Path: utzoo!attcan!uunet!lll-winken!ames!ncar!boulder!sunybcs!rutgers!bellcore!faline!thumper!gamma!pyuxp!pyuxe!pyuxf!asg
From: asg@pyuxf.UUCP (alan geller)
Newsgroups: comp.sys.mac
Subject: Re: multitasking and IPC (was: System 8.0: no more DA's.)
Message-ID: <482@pyuxf.UUCP>
Date: 10 Jan 89 23:23:48 GMT
References: <1988Dec16.191309.21623@cs.rochester.edu> <326@internal.Apple.COM> <23343@apple.Apple.COM>
Organization: Bell Communications Research
Lines: 43

In article <23343@apple.Apple.COM>, dwb@Apple.COM (David W. Berry) writes:
> 
> ... first, an excellent explanation of 'task switches' versus
>     'layer switches', and how they're related to GetNextEvent. ...
>
> 	Task switches may occur while a modal dialog is displayed.
> Layer switches can't.  Thus, background tasks, including inactive
> layers, do get execution time while a modal dialog is displayed,
> they just can't be made the current front layer.
> 
> David W. Berry
> apple!dwb@sun.com	dwb@apple.com	973-5168@408.MaBell


Thanks for the explanation -- it was very clear.

My question, though, is:
	WHY can't layer switches happen when a modal dialog is displayed?
	Or, alternatively, would it be possible to create a type of dialog
		that would be 'layer modal' rather than 'system modal'; 
		that is, if such a dialog were displayed, then no other
		window/dialog/menu/control/etc. in the owning layer and
		task would be available to the user, but the user could
		switch layers (to a different task) if so desired?

	In other words, could modal dialogs just lock their owning task,
	rather than prohibit layer switches?  It doesn't seem to me that
	this should be difficult; in some sense, it should be easier, as
	then the modal dialog manager doesn't have to be multi-finder
	aware (actually, I guess it isn't, it just hogs the mouse and
	keyboard; at least, I hope that's how it works).

	Of course, in some circumstances, you might want to prohibit layer
	switches (serious system errors, for example); in that case, why
	not have 'Layer Modal' and 'System Modal' dialogs?

	Or am I missing something?  Likely I am; while I've used the Mac
	a good deal, I'm a novice at MacOS programming, so if I've missed
	something major and obvious, please, just explanations, no flames!

Alan Geller
Bellcore
...!{princeton|rutgers}!bcr!asg