[gambit-list] Gambit and event-based systems

Estevo euccastro at gmail.com
Thu Aug 7 12:37:43 EDT 2014

> Okay, it wasn't that easy. I *do* need to have a Gambit thread running a
> loop in the background as well. How can I keep that loop running?

What do you need to do in this thread exactly?

I imagine that loop will keep running as long as you are in the Scheme
world, that is, handling any type of event.  Unfortunately, in an
event-based system your app will by default spend much of the time in the C
world, waiting for things to happen.  A way to work around that is
suggested below.

> When you said "install a timer in the host system and keep calling" you
> mean that I need to break up that loop into a callback which is called with
> a repeating timer? (I think this is not a good solution).

If you really prefer to implement your background task as a loop in a
gambit thread, you can communicate between the 'main' (event-system
controlled) and 'worker' gambit threads via gambit thread mailboxes.  So
your timer handler just sends a message to the worker thread and waits for
a message in response.  So the worker loop blocks waiting for a message at
the beginning of each iteration and sends a message back at the end.  The
content of these messages can be bogus and ignored both ways.  What this
achieves is setting apart some time to give your logic a chance to run.

Also, many event-based systems have an 'on idle' event type such that if
you register to listen for it, your handler will get triggered whenever the
application is doing nothing.  So if you don't really need your background
logic to happen in regularly timed ticks, using the idle handler instead
might make better use of processing time left over from event handling and

As a step further in this direction, you can have *all* your event handlers
works like this, just delegating the real work to the "worker" gambit
thread through mailboxes, and then you can make your code feel pretty much
as though you controlled the main event loop.  In this case, the messages
sent to mailboxes would contain information about the events.  So the
worker thread can switch over event type, just as in plain SDL.  Note you
still need a timer or idle handler to have stuff happen outside of user
input and redraw events.

In this case your "worker" thread looks and feels pretty much like a "main"
thread.  The event handlers themselves only contain stubs.

>  And then, where would you create and start that thread?

I don't know the specifics of your platform.  In some, starting the event
loop is done explicitly, so you can create the thread before you do that.
In others, there is an 'init' event type.  Where would you, for example,
initialize OpenGL state that will not change throughout your application?
I bet somewhere around there you might be able to launch that thread too.

As a last resort, you can just do it the first time that any event handler
gets called.
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