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From: grogers@uiucdcsm.cs.uiuc.edu
Newsgroups: comp.windows.x
Subject: Event Questions
Message-ID: <26900019@uiucdcsm>
Date: Tue, 10-Nov-87 16:47:00 EST
Article-I.D.: uiucdcsm.26900019
Posted: Tue Nov 10 16:47:00 1987
Date-Received: Fri, 13-Nov-87 22:05:35 EST
Lines: 53
Nf-ID: #N:uiucdcsm:26900019:000:1958
Nf-From: uiucdcsm.cs.uiuc.edu!grogers    Nov 10 15:47:00 1987


I am writing an application that cannot follow the usually X11 model of polling
for events, e.g.
	
	for(;;) {
		while (XPending(dpy)) {
			XNextEvent( dpy, &xevent);
			switch( xevent.type) {
			...
			 process events
			...
			}
		}
		do non-event processing.
	}

Yet, I still need to know when some events happen (e.g. expose).  So, what I
am currently doing is using SIGIO on the dpy->fd.  This all seems to work ok,
but I have a few questions:

- Is there a better solution than using SIGIO?


(Here's the really important one...)
- Does the server send ALL events to the client and let the Xlib filter out the
	events not requested by the application or does the mask get applied
	at the server level?  Really I'm asking if it costs me anything to
	enable events and then ignore them (especially expensive if the event
	causes a SIGIO interrupt and then is ignored).


- I'm I supposed to be able to change the event mask after a ButtonPress event
	but before a ButtonReleased event?  For example, after a ButtonPress
	I want to drag something across the screen by watching the MotionNotify
	events until a ButtonReleased event.  My problem is this, after the
	ButtonPress event, if I change the event mask via
	XGetWindowAttributes, and XSelectInput or XChangeWindowAttributes I
	do not get any MotionNotify events.  [I've verified that the new mask
	is set correctly by a second call to XGetWindowAttributes.]  However,
	if I accept MotionNotify events before the ButtonPress event comes, 
	everything works as expected (i.e. I get the events.) 

	Anyway, Since I'm using ButtonMotionMask it doesn't really matter
	if this mask is enabled before a ButtonPress event because MotionEvents
	will only come when the button is down and that's just what I want.

Greg Rogers
University of Illinois at Urbana-Champaign
Department of Computer Science

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