Re: Callbacks from modules

[Ivan Andrus <darthandrus@gmail.com>]

On Nov 26, 2015, at 8:43 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> 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.

I did a version of this (in core) some time ago by adding a new event type.  I wasn’t ever happy with it and for various reasons I think it makes more sense as a module.

> Anyway, I assume you have a C callback in your module that is
> triggered by the speechSynthesizer:didFinishSpeaking message, is that
> right?  

Yes.

> 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.

I was hoping to avoid polling, since the use case I have would like to start reading again as soon as the previous version finishes.

> 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.

I don’t understand how I would implement this option, though it sounds like what I was hoping for.  In order to call a lisp function, don’t I need an emacs_env?

> Would any of these do the job?

I hope so.  :-)  I’ll probably try the timer option.

Thanks,
Ivan