Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!mailrus!tut.cis.ohio-state.edu!bloom-beacon!apple!dwb
From: dwb@Apple.COM (David W. Berry)
Newsgroups: comp.sys.mac
Subject: Re: multitasking and IPC (was: System 8.0: no more DA's.)
Message-ID: <23343@apple.Apple.COM>
Date: 4 Jan 89 23:49:48 GMT
References: <1988Dec16.191309.21623@cs.rochester.edu> <326@internal.Apple.COM> <807@esl.UUCP> <747@lts.UUCP> <34550@think.UUCP>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 33

In article <34550@think.UUCP> barmar@kulla.think.com (Barry Margolin) writes:
>I have one question about this whole issue:

	Perhaps the whole situation can be explained by laying out two
separate but connect entities.  There are tasks and layers.  A task is
much like a UNIX process, they can even be run in the background.  A
layer on the other hand is a collection of windows belonging to a single
task.  Note that not all tasks own a layer, applications which have
the "background only" bit of the SIZE resource don't get layers created
and thus can't have windows.

	By extension then, there are task switches and there are layer
switches.  Task switches change the active task and layer switches change
the active layer.

	Both task and layer switches happen during GetNextEvent.
	
	Layer switches are a direct result of user action, a mouse
click somewhere.  Task switches are a result of user inaction, no
user events have occurred so let's see if anybody wants to do anything
in the background.

	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.


Opinions:  MINE, ALL MINE! (greedy evil chuckle)

David W. Berry
apple!dwb@sun.com	dwb@apple.com	973-5168@408.MaBell