume you have a C callback in your module that is
triggered by the speechSynthesizer:didFinishSpeaking message, is that
right?  Then one way would be to have that callback set a flag,
provide a Lisp-callable function that returns the flag, and then start
a timer that will test the flag and call your Lisp callback when the
flag is set.

Another, perhaps simpler, possibility would be to have the module
provide a Lisp-callable function that will register a Lisp callback.
Then your C callback will simply call that Lisp function.

Would any of these do the job?