Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!van-bc!ubc-cs!uw-beaver!cornell!rochester!kodak!ispd-newsserver!ism.isc.com!ico!auto-trol!marbru
From: marbru@auto-trol.com (Martin Brunecky)
Newsgroups: comp.windows.x
Subject: Re: BUG: XtSetValues assumes all non-widgets are RectObjs
Message-ID: <1991Apr19.153005.22266@auto-trol.com>
Date: 19 Apr 91 15:30:05 GMT
References: <1991Apr17.073327.12784@melba.bby.oz.au>
Reply-To: marbru@auto-trol.UUCP (Martin Brunecky)
Organization: Auto-trol Technology, Denver
Lines: 60

In article <1991Apr17.073327.12784@melba.bby.oz.au> gnb@bby.oz.au (Gregory N. Bond) writes:
>[This has been sent to xbugs]
>
>	The XtSetValues handling of redraws is flawed for Objects that
>	are not widgets and not subclasses of RectObj.  When a widget
>	set_values routine returns True, XtSetValues forces a redraw
>	by doing an XClearArea on the widget window.  If the object is
>	not a widget, XtSetValues finds the nearest widget ancestor
>	and, treating the object as a RectObj, clears the portion of
>	the ancestor window that is "covered" by the RectObj.  If the
>	object is not a RectObj, this will be wrong and a random part
>	of the window will be cleared.  If the data wrongly read as
>	the rectangle coords is sufficiently perverse, a BadValue
>	error may be generated (I think, I've not actually seen one).
>
>	It doesn't seem to be actually causing any problems for me,
        ....

     I had more than my share of troubles with SIMILAR bug in Motif 1.0
     "flavor" of XtIntrinsics. We ended-up padding all our "objects"
     with "RectObj" fields, initialized appropriately. ick, ick, ick.

     However, this WAS Motif R3++ code, and it WAS really bad. I looked
     at the R4 code several times, and it does NOT seem to have this
     problem (at least not THAT bad).
     UNLESS there is a "buggy" widget. The MIT code assumes that "Objects"
     will be "reasonable" and set_values will return redisplay = FALSE.
     It also assumes that constraint widgets will be "reasonable", and
     for "Object" children their constraint_set_values will return FALSE.
     So, if the objects in question are not buggy - no problem, as far
     as I can tell.

     However, the world is not a perfect place. Though I would like all
     the widgets to be perfect, I doubt it will ever happen. AS in this
     case XtIntrinsics can compensate for "buggy" widgets, I'd recomend
     to change the code as suggested.

>	It is not clear what the correct action is in this case.  One
>	approach is to generate a synthetic expose event on the window
>	with an empty region.  
     
     Why ? If there is a window/rectangle, XClearArea will do the job.
     
     There, however, is another, related problem. SetValues descends the
     class hierarchy top-down. If my widget's superclass said "yes, I
     need to redisplay", my widget has no way to change that - even though
     because of changed semantics my class in fact may not need to redisplay
     at all.
     So what I need is a tri-state return from SetValues:
        TRUE  = this class does require redisplay
        FALSE = this class does not require redisplay
        UNDO  = even if superclass required redisplay, it is not needed



-- 
=*= Opinions presented here are solely of my own and not those of Auto-trol =*=
Martin Brunecky                           {...}sunpeaks!auto-trol!marbru
(303) 252-2499                        (sometimes also:  marbru@auto-trol.COM )
Auto-trol Technology Corp. 12500 North Washington St., Denver, CO 80241-2404