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From: beard@ux5.lbl.gov (Patrick C Beard)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Problem with GetNextEvent trap patch in MultiFinder
Message-ID: <9093@dog.ee.lbl.gov>
Date: 17 Jan 91 03:39:07 GMT
References: <1991Jan15.200340.28431@bronze.ucs.indiana.edu>
Reply-To: beard@ux5.lbl.gov (Patrick C Beard)
Organization: Berkeley Systems, Inc.
Lines: 43
X-Local-Date: Wed, 16 Jan 91 19:39:07 PST

In article <1991Jan15.200340.28431@bronze.ucs.indiana.edu> cmcclary@bronze.ucs.indiana.edu (Charles McClary) writes:
#I am trying to patch the GetNextEvent trap A970.  My test patch 
#seems to work correctly in Finder, but not in MultiFinder.  In 
#MultiFinder, when a desktop icon (disk icon) is covered up by a 
#volume or folder window and the window is then moved to uncover 
#the icon, the portion of the desktop icon which was covered remains 
#invisible.  This does not happen in finder.  Below is the source code 
#for the patch.  I am using Think C 4.0.  What an I leaving out?

You, sir, are tail-patching GetNextEvent, which is most certainly a
no-no.  (I know, I've attempted it before, and had EXACTLY this behaviour.)

#pascal Boolean NewGetNextEvent(eventMask, theEvent)
#inteventMask;
#EventRecord* theEvent;
#{
#SetUpA4();
#
#CallPascalB(eventMask, theEvent, oldGetNextEvent);

essentially:

	asm {
		move.l oldGetNextEvent, a0
		move.w eventMask,-(sp)
		pea theEvent
		jsr (a0)
	}

#
#RestoreA4();
#}

The only way to patch GetNextEvent safely, that I've found, is to 
use a head patch, which basically means, do your patch behaviour, then
jump directly to the old address.  This part obviously has to be done
in assembly language.  Exercise for the reader.

--
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-  Patrick Beard, Macintosh Programmer                        (beard@lbl.gov) -
-  Berkeley Systems, Inc.  ".......<dead air>.......Good day!" - Paul Harvey  -
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