GUI applications written in XPCE can benefit from Prolog threads if
they need to do expensive computations that would otherwise block the
UI. The XPCE message passing system is guarded with a single mutex,
which synchronises both access from Prolog and activation through the
GUI. In MS-Windows, GUI events are processed by the thread that created
the window in which the event occurred, whereas in Unix/X11 they are
processed by the thread that dispatches messages. In practice, the most
feasible approach to graphical Prolog implementations is to control XPCE
from a single thread and deploy other threads for (long) computations.
Traditionally, XPCE runs in the foreground (main)
thread. We are working towards a situation where XPCE can run
comfortably in a separate thread. A separate XPCE thread can be created
using pce_dispatch/1.
It is also possible to create this thread as the library(()pce)
is loaded by setting the xpce_threaded to true.
Threads other than the thread in which XPCE runs are provided with
two predicates to communicate with XPCE.
- [det]in_pce_thread(:Goal)
- Assuming XPCE is running in the foreground thread, this call gives
background threads the opportunity to make calls to the XPCE thread. A
call to in_pce_thread/1
succeeds immediately, copying Goal to the XPCE thread. Goal
is added to the XPCE event queue and executed synchronous to normal user
events like typing and clicking.
- [semidet]in_pce_thread_sync(:Goal)
- Same as in_pce_thread/1,
but wait for Goal to be completed. Success depends on the
success of executing Goal. Variable bindings inside Goal
are visible to the caller, but it should be noted that the values are
being copied. If Goal throws an exception, this
exception is re-thrown by in_pce_thread/1.
If the calling thread is the‘pce thread’, in_pce_thread_sync/1
executes a direct meta-call. See also in_pce_thread/1.
Note that in_pce_thread_sync/1
is expensive because it requires copying and thread communication. For
example, in_pce_thread_synctrue runs at approximately
50,000 calls per second (AMD Phenom 9600B, Ubuntu 11.04).
- pce_dispatch(+Options)
- Create a Prolog thread with the alias name
pce for XPCE
event handling. In the X11 version this call creates a thread that
executes the X11 event-dispatch loop. In MS-Windows it creates a thread
that executes a windows event-dispatch loop. The XPCE event-handling
thread has the alias pce. Options specifies the
thread attributes as thread_create/3.
