Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!atheybey@PTT.LCS.MIT.EDU
From: atheybey@PTT.LCS.MIT.EDU
Newsgroups: comp.sys.amiga.tech
Subject: Re: Double click detection?
Message-ID: <8901301541.AA05371@PTT.LCS.MIT.EDU>
Date: 30 Jan 89 15:41:53 GMT
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: atheybey@PTT.LCS.MIT.EDU
Lines: 57
To: nntp-poster@PTT.LCS.MIT.EDU
Repository: PTT
Originating-Client: flower

shimoda@infohh.rmi.de (Markus Schmidt) writes:
%Four doubleclicks I use this logic:
%
%while (1) {
%
%      imsg= GetMsg ( ...
%      if (imsg->Class=  /* endevent */) 
%	 break;
%
%
%      if (imsg->Class==SELECTDOWN)
%      {
%	 static Seconds= 0, 
%	       Micros= 0;
%
%	 if ((Seconds!=0 || Micros!=0) &&
%	    DoubleClick(Seconds,Micros,
%		     imsg->Seconds, imsg->Micros)) {
%
%		  /* Your Stuff ... */
%
%		  Seconds= Micros= 0;
%	 }
%	 else {
%	    Seconds= imsg->Seconds;   Micros= imsg->Micros;
%	 }
%      }
%
%}
%
%Please notice that ReplyMsg is not handled. I just hacked that down
%online. 
%
%.-----------------------------------------------------------------.
%|  shimoda@rmi.de       `---'   The last remaining adventure      |
%|  shimoda@gopnbg.UUCP  |O o|   since there are no more tigers:   |

Please correct me if I am wrong, but doesn't this code fragment
contain two Bad Things?  No Guru has yet responded, so I thought I
would take a shot at sticking my foot in my mouth.

It busy-waits.  There is no call to Wait(), or WaitPort() before the
GetMsg().  Even  if you are busy-waiting only while looking for double
clicks, you shouldn't.

GetMsg() will return NULL if no message has yet arrived.  With the
various 'if (imsg-> ... )' tests, you are then indirecting through
address 0, which while not necessarily fatal, is sloppy and will quite
likely break on any system with memory protection (unless it's a VAX :-).
Even if you do call Wait(), you should not depend on GetMsg()
returning something non-NULL.

---------------------------------------------------------------------------
Andrew Heybey, atheybey@ptt.lcs.mit.edu     Room 509, 545 Technology Square
Advanced Network Architecture Group   	 	       Cambridge, MA  02139
MIT Laboratory for Computer Science    			     (617) 253-6011
---------------------------------------------------------------------------