Howto:Start worker threads using listeners in Nasal

The following snippet of code demonstrates how to run a Nasal function (named "hello") in a dedicated background thread (i.e. asynchronously) once a property listener is fired.

Once invoked, the hello function keeps calling itself asynchronously using a settimer() call at an interval of 5 seconds. After 60 seconds, it is getting terminated by the second listener that registered the "kill_thread" handler to be invoked by settimer after 60 seconds, via another Nasal thread.

# async_demo.nas
var cancel_thread=0;
var l = nil;
var r = nil;

var hello = func {
  print("Hello world from the background thread\n");
  if(!cancel_thread) settimer(hello,5,1);
}
var spawn = func(c) return func {thread.newthread(c)};
var setlistener_job = func(p,f) setlistener(p, spawn(f) );

var kill_thread = func {
  removelistener(l);
  print("Killing background thread");
  cancel_thread=1;
  removelistener(r);
}

_setlistener("/sim/signals/nasal-dir-initialized", func {
 l=setlistener_job("/nasal-threading/test", hello);
 r=setlistener_job("/nasal-threading/test", func { settimer(kill_thread, 60, 1); });
 print("async_demo.nas loaded");
});

To run the demo, copy/paste the code to a file in $FG_ROOT/Nasal, start up fgfs and modify the property "/nasal-threading/test" using the built-in property browser.