Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!bloom-beacon!mit-vax!josh
From: josh@mit-vax.LCS.MIT.EDU (Joshua Marantz)
Newsgroups: comp.windows.x
Subject: Re: Detecting/Compressing Multiple Exposure Events - how ?
Message-ID: <4652@mit-vax.LCS.MIT.EDU>
Date: 31 Aug 88 01:30:19 GMT
References: <126@tityus.UUCP>
Organization: MIT LCS, Cambridge, MA
Lines: 34
In-reply-to: jim@athsys.uucp's message of 30 Aug 88 17:54:08 GMT


This article brings up a good point.  The existing Xlib procedures for
manipulating the event queue need to be augmented.  Another way to
solve the redundant exposure problem would be to provide two Pending
routines that do not block, and do not remove matched events from the
queue.  These would complement the functionality of XCheckIfEvent and
XCheckMaskEvent.  They might be called:

	XIfPending (display, event_return, predicate, arg);
	XMaskPending (display, event_return, event_mask, arg);

Currently, I have to emulate that functionality with an XCheckIfEvent followed
by an XPutBackEvent, which has the annoying side effect of rearranging the
event queue.

The only solution I can think of that does not require a change to Xlib is
to create your own event queue and move all of X's events into it whenever
there are X events pending.  This permits you to define your own methods of
access to the event queue.

Another advantage of defining your own event queue is that you can efficiently
filter out uninteresting events, check for user interrupts (like Control-C), 
and so on.  This is more efficient than using the Xlib event manipulation
functions (even with the addition of XIfPending and XMaskPending) because
each event will be examined once by a filter that may flag it as either very
interesting (such as an interrupt character), or very uninteresting (a
redundant expose or pointer-motion event).  If you leave the events in X's
event queue, you must rescan the entire event queue each time you want to
check for an interrupt.

Note that the auxiliary event queue can utilize X's event structures, which
simplifies the implementation and elliminates extraneous data copying.

-Joshua Marnatz
Viewlogic Systems, Inc.