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From: glennw@hercules.UUCP
Newsgroups: comp.windows.x
Subject: Re: XUpdateMouse()
Message-ID: <1786@hercules.TEK.COM>
Date: Thu, 26-Feb-87 10:29:12 EST
Article-I.D.: hercules.1786
Posted: Thu Feb 26 10:29:12 1987
Date-Received: Sun, 1-Mar-87 08:15:11 EST
References: <8702231839.AA02342@ingres.Berkeley.EDU>
Organization: Tektronix Inc., Beaverton, Or.
Lines: 116
Summary: the secrets of XUpdateMouse revealed (and improved)

Fcc:x

In article <8702231839.AA02342@ingres.Berkeley.EDU>, dcmartin@INGRES.BERKELEY.EDU (David C. Martin) writes:
> The manual says that XUpdateMouse() "eliminates any mouse moved events at the
> head of the queue..."
> 
> Does this mean that all mouse moved events are flushed from the queue, with
> the exception of the last, or are all mouse moved events flushed?

   XUpdateMouse only flushes the contiguous block of MouseMoved events at
the head of the queue, that is, flushing ceases when the first non-mouse
event is encountered.  

   In the little project I am doing now, I have found this to be less
than optimal.  For one thing, there is no good way to be sure that the
pointer is not in another window overlapping yours, because there is no
way to find out if any mouse events actually occured.  For another, there
is no reason to leave any Mouse events lying around in the queue once the
mouse has been queried.

  Also, I have found it useful to have the one routine missing from the
set, XUpdateMouseButtons.  

   So I have written a new Mouse polling routine, XUpdatePointer
(V11-style name), which adds a return argument to XQueryMouseButtons
giving the number of events removed from the queue, and deletes all
MouseMoved and Button events throughout the queue.  The calling routine
uses the last mouse state if no events are found.  This routine
significantly improves performance when mouse buttons and keys are
intermixed in the queue.  The source is enclosed.

   A correct XUpdateMouse or XUpdateMouseButtons is a simple reduction
from this routine.

   I hit one minor mystery which I coded around, not having time to
figure it out.  Apparently, either GetReq() or _XReply allows new events
to enter the queue, so I had to move the XPending call after these calls
to get an accurate count of the events removed from the queue.

   One annoying problem with getting buttons with QueryButton is that it
is easy to miss a fast button press.  I am thinking about a further hack
on this routine which would return the state of each button event, if any
instead of polling the pointer, guaranteeing accurate capture of the
button presses and additionally reducing the IPC overhead a bit.
-----------
#include <X/mit-copyright.h>

/* $Header: XUpdatePointer.c,v 1.1 87/02/23 06:31:02 glennw Exp $ */
/* Copyright    Massachusetts Institute of Technology    1985	*/

#include "XlibInternal.h"

extern _QEvent *_qfree;

/*  Like XQueryButtons, but also reads pending events and flushes any 
 * MouseMoved, ButtonPressed, or ButtonRelease events anywhere in the queue.  
 * It returns the number of events flushed in the removed argument.  
 * A good way to track the mouse is to use a MouseMoved or Button event
 * as a "hint", by calling this routine to get up to date coordinates.
 */

Status XUpdatePointer (w, x, y, subw, state, removed)
	Window w;
	Window *subw;
	int *x, *y;
	short *state;
	int	*removed;
{
	register Display *dpy;
	register XReq *req;
	register _QEvent *qelt, *qprev;
	_QEvent qdummy;
	XRep rep;
	int quelen;

	GetReq(X_QueryMouse, w);
	if (!_XReply(dpy, &rep))
	    return(0);
	quelen = XPending();
	    /* read all events in socket's kernel buffer, flush output */
		/* note - apparently GetReq or XReply allows more events to get in,
		   so XPending is moved after them to get a correct queue length */
	qprev = &qdummy;		/* temporary stash for next ptr while at head */
	qelt = dpy->head;
	while (qelt) {
		if ((qelt->event.type & (MouseMoved | ButtonPressed | ButtonReleased)) != 0)
		{
			qprev->next = qelt->next;	/* stash next ptr or splice cur out */
		    qelt->next = _qfree;		/* and into free list */
		    _qfree = qelt;
			qelt = qprev->next;			/* move to next item */
		    dpy->qlen--;
		}
		else
		{
			if (qprev == &qdummy)	/* first non-Mouse event; reset head */
			{
				dpy->head = qelt;
			}
			qprev = qelt;
			qelt = qelt->next;		/* skip over non-Mouse events */
		}
	}
	if(qprev == &qdummy)		/* found no non-Mouse event; queue is empty */
		dpy->head = dpy->tail = NULL;
	else
		dpy->tail = qprev;	/* in case we removed events from end of queue */

	*x = rep.param.s[2];
	*y = rep.param.s[3];
	*subw = rep.param.l[0];
	*state = rep.params4;
	*removed = quelen - dpy->qlen;
	return (1);
}

-- 
Glenn Widener